3 Fe2(SO4)2 + K2Cr2O7 + 7 H2SO4 → 3 Fe2(SO4)3 + Cr2(SO4)3 + 7 H2O + K2SO4
This is an oxidation-reduction (redox) reaction:
6 FeII - 6 e- → 6 FeIII (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
Fe2(SO4)2 is a reducing agent, K2Cr2O7 is an oxidizing agent.