Cr2(SO4)3 + 3 Fe2(SO4)3 + K2SO4 + 7 H2O → K2Cr2O7 + 6 FeSO4 + 7 H2SO4
This is an oxidation-reduction (redox) reaction:
2 CrIII - 6 e- → 2 CrVI (oxidation)
6 FeIII + 6 e- → 6 FeII (reduction)
Cr2(SO4)3 is a reducing agent, Fe2(SO4)3 is an oxidizing agent.