9 Na2S + 2 K2Cr2O7 + 9 H2SO4 → 10 S + 18 NaOH + 2 Cr2(SO4)3 + 2 K2SO4
This is an oxidation-reduction (redox) reaction:
9 S-II - 18 e- → 9 S0 (oxidation)
SVI + 6 e- → S0 (reduction)
4 CrVI + 12 e- → 4 CrIII (reduction)
Na2S is a reducing agent, H2SO4 is an oxidizing agent, K2Cr2O7 is an oxidizing agent.