PREPARATION OF SALTS YOU SHOULD BE ABLE TO: QUICK POINTS PREPARATION OF SALTS METHODS OF PREPARING SOLUBLE SALTS METHOD 1 EXAMPLE
■ Investigate the reactions of non-oxidising acids with metals, carbonates, hydrogen carbonates, bases.
■ Investigate the reaction of bases with ammonium salts.
■ Identify an appropriate method of salt preparation based on the solubility of the salt.
■ Distinguish between acid salts and normal salts.
A salt is formed when the hydrogen ion in an acid is replaced by a metal or ammonium ion.
Salts can be soluble or insoluble.
A variety of salts can be formed using the methods outlined below. The salt formed may be soluble or insoluble. A soluble salt is usually prepared by reacting the acid solution with the base, metal or carbonate. Heat that solution to remove excess water, filtering and leaving a small quantity of the filtrate to evaporate.
Crystals formed in this way usually have their water of crystallization and are termed hydrated salts. If they are heated to dryness and all the water of crystallization are removed, the salts become anhydrous.
Soluble salts can be produced by different means.
Acid + Base ––––– Salt + Water
Acids react with metallic oxides or hydroxides (commonly called bases) to produce a salt and water. Soluble metallic hydroxides are called alkalis.
Sulphuric acid + zinc oxide ––– zinc sulphate + water
H2SO4 (aq) + ZnO (s) –––– ZnSO4 (aq) + H2O (l)
ZnO is the base.
Hydrochloric acid + potassium hydroxide –––––– potassium chloride + water
HCl (aq) + KOH (aq) –––––––– KCl (aq) + H2O (l)
KOH is the alkali (also a base)
METHOD 2
Acid + Metal carbonate ––––––– Salt + Water + Carbon dioxide
Acids react with carbonates to produce salts. Carbon dioxide is released from the carbonates during the process.
EXAMPLE
Nitric acid + magnesium carbonate –––– magnesium nitrate + water + carbon dioxide
2HNO3 (aq) + MgCO3 (s) ––––––– Mg(NO3)2 (aq) + H2O (l) + CO2 (g)
METHOD
Acid + Metal ––––––– Salt + Hydrogen Acids react with metals to produce a salt and release hydrogen gas.
Some less reactive metals will not react with dilute acids.
EXAMPLE
Hydrochloric acid + calcium ––––––– calcium chloride + hydrogen
2HCl (aq) + Ca (s) ––––––– CaCl2 (aq) + H2 (g)
QUICK CHECK
How would you test for the gas produced when metals react with acids?
Insoluble salts are usually prepared by mixing two solutions of soluble salts together, then filtering, washing and drying the precipitate formed. This is also called a double decomposition reaction.
EXAMPLE
Lead nitrate + sodium chloride ––––––– lead chloride + sodium nitrate
Pb (NO3)2 (aq) + 2 NaCl (aq) ––––––– PbCl2 (s) + 2 NaNO3 (aq)
The lead chloride formed is the insoluble salt, which would be seen as a precipitate.
A typical method is shown below.
Salts can also be termed normal or acid salts. If all the hydrogen ions are replaced in the acid, a normal salt is formed. However, if the hydrogen ions were only partially replaced, an acid salt is formed. Acid salts are formed from dibasic and tribasic acids.
EXAMPLE
H2CO3 – Carbonic acid is dibasic (has 2 hydrogen ions available to be replaced)
If carbonic acid reacts with sodium hydroxide, two salts can be formed.
2 NaOH (aq) + H2CO3 (aq) ––––––– Na2CO3 (aq) + 2 H2O (l)
– normal salt sodium carbonate is formed.
NaOH (aq) + H CO (aq) ––––––– NaHCO (aq) + 2 3 3 H2O (l)
– acid salt sodium hydrogen carbonate or sodium bicarbonate is formed
Other methods of preparing salts include direct combination; for example, with the preparation of iron III chloride. Iron and chlorine are heated directly to produce the salt.
FURTHER PRACTICE
(i) Outline the most suitable method of preparing the following salts:
(a) Magnesium sulphate
(b) Copper carbonate
(c) Aluminium chloride
Write equations for all reactions.
(ii) Research the uses of the following salts: (a) Calcium sulphate
(b) Sodium nitrate
(c) Sodium hydrogen carbonate