3 C2H2 + 5 K2Cr2O7 + 20 H2SO4 → 5 Cr2(SO4)3 + 5 K2SO4 + 6 CO2 + 23 H2O
This is an oxidation-reduction (redox) reaction:
6 C-I - 30 e- → 6 CIV (oxidation)
10 CrVI + 30 e- → 10 CrIII (reduction)
C2H2 is a reducing agent, K2Cr2O7 is an oxidizing agent.