K2Cr2O7 + 6 NaCl + 7 H2SO4 → 3 Na2SO4 + Cr2(SO4)3 + K2SO4 + 3 Cl2 + 7 H2O
This is an oxidation-reduction (redox) reaction:
6 Cl-I - 6 e- → 6 Cl0 (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
NaCl is a reducing agent, K2Cr2O7 is an oxidizing agent.