Cr2O72− + 6Fe2+ + 14H+ to 2Cr3+ + 6Fe3+ + 7H2O
The oxidation of Cr in Cr2O72− :
Let the oxidation of Cr = x;
2x + (-2 x 7) = -2 implies 2x - 14 = -2
2x = 12 ; x = +6
Hence, the change in oxidation of Cr = +6 to +3