5e. Salts

Question 1

How is a salt formed?

Answer 1

A salt is formed when the hydrogen ions, H⁺, of the acid is partially or completely replaced by a metal ion or an ammonium ion.

Question 2

Water of Crysallisation

Answer 2

Anhydrous salts: salts that do not contain water of crystallisation

• CuSO₄.H₂O (blue crystals)
• CoCl₂.6H₂O (pink crystals)

Hydrated salts: salts that contain water of crysallisation

• CuSO₄ (white powder)
• CoCl₂ (blue crystals)

Water of crystallisation can be lost by heating. Evaporation to dryness should not be used to obtain hydrated crystals.

Question 3

Preparation of Insoluble Salts

Steps

Answer 3

Preparation of insoluble salts is by precipitation, involving mixing 2 aqueous solutions to obtain an insoluble salt.
e.g. BaSO₄(s), AgCl(s), PbI₂(s)

Example: Preparation of barium sulfate (with aqueous barium nitrate and aqueous sodium sulfate)

1. Mix the two aqueous solutions in a beaker.
2. Stir well. A white precipiate (insoluble salt) would be formed.
3. Filter the mixture to obtain barium sulfate as the residue.
4. Wash the residue with deionised water.
5. Dry the residue between sheets of filter paper.

Question 4

Why is an insoluble reactant (e.g. calcium carbonate) unsuitable for preparing an insoluble salt (e.g. calcium sulfate)?

Calcium carbonate + sulfuric acid

Answer 4

Calcium carbonate reacts with sulfuric acid to form a layer of insoluble salt, calcium sulfate, which will coat onto the surface of the calcium carbonate, which prevents further reactoin between the calcium carbonate and sulfuric acid.
This means that the calcium sulfate salt formed will be contaminated with unreacted calcium carbonate. The calcium sulfate salt prepared is not pure.

Question 5

Preparation of Soluble Non-S.P.A. Salts

Steps

Answer 5

Preparation of soluble non-S.P.A. salts is by reaction of acid with excess insoluble carbonate, insoluble base, or moderately reactive metal (Note: only moderatly reactive metals, such as Mg, Zn, Fe, should be used. Reactive metals such as Na, K, will react explosively with acids. Metals below hydrogen in the reactivitiy series such as Cu, Ag, Au, will not react with acid).
e.g. ZnSO₄(aq), ZnCl(aq), ZnNO₃(aq), CuSO₄(aq), MgCl₂(aq)

Example: Preparation of zinc sulfate (with zinc metal and dilute sulfuric acid)

1. Heat the acid in a beaker. (To speed up rate of reaction.)
2. Add excess insoluble base / insluble carbonate / moderately reactive metal to the acid. (To ensure that all the acid has completely reacted with the insoluble base / insoluble carbonate / moderately reactive metal. This will ensure that the resulting salt solution is free from acid contamination.)
3. Stir the mixture well. (To mix the reactants well for complete reaction.)
4. Filter the mixture to remove excess insoluble base / insoluble carbonate / metal as the residue.
5. Collect the filtrate which is the soluble salt solution needed.
6. Heat the filtrate to obtain a hot, saturated solution.
7. Cool the hot, saturated solution to obtain the crystals. (As temperature decreases, solubility of salt decreases. Hence, salt will crystalise from the solution)
8. Filter the mixture to collect crystals as the residue.
9. Dry the crystals between sheets of filter paper.

Question 6

Why do we not prepare soluble S.P.A salt (e.g. sodium sulfate) the same way as soluble non-S.P.A salt (e.g. zinc sulfate)?

Answer 6

Acid + metal: H₂SO₄(aq) + 2Na(s) → Na₂SO₄(aq) + H₂(g)

For acid + metal reaction, the reaction between Group 1 metals and acids is highly explosive and not exeprimentally feasible to carry out safely.

Acid + excess carbonate: H₂SO₄(aq) + Na₂CO₃(s) → Na₂SO₄(aq) + H₂O(l) + CO₂(g)

Acid + excess base: H₂SO₄(aq) + 2NaOH(s) → Na₂SO₄(aq) + H₂O(l)

For acid + excess carbonate and acid + excess base reaction, when excess soluble base / soluble carbonate is added to acid, it dissolves, and no residue will be observed. This excess soluble reactant cannot be removed by filtration.

Question 7

Preparation of Soluble S.P.A. Salts

Steps

Answer 7

Preparation of soluble S.P.A. salts is by titration, involving of acid with alkali or soluble carbonate.
e.g. NaCl(aq), KNO₃(aq), (NH₄)₂SO₄(aq)

Example: Preparation of sodium sulfate (with aqueous sodium hyroxide and dilute sulfuric acid)

1. Pipette a known volume of alkali / soluble carbonate into a conical flask.
2. Add a few drops of indicator.
3. Add acid from burette to alkali / souble cabonate until the end-point. End-point is reached when the indicator changes colour.
4. Record the volume of acid needed to neutralise the alali / souble carbonate.
5. Repeat titration until consistent results are obtained.
6. Repeat the experiment without adding indicator by adding the recorded volume of acid into the conical flask containing the known volume of alkali / soluble carbonate.
7. Heat the salt solution to obtain a hot, saturated solution.
8. Allow hot, saturated soluiton to cool for the salt crystals to form.
9. Filter the mixture to collect the crystals as residue.
10. Dry the crystals between sheets of filter paper.