Question

Which of these reactions are redox reactions? Check all that apply.

Cd + HCl → CdCl2 + H2

CuCl2 + Na2S → 2NaCl + CuS

CaCO3 → CaO + CO2

2ZnS + 3O2 → 2ZnO + 2SO2

CH4 + 2O2 → CO2 + 2H2O

Answer 1

Redox reactions: 1, 4, and 5.

Explanations (oxidation-state changes):

1) Cd + HCl → CdCl2 + H2
- Cd: 0 → +2 (oxidized)
- H: +1 (in HCl) → 0 (in H2) (reduced)
→ Redox

2) CuCl2 + Na2S → 2NaCl + CuS
- Cu: +2 → +2 (no change)
- S: -2 → -2 (no change)
→ No redox (double‑displacement / precipitation)

3) CaCO3 → CaO + CO2
- Ca: +2 → +2
- C: +4 → +4
- O: -2 → -2
→ No redox (decomposition with no change in oxidation states)

4) 2ZnS + 3O2 → 2ZnO + 2SO2
- Zn: +2 → +2
- S: -2 → +4 (oxidized)
- O: 0 → -2 (reduced)
→ Redox

5) CH4 + 2O2 → CO2 + 2H2O
- C: -4 → +4 (oxidized)
- O: 0 → -2 (reduced)
→ Redox