6 CuI + 8 H2SO4 → 6 CuSO4 + 2 S + 3 I2 + 8 H2O
This is an oxidation-reduction (redox) reaction:
6 CuI - 6 e- → 6 CuII (oxidation)
2 SVI + 12 e- → 2 S0 (reduction)
6 I-I - 6 e- → 6 I0 (oxidation)
CuI is a reducing agent, H2SO4 is an oxidizing agent, CuI is a reducing agent.