The decomposition of silver chloride in sunlight and the displacement of copper from copper sulphate solution by zinc are both redox reactions. For each reaction: (i) write the balanced chemical equation, and (ii) identify which substance is oxidised and which is reduced, justifying your identification in terms of gain or loss of oxygen or hydrogen.
Generated by claude-sonnet-4-6 · 2026-06-26 00:56 · grounding rag
Model Answer
Reaction 1: Decomposition of Silver Chloride
(i) Balanced Chemical Equation:
$$2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)$$
(ii) Identification of Oxidation and Reduction:
- Silver chloride (AgCl) is reduced — Ag⁺ in AgCl loses oxygen/chlorine and silver metal is formed; alternatively, AgCl loses a more electronegative partner. In terms of the textbook definition: Ag gains no oxygen, so it undergoes reduction (loss of the combined state to free metal, i.e., loss of oxygen/non-metal).
- Chloride is oxidised — chlorine is released as Cl₂ (loss of hydrogen / gain of a partner that reduces it is reversed); Cl⁻ loses electrons to form Cl₂.
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Reaction 2: Displacement of Copper by Zinc
(i) Balanced Chemical Equation:
$$\text{Zn}(s) + \text{CuSO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{Cu}(s)$$
(ii) Identification of Oxidation and Reduction:
- Zinc (Zn) is oxidised — Zinc loses electrons and combines with the sulphate, i.e., it goes from free element to a compound (gains oxygen/sulphate group). It is oxidised.
- Copper (Cu) is reduced — Copper in CuSO₄ is in combined form (with oxygen-containing sulphate); it is displaced as free copper metal, meaning it loses oxygen/combined state. Hence, CuSO₄ is reduced.
Source: Chapter 1, Section on Oxidation and Reduction (Gain or Loss of Oxygen)
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Explanation
- The textbook defines oxidation as gain of oxygen or loss of hydrogen, and reduction as loss of oxygen or gain of hydrogen — always justify using these exact terms.
- For the silver chloride reaction, sunlight supplies energy (photochemical decomposition); remember to show the sunlight condition above the arrow.
- For the zinc-copper reaction, zinc displaces copper because zinc is more reactive — this is a classic displacement + redox example.
- Examiners award marks for: (a) correct balanced equation, (b) correct identification of oxidised substance, and (c) correct identification of reduced substance with brief justification. Missing any one loses a mark.