2 Cr213/2O7- + 28 H3O+ + 14 (Br-I)- 4 (CrIII)3+ + 42 H2O + 7 Br20

This is an oxidation-reduction (redox) reaction:

14 Br-I - 14 e- 14 Br0 (oxidation)

4 Cr13/2 + 14 e- 4 CrIII (reduction)

Change the oxidation states