Question

In chromyl chloride test for confirmation of Cl^-, a yellow solution is obtained. Acidification of the solution and addition of amyl alcohol and 10% H₂O₂​ turns organic layer blue indicating formation of chromium pentoxide. The oxidation state of chromium in that is

• A. \(+6\)
• B. \(+5\)
• C. \(+10\)
• D. \(+3\)

Answer 1

The Correct Option is A

The chromyl chloride test is used to confirm the presence of chloride ions (\( \text{Cl}^- \)) in a sample. During this test, the sample reacts to form a yellow solution. This solution is then acidified, and the addition of amyl alcohol and 10% hydrogen peroxide (\( \text{H}_2\text{O}_2 \)) leads to the formation of an organic layer that turns blue. This color change indicates the formation of chromium pentoxide (\( \text{CrO}_5 \)).

Let's break down the formation and characteristics of chromium pentoxide: 

1. Chromium pentoxide (\( \text{CrO}_5 \)) forms when chromium in the solution is oxidized in the presence of peroxide.
2. \( \text{CrO}_5 \) exhibits a distinctive blue color due to its structure, which involves multiple bonds with oxygen.
3. The oxidation state of chromium in chromium pentoxide (\( \text{CrO}_5 \)) can be determined by analyzing its chemical structure. Each oxygen atom has an oxidation state of \( -2 \).

Let's calculate the oxidation state of chromium in \( \text{CrO}_5 \):

• \( \text{CrO}_5 \) has 5 oxygen atoms, each with an oxidation state of \( -2 \), contributing a total of \( -10 \) (i.e., \( 5 \times -2 = -10 \)).
• Let the oxidation state of chromium be \( x \). The total charge should be zero for a neutral compound:
• \( x + (5 \times -2) = 0 \)
• Simplifying, \( x - 10 = 0 \), thus \( x = +10 \).

However, in our experience with chromate configurations and the given options, we know the stable oxidation state of chromium in \(\text{CrO}_5 \) should actually be \( +6 \) due to peroxo bonds counteracting to effectively reduce the apparent state.

Therefore, the correct option is:

\(+6\)

Answer 2

- During the chromyl chloride test, the following reaction occurs:

Cl⁻ + K₂Cr₂O₇ + H₂SO₄ → CrO₂Cl₂ + Cl⁻

- In basic medium, this results in a yellow solution due to CrO₄²⁻.

- Upon acidification, addition of amyl alcohol, and 10% H₂O₂, a blue organic layer forms due to the formation of CrO₅ (chromium pentoxide).

- The oxidation state of chromium in CrO₅ is +6.

So, the correct answer is: +6