Question

Which reactions are oxidation-reduction reactions? Check all that apply.

FeS(s) + 2HCl(aq) → H2S(g) + FeCl2(g)

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3

2C3H6(g) + 9O2(g) → 6CO2(g) + 6H2O(g)

Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)

Answer 1

Redox reactions: 3 and 4.

Brief reasoning (oxidation numbers):

1) FeS + 2HCl → H2S + FeCl2
- Fe: +2 → +2 (no change)
- S: −2 → −2 (no change)
- H: +1 → +1 (no change)
No oxidation-state changes → not a redox reaction (acid–base / protonation).

2) AgNO3 + NaCl → AgCl + NaNO3
- Ag: +1 → +1
- Na: +1 → +1
- Cl: −1 → −1
No oxidation-state changes → not redox (precipitation).

3) 2C3H6 + 9O2 → 6CO2 + 6H2O
- C in C3H6: average −2 → +4 in CO2 (oxidation)
- O: 0 in O2 → −2 in CO2/H2O (reduction)
Oxidation and reduction occur → redox.

4) Fe2O3 + 3CO → 2Fe + 3CO2
- Fe: +3 → 0 (reduction)
- C in CO: +2 → +4 in CO2 (oxidation)
Oxidation and reduction occur → redox.