Which of the following reaction is an example of autoreduction?

The correct answer is Option (4) → \(Cu_2O + \frac{1}{2}Cu_2S \longrightarrow 3Cu + \frac{1}{2}SO_2\)

In autoreduction (self-reduction) a metal is reduced without any external reducing agent: the ore reduces itself. Copper is extracted from its sulphide ore in this way. The sulphide is first partly roasted to the oxide:

    \( \displaystyle \ce{2Cu2S + 3O2 -> 2Cu2O + 2SO2} \)

The copper(I) oxide formed then reacts with the remaining copper(I) sulphide. The sulphide ion (oxidation state −2) is oxidised to \( \ce{SO2} \) (+4), and copper(I) (+1) is reduced to copper metal (0):

    \( \displaystyle \ce{2Cu2O + Cu2S -> 6Cu + SO2} \)

Halving every coefficient gives the equation in Option (4). Since copper is reduced by the sulphur of its own ore, this is autoreduction.

Why the other options are incorrect

• Option (1) — Incorrect. Iron oxide is reduced by an external reducing agent, carbon monoxide, as in the blast furnace.

• Option (2) — Incorrect. Copper(I) oxide is reduced by an external reducing agent, coke (carbon).

• Option (3) — Incorrect. This is displacement of copper from solution by a more reactive metal (hydrometallurgy). Iron is the reducing agent here; with iron the usual product is \( \ce{Fe^{2+}} \):

      \( \displaystyle \ce{Cu^2+(aq) + Fe(s) -> Cu(s) + Fe^2+(aq)} \)

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Extraction of Copper from Cuprous Oxide – …