K2Cr2O7 + 10 KI + 7 H2SO4 → 6 K2SO4 + Cr2(SO4) + 5 I2 + 7 H2O
This is an oxidation-reduction (redox) reaction:
SVI + 4 e- → SII (reduction)
10 I-I - 10 e- → 10 I0 (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
H2SO4 is an oxidizing agent, KI is a reducing agent, K2Cr2O7 is an oxidizing agent.