3 Na2S + 4 K2Cr2O7 + 16 H2SO4 → 3 Na2SO4 + 4 Cr2(SO4)3 + 4 K2SO4 + 16 H2O
This is an oxidation-reduction (redox) reaction:
3 S-II - 24 e- → 3 SVI (oxidation)
8 CrVI + 24 e- → 8 CrIII (reduction)
Na2S is a reducing agent, K2Cr2O7 is an oxidizing agent.