2 FeSO3 + K2Cr2O7 + 5 H2SO4 → Cr2(SO4)3 + Fe2(SO4)3 + K2SO4 + 5 H2O
This is an oxidation-reduction (redox) reaction:
2 SIV - 4 e- → 2 SVI (oxidation)
2 FeII - 2 e- → 2 FeIII (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
FeSO3 is a reducing agent, FeSO3 is a reducing agent, K2Cr2O7 is an oxidizing agent.