EXPERIMENTAL: PREPARATION OF WATER-SOLUBLE SALTS USING NEUTRALIZATION
AND METAL DISPLACEMENT REACTIONS
Introduction
Reactions used to prepare salts, which are ionic compounds made up of
ions other than H1+, OH1-, or O2-, include neutralization (i.e., the
reaction between an acid and a base), displacement (i.e., when a more
reactive element displaces a less reactive element from its compound),
and precipitation (i.e., when two soluble ionic compounds react to form
a precipitate). Shown below are equations which summarize exemplars of
these three reaction types; neutralization (1 - 4), displacement (5 and
6), and precipitation (7).
(1) NiO(s) + 2HCl(aq) ——————————————® NiCl2(aq) + H2O(l)
(2) Mg(OH)2(s) + H2SO4(aq) ——————————® MgSO4(aq) + 2H2O(l)
(3) PbCO3(s) + 2HNO3(aq) ————————————® Pb(NO3)2(aq) + H2O(l) + CO2(g)
(4) 2NH3(aq) + H33PO4(aq) ———————————® (NH4)2HPO4(aq)
(5) Zn(s) + H2SO4(aq) ———————————————® ZnSO4(aq) + H2(g)
(6) 2Al(s) + 3Zn(NO3)2(aq) ——————————® 2Al(NO3)3(aq) + 3Zn(s)
(7) 2KOH(aq) + FeSO4(s) —————————————® 2Fe(OH)2(s) + K2SO4(aq)
Whereas a precipitation reaction is commonly used for the preparation
of an insoluble salt, neutralization and metal displacement reactions
are often those of choice for the preparation of water-soluble salts.
[... K > Ca > Na > Mg > Al > Mn > Zn > Fe > Co > Pb > (H) > Cu > Ag ..]
First, using a neutralization reaction, you are required to prepare
crystals of either copper(II) ethanoate or copper(II) chloride, using
appropriate starting materials from: aqueous ethanoic acid; aqueous
sulfuric acid; aqueous hydrochloric acid; copper; copper(II) oxide;
copper(II) carbonate; copper(II) sulfate; and copper(II) nitrate.
And second, using a metal displacement reaction, you are required to
prepare crystals of a second ethanoate or chloride. One starting
material should be some of your freshly prepared copper(II) ethanoate
or copper(II) chloride; and the other should be a suitably correct
choice from: cobalt; cobalt(II) carbonate; manganese; manganese(III)
carbonate; silver; silver(I) carbonate; zinc; and zinc carbonate. |
Before starting your experimental work, be sure to complete this flow
chart - which should summarize your choice of reactants and products.
Method (1) - via a Neutralization Reaction
1. Collect the following (for Methods 1 and 2): safety glasses; Bunsen
burner kit; 100 cm³ glass beaker; measuring cylinder (25 cm³); stirring
rod; filter funnel; 2 filter papers; retort stand and clamp; 2 splints;
evaporating dish; 2 petri dishes; pH paper; and starting materials.
2. Set up, ready for use, the Bunsen burner kit and the filtration
apparatus; until required, the evaporation and petri dishes should be
lying face downwards on tissue paper.
3. Using the measuring cylinder, place 25 cm³ of dilute acid into the
beaker equipped with a stirring rod; check the pH of the acid.
4. Warm the acid using a small Bunsen flame (or a boiling water-bath
if a metal is used as one of the starting materials).
5. [Throughout this step, you should be prepared to switch off the
Bunsen if the reaction becomes too vigorous. Furthermore, you should
try and maintain a volume of about 25 cm³ for the reaction mixture, by
adding small volumes of water as and when required.] To the warm acid,
add the solid starting material in small portions until it is in slight
excess; that is, until no more dissolves and the pH paper no longer
turns red.
6. [The reaction mixture contains an excess of insoluble starting
material and the required salt dissolved in water.] Dampen the filter
paper and, using the stirring rod as a guide, filter the warm reaction
mixture into the evaporating dish; it is good technique to wash both
the beaker and filter paper with a little water.
7. [The filtrate (and washings) should contain only the required salt
dissolved in water.] Transfer half the filtrate to a petri dish; cover
this dish with paper, (labeled with name, contents, and date), and
allow the solution to crystallize at room temperature.
8. Using Scotch-tape, attach a small sample of these crystals to this
worksheet; to be safe, you must assume the compound is toxic.
Method (2) - via a Displacement Reaction
1. To the second half of the filtrate (see step 7 above), stir in the
metal powder in small portions until in slight excess; that is, until
there are no more changes to the supernatant.
2. Dampen the filter paper and, using the stirring rod as a guide,
filter this reaction mixture into a second petri dish ....
3. Cover this dish with paper, (labeled with name, contents, and
date), and then allow the solution to crystallize at room temperature.
4. Using Scotch-tape .......... you must assume the compound is toxic.
Notes
Method 1 can be used to prepare most water-soluble salts, providing the
solid starting material, (i.e., metal, oxide, hydroxide, or carbonate),
is insoluble in water. However, a 'titration' method must be used if
the starting material is soluble in water (i.e., all Group 1 compounds,
ammonium compounds, nitrates, and ethanoates).
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