CAPF AC 2023 Q11: Science & Technology – Chemistry
- (a) Cu(I) and S of Cu₂S
- (b) Cu and S of SO₂
- (c) Cu and O of SO₂
- (d) Cu(I) and O of SO₂
Answer: (a) Cu(I) and S of Cu₂S
Explanation
Oxidant and reductant in copper smelting
Given: 2Cu₂O (s) + Cu₂S (s) → 6Cu (s) + SO₂ (g); identify the oxidant and the reductant.
How to think: write the oxidation number of each element before and after.
The species that is reduced is the oxidant.
The species that is oxidised is the reductant.
| Element | Before | After | Change | Role |
|---|---|---|---|---|
| Cu (in Cu₂O and Cu₂S) | +1 | 0 | Reduced | Oxidant |
| S (in Cu₂S) | −2 | +4 (in SO₂) | Oxidised | Reductant |
| O | −2 | −2 | No change | Neither |
So Cu(I) is the oxidant and the sulphur of Cu₂S is the reductant.
Where this reaction occurs: in the Bessemer converter during the extraction of copper.
It is called auto-reduction, and the product is blister copper.
Answer: (a) Cu(I) and S of Cu₂S.
Why each option is right or wrong
- (a) CORRECT. Cu(I) gains electrons (oxidant) and S of Cu₂S loses them (reductant).
- (b) Incorrect. Copper metal and the sulphur in SO₂ are products, not the reacting species.
- (c) Incorrect. Oxygen stays at −2 throughout; it is neither oxidant nor reductant.
- (d) Incorrect. Cu(I) is right for the oxidant, but oxygen does not act as the reductant.
Key point
Oxidant = species reduced (oxidation number falls). Reductant = species oxidised (oxidation number rises). 2Cu₂O + Cu₂S → 6Cu + SO₂: Cu +1 → 0, S −2 → +4. Auto-reduction gives blister copper.
Remember
Cu⁺ → Cu (oxidant); S²⁻ → S⁴⁺ (reductant).
Common mistakes
- Naming the products (Cu and SO₂) as oxidant and reductant.
- Reversing the terms: the oxidant is the one that gets reduced.
- Thinking oxygen must be involved because the product is an oxide.
Exam tip
Educated guess: an oxidant and a reductant must be among the reactants. Only option (a) names two reacting species; the others point to SO₂, a product.
Sources
- NCERT Class XI Chemistry — Redox Reactions
- NCERT Class XII Chemistry — General Principles and Processes of Isolation of Elements (extraction of copper)