METALS: NICKEL
Nickel, which is relatively rare in the Earth's crust (0.008%), is
usually found as a sulfide (e.g., in the ore millerite). This element 
is a typical transition metal, as evinced by its high melting point 
(1535°C), high density (8.91 g cm-³), variable oxidation states [e.g., 
Ni(II) and Ni(III)], formation of coloured compounds (which are often 
green), and catalytic activity (e.g., it is used in the hydrogenation 
and dehydrogenation of organic compounds).
[.. K > Cs > Ca > Na > Mg > Al > Fe > Ni > Sn > (H) > Cu > Hg > Ag ..]
1. Suggest how nickel can be extracted from nickel(II) oxide, which is obtained by roasting its sulfides in air. By chemical reduction with dihydrogen / carbon (... lower tendency to form carbides) / aluminium [1] 2. Perhaps coincidentally, the thermal stabilities of nitrates appear to parallel the reactivity series. For example, mercury(II) nitrate decomposes on very gentle heating, to give a silvery liquid, nitrogen dioxide, and dioxygen: whereas, nickel(II) nitrate decomposes only on moderately strong heating, to give a black solid, nitrogen dioxide, and dioxygen: and, typical of Group 1 nitrates, caesium nitrate decomposes only on very strong heating, to give a pale-yellow solid and dioxygen. (a) Construct the symbol equation for each of these decompositions.
                                                                    [6]
(b) The volume (V1) of one mole of any gas at room temperature (25°C =
298 K; T1) and pressure (100 kPa; P1) is 24000 cm³; furthermore, the
following relationship holds true for gases:
                          P1 × V1     P2 × V2
                         ¾¾¾¾¾ = ¾¾¾¾¾
                             T1         T2
Determine the volume (V2) of nitrogen dioxide, at room temperature and
low pressure (5 kPa; P2), obtained from the thermal decomposition of
3.66 g of nickel(II) nitrate - as follows.
Calculate the molar mass of nickel(II) nitrate.  Molar mass (M) of
NiN2O6 = (1 × 59) + (2 × 14) + (6 × 16) = 59 + 28 + 96 = 183 g mol-¹
Calculate the number of moles of nickel(II) nitrate in 3.66 g of the
compound.  Moles (n) = mass (m) ÷ molar mass (M) = 3.66 ÷ 183 = 0.02
Using the symbol equation, determine the number of moles of nitrogen
dioxide obtained from this number of moles of nickel(II) nitrate.
0.02 moles of the nitrate will give 0.04 moles of nitrogen dioxide.
Calculate the volume (V1) of gas obtained at room temperature (T1) and
pressure (P1).  V1 = 0.04 × 24000 cm³ = 960 cm³
And finally, using the above relationship, calculate the volume (V2) of
gas at the decreased pressure (P2).
                                                                    [9]
3.  When powdered nickel is added to aqueous mercury(II) nitrate, the
grey solid rapidly dissolves, the colourless solution changes to green,
a silvery liquid forms, and the temperature of the solution increases.
Construct the net ionic equation for this redox reaction, complete with 
a qualitative indication of the heat energy change.
                                                                    [3]
4.  Suggest one reason why aqueous solutions of metal nitrates should
not be discharged into the environment.  Soluble nitrate ions can lead
to eutrophication. / Metal ions can accumulate up the trophic levels.
                                                                    [1]

5.  Shown below is a diagram of an electrolytic cell used to nickel-
plate an aluminium object.
[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) The mass (m) of the object increased by 0.383 g in 20 minutes. Write an ionic equation for the reaction which occurs at the cathode.
Calculate the number of moles (n) of nickel deposited at the cathode.  
n = mass (m) ÷ molar mass (M) = 0.383 ÷ 59 = 0.00649
Calculate the quantity of electricity (Q) required to deposit this 
number of moles.  Q = n × z × F = 0.00649 × 2 × 96500 = 1253 C
And finally, calculate the current (I) that flowed in the circuit.
I = Q ÷ t = 1253 ÷ (20 × 60) = 1.04 A
                                                                    [7]
(b) State and explain what would be observed for each of the following, 
if the polarities of the above circuit were reversed.
Nickel strip  A grey solid would be deposited, as the aqueous Ni(II)
ions are reduced.
Aluminium object  This would become duller / be coated thinly with a
white solid (... i.e., anodized), as oxygen gas evolves from the anode.
Electrolyte  The solution would change from green to colourless, as the
concentration of Ni(II) ions decreases, leaving dilute nitric acid.
                                                                    [6]
6.  Nickel's use as a catalyst is exemplified by the dehydrogenation of
ethylbenzene to phenylethene. [This alkene, better known as styrene, is
the monomer in the manufacture of poly(phenylethene).]
Suggest and explain two advantages in using a high temperature in the
above reaction.  It increases the speed of reaction, because the
increased kinetic energies of the particles means that more have the 
required energy for successful collisions; and it increases the yield 
of phenylethene, because the position of equilibrium moves to oppose 
the temperature decrease of this endothermic reaction.
                                                                    [4]
Dr. R. Peters Next Contents' List & Teacher's Notes