AUFBAU1 [METALS]
[01] INTRODUCTION - REACTIVITY (1)
[02] INTRODUCTION - REACTIVITY (2)
[03] DISPLACEMENT REACTIONS (1)
[04] EXTRACTION of SODIUM
[05] CALCIUM & MAGNESIUM
[06] EXTRACTION of ALUMINUM
[07] EXTRACTION of ZINC
[08] EXTRACTION of IRON
[09] COBALT
[10] TIN & LEAD
[11] EXTRACTION of COPPER
[12] DISPLACEMENT REACTIONS (2)
[13] DISPLACEMENT REACTIONS (3)
[14] PRECIPITATION REACTIONS
[15] LIGAND-EXCHANGE REACTIONS
[16] DISPLACEMENT REACTIONS (4)
[17] SODIUM COMPOUNDS
[18] ALUMINUM & ZINC COMPOUNDS
[19] IRON COMPOUNDS
[20] COPPER COMPOUNDS (1)
[21] COPPER COMPOUNDS (2)
[22] MERCURY
[23] SILVER
[24] STRONTIUM
[25] CADMIUM
[26] MOLYBDENUM
[27] VANADIUM
[28] TITANIUM
[29] NICKEL
[30] MANGANESE
[31] CHROMIUM
[32] PLATINUM
METALS: INTRODUCTION - REACTIVITY (1)
An element's metallic character can be precisely defined in terms of
electrical conductivity, as exemplified by the sub-set shown below:
[Ag > Cu > Al > Ca > Mg > Na > Zn > K > Fe > Sn > Pb > Hg]
However, an element's metallic character is more usually considered in
terms of chemical reactivity; and, as exemplified by the same sub-set,
there is no apparent correlation between conductivity and reactivity:
[K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > Cu > Hg > Ag]
There is just the 'small' matter of defining chemical reactivity ...
Bon gré, mal gré?
The addition of a small quantity of solid potassium to water results in
the molten metal whizzing over the water's surface and the evolved gas
catching fire with a lilac flame. By contrast, a similar experiment with
solid lithium merely results in steady evolution of the gas (dihydrogen).
[Scene. A (kangaroo?) court in the state capital of Laputa]
Prosecutor: M'Lud. These results prove, beyond any reasonable doubt,
that Mr. Potassium is more reactive than the defendant.
Judge: Thank you kindly, Sir. [He addresses the prosecutor.] A
splendid case for the prosecution, if I may so say. Has
the defendant anything to add before I pass sentence?
Mr. Lithium: Yes, my Lord. I do admit that I have a higher activation
energy: but, my reaction with water is more exothermic
than Mr. Potassium's.
Judge: Stop! This Court will not be blinded by science ...
Mr. Lithium: But ...
Judge: Silence! You are in contempt of this Court. [He places
a black handkerchief over his grubby-looking, moth-eaten
wig.] Mr. Lithium, I sentence you, for the rest of your
natural life, to a position below Mr. Potassium in the
Reactivity Series. [Cheers from the public gallery.] |
The explanation for the observations above is as follows. Potassium has
a low melting point, 63°C, and so the heat of reaction is sufficient to
make it melt; the molten metal spreads out to expose a larger surface
area, and so it reacts even faster: as a result, heating in situ causes
the dihydrogen gas to catch fire. By contrast, lithium reacts much more
slowly, because it has a higher melting point, 181°C, and so there are
fewer collisions between the particles; its reaction with water is more
exothermic, but this heat energy is released more slowly. These energy
level diagrams provide an alternative summary of this explanation.
[Scene. The re-trial in the same (kangaroo?) court of Laputa]
Counsel: M'Lud. These energy level diagrams are important new
evidence; they prove, beyond any reasonable doubt, that
my client is more reactive than Mr. Potassium.
Judge: Thank you most kindly, Sir. [He addresses counsel.] I am
obliged to say that you have presented a waterproof case
for the defence. [The legal beavers humor the judge by
smiling weakly.] A clear miscarriage of justice. Has the
defendant anything to add before I overturn the original
verdict?
Mr. Lithium: Indeed I do, my Lord. In contrast to Mr. Potassium, I
react readily with dinitrogen. Nevertheless, with most
other reactants, I am less ...
Judge: Please do stop! Time, or at least my time, is valuable.
[He glances briefly, but wistfully, at his golf clubs.]
This Court will not be blinded by yet more science ...
Mr. Lithium: But ...
Judge: Silence! You are in contempt of this Court. [He places
a white handkerchief over his (now) grubbier-looking,
moth-eaten wig.] Mr. Lithium, I place you in a position
above Mr. Potassium in the Reactivity Series. [Cheers
from the (clearly undiscriminating) public gallery.] |
Indirectly, the reactions of lithium and potassium 'crystallize' two
problems which bedevil chemists, whether they be putative or mature.
First, the almost irresistible tendency to blur the distinction between
the rate and the energy change of a reaction. This blurring appears to
establish strong roots at an early stage in one's scientific career: so
much so, that authors of university and specialist textbooks invariably
feel the need to remind their readers of the distinction. *
And second, perhaps in the desire 'to hammer the subject into shape', a
tendency to assume that a limited number of reactions will necessarily
establish a rule which can be applied willy-nilly (bon gré, mal gré).
Neither of these problems are likely to disappear in the near future:
but their adverse effects can certainly be minimized by bearing in
mind four principles.
First, there is no connection between the activation energy and the
heat energy change (DH) for a chemical reaction.
Second, to ensure consistency, chemical reactivities should be compared
only in terms of energy (DE) or heat energy (DH) changes. #
Third, despite inevitable limitations in its use, a (metal) reactivity
series based on standard redox potentials is inherently self-consistent
because it is derived from measured physical data.
And fourth, a metal (M) reactivity series, based on redox potentials in
aqueous solutions, and a Periodic Table of the Elements, which is just
a method of summarizing the ground-state electronic structures of
gaseous atoms, both provide a suitable focus for correlation: but,
neither should be considered as a substitute for experimental facts.
* See, for example, F. A. Cotton and G. Wilkinson, Advanced Inorganic
Chemistry, Wiley, New York, 1988; P. W. Atkins, Physical Chemistry,
Oxford University Press, Oxford, 1989; and, P. J. Sykes, A Guidebook
to Mechanism in Organic Chemistry, Longman, London, 1986.
# Acceptance of this principle does not preclude comparing the rates
of reactions. [In advanced studies, reactivities are compared in terms
of kinetic and thermodynamic stabilities, which are discussed in terms
of activation energies and free energy changes (DG), respectively.]
METALS: INTRODUCTION - REACTIVITY (2)
A healthily critical attitude to reactivity can be acquired from the
careful consideration of the following example of hypothesis testing.
[Li > K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag]
Quelle horreur!
Hypothesis: 'Within each group of the Periodic Table, the reactivities
of the metallic elements (M) increase with increasing atomic number.'
Data: A. J. Bard et al., Standard Potentials in Aqueous Solutions,
Marcel Dekker, New York, 1985.
Results: Standard oxidation potentials derived from this source of data
show that the ease of oxidation for the metallic elements of six groups
is as follows.
Group 1: 3Li > 37Rb ³ 19K ³ 55Cs > 11Na
Group 2: 56Ba > 38Sr > 20Ca > 12Mg > 4Be
Group 3: 57La > 39Y > 21Sc
Group 11: 29Cu > 47Ag > 79Au
Group 12: 30Zn > 48Cd > 80Hg
Group 13: 13Al > 31Ga > 49In > 61Tl
Sources of Error: Surprisingly, reference books reveal a wide variation
in the numerical values of redox potentials. Nevertheless, in terms of
the ordering within groups, such books are concordant with Bard et al.
Conclusion: As adjudged by standard oxidation potentials, in a study
limited to the elements in Periods 2 to 6, the data provide no general
support for the hypothesis. Thus, Group 1 is irregular; the ease of
oxidation does increase as the atomic number increases in both Groups 2
and 3 (i.e., as each group is descended): but, the ease of oxidation
decreases as the atomic number increases in Groups 11, 12, and 13.
However, the use of different criteria for reactivity may well provide
support for this hypothesis. *
After due consideration of the above example of hypothesis testing, a
student may well exclaim: "Quelle horreur! Is there no method to this
madness?" The answer is yes: if, and only if, one accepts that many
patterns in Chemistry are complex, and that they start to emerge only
after focusing closely on the details.
Plus horreur?
A preliminary insight into some of the variables which determine the
reactivity of metals can be gained by placing a specific reaction type
'under the microscope'; e.g., reaction of a solid metal with an aqueous
solution of acid to form a hydrated metal cation and dihydrogen gas:
M(s) + H1+(aq) ———————————® M1+(g) + ½H2(g)
This reaction can be divided into several ergonic processes; the Table
below shows typical values of these processes for four metals known to
form uni-positive cations.
Ergonic process / kJ mol-1
|
Li |
K |
Na |
Ag |
M(s) ———————————® M(g) DH1 |
161 |
90 |
109 |
289 |
M(g) ———————————® M1+(g) + e- DH2 |
520 |
419 |
496 |
731 |
M1+(g) ——————————® M1+(aq) DHH |
-523 |
-331 |
-419 |
-464 |
H1+(aq) + e- ————® ½H2(g) DHR |
-432 |
-432 |
-432 |
-432 |
M(s) + H1+(aq) ——® M1+(aq) + ½H2(g) DH |
-274 |
-254 |
-246 |
+124 |
DH = DHS + DH1 + DH2 + DHR, where: DH1 is the heat of sublimation;
DH2 is the 1st ionization energy; DHH is the heat of hydration of
the metal cation; and, DHR is the heat of reduction of the aqueous
hydrogen ion to molecular dihydrogen. |
A complete explanation of these tabulated data is well beyond the scope
of this text: but even a partial explanation should be illuminating ...
Two processes are always endothermic, the heat of sublimation (DH1) and
the 1st ionization energy (DH2), whereas two are always exothermic, the
heat of hydration of the metal cation (DHH) and the heat of reduction
of the aqueous hydrogen ion to molecular dihydrogen (DHR). Because the
value of DHR is constant, the differences in the overall heat energy
change (DH) depend upon the relative magnitudes of DH1, DH2, and DHH.
The heat of sublimation reflects the strength of metallic bonding in the
solid state. However, the relationship between this bonding and ground-
state electronic structure of a gaseous atom is very complex indeed. Be
that as it may, uniform periodicity is clearly not observed; thus,
3Li > 11Na > 19K (Group1), but 79Au > 29Cu > 47Ag (Group 11).
The 1st ionization energy is the only ergonic process which is directly
related the ground-state electronic structure of a gaseous atom.
Nevertheless, here also, uniform periodicity is not observed; thus,
3Li > 11Na > 19K (Group1), but 79Au > 29Cu > 47Ag (Group 11).
The heat of hydration reflects the electronic structure of the gaseous
cation and its subsequent bonding with a (variable) number of water
molecules to form the hydrated cation. Unfortunately, a clear statement
of periodicity is precluded because, in contrast to those in Group 1,
the metals in Group 11 show variable oxidation states.
And finally, ... These data would support this principle: 'high metal
reactivity is favored by a low heat of sublimation, a low ionization
energy, and a high heat of hydration of the metal cation'. The overall
heat energy change (DH) is negative for each Group 1 metal, and the
relative magnitudes of DH parallel the measured energy changes of their
reactions with water (i.e., Li > K > Na); the 'anomalous' position of
lithium is attributable to the higher heat of hydration of the lithium
cation. In contrast, largely because of silver's much higher ionization
energy, DH is positive for this Group 11 metal; this is consistent with
the observed lack of reaction between silver and dilute acid or water.
[Scene. A court in the state capital of Poppermania]
Mermaid: Plus horreur! ... M'Lud ... [The judge holds up his hand.]
Judge: My Lord is the correct method of address, if you please.
Mermaid: Sorry, my Lord. These results and explanations are all well
and good: but, they can have little relevance to solid ionic
compounds, such as chlorides, oxides, and nitrides, surely?
Judge: [He smiles tolerantly at this less-than-humble student.]
I do admit, strictly speaking, only parts are; for example,
the DH1 and DH2 terms. However, for other reactants, there
should be analogous terms to DHH and DHR. Perhaps you would
care to note the relationship of DHH to the lattice energy?
Mermaid: Duly noted, M'Lud ... I mean, my Lord. Please no more ...
My time is valuable! [She states petulantly before glancing
briefly, but proudly, at her varnished finger-nails.]
Judge: Oh dear, in my day ... no, never mind. [He sighs.] Remember,
Rome was not built in a day: although it may well have burnt
down in one! [He glances at his fiddle before dabbing some
eau-de-cologne on his pristine wig.] Mermaid, I ... advise
you to spend at least part of your free-time giving the gray
cells some aerobic exercise. [Sighs of relief from the rest
of the synchronized-swimming team in the public gallery.]
Mermaid: My Lord, perhaps I should emigrate to Laputa? [... but the
venerable judge has fallen asleep (bless his silken socks).] |
* The student is encouraged, albeit gently, to consider testing this
hypothesis, or a similar one which focuses on Periods, using different
physical data (e.g., heats of sublimation, ionization energies, energy
changes for the formation of solid ionic compounds, ...); such data
should be abstracted from reference books.
METALS: DISPLACEMENT REACTIONS (1)
From a consideration of a reactivity series based on redox potentials,
one might reasonably predict that a metal should displace hydrogen from
either water or dilute acids if it is more reactive than hydrogen ...
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. The Table summarizes the results of experiments which examined the
reactions of eleven metals with water and/or steam, whereas the diagram
shows the apparatus used for the experiments with steam.
|
Observations of reactions of metals with water and/or steam * |
K |
Violent reaction with cold water; metal floated on water as
a small molten ball; evolved gas burnt spontaneously with a
lilac flame; resulting solution turned universal indicator
paper blue-purple. |
Ca |
Very vigorous reaction with cold water; evolved gas produced
an explosive pop with a lighted splint; resulting milky-white
suspension, which was very hot, turned universal indicator
paper dark-blue. |
Na |
Very vigorous reaction with cold water; metal floated on water
as a small molten ball; evolved gas did not burn spontaneously,
but produced an explosive pop with a lighted splint; resulting
solution turned universal indicator paper blue-purple. |
Mg |
Little or no reaction with cold water. Very slow reaction with
hot water (containing traces of universal indicator solution);
the solution changed slowly from green to blue over a period
of two hours (but no precipitate formed). Rapid reaction with
steam; white solid formed; evolved gas burnt. |
Al |
No reaction with boiling water: although reaction with boiling
water containing a catalytic quantity of sodium chloride;
evolved gas burnt with an explosive pop. Fairly rapid reaction
with steam; white solid formed; evolved gas burnt. |
Zn |
No reaction with boiling water. Fairly slow reaction with
steam; initially formed yellow solid cooled to a white solid;
evolved gas burnt intermittently. |
Fe |
Very slow reaction with steam; black solid formed; gas, if
evolved, not in sufficient quantity to be burnt. |
Sn |
Little or no reaction with steam; trace of white solid formed. |
Pb |
Little or no reaction with steam; trace of white solid formed. |
Cu |
No (evidence for) reaction with steam. |
Ag |
No (evidence for) reaction with steam. |
* Caveat This Table presents descriptors which indicate both relative
reaction speeds & energy changes; this conflation must not be allowed
to obscure the fact that there is no connection between the activation
energy & the heat energy change (DH) for a chemical reaction. |
(a) In common with similar presentations in textbooks, these results
do not refer to controlled experiments. Thus, although the qualitative
independent variable chosen was 'metal', no attempts were made to keep
other variables constant; e.g., the volume of water or the absence of
catalysts. Name the two physical quantities of the metals which should
have been held constant. ______________________________________________
[2]
(b) The symbol equation for the reaction between potassium and water is
Noting that calcium and sodium react with water similarly, construct
the symbol equation for the reaction between calcium and water. _______
_______________________________________________________________________
[2]
Calcium hydroxide is an alkali, because it is a base which is soluble
in water (albeit only slightly soluble). What is the common name for
aqueous calcium hydroxide? ____________________________________________
[1]
(c) Magnesium reacts with steam to form the hydroxide,
but this thermally decomposes at the temperature of the experiment,
so the reaction between magnesium and steam is usually summarized as:
Noting that aluminum and zinc react with steam similarly, construct
the (overall) symbol equation for the reaction between zinc and steam.
_______________________________________________________________________
[1]
(d) Iron has been shown to react reversibly with steam; i.e.,
Note, however, that these particular experiments could not determine
the reversible character of these reactions, because the apparatus used
was an 'open system'. Name one substance which is transferred to the
surroundings in these experiments. ____________________________________
[1]
[Neither tin nor lead react with water or steam: but both react with
dilute acids. The explanation for these observations is, regrettably,
beyond the scope of this text; nevertheless, one should be aware that
redox potentials are dependent on pH.]
2. Readily duplicated investigations have shown that, under comparable
conditions, metals react faster with dilute acids than with water; a
partial explanation for this difference is as follows. Water undergoes
very slight dissociation (or ionization); i.e.,
Because the position of the above equilibrium lies almost completely to
the left, the concentration of hydrogen ions in water is low [roughly
10-7 mol dm-³ (i.e., pH 7)]. By contrast, dilute acids undergo either
partial or complete dissociation; e.g.,
Therefore, the concentration of hydrogen ions in dilute acids is higher
[ranging from 10-1 to 10-6 mol dm-³ (pH 1 to 6)]. In consequence, there
will be more collisions between metal atoms and these ions in dilute
acids than in water, and so faster reactions.
Construct the symbol equation for the reaction between:
Sodium and dilute hydrochloric acid (dangerously explosive) ___________
_______________________________________________________________________
Magnesium and dilute sulfuric acid (vigorous) _________________________
_______________________________________________________________________
Aluminum and dilute sulfuric acid (fairly vigorous) ___________________
_______________________________________________________________________
Iron and dilute sulfuric acid (which, in part, is analogous to that
between steel and this component of 'acid rain') ______________________
_______________________________________________________________________
[8]
3. Neither copper nor silver react with water, steam, or dilute acids.
Name another metal which is similarly unreactive. _____________________
[1]
METALS: EXTRACTION of SODIUM
Sodium, the fourth most abundant metal in the Earth's crust (2.4%), is
usually found as the chloride (e.g., in the ore rock salt or dissolved
in sea water). Sodium has a low melting point (98°C) and a low density
(0.97 g cm-³), which are characteristics of Group 1 metals, and forms
(usually) white or colorless compounds in only one oxidation state
(I), which is typical of main group metals.
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. Sodium is extracted by the electrolytic reduction of purified,
molten rock salt; the ionic equations for the reactions occurring at
the electrodes are:
(a) Label this diagram of a Downs electrolytic cell with: Carbon anode;
Chlorine gas; Insulator; Molten electrolyte; Molten sodium; and, Steel
cathode.
(b) To lower the melting point of sodium chloride from 774°C to about
600°C, and so reduce the energy costs, calcium chloride is added to the
rock salt. As a result of this added impurity, a mixture of molten
sodium and solid calcium is obtained. Write an ionic equation for the
reduction of calcium ions at the cathode.
_______________________________________________________________________
[1]
(c) Water must not be present in the Downs cell, partly because a
different cathodic reaction occurs. Thus, hydrogen gas is evolved at
the cathode from the electrolysis of an aqueous solution containing
Na1+(aq), Cl1-(aq), H1+(aq), and OH1-(aq) ions. Write an ionic equation
for the reduction of hydrogen ions at the cathode.
_______________________________________________________________________
[1]
(d) After manufacture, sodium is stored under oil because it is rapidly
oxidized by atmospheric oxygen. Furthermore, sodium reacts explosively
with many other non-metallic elements and compounds. Construct the
symbol equation for the reaction of sodium with:
Difluorine ____________________________________________________________
Dioxygen ______________________________________________________________
Water _________________________________________________________________
[6]
Sodium's high reactivity has necessarily restricted its use to a few
applications. Thus, aside from its incorporation into specialized
alloys (e.g., Na-Hg amalgam), it is used in street vapor lamps, as a
coolant in nuclear reactors, and as a chemical reducing agent.
2. Sodium's use as a coolant in nuclear reactors depends on three of
the metal's properties. First, it is a liquid at temperatures between
98°C and 883°C, so the metal flows easily around the hot reactor core
(which operates at a temperature of about 660°C). Second, it is a good
thermal conductor (because of its free moving delocalized electrons),
so excess heat generated in the core is conducted away efficiently.
And third, it has a relatively high thermal capacity, so large amounts
of heat energy are absorbed for any given temperature rise.
[Q = n × z × F and Q = I × t, where: Q, measured in coulombs (C), is
the quantity of electricity; n is the number of moles of substance
evolved at the electrode; z is the charge on the ion; F is a constant,
with a value of 96500 C mol-¹; I, measured in amps (A), is the current;
and t, measured in seconds (s), is the time.]
(a) A fast-breeder reactor under construction required 690 tonnes of
sodium. A typical Downs electrolytic cell operates continuously at a
current of 25 kA. Determine the time (t) it would take to obtain the
required mass of sodium - as follows.
Convert this mass (m) of sodium from tonnes to grammes (where
1 tonne = 1000 kg). ___________________________________________________
Calculate the number of moles (n) in this mass (m) of sodium. _________
_______________________________________________________________________
Calculate the quantity of electricity (Q) required to deposit this
number of moles (n) of sodium at the cathode. _________________________
_______________________________________________________________________
And finally, calculate the time (t) taken for this quantity of
electricity (Q) to be used. ___________________________________________
_______________________________________________________________________
[7]
(b) The volume (V1) of one mole of any gas at room temperature (25°C =
298 K; T1) and pressure (0.1 MPa; P1) is 24 dm³; furthermore, the
following relationship holds true for gases:
P1 × V1 P2 × V2
¾¾¾¾¾ = ¾¾¾¾¾
T1 T2
Determine the volume (V2) of chlorine gas obtained, at room temperature
and high pressure (20 MPa; P2), during the production of this mass of
mass of sodium - as follows.
State the number of moles (n) of chlorine atoms, Cl(g), formed at the
anode. ________________________________________________________________
State the number of moles of chlorine gas, Cl2(g), evolved at the
anode. ________________________________________________________________
Calculate the volume of gas (V1) obtained at room temperature (T1) and
pressure (P1). ________________________________________________________
Then, using the above relationship, calculate the volume (V2) of gas
at the increased pressure (P2). _______________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[6]
3. Sodium metal reduces liquid ammonia to sodium amide, NaNH2(s), a
sparingly used fertilizer. Construct the symbol equation for this redox
reaction, which is executed under anhydrous and anaerobic conditions.
_______________________________________________________________________
[2]
METALS: CALCIUM & MAGNESIUM
Calcium, (m.pt. = 839°C; r = 1.54 g cm-³), is the third and magnesium,
(m.pt. = 649°C; r = 1.74 g cm-³), is the fifth most abundant metal in
the Earth's crust (4.2% and 2.3%, respectively); as the Table below
shows, they occur in a variety of minerals and ores. In common with
several other Group 1 and 2 elements, both metals are often extracted
by the electrolytic reduction of their molten chlorides.
Name of mineral or ore |
Composition or principal component |
Gypsum |
CaSO4.2H2O |
Limestone, calcite, chalk, marble |
CaCO3 |
Dolomite |
CaCO3.MgCO3 |
Magnesite |
MgCO3 |
Epsomite |
MgSO4.7H2O |
Despite their relative abundances, neither metal is used extensively in
industry, although both are used in alloys and for chemical reductions:
nevertheless, in the long term, magnesium may replace aluminum as the
low-density structural metal of choice because the supply available in
seawater is virtually unlimited. By way of contrast, both calcium and
magnesium ions have been shown to be essential to all living organisms,
and both affect the 'hardness' of water (one aspect of water quality
considered to be important in developed countries).
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. The presence of nitrate ions (leached from farms), or toxic heavy
metal ions (leached from mines), or calcium ions (which contribute to
hardness), is rarely an important consideration of water quality in
developing countries - where survival frequently depends on obtaining
enough water which is sufficiently pathogen-free. When favorable
conditions prevail, water is purified by boiling (a process which kills
pathogens by irreversibly denaturing their enzymes). Name one pathogen
which is often present in water contaminated with untreated sewage.
_______________________________________________________________________
[1]
2. This diagram shows a simplified plan of a typical water-treatment
works in a developed country.
(a) Name one urban water source. ______________________________________
[1]
(b) Suggest the purpose of the:
Sand filtration beds __________________________________________________
Chlorinator unit ______________________________________________________
[2]
(c) Each storage tank must be covered. Suggest one biological reason
why sunlight should be prevented from reaching the water. _____________
_______________________________________________________________________
[1]
3. 'Hard water', which is water that does not readily form a lather
with soap, is caused by the presence of any dissolved Group 2 ions -
though the focus here is on those of calcium [i.e., aqueous Ca(II)] ...
Unpolluted rainwater is effectively a dilute solution of carbonic acid,
H2CO3(aq), and 'temporary' hard water results from the slow reaction of
rainwater with minerals or ores containing calcium carbonate:
Water containing soluble calcium hydrogencarbonate is considered to be
temporarily hard because the hydrogencarbonate is thermally unstable;
thus, when it is thermally decomposed, soluble calcium ions are removed
from solution as a precipitate - commonly known as 'scale' or 'fur':
'Permanent' hard water results from the slow (and slight) dissolution
of the calcium sulfate present in minerals and ores:
Water containing dissolved calcium sulfate is considered to be
permanently hard because this type of hardness cannot be removed by
boiling; indeed, removal of the soluble calcium ions requires the use
of an 'ion exchanger' or a 'water-softener' (e.g., sodium carbonate):
(a) Construct a single symbol equation, involving naturally occurring
magnesite, for the formation and removal of temporary hard water. _____
_______________________________________________________________________
[2]
(b) The structural formula of a typical soap, sodium hexadecanoate, is:
The anions of this soap, better known as sodium palmitate, react with
the calcium ions present in permanent hard water to form a precipitate
of 'scum':
Complete a similar equation for the reaction of these anions with the
calcium ions present in temporary hard water:
_______________________________________________________________________
[2]
(c) In hard water areas, scale precipitated in pipes and in tanks can
be removed safely by neutralization with an aqueous solution of a weak
acid [e.g., HCOOH(aq)]. Suggest and explain, using a symbol equation,
why farmers usually prefer to remove scale with dilute nitric acid. ___
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[4]
4. State two biologically important roles of calcium ions, apart from
the formation of exo- and endo-skeletons. _____________________________
_______________________________________________________________________
[2]
5. Name and state the function of one biological molecule that
contains magnesium ions. ______________________________________________
_______________________________________________________________________
[2]
METALS: EXTRACTION of ALUMINUM
Aluminum, the commonest metal in the Earth's crust (8.3%), is usually
found as the oxide (e.g., in the ore bauxite). Typical of main group
metals, aluminum has a fairly low melting point (660°C), a low density
(2.7 g cm-³), and forms (usually) white or colorless compounds in only
one oxidation state (III).
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. Aluminum is extracted by the electrolytic reduction of purified
molten bauxite; the ionic equations for the reactions occurring at the
electrodes are:
(a) Label this diagram of an industrial electrolytic cell with:
Graphite anode; Graphite cathode; Insulator; Molten aluminum; Molten
electrolyte; and, Solid crust of electrolyte.
(b) To lower the melting point of aluminum oxide from about 1500°C to
1000°C, and so reduce energy costs, cryolite (Na3AlF6) is added to the
bauxite. Explain why (pure) aluminum oxide has a high melting point.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[3]
(c) Name one toxic gas that might arise from the thermal decomposition
of cryolite. __________________________________________________________
[1]
(d) The anode needs to be replaced periodically because the carbon is
completely oxidized by the evolving dioxygen. Name the toxic pollutant
formed by partial oxidation of the carbon anode. ______________________
[1]
[Q = n × z × F and Q = I × t, where: Q, measured in coulombs (C), is
the quantity of electricity; n is the number of moles of substance
evolved at the electrode; z is the charge on the ion; F is a constant,
with a value of 96500 C mol-¹; I, measured in amps (A), is the current;
and t, measured in seconds (s), is the time.]
2. In industry, a typical electrolytic cell operates continuously at a
current of 4000 kA. Determine the mass (m) of aluminum that forms at
the cathode every day - as follows.
Calculate the quantity of electricity (Q) used every 24 hours. ________
_______________________________________________________________________
Calculate the number of moles (n) of aluminum formed at the cathode.
_______________________________________________________________________
And finally, calculate the mass (m) of aluminum formed at the cathode.
_______________________________________________________________________
[6]
3. Electrolytic reduction, which is often used for the extraction of
the most reactive metals, requires expensive electrical energy: by
contrast, chemical reduction with carbon uses a relatively cheap source
of chemical energy. Which method of generating electrical power is the
cheapest and most environmentally acceptable? _________________________
[1]
297 kJ of energy are required to produce 1 mole of aluminum metal by
electrolysis, whereas only 26 kJ of energy are required to recycle
1 mole. Calculate the percentage energy saved by recycling aluminum.
_______________________________________________________________________
[2]
4. Aluminum's high natural abundance, high resistance to corrosion,
high thermal conductivity, low density, and low electrical resistance,
has resulted in its widespread use (e.g., in window frames, cooking
utensils, food and drink cans, aircraft, and electrical power lines).
The metal surface is invariably covered by a very thin layer of oxide,
because pure aluminum is rapidly oxidized by atmospheric oxygen;
This layer is non-porous and protects the metal from further corrosion,
providing it is not exposed to aqueous chloride ions.
(a) Paralleling aluminum's increased use in cooking utensils and in
food containers, there has been an increase in the number of people who
suffer from Alzheimer's disease. This parallel may indeed be a most
unfortunate coincidence, rather than a correlation, but one partial
explanation could be as follows. Food is often preserved in brine and
cooked with salt; so, should the protective layer of aluminum oxide in
the containers be removed by aqueous chloride ions, soluble aluminum
ions could be formed, ingested, and then absorbed into the bloodstream.
Construct an explanation, complete with two symbol equations, for the
assertion that it could be hazardous to use aluminum foil when cooking
salted meat in the presence of steam. _________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[4]
(b) When the thin layer of oxide is made thicker, by anodizing, then
it can easily absorb variously colored dyes. Shown below is a diagram
of a typical electrolytic cell used to anodize aluminum objects.
Oxygen gas is evolved at the anode from the electrolysis of an aqueous
solution containing H1+(aq), OH1-(aq), and SO42-(aq) ions; the ionic
equations for the reactions occurring at the electrodes are:
The aluminum object immediately combines with this evolved oxygen gas
to form more aluminum oxide on its surface. Suggest two reasons why
aluminum is anodized. _________________________________________________
_______________________________________________________________________
[2]
METALS: EXTRACTION of ZINC
Zinc, which is relatively rare in the Earth's crust (0.007%), occurs
mainly as the sulfide or the carbonate (e.g., in the ores zinc blende
and calamine, respectively). This element is not considered to be a
transition metal, despite its position on the Periodic Table, partly
because it forms compounds which are (usually) white or colorless and
in only one oxidation state (II).
[.. K > Ca > Na > Mg > Al > Zn > Cd > Fe > Sn > Pb > (H) > Hg > Cu ..]
1. Extracting natural resources invariably produces a conflict between
economic advantages and conservation of the environment. For example,
open-cast mining of zinc ores usually provides employment for the local
population: but also produces large quantities of waste rock (known as
spoil), as well as the disruption or destruction of delicately balanced
ecosystems. Suggest one conservation measure which should be undertaken
when resources are extracted. _________________________________________
_______________________________________________________________________
[1]
2. Zinc is extracted and purified by the processes summarized in this
flow diagram.
(a) Combustion of crude zinc blende usually results in a mixture of
metal and silicon oxides, because the sand-bearing ores of zinc, lead,
and cadmium often occur together. Construct the symbol equation for the
complete combustion of (pure) zinc sulfide. ___________________________
_______________________________________________________________________
[2]
State the most important use of the by-product (i.e., sulfur dioxide).
_______________________________________________________________________
[1]
(b) Construct the symbol equation for the neutralization reaction of
zinc carbonate and dilute sulfuric acid. _____________________________
_______________________________________________________________________
[2]
(c) In the electrolytic reduction of aqueous zinc sulfate, which
contains the ions Zn2+(aq), SO42-(aq), H1+(aq), and OH1-(aq), dioxygen
is evolved at the carbon-graphite anode and zinc is deposited at the
carbon-graphite cathode. Write an ionic equation for the reaction which
occurs at the:
Anode _________________________________________________________________
Cathode _______________________________________________________________
[3]
Suggest and explain one reason why zinc is not extracted by the
electrolytic reduction of zinc oxide. _________________________________
_______________________________________________________________________
[2]
(d) Impure zinc is obtained when a mixture of oxides, obtained from the
combustion of crude zinc blende, is heated with coke and limestone in a
furnace at a temperature of about 1000°C. Fairly pure zinc, separated
from impurities by physical methods (e.g., fractional distillation), is
then either used directly or further purified electrolytically.
Coke, the source of chemical energy in the blast furnace, is burnt both
to release heat energy and to provide the main reducing agent:
Calcium oxide, formed by thermal decomposition of limestone, reacts
with silicon oxide to form slag (which is less dense than molten lead):
|
Zinc |
Cadmium |
Lead |
Melting point / °C |
420 |
321 |
328 |
Boiling point / °C |
908 |
765 |
1751 |
Density (r) / g cm-³ |
7.14 |
8.64 |
11.34 |
Noting the properties given in the Table, label this diagram of a zinc
blast furnace with: CaSiO3(l); Cd(l); Condenser; Hot air blast; Pb(l);
Reactants (oxides, coke, and limestone); Waste gases; and, Zn(l).
Construct an explanation, complete with two symbol equations, why
calamine could be used directly as a raw material in the blast furnace.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[4]
What two physical processes are involved in fractional distillation?
_______________________________________________________________________
[2]
3. Zinc is used in alloys, in batteries, and to galvanize iron. If the
trends in present-day use of zinc continues, it has been guesstimated
that the known reserves of zinc ores will last for perhaps no more than
20 years. Suggest two ways of overcoming this potential shortage of
zinc. _________________________________________________________________
_______________________________________________________________________
[2]
METALS: IRON
Iron, the second most abundant metal in the Earth's crust (5.6%), is
usually found as an oxide (e.g., Fe2O3 and Fe3O4 in the ores hematite
and magnetite, respectively). This element, which has been shown to be
essential to all biological species, is a typical transition metal.
Thus, iron has a high melting point (1535°C) and a high density
(7.87 g cm-³), forms colored compounds (which are often pale-green or
brown), and shows variable oxidation states [e.g., Fe(II) and Fe(III)].
Furthermore, iron and many of its compounds show catalytic activity
(e.g., iron powder is used in the Haber synthesis of ammonia, and
several iron-containing enzymes are involved in nitrogen-fixation, in
photosynthesis, and in respiration).
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. Iron is extracted from its ores by the chemical reduction of iron
oxides with carbon in a furnace at a temperature of about 800°C;
overall, the processes can be summarized by these equations:
Coke, the source of chemical energy in the blast furnace, is burnt both
to release heat energy and to provide the main reducing agent:
Calcium oxide, formed by thermal decomposition of limestone, reacts
with the silicon oxide present in sand, a major impurity in iron ores,
to form slag (which is less dense than molten iron):
(a) Label this diagram of an iron blast furnace with: Blast of hot air;
Molten iron; Molten slag; Outlet for iron; Outlet for slag; Reactants
(iron ore, coke, and limestone); and, Waste gas outlet.
(b) Various iron oxides are formed within the furnace. Construct the
symbol equation for the reduction of iron(II) oxide by carbon monoxide.
_______________________________________________________________________
[2]
(c) Name two of the waste gases. ______________________________________
[2]
2. Nearly all metals corrode: but only iron 'rusts'. The rusting of
iron, which starts with the anodic reaction Fe(s) ——® Fe2+(aq) + 2e-,
involves a complicated series of redox and precipitation reactions that
can be loosely summarized by the following equation (where, n and x are
variable numbers, and z and y are constants):
(a) Laboratory analysis showed that a sample of rust contained 5.6 g of
iron, 2.4 g of oxygen, and 2.7 g of water. The steps involved in this
analysis included strong heating of the hydrated oxide, to remove its
water of crystallization, followed by strong heating of the anhydrous
oxide in a stream of dihydrogen, to effect its reduction; i.e.,
|
Fe |
O |
H2O |
Mass combining (m) / g |
|
|
|
Molar mass (M) / g mol-¹ |
|
|
|
Number of moles combining (m ÷ M)
|
|
|
|
Simplest ratio of number of moles |
|
|
|
Complete the Table above, so as to determine the empirical formula of
this sample of rust. __________________________________________________
[5]
Rusting occurs faster in the presence of aqueous chloride ions; a
partial explanation for this observation is as follows. Corrosion is an
electrochemical process, in which one part of the metal surface acts as
the anode, another part acts as the cathode, and an aqueous electrolyte
completes the electrical cell. Electron flow from anode to cathode is
facilitated by migration of ions such as Fe2+(aq) in the electrolyte;
and, because pure water is only very slightly ionized, an aqueous
solution of chloride ions is clearly a better electrolyte than water.
(b) State two ions present in 'acid rain' that might also increase the
rate of rusting / corrosion. __________________________________________
[2]
Iron's widespread use as a structural material is due to its low cost
and high tensile strength. Unfortunately, rust weakens structures;
moreover, it cannot protect the metal from further corrosion because it
is porous. Accordingly, apart from important safety considerations, the
costs of possible replacements need to be minimized. And so, when iron
(or steel) is likely to rust, it is: painted, coated with plastic, or
plated with another metal, all of which provide a physical barrier; or
attached to a more reactive metal, which acts as a 'sacrificial anode'.
(c) Zinc is more easily oxidized than iron. Therefore, in an electrical
cell, the preferred anodic reaction Zn(s) ——® Zn2+(aq) + 2e- results in
electron flow to an iron cathode, and so prevents the anodic reaction
Fe(s) ——® Fe2+(aq) + 2e-. Thus zinc acts as a sacrificial anode, and so
blocks of zinc attached to iron structures provide effective protection
against rusting. Explain, in similar terms, the effectiveness of:
Magnesium blocks ______________________________________________________
_______________________________________________________________________
_______________________________________________________________________
Tin blocks ____________________________________________________________
_______________________________________________________________________
[5]
(d) Iron coated with a layer of zinc is known as 'galvanized iron'.
Galvanization protects iron from rusting, as follows: the layer acts as
a physical barrier to dioxygen and water; and then the zinc acts as a
sacrificial anode when this layer is breached. Suggest one disadvantage
in galvanizing iron. __________________________________________________
[1]
METALS: COBALT
Cobalt, usually found in the Earth's crust as a sulfide (e.g., in the
ore cobaltite), is the second rarest (0.003%) of the four elements that
can be magnetized; the other three are iron (5.6%), nickel (0.008%),
and gadolinium (0.0005%). This relative rarity means that cobalt is not
used for structural purposes but for the formation of specialized
alloys (e.g., Alnico, which is used to manufacture permanent magnets).
This transition metal, which has a high melting point (1495°C) and a
high density (8.89 g cm-³), forms compounds in several oxidation states
[e.g., (blue) cobalt(II) chloride and (brown) cobalt(III) chloride].
[.. K > Ca > Na > Mg > Al > Zn > Fe > Co > Sn > (H) > Cu > Hg > Ag ..]
1. The extraction of cobalt from its ores is complex, but the final
process usually involves chemical reduction of cobalt(II) dicobalt(III)
oxide (Co3O4). Construct the symbol equation for the reduction of this
tetroxide with each of these reducing agents used in industry:
Aluminum ______________________________________________________________
Carbon ________________________________________________________________
Dihydrogen ____________________________________________________________
[6]
Cobalt can be electrolytically purified, using impure cobalt as the
anode, pure cobalt as the cathode, and aqueous cobalt(II) chloride as
the electrolyte. Explain why solid cobalt(II) chloride would not be
suitable as an electrolyte. ___________________________________________
_______________________________________________________________________
_______________________________________________________________________
[2]
2. Vitamin-B12, which is synthesized exclusively by bacteria (e.g.,
Methanobacteria), has a structure similar to both chlorophyll-a and
hemoglobin.
This compound is involved in the bacterial biosynthesis of methane and
organometallic compounds of heavy metals (e.g., dimethylmercury).
Vitamin-B12 is also a vital cofactor in the synthesis of mammalian red
blood cells; it is acquired from dietary sources and from symbiotic
bacteria residing in the alimentary canal. Name the human deficiency
disease caused by a lack of this vitamin. _____________________________
[1]
3. Cobalt(II) chloride is used as an indicator of water; thus, in the
presence of this compound, blue 'cobalt(II) chloride paper' turns pink:
State and explain, using Le Chatelier's Principle, the effect of an
increase in temperature on this reaction. _____________________________
_______________________________________________________________________
_______________________________________________________________________
[2]
4. Anhydrous cobalt(II) chloride is an effective catalyst for the
synthesis of alkanes (in non-aqueous solvents); a typical scheme is:
(a) Construct a similar scheme for the synthesis of pentane (C5H12).
_______________________________________________________________________
_______________________________________________________________________
[2]
(b) Complete and label the energy level diagram for this synthesis of
pentane.
Energy ___
___
___________________ _ _ _ _ _ _ _ _ _ _ _ _
___________________
_______________
_______________
___________________________________________
Path of reaction
[5]
(c) Compounds with the same molecular formula but different structural
formulae are known as isomers. Complete these structural formulae to
show the three alkane isomers which have a molecular formula of C5H12.
C C
C C C C C C C C C C C C
C
pentane 2-methylbutane 2,2-dimethylpropane
[3]
(d) Suggest one physical method which could be used to distinguish
between these three isomers. __________________________________________
[1]
5. One radioactive isotope of cobalt, Co-60, has been used in the
treatment of cancer; it has a half-life of 5.3 years; and spontaneously
emits both b-particles and g-rays (i.e., high speed electrons and high
frequency photons, respectively). Determine how many protons, neutrons,
and electrons there are in each of the following.
Co-59 _________________________________________________________________
Co-60 _________________________________________________________________
Co(III)-60 ____________________________________________________________
[3]
METALS: TIN & LEAD
Tin, a rare element in the Earth's crust (0.0002%), occurs mainly in
the ore cassiterite [impure tin(IV) oxide]. The metal was obtained
first in the Bronze Age, when a mixture of coal and rocks was heated
in air; so, presumably, one of the reactions that occurred was:
Modern technology uses coke and purified cassiterite in a furnace: but,
otherwise, the extractive method used to obtain tin, as well as copper
and lead, has not fundamentally changed over 7000 years.
[.. K > Ca > Mg > Al > Zn > Fe > Co > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. The principal ore of lead, a relatively rare element (0.001%) in
Earth's crust, is galena [impure lead(II) sulfide]. Lead(II) oxide is
formed when this ore is heated in air, and the metal is extracted by
the chemical reduction of this oxide with carbon in a furnace.
(a) Construct the symbol equation for each reaction in this process.
_______________________________________________________________________
_______________________________________________________________________
[4]
(b) Carbon and carbon monoxide are the commonest reducing agents used
industrially to reduce metal oxides, but others are also used (e.g.,
aluminum and dihydrogen). In the laboratory, reductions with hydrogen
gas can be attempted using the apparatus shown in this diagram.
This is an open system in which equilibrium is never reached; thus, as
predicted by Le Chatelier's Principle, the stream of dihydrogen ensures
that the position of equilibrium continually 'shifts to the right'.
The Table below shows the calculated heat energy changes for the
reduction of various metal(II) oxides with dihydrogen; i.e.,
State and explain the effect of a temperature increase on the:
Rate of each reaction _________________________________________________
_______________________________________________________________________
_______________________________________________________________________
Yields of products in each endothermic reaction _______________________
_______________________________________________________________________
_______________________________________________________________________
Yields of products in each exothermic reaction ________________________
_______________________________________________________________________
_______________________________________________________________________
[6]
2. Tin, (m.pt. = 232°C; r = 7.29 g cm-³), and lead, (m.pt. = 327°C; r
= 11.34 g cm-³), are both used extensively in alloys. Name two alloys
which contain either or both metals. __________________________________
[2]
3. Tin is used to manufacture 'tin-plate' (i.e., steel plated with a
layer of tin). Explain how tin protects steel from corrosion. _________
_______________________________________________________________________
_______________________________________________________________________
[2]
4. Approximately half of the lead produced is used in the manufacture
of storage batteries; in turn, these are recycled to provide about 40%
of industry's requirements of lead. A lead storage battery consists of
six cells connected in series; each 2 V cell, which has a lead anode, a
cathode made of lead(IV) oxide, and an electrolyte of aqueous sulfuric
acid, delivers high currents for short periods of time.
(a) When a battery discharges (i.e., when in use providing the required
voltage), the reactions occurring at the electrodes are:
State the energy change which occurs during discharge. ________________
_______________________________________________________________________
[2]
(b) Suggest and explain one reason why such batteries are labeled with
the hazard symbol 'No smoking'. _______________________________________
_______________________________________________________________________
[2]
(c) Recharging the battery involves applying an external voltage to the
anode and cathode. What name is given to this reversal of the above
electrochemical reactions? ____________________________________________
[1]
5. Compounds of tin and lead have few (if any) beneficial functions in
living organisms. However, lead compounds are known to be particularly
toxic, because lead(II) ions inhibit the 'active sites' of a number of
enzymes; furthermore, as with so many non-biodegradable biocides, they
accumulate up the trophic levels.
(a) Name two sources of the lead ions introduced into the environment,
apart from lead storage batteries and mine-workings. __________________
_______________________________________________________________________
[2]
(b) The diagram below represents one food chain in the complex food web
of a typical slow-moving river in a temperate region.
Estimate the amount of lead(II) ions which could be accumulated by one
otter, assuming: that each consumer in this food chain obtains its
chemical energy by eating 10 individuals of the organism at the trophic
level immediately below it; that none of the consumers egest or excrete
these ions; and that each duckweed plant absorbed 0.01 mg of these
ions. _________________________________________________________________
[2]
Name the source of energy for the producer in this food chain. ________
[1]
METALS: EXTRACTION of COPPER
Copper, which is relatively rare in the Earth's crust (0.006%), occurs
mainly as a sulfide (e.g., the ore chalcopyrite contains CuFeS2). This
metal, which has been shown to be essential to all biological species,
is a typical transition element. Thus, copper has a high melting point
(1083°C) and a high density (8.92 g cm-³), forms colored compounds
(which are often blue or green), and shows variable oxidation states
[e.g., Cu(I) and Cu(II)]. Furthermore, copper and many of its compounds
show catalytic activity [e.g., copper(II) oxide is used as a catalyst
in the manufacture of methanol, and several copper-containing enzymes
are involved in photosynthesis and in respiration].
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. There are several routine methods for the extraction of copper, but
the processes used, which can be fairly complex, are dependent on both
the composition and quality of the ore. However, the final steps in one
method can be summarized by the following pair of equations:
In industry, copper is electrolytically purified using slabs of impure
copper as anodes, thin sheets of pure copper as cathodes, and solutions
of copper(II) sulfate as electrolytes; this industrial process can be
modelled in the laboratory using the electrical circuit shown as Cell 1
in this diagram.
Aqueous copper(II) sulfate contains Cu2+(aq), SO42-(aq), OH1-(aq), and
H1+(aq) ions. When this solution is electrolyzed, the cathodic reaction
always results in copper being deposited, but the anodic reaction is
dependent on the conductor acting as the anode: aqueous copper(II) ions
are formed using a copper anode (Cell 1), whereas oxygen gas is evolved
using either a platinum or carbon-graphite anode (Cell 2).
(c) In cell 1, providing the anode is replaced periodically, continued
electrolysis results in the electrolyte remaining blue-colored. In
cell 2, by contrast, continued electrolysis results in the electrolyte
becoming colorless [as the concentration of Cu2+(aq) ions decreases].
Suggest what this colorless solution contains. ________________________
[1]
2. Copper is used extensively in water piping, alloys, electroplating,
and in electrical wiring. Suggest two reasons for this widespread use.
_______________________________________________________________________
[2]
3. An industrial chemist investigated the following hypothesis: 'For
the electrolysis of aqueous copper(II) sulfate, the number of moles
(n) of copper deposited at a cathode increases in direct proportion to
time (t); i.e., n = k × t'; the Table shows a summary of the chosen
conditions and raw data.
Constants: aqueous copper(II) sulfate (600 cm³; 0.20 mol dm-³); direct
current (5.35 A); room temperature (17°C); surface area and distance
between copper electrodes (values measured but not recorded).
Time (t) / s |
0 |
60 |
120 |
180 |
180 |
180 |
240 |
300 |
Cu deposited / mg |
0 |
110 |
210 |
250 |
315 |
325 |
430 |
530 |
Cu deposited (n) / mmol |
0.0 |
1.7 |
3.3 |
3.9 |
4.9 |
5.1 |
6.7 |
8.3 |
(a) A value of a dependent variable which does not follow the general
pattern is usually referred to as an 'anomalous result'. In this
investigation, for example, the first value of n when t = 180 s appears
anomalous. Suggest one possible reason for this anomaly. ______________
_______________________________________________________________________
[1]
(b) Noting that the independent variable is on the horizontal axis,
plot all nine data points on this fully labeled graph paper, and then
draw a best straight line through as many points as is sensible.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Cu 8_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
d 7_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
e |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
p 6_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
o |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
s 5_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
i |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
t 4_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
e |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
d 3_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
(n) 2_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
/ 1_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
mmol 0_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
| | | | | |
0 60 120 180 240 300
Time (t) / s
[3]
This graph shows direct proportionality between the two variables,
because the straight line passes through the origin. Determine the
gradient of the graph; this value, 'k', is the proportionality constant
in the directly proportional relationship n = k × t.
(y2 - y1)
k = ————————— =
(x2 - x1)
[3]
constants). 'When a direct current of _______ was passed through an
electrolysis cell containing _______ electrodes and 600 cm³ of aqueous
copper(II) sulfate (0.20 mol dm-³), at room temperature (17°C), the
number of moles (n) of copper deposited at the cathode increased in
direct proportion to the time (t) of the electrolysis (within the
range __________); i.e., n = k × t, where k = ________________.'
[4]
METALS: DISPLACEMENT REACTIONS (2)
A displacement reaction is the chemical change which occurs when a more
reactive element displaces a less reactive element from its compound;
In this redox reaction, electrons are transferred from zinc atoms to
copper(II) ions because zinc is a better reducing agent (i.e., it is
more easily oxidized); the redox half equations are:
The aqueous nitrate ions are spectators, so the net ionic equation is:
[.. K > Sr > Ca > Mg > Be > Al > Zn > Fe > Sn > (H) > Cu > Hg > Ag ..]
1. Temperature rises (DT) were measured for displacement reactions
involving beryllium and aqueous solutions of metal nitrates; this first
Table shows a summary of the chosen conditions and raw data.
Constants: volume (25 cm³), concentration (0.25 mol dm-³), and starting
temperature (17°C) of aqueous nitrate; beryllium powder (an excess);
thermometer (0-100°C); reaction vessel (an insulated plastic cup).
Nitrate(aq) |
Mg(II) |
Mg(II) |
Al(III) |
Zn(II) |
Pb(II) |
Cu(II) |
DT / °C |
0 |
0 |
6 |
14 |
32 |
45 |
(a) Beryllium compounds are extremely toxic. Apart from wearing safety
glasses, suggest one other vital safety precaution adopted for these
experiments. __________________________________________________________
[1]
(b) For the reaction between beryllium and aqueous copper(II) nitrate,
construct the symbol equation and state two other observable changes.
_______________________________________________________________________
_______________________________________________________________________
[3]
(c) State why no temperature rise was observed with aqueous magnesium
nitrate. ______________________________________________________________
[1]
(d) Use the results in this first Table to predict the temperature rise
(DT) that would be obtained, using the same set of constants, with
these aqueous nitrates: Sr(II) _______ Fe(II) _______ Ag(I) _______
[3]
2. A research chemist investigated the following hypothesis: 'The
speed (S) of reaction between beryllium and aqueous hydrochloric acid
increases in direct proportion to the concentration (C) of the acid;
i.e., S = k × C'; this second Table shows a summary of the chosen
conditions and raw data.
Constants: length (6.0 cm) and surface area (6.0 cm²) of beryllium
ribbon; volume of solution (25 cm³); room temperature (21°C); absence
of catalysts; volume of dihydrogen collected via gas syringe (25 cm³).
Concentration (C) / mol dm-³ |
0.0 |
0.0 |
0.15 |
0.30 |
0.45 |
0.60 |
0.75 |
Reaction time (t) / s |
»900 |
»900 |
120 |
58 |
42 |
32 |
25 |
Reaction speed (S) / ms-¹ |
0.0 |
0.0 |
|
|
|
|
40 |
(a) Construct the net ionic equation for the displacement reaction
investigated in the above hypothesis. _________________________________
_______________________________________________________________________
[2]
(b) Calculate the missing values for the reaction speed, and insert
these data in this second Table above.
[2]
(c) Label both axes, and then plot all seven data points.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
40_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
30_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
20_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_| Beryllium |_|_|_|_|_|_|_|_|_|_|_|
10_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
0_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
| | | | | | | |
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
[4]
Draw a best straight line through as many points as is sensible, and
then determine the gradient of the (beryllium) graph; this value, 'k',
is the proportionality constant in the directly proportional
relationship S = k × C. _______________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[2]
Write a precisely worded conclusion based on the (beryllium) graph.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[5]
(d) Strictly speaking, to extrapolate a relationship outside the range
of the independent variable examined is scientifically flawed: but to
do so has practical advantages with respect to safety considerations,
time, and costs. Use the equation, and yor value of 'k', to estimate
the reaction time if concentrated hydrochloric acid was used (i.e.,
C = 10 mol dm-³; pH -1). ______________________________________________
_______________________________________________________________________
[2]
(e) Sketch the graphs that would be obtained if Be was replaced with Ca
and with Ag. Recalling that there is no connection between the energy
change and the activation energy, suggest and explain using a symbol
equation, why each graph differs from the one obtained for Be.
Calcium _______________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[6]
Silver ________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[3]
METALS: DISPLACEMENT REACTIONS (3)
A catalyst is defined as 'a substance which increases the rate of a
chemical reaction, but which is itself chemically unchanged at the end
of the reaction'. Although many substances have catalytic activity
attributed to them, because they speed up chemical reactions, the
absence of permanent chemical change is often assumed ...
Zinc is a better reducing agent than hydrogen, and so displaces this
less reactive element from its compounds; e.g.,
Because the above reaction is too slow to be useful as a convenient
laboratory preparation of dihydrogen, a few drops of aqueous copper(II)
sulfate are often added to increase the speed of reaction; and so the
preparative method is usually described by this equation:
However, as the aqueous sulfate ions are spectators in this reaction
mixture, and as zinc is also a better reducing agent than copper, there
will be (at least) two competing reactions; i.e.,
In principle, because copper(II) ions are better oxidizing agents than
hydrogen ions, zinc should preferentially displace copper; if this is
so, then the method might be better described by this equation:
Although this description is supported by the presence of copper powder
at the end of the reaction, alternatives are feasible - particularly
when one considers the possible involvement of copper(I) ions.
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. One consequence of the above definition is that a catalyst has no
effect on the yield of products (i.e., the position of equilibrium in a
closed system). Although the laboratory preparation of hydrogen gas is
normally executed in an open system, the yield does appear to decrease
as the amount of copper(II) ions increases. To examine quantitatively
this effect, the following hypothesis was investigated: 'For the
reaction of zinc with dilute sulfuric acid, the volume (V) of hydrogen
gas evolved decreases in linear proportion to the amount (M) of aqueous
copper(II) ions added; i.e., V = k × M + c'; the Table shows a summary
of the chosen conditions and raw data (no duplicate experiments were
executed).
Constants: zinc granules (0.260 g = 4.00 mmol); aqueous sulfuric acid
(1.00 mol dm-³; 100 cm³); aqueous copper(II) sulfate (1.00 mol dm-³);
room temperature (298 K) and pressure (100 kPa); thermostatted water-
bath; hydrogen gas collected with a 100 cm³ gas syringe.
Amount of Cu2+ added (M) / mmol |
0.50 |
1.00 |
1.50 |
2.00 |
2.50 |
Volume of H2 evolved (V) / cm³ |
84 |
73 |
61 |
47 |
36 |
(a) Noting that hydrogen gas was evolved in these experiments, suggest
one important safety precaution adopted. ______________________________
[1]
(b) Suggest one reason why higher values of the independent variable
were not examined. ____________________________________________________
[1]
Explain what would happen to the values of the dependent variable if,
during the investigation, the temperature increased (but the pressure
remained constant). ___________________________________________________
_______________________________________________________________________
[2]
(c) Plot all five data points, and then draw the best straight line.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
V |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
o 90_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
l |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
u |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
m |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
e |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
70_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
o |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
f |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
gas |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
50_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
(V) |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
/ |_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
cm³ 30_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
\_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
0_/_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
| | | | | | |
0 0.5 1.0 1.5 2.0 2.5 3.0
Amount of aqueous Cu(II) added (M) / mmol
[3]
Extrapolate this straight line to the vertical axis; the intercept on
this axis, which corresponds to the maximum volume of hydrogen gas that
can be obtained, is the value 'c' in the linearly proportional
relationship V = k × M + c. ___________________________________________
[1]
Determine the gradient of the graph; this (negative) value, 'k', is the
proportionality constant in the linearly proportional relationship
V = k × M + c. ________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[3]
Write a precisely worded conclusion based on the graph. _______________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[5]
(d) This linear relationship rearranges to M = (V - c) ÷ k. Determine
the amount of copper(II) ions which results in no hydrogen gas being
evolved (i.e., V = 0 cm³), by using yor values of 'k' and 'c' in this
rearranged equation. __________________________________________________
[2]
(e) Experiments were also executed using different metals, but with a
similar set of constants; the aqueous copper(II) ions were omitted.
Thus, when 4.00 mmol of magnesium was used, instead of zinc, the same
maximum volume of hydrogen gas was obtained. Explain briefly what
theoretical volume of dihydrogen would be expected using 4.00 mmol of:
Cadmium _______________________________________________________________
Strontium _____________________________________________________________
Lithium _______________________________________________________________
Gallium _______________________________________________________________
[8]
METALS: PRECIPITATION REACTIONS
A precipitation reaction is the chemical change which occurs when two
ionic reactants give an insoluble product from an aqueous solution;
Because an ionic compound dissociates into (hydrated) cations and
anions when dissolved in water, a better perspective of a precipitation
reaction is gained by constructing an ionic equation; i.e.,
Those ions that are not involved in the overall reaction, here Cu2+(aq)
and Cl1-(aq), are referred to as spectator ions; and, so as to focus on
the change that actually occurs, a net ionic equation is often written;
In order to be able to predict whether a precipitate will be formed
when two aqueous solutions are mixed, one needs to know the solubility;
i.e., the maximum amount of solute that can be dissolved in the solvent
water at a specific temperature. Extensive compilations of solubility
data are available in reference books (e.g., The Handbook of Physics
and Chemistry), though it is common practice to divide compounds into
three broad categories based on their solubilities in water at 20°C;
i.e., 'soluble' (> 10 g kg-¹), 'slightly soluble' (1 - 10 g kg-¹), and
'insoluble' (< 1 g kg-¹). These categories are then used to formulate
'solubility rules' - as exemplified by A ————®
A. All Group 1 and ammonium compounds are soluble.
B. All nitrates, hydrogencarbonates, and ethanoates are soluble.
C. Most chlorides, bromides, and iodides are soluble; the exceptions
are those containing silver(I) or lead(II) ions.
D. Most sulfates are soluble; CaSO4, SrSO4, and Ag2SO4 are slightly
soluble, whereas BaSO4 and PbSO4 are insoluble.
E. Most carbonates, phosphates, and sulfides are insoluble; the
exceptions are those containing Group 1 or ammonium ions.
F. Most hydroxides are insoluble; Ca(OH)2 and Sr(OH)2 are slightly
soluble, whereas Ba(OH)2 and Group 1 hydroxides are soluble.
1. Probably the most familiar example of a precipitation reaction is
that observed in the limewater test for carbon dioxide; the formation
of the characteristic milky-white precipitate is usually summarized as:
However, this symbol equation clearly requires explanation ... The gas
carbon dioxide, a covalently bonded compound, dissolves slightly in
water to form carbonic acid, H2CO3(aq), which partially dissociates;
Le Chatelier's Principle predicts that both equilibria will shift to
the right if aqueous carbonate ions are removed from solution; and, as
the ionic equation below shows, this will occur by precipitation:
(a) This ionic equation reveals a precipitation and a neutralization
reaction. Construct a net ionic equation for each reaction. ___________
_______________________________________________________________________
_______________________________________________________________________
[2]
(b) Continued bubbling of carbon dioxide through limewater results in
the (initially formed) precipitate redissolving. Construct the symbol
equation for this reaction. ___________________________________________
_______________________________________________________________________
[2]
2. The precipitation reaction between aqueous solutions of potassium
iodide and lead(II) nitrate was investigated quantitatively.
Using a pipet, 10.0 cm³ of aqueous lead(II) nitrate (0.100 mol dm-³)
was added to a test tube, and then, using a buret, 3.0 cm³ of an
aqueous solution of potassium iodide was added. The reaction mixture
was shaken thoroughly, and then, in the absence of a centrifuge,
allowed to stand for 1 hor before the height of the yellow precipitate
was measured (to within 0.5 mm). This procedure was repeated several
times, except different volumes of aqueous potassium iodide were used;
the Table shows a summary of the results.
Vol. KI(aq)/cm³ |
0.0 |
3.0 |
6.0 |
9.0 |
12.0 |
15.0 |
18.0 |
21.0 |
24.0 |
27.0 |
Ht. ppte./mm |
|
4.5 |
8.0 |
11.5 |
16.5 |
20.0 |
24.0 |
27.0 |
27.5 |
27.0 |
(a) Insert the missing value in the above Table, and then plot all ten
data points. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
27_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
24_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
21_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
Height of 18_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
precipitate 15_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
/ 12_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
mm 9_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
6_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
3_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
0_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
| | | | | | | | | |
0 3 6 9 12 15 18 21 24 27
Volume of KI(aq) / cm³
[3]
(b) Draw the two best straight lines, and then, by intersecting these
lines, determine the minimum volume of aqueous potassium iodide
required to react completely with 10.0 cm³ of aqueous lead(II) nitrate
(0.100 mol dm-³). _____________________________________________________
[3]
(c) Use this value, and the symbol equation, to state the concentration
of the aqueous potassium iodide solution used in this investigation.
_______________________________________________________________________
[1]
3. Soluble lead(II) ions, which inhibit the active sites of a number
of enzymes, are absorbed by diffusion across semi-permeable membranes;
so, soluble lead(II) nitrate is obviously more toxic than any insoluble
lead(II) compound. Nevertheless, the accidental ingestion of aqueous
lead(II) nitrate is rarely fatal, in humans at least, because insoluble
lead(II) ions are precipitated from the reaction between this solution
and the hydrochloric acid present in gastric juice. Construct both the
symbol and net ionic equations for the precipitation reaction between
aqueous solutions of lead(II) nitrate and hydrochloric acid. __________
_______________________________________________________________________
_______________________________________________________________________
[3]
METALS: LIGAND-EXCHANGE REACTIONS
A ligand-exchange reaction is the chemical change which occurs when one
ligand is replaced by another; a ligand (L) is a molecule or an ion
(coordinatively) bonded to a central atom or ion. This type of reaction
is exemplified by reactions summarized by this general equation:
Hydrated metal ions, formed when an ionic compound dissolves in water,
can be represented either as Mn+(aq) or as [M(H2O)6]n+(aq) - because
each metal ion is usually coordinated to six water molecules acting as
ligands. The rates of ligand-exchange between coordinated and solvent
water molecules have been investigated using labeled water; i.e.,
No attempt is made here to summarize the extensive research directed at
measuring and explaining the rates of these particular ligand-exchange
reactions; suffice to note that, as this order illustrates, .. Pb(II) >
K(I) > Cu(II) > Na(I) > Ca(II) > Zn(II) > Fe(II) > Mg(II) > Al(III) ..,
these rates do not correspond to the metal reactivity series.
[.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. Most molecules in living organisms are structurally very complex;
accordingly, in order to provide insight into some of their biological,
chemical, and physical properties, scientists often study simpler or
'model' compounds. Shown below are the structural formulae of two such
model compounds, which are readily prepared in two or three steps from
inexpensive reactants.
In each compound, the for nitrogens coordinated to the Cu(II) ion form
a square plane. Suggest the geometry of the compounds formed when:
A ligand is added above the plane. ____________________________________
Ligands are added above and below the plane. __________________________
[2]
Suggest what metal ion should replace Cu(II) in the above compounds for
either to be considered as a (marginally acceptable) model for studying
ligand-exchange reactions in:
Chlorophyll-b __________ Hemoglobin ___________ Vitamin-B12 ___________
[3]
2. Probably the most familiar example of ligand-exchange reactions is
that observed in the standard test for the hydrated copper(II) ion with
(excess) aqueous ammonia; formation of the characteristic 'royal-blue'
solution is usually summarized as:
(a) Color is an example of a qualitative variable; i.e., one where the
data are non-numerical or purely descriptive. In investigations, such a
variable can present problems in description (and, subsequently, in
interpretation), because of the inevitable element of personal opinion;
e.g., all ions corresponding to the formula [Cu(H2O)x(NH3)y]²+(aq) are
shades of blue, and the [Cu(H2O)2(NH3)4]²+(aq) ion has been variously
described as dark-blue, deep-blue, royal-blue, and indigo. Fortunately,
providing a suitable instrument is used, color can be expressed as a
quantitative variable; i.e., one where the data are collected either by
counting or by measuring. Complete the Table below, by calculating the
frequency of the photon absorbed by each ion, using the formula n = fl,
where: n is the speed of light (3 × 108 ms-1); f is the frequency; and
l is the wavelength.
Aqueous ion Color |
Wavelength of
photon absorbed
(l) / × 10-9 m |
Frequency of
photon absorbed
(f) / × 1014 Hz |
[Cu(H2O)6]2+ blue |
820 |
|
[Cu(H2O)5(NH3)]2+ ?-blue |
740 |
|
[Cu(H2O)4(NH3)2]2+ ?-blue |
675 |
|
[Cu(H2O)3(NH3)3]2+ ?-blue |
630 |
|
[Cu(H2O)2(NH3)4]2+ ?-blue |
585 |
|
[Cu(H2O)(NH3)5]2+ ?-blue |
635 |
|
(b) The symbol equation above summarizes a series of reversible ligand-
exchange reactions that involve the successive replacement of four
molecules of water with those of ammonia. Construct the symbol equation
for one of these reversible ligand-exchange reactions. ________________
_______________________________________________________________________
[2]
For a series of reversible reactions, using Le Chatelier's Principle to
predict the overall position of equilibrium is appropriate but not
straightforward. Usually, in investigating such series, the first step
is to measure various physical quantities; e.g., the concentrations of
reactants and products. Calculate the concentration of pure water. ____
_______________________________________________________________________
_______________________________________________________________________
[2]
(c) In attempting to extend the scope of any given reaction, scientists
often consider variations (or develop hypotheses) based on the patterns
inherent in the Periodic Table; e.g., variants of Cu(II), H2O, and NH3
might reasonably include Ag(II), H2S, and PH3, respectively. An equally
rich source of variation lies in the patterns of organic chemistry ...
Each homologous series can be represented by a general formula; e.g.,
CnH2n+2 for chain alkanes. The so-called 'trivial' member of a series
is the compound whose formula corresponds to n = 0; this is dihydrogen
for the alkanes, whose first true member is methane. Several members of
an homologous series, which contains ammonia as the trivial member, act
as ligands in similar exchange reactions. Determine the general formula
of this series, and draw the structural formulae of its first two true
members. ______________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[3]
METALS: DISPLACEMENT REACTIONS (4)
Pure water is a covalently bonded compound with some unusual physical
properties; these include a high boiling point, a high surface tension,
a maximum density as a liquid, a high latent heat of vaporization, and
the highest heat capacity of any liquid. All of these properties are
partially attributable to water's extended structure, which involves
'hydrogen bonding'; this type of bonding is present in many molecules,
(for example, and in particular, between the strands of DNA double
helices), but its extent in pure water appears to be truly exceptional.
However, pure water is rarely encountered in Nature because it is such
an excellent solvent in which many solutes dissolve to varying degrees.
Furthermore, the resulting aqueous solutions might quite reasonably be
expected to have different properties; e.g., heat capacities ...
1. A research chemist decided to use a metal displacement reaction,
to examine this hypothesis: 'As the concentration (C) of chloride ions
increases, the specific heat capacities (c) of the aqueous solutions
decrease in linear proportion; i.e., c = k × C + i'; the Table shows a
summary of the chosen reaction conditions and raw data.
Constants: heat energy supplied (H = 0.540 kJ); powdered zinc (excess);
aqueous copper(II) chloride (0.25 mol dm-³; 10.0 cm³ via bulb pipet);
starting temperature of reaction mixture (20.0°C ± 0.2°C); thermometer
(0-50°C, graduated to 0.1°C); reaction vessel (thermos flask).
NaCl(s)
added /
g |
Concentration
of Cl1- (C) /
mol dm-³ |
Density
r /
kg dm-³ |
Total mass
m /
kg |
Temperature
rise (DT) /
K |
Specific heat
capacity (c) /
kJ kg-¹ K-¹ |
0.00 |
0.50 |
1.02 |
0.0102 |
13.8 |
|
0.59 |
1.50 |
1.05 |
0.0105 |
14.9 |
|
1.17 |
2.50 |
1.08 |
0.0108 |
14.8 |
3.38 |
1.17 |
2.50 |
1.08 |
0.0108 |
15.3 |
3.27 |
1.17 |
2.50 |
1.08 |
0.0108 |
15.5 |
3.25 |
1.76 |
3.50 |
1.11 |
0.0110 |
15.7 |
|
2.34 |
4.50 |
1.14 |
0.0113 |
15.7 |
|
(a) The standard equation H = m × c × DT rearranges to c = H ÷ m × DT.
Using this rearranged equation, calculate the missing values for the
specific heat capacity (c), and insert these data in the Table.
[3]
(b) The first specific heat capacity for C = 2.50 mol dm-³ appears to
be an anomalous result. Suggest one possible reason for this anomaly.
_______________________________________________________________________
[1]
(c) Label both axes, plot all seven data points, and then draw a best
curve through as many points as is sensible.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
3.8_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
3.6_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
3.4_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
3.2_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
3.0_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
0.0_/_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|
| | | | | | | | | |
0.0 1.0 2.0 3.0 4.0
[5]
Complete this precisely worded conclusion, based on the tabulated and
graphical data. 'As determined from displacement reactions at room
temperature (20°C), using excess zinc powder and 10.0 cm³ of aqueous
copper(II) chloride (0.25 mol dm-³), specific heat capacities (c) of
aqueous solutions of sodium chloride decreased as the concentration (C)
of chloride ions __________ (within the range ___________________). The
curve indicates that these two variables are not in ___________________
to each other; i.e., ________________'
[4]
(d) Here, unfortunately, values of the dependent variable outside the
range of the independent variable examined cannot be obtained either
by calculation, because of the non-linear relationship between the
variables, or by extrapolation, because of the graph's limited range.
Nevertheless, from inspection of the tabulated data, guesstimate a
value for the specific heat capacity (c) of saturated aqueous sodium
chloride (C = 6.5 mol dm-³). __________________________________________
[1]
(e) Complete the following paragraph, which incorporates one partial
interpretation and one possible consequence of the results of the above
investigation. 'The specific heat capacity of an aqueous solution
decreases as the concentration of chloride ions __________, presumably
because the amount of hydrogen bonding __________. In consequence, its
ability to act as a heat buffer, in intra-cellular or extra-cellular
environments, will _________; furthermore, because all biochemical
reactions are controlled by temperature-sensitive enzymes, this may
result in __________ efficiency of these biological catalysts.'
[4]
METALS: SODIUM COMPOUNDS
The commercial applications of sodium compounds in developed countries
are so numerous that any brief overview has obvious limitations. But,
as a starting point, it does appear appropriate to state that their
extensive use is attributable to the following four aspects. Firstly,
sodium compounds are cheap to manufacture; in part, this reflects the
abundance and accessibility of the raw materials (e.g., sea water and
rock salt both contain sodium chloride). Secondly, sodium compounds are
usually soluble in water and have high thermal stability. Thirdly, in
most applications, it is the chemical properties of the anions which
are important; the sodium cations are usually mere spectators. And
fourthly, sodium cations are (relatively) non-toxic.
1. Brine is a concentrated aqueous solution of sodium chloride. Three
useful products are obtained from the electrolysis of brine, as this
overall symbol equation shows:
The ionic equations for the reactions occurring at the electrodes are:
(a) Label this diagram of a (Diaphragm) electrolytic cell with: Cl2(g);
H2(g); NaOH(aq); Saturated brine; Steel cathode; and, Titanium anode.
(b) A typical electrolytic cell used in industry, which operates
continuously at 1.5 kA and 4 V, must have at least five hazard symbols
clearly visible: 'Corrosive' [i.e., NaOH(aq)]; 'Explosive' [H2(g)];
'Highly flammable' [H2(g)]; 'Oxidizing' [Cl2(g)]; and 'Toxic' [Cl2(g].
State three items of safety clothing necessary for workers or visitors.
_______________________________________________________________________
[3]
(c) The diaphragm is selectively permeable, allowing hydroxide (but not
chloride) ions to enter the cathodic compartment. Suggest and explain
one advantage in this selectivity. ____________________________________
_______________________________________________________________________
_______________________________________________________________________
[2]
The diaphragm also prevents mixing of the gaseous electrode products,
dihydrogen and dichlorine, which together react explosively. Construct
the symbol equation for this reaction. ________________________________
_______________________________________________________________________
[1]
(d) Suggest and explain one reason why electrodes made of iron would be
unsuitable. ___________________________________________________________
_______________________________________________________________________
[2]
2. Aqueous sodium carbonate neutralizes aqueous hydrochloric acid:
As this ionic equation shows, it is the carbonate (and hydrogen) ions
which are the reactants: the sodium (and chloride) ions are spectators.
Construct a similar ionic equation for the precipitation reaction
between aqueous solutions of sodium hydroxide and iron(II) sulfate.
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
[2]
3. Sodium carbonate is thermally very stable; indeed, it melts before
it decomposes at temperatures above 1000°C:
By contrast, sodium hydrogencarbonate is thermally much less stable;
thus, at temperatures above 90°C, it decomposes to give three products.
Construct the symbol equation for this thermal decomposition. _________
_______________________________________________________________________
_______________________________________________________________________
[2]
4. The Table below presents one illustrative application for eight
sodium compounds. Complete this Table by inserting the correct formula
from this list: [CH3(CH2)16COO]Na ; NaCl ; NaHCO3 ; NaNH2 ; NaOCl ;
NaOH ; Na2[B2(O2)2(OH)4] ; Na2CO3.
Formula of compound
(Application) |
Property of compound or anion which
is important in specified application |
(Strong base) |
Completely dissociates in water; the hydroxide
ions neutralize aqueous hydrogen ions. |
(Baking powder) < |