Name and state in brief the process which is used to prepare sodium hydroxide from sodium chloride. In this process along with the main product two gases 'X' and 'Y' are also given off at the two electrodes. Name 'X' and 'Y' specifying the name of their respective electrode at which each gas is obtained. One of these gases when reacts with dry calcium hydroxide produces a compound 'Z' which is widely used in water treatment plants and textile industries. Name Z and write chemical equation for the reaction involved in its formation.
Generated by claude-sonnet-4-6 · 2026-06-15 06:46 · grounding rag
Model Answer
Process: Chlor-Alkali Process
When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes to give sodium hydroxide. This process is called the chlor-alkali process.
$$2\text{NaCl(aq)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{Cl}_2\text{(g)} + \text{H}_2\text{(g)}$$
Gas X — Chlorine (Cl₂): obtained at the anode.
Gas Y — Hydrogen (H₂): obtained at the cathode.
Compound Z — Bleaching Powder [Ca(ClO)₂]
Chlorine gas (X) reacts with dry slaked lime [Ca(OH)₂] to produce bleaching powder. It is widely used in water treatment plants (to kill germs) and in the textile industry (for bleaching).
Chemical equation:
$$2\text{Ca(OH)}_2 + 2\text{Cl}_2 \rightarrow \text{Ca(ClO)}_2 + \text{CaCl}_2 + 2\text{H}_2\text{O}$$
Source: Chapter 2, Section 2.4.3
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Explanation
- The examiner expects the name and brief definition of the chlor-alkali process, the balanced equation, correct identification of both gases with their respective electrodes, the name of compound Z (bleaching powder), its two uses, and its formation equation — all five elements map to the 5 marks.
- A common mistake is reversing the electrodes: Cl₂ → anode, H₂ → cathode.
- Write the bleaching powder equation exactly as given in the textbook; the examiner checks it carefully.
- "Dry calcium hydroxide" = dry slaked lime = Ca(OH)₂ — use this term in your answer.