Question

The reaction for the combustion of acetylene is shown. Explain why this reaction is a redox reaction by identifying the oxidation numbers that change, what is oxidized, what is reduced, and what happens to the electrons during the reaction.

2C2H2 + 5O2 → 4CO2 + 2H2O


Which explanations did you include in your response? Check all that apply.

The oxidation number of carbon changes from -1 to +4.

The oxidation number of oxygen changes from 0 to -2.

Carbon is oxidized.

Oxygen is reduced.

Electrons are transferred from carbon to oxygen.

Answer 1

All five apply.

Brief justification:
- In C2H2 each H is +1, so each C must be -1 (2C + 2(+1) = 0 → C = -1). In CO2 carbon is +4. So the oxidation number of carbon changes from -1 to +4.
- In O2 oxygen is 0; in CO2 and H2O oxygen is -2. So the oxidation number of oxygen changes from 0 to -2.
- Carbon’s oxidation number increases (−1 → +4), so carbon is oxidized (it loses electrons).
- Oxygen’s oxidation number decreases (0 → −2), so oxygen is reduced (it gains electrons).
- Electron accounting: each C goes from −1 to +4 (loses 5 e−); 4 C atoms lose 4×5 = 20 e−. There are 10 O atoms (5 O2) each going 0 → −2 (gaining 2 e−), so they gain 10×2 = 20 e−. Electrons are transferred from carbon to oxygen, confirming this is a redox reaction.