Cr2_22O72−_{7}^{2-}72− + 6Fe2+^{2+}2+ + 14H+^{+}+ →\to→ 2Cr3+^{3+}3+ + 6Fe3+^{3+}3+ + 7H2_22O
The oxidation of Cr in Cr2_22O72−_{7}^{2-}72− :
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
