4 CuS + 7 O2 → 2 Cu2O + 4 SO3
This is an oxidation-reduction (redox) reaction:
4 CuII + 4 e- → 4 CuI (reduction)
4 S-II - 32 e- → 4 SVI (oxidation)
14 O0 + 28 e- → 14 O-II (reduction)
CuS is an oxidizing agent, CuS is a reducing agent, O2 is an oxidizing agent.