Question

Acidic dichromate ion reacts with hydrogen peroxide to the formation of deep blue colour. This is due to the formation of

• A. CrO$_4$
• B. CrO$_5$
• C. Both (a) and (b)
• D. None of (a) and (b)

Hint:

Blue colour in chromium reactions usually indicates peroxo complex formation.

Answer 1

The Correct Option is B

The question asks about the reaction of acidic dichromate ion with hydrogen peroxide leading to the formation of a deep blue color. Let's examine this reaction and identify the correct product responsible for this color change.

The reaction involved is as follows:

\(2 \text{K}_2\text{Cr}_2\text{O}_7 + 3 \text{H}_2\text{SO}_4 + 3 \text{H}_2\text{O}_2 \rightarrow 2 \text{CrO}_5 + 3 \text{K}_2\text{SO}_4 + 4 \text{H}_2\text{O}\)

In this reaction:

• Acidic dichromate ion (\( \text{Cr}_2\text{O}_7^{2-} \)) reacts with hydrogen peroxide (\( \text{H}_2\text{O}_2 \)).
• This results in the formation of chromium peroxide (\( \text{CrO}_5 \)), which is known for its deep blue color.

The deep blue color observed is specifically due to the formation of chromium peroxide (\( \text{CrO}_5 \)), which is a transient and unstable compound formed in an acidic medium.

Let's analyze the options:

• Option A: \( \text{CrO}_4 \) - This is incorrect because chromate ions (\( \text{CrO}_4^{2-} \)) do not form a deep blue color with hydrogen peroxide.
• Option B: \( \text{CrO}_5 \) - This is correct, as the reaction product \( \text{CrO}_5 \) is responsible for the deep blue color.
• Option C: Both (a) and (b) - This is incorrect because only \( \text{CrO}_5 \) is responsible for the deep blue color.
• Option D: None of (a) and (b) - This is incorrect as \( \text{CrO}_5 \) is the correct answer.

Thus, the correct answer is that the deep blue color is due to the formation of \( \text{CrO}_5 \).