Question

Acidified solution of chromic acid on treatment with ${{\text{H}}_2}{{\text{O}}_2}$ yield:
A)${\text{Cr}}{{\text{O}}_3}{\text{ + }}{\text{ }}{{\text{H}}_2}{\text{O + }}{{\text{O}}_2}$
B)${\text{C}}{{\text{r}}_2}{{\text{O}}_3}{\text{ + }}{\text{ }}{{\text{H}}_2}{\text{O + }}{{\text{O}}_2}$
C)${\text{Cr}}{{\text{O}}_5}{\text{ + }}{\text{ }}{{\text{H}}_2}{\text{O}}$
D)${{\text{H}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}{\text{ + }}{\text{ }}{{\text{H}}_2}{\text{O + }}{{\text{O}}_2}$

Answer 1

To solve this question, it is required to have knowledge about the different reactions of chromic acid and the products formed. Chromic acid is a mixture made by mixing concentrated sulphuric acid to a dichromate. Chromic acid is used as an oxidising agent in conversion of alcohol or aldehydes to carboxylic acid.

Complete step by step answer:
As we know that, chromic acid is present as ${{\text{H}}_2}{\text{Cr}}{{\text{O}}_4}$ . It can also be prepared by the reaction of chromium trioxide to water. Chromic acid is present as dichromic acid and is formed when chromic acid is added to chromium trioxide.
Acidified solution of chromic acid is prepared by the reaction of potassium dichromate with sulphuric acid. It forms dichromic acid and potassium sulphate. The reaction will occur as follows:
${{\text{K}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}{\text{ + }}{\text{ }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {{\text{K}}_2}{\text{S}}{{\text{O}}_4}{\text{ + }}{{\text{H}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}$
The dichromic acid on reaction with hydrogen peroxide will form chromium peroxide and water. The reaction will occur as follows:
${{\text{H}}_2}{\text{C}}{{\text{r}}_2}{{\text{O}}_7}{\text{ + }}{{\text{H}}_2}{{\text{O}}_2} \to {\text{Cr}}{{\text{O}}_5}{\text{ + }}{{\text{H}}_2}{\text{O}}$
Thus, the reaction of acidified chromic acid with hydrogen peroxide will form chromium peroxide and water.
$\therefore$ The correct option is option C, i.e. ${\text{Cr}}{{\text{O}}_5}{\text{ + }}{\text{ }}{{\text{H}}_2}{\text{O}}$ .

Additional Note: Chromic acid is an intermediate in chromium plating. It is also used in cleaning of glassware, to dissolve otherwise insoluble residues. Chromium peroxide is a highly unstable compound. It is a better oxidising agent than dichromate ion, but due to its reactivity it cannot be used in organic synthesis reactions.

Note: In the question, acidified chromic acid is a good oxidising agent. But on reaction with hydrogen peroxide, it forms a peroxide instead of reducing itself. The addition of acid on chromic acid or potassium dichromate will result in protonation of the dichromate ion.