Question

(a) State what happens when:
(i) Copper sulphate solution is electrolysed using a platinum anode.
(ii) Excess ammonia is passed through an aqueous solution of lead nitrate.
(iii) Washing soda is left exposed to dry air for long time.
(iv) Silver nitrate solution is added to dil. HCl.
(v) Hot concentrated sulphuric acid is added to sodium chloride crystals.

Answer 1

(i) When copper sulphate solution is electrolysed using a platinum anode, blue colour of the solution fades and sulphuric acid is formed. The electrolytic reaction occur as follows:
Electrolytic dissociation occur as follows:
CuSO₄ ⇌ Cu²⁺ + SO₄^2-
H₂O ⇌ H⁺ + OH^-
At cathode: Cu²⁺ and H⁺ migrate towards the cathode. Cu²⁺ ions are preferentially discharged than H⁺ ions and form neutral copper atoms.

Cu²⁺ + 2e^- → Cu

At anode: SO₄^2- and OH^- ions migrate towards anode. OH^- ions are preferentially discharged to form neutral OH which further react with each other to give water and oxygen.

OH^- - e^- → OH

4OH → 2H₂O + O₂

(ii) When excess ammonia is passed through an aqueous solution of lead nitrate, a white precipitate of lead hydroxide is formed.

Pb(NO₃)₂ + 2NH₄OH → Pb(OH)_2↓ + 2NH₄NO₃

(iii) When washing soda is left exposed to the dry air for a long time, it loses its water of crystallisation and turns amorphous. This property of washing soda is known as efflorescence.

(iv) When silver nitrate solution is added to dil. HCl, a curdy-white precipitate of silver chloride is obtained which is soluble in ammonium hydroxide and insoluble in dil. nitric acid.

AgNO₃ + HCl → AgCl ↓ + HNO₃

(v) When hot concentrated sulphuric acid is added to sodium chloride crystals at temperature below 200⁰C, sodium hydrogen sulphate and hydrogen chloride are formed. The latter escapes as a gas.

NaCl + H₂SO₄ → NaHSO₄ + HCl↑