Question

When hydrogen peroxide is treated with a cold acidified ${K_2}C{r_2}{O_7}$ solution containing ether, blue colour is obtained. This is due to:
A. Perchromic acid
B. Potassium chromate
C. Chromium sulphate
D. Chromium trioxide

Answer 1

Potassium dichromate or ${K_2}C{r_2}{O_7}$ is an orange coloured oxidizing agent, used in various reactions. On the other hand, hydrogen peroxide or ${H_2}{O_2}$ is a pale blue viscous liquid. They react with each other and give blue colour compounds.

Complete answer:
As we know, potassium dichromate $\left( {{K_2}C{r_2}{O_7}} \right)$ is orange in colour and on reaction with pale blue hydrogen peroxide, it yields blue colour compound. To identify the blue colour compound we must write the chemical reaction as follows:
${K_2}C{r_2}{O_7} + 4{H_2}{O_2} + {H_2}S{O_4}\xrightarrow{{{\text{Ether}}}}Cr{O_5} + {K_2}S{O_4} + 5{H_2}O$
The reaction proceeds in three different steps:
Step I: ${K_2}C{r_2}{O_7} + {H_2}S{O_4} \to {K_2}S{O_4} + {H_2}C{r_2}{O_7}$
Step II: $4{H_2}{O_2} \to 4{H_2}O + 4\left( O \right)$
Step III: ${H_2}C{r_2}{O_7} + 4\left( O \right) \to 2Cr{O_5} + {H_2}O$
Hence the three products formed are $Cr{O_5}$, ${K_2}S{O_4}$ and ${H_2}O$. Among these, $Cr{O_5}$ or chromium pentoxide has a blue colour in ether.
In the above reaction, the oxidation state of hydrogen peroxide changes from $- 1$ to $- 2$ as it acts as an oxidizing agent in the reaction.
In less acidic conditions, reaction of hydrogen peroxide with potassium dichromate gives a deep blue-violet coloured compound. Whereas, in an alkaline solution this reaction gives a red-brown coloured compound.
Hence the correct option is A, per-chromic acid.

Additional information:
$Cr{O_5}$ or chromium pentoxide is also known as perchromic acid and it has a structure of butterfly.

The structure is so, due to two peroxy linkages in the compound.

Note:
Potassium dichromate is an oxidizing agent that oxides aldehydes and alcohols (primary and secondary). It is toxic in nature and also known to be a carcinogen and a mutagen. Therefore, it can be fatal for the environment. It is also a common reason behind dermatitis, especially on arms and hands.