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Question: Balance the redox reaction by oxidation number method. It occurs in acidic medium. \({H_2}{O_2} + ...

Balance the redox reaction by oxidation number method. It occurs in acidic medium.
H2O2+Cr2O7−2→O2+Cr+3{H_2}{O_2} + C{r_2}{O_7}^{ - 2} \to {O_2} + C{r^{ + 3}}

Explanation

Solution

As we know that the chemical equation given is an oxidation-reduction (redox) reaction. In this chemical equation Cr2O72−C{r_2}{O_7}^{2 - }is an oxidising agent and H2O2{H_2}{O_2}is a reducing agent. In an acidic medium we will try to balance the chemical equation by using H2O{H_2}O and H+{H^ + }.

Complete step by step answer:
As we can see that in the given chemical equation , both oxidation and reduction is going on. The oxygen in the hydrogen peroxide in the reactant side has an oxidation number of −1 - 1. And the oxygen in the product side has an oxidation state of 00 . So oxygen is undergoing an oxidation reaction.
6O−1→6O0+6e−6{O^{ - 1}} \to 6{O^0} + 6{e^ - }
The chromium has an oxidation state of +6 + 6 in the Cr2O72−C{r_2}{O_7}^{2 - } in the reactant side and it has an oxidation state of +3 + 3in the product side. So chromium is undergoing a reduction reaction. The reduction reaction can be written as 2Cr+6+6e−→2Cr+32C{r^{ + 6}} + 6{e^ - } \to 2C{r^{ + 3}} . The complete reaction can be written as : Cr2O7−2+3H2O2+8H+→2Cr+3+3O2+7H2OC{r_2}{O_7}^{ - 2} + 3{H_2}{O_2} + 8{H^ + } \to 2C{r^{ + 3}} + 3{O_2} + 7{H_2}O . The H+{H^ + } and H2O{H_2}O is used to balance the hydrogen and oxygen atoms in an acidic medium.
So the complete balanced equation in acidic medium is: Cr2O7−2+3H2O2+8H+→2Cr+3+3O2+7H2OC{r_2}{O_7}^{ - 2} + 3{H_2}{O_2} + 8{H^ + } \to 2C{r^{ + 3}} + 3{O_2} + 7{H_2}O
Additional information: Hydrogen peroxide H2O2{H_2}{O_2} is also known as Dioidane, Oxidanyl and per hydroxic acid. In pure form it is a pale blue liquid. It is slightly more viscous than water.
Cr2O72−C{r_2}{O_7}^{2 - } has an orange colour. The Cr+3C{r^{ + 3}}has a green colour.

Note:
Always remember that Cr2O72−C{r_2}{O_7}^{2 - } is a very strong oxidising agent . H2O2{H_2}{O_2} can act as both oxidising agent and reducing agent according to the situation. In an acidic medium we use H+{H^ + } and H2O{H_2}Oto balance the oxygen and hydrogen atoms . The Cr2O72−C{r_2}{O_7}^{2 - }is orange coloured and Cr+3C{r^{ + 3}} is green coloured.