(a) Explain chlor-alkali process with chemical equation. Name the products formed at anode and cathode. OR (b) Write the preparation of following compounds with balanced chemical equation: (i) Baking soda (ii) Bleaching powder (iii) Plaster of Paris

This is an internal-choice question; either part (a) OR part (b) is to be attempted fully for 3 marks.

(a) Chlor-alkali process

When electricity is passed through a concentrated aqueous solution of sodium chloride (called brine), it decomposes to give sodium hydroxide. The process is called the chlor-alkali process because of the products formed — 'chlor' for chlorine and 'alkali' for sodium hydroxide:

$$2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{electricity}} 2NaOH(aq) + Cl_2(g) + H_2(g)$$

• Chlorine gas ($Cl_2$) is liberated at the anode.
• Hydrogen gas ($H_2$) is liberated at the cathode.
• Sodium hydroxide (NaOH) solution is formed near the cathode.

All three products are useful: $Cl_2$ is used to manufacture bleaching powder and in water treatment/PVC; $H_2$ is used as a fuel and in the manufacture of ammonia and margarine; NaOH is used in soap and detergent manufacture, paper making, and in the chlor-alkali based chemical industry.

(b) Preparation of important chemicals from common salt

(i) Baking soda (sodium hydrogencarbonate, $NaHCO_3$) is prepared from sodium chloride, water, carbon dioxide and ammonia (Solvay-type process):

$$NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$$

It is a mild, non-corrosive basic salt used in cooking, in antacids, and in soda-acid fire extinguishers.

(ii) Bleaching powder ($Ca(ClO)_2$) is produced by the action of chlorine gas (obtained from the chlor-alkali process) on dry slaked lime, $Ca(OH)_2$:

$$2Ca(OH)_2 + 2Cl_2 \rightarrow Ca(ClO)_2 + CaCl_2 + 2H_2O$$

It is used for bleaching cotton and linen in textile industries and wood pulp in paper factories, as an oxidising agent in chemical industries, and to disinfect drinking water.

(iii) Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$, calcium sulphate hemihydrate) is obtained by heating gypsum ($CaSO_4 \cdot 2H_2O$) at 373 K, which drives off part of its water of crystallisation:

$$CaSO_4 \cdot 2H_2O \xrightarrow{373\,K} CaSO_4 \cdot \tfrac{1}{2}H_2O + 1\tfrac{1}{2}H_2O$$

It is a white powder that, when mixed with water, changes back into hard gypsum, so it is used by doctors for supporting fractured bones and for making toys and decorative materials.