3 Cr2O3 + 11 KNO3 + 4 K3CO3 → 6 K2CrO4 + 4 CO2 + 11 KNO2
This is an oxidation-reduction (redox) reaction:
4 CIII - 4 e- → 4 CIV (oxidation)
11 NV + 22 e- → 11 NIII (reduction)
6 CrIII - 18 e- → 6 CrVI (oxidation)
K3CO3 is a reducing agent, KNO3 is an oxidizing agent, Cr2O3 is a reducing agent.