Problem 19 - Chemistry in 30 days

Question

On shaking \(\mathrm{H}_{2} \mathrm{O}_{2}\) with acidified potassium dichromate and ether, ethereal layer becomes (a) green (b) red (c) blue (d) brown

Step-by-Step Solution

Answer

The ethereal layer becomes green.

Step 1: Identifying the Reaction Components

In this exercise, we are dealing with hydrogen peroxide ( H_2O_2 ), acidified potassium dichromate ( K_2Cr_2O_7 ), and ether. The reaction is an oxidation-reduction one, with potassium dichromate acting as the oxidizing agent.

Step 2: Understanding the Reaction Mechanism

Acidified potassium dichromate ( K_2Cr_2O_7 ) in an acidic medium usually contains the strongly oxidizing Cr_2O_7^{2-} ions, which can be reduced to Cr^{3+} ions. Hydrogen peroxide ( H_2O_2 ) acts as a reducing agent.

Step 3: Determining the Color Change

When Cr_2O_7^{2-} n ions are reduced to Cr^{3+} in the presence of H_2O_2 , the color change observed is from the orange color of Cr_2O_7 to the green color of Cr^{3+} ions. This change can cause the ethereal layer to become green due to the presence of Cr^{3+} .

Key Concepts

Hydrogen Peroxide Reactions

Hydrogen peroxide (H₂O₂) is a common chemical that plays a significant role in oxidation-reduction reactions. It is well-known as a bleaching agent and disinfectant. However, in chemical reactions, it often acts as a reducing agent.
When H₂O₂ reacts, it converts into water and oxygen. This breakdown is generally catalyzed by various agents, leading to its role as a reducing agent. In an acidic solution, H₂O₂ is known to reduce other compounds while it gets oxidized. This makes it quite versatile in chemical reactions.

• H₂O₂ is often reduced to water (0 + 2H⁺ + 2e⁻ → 2H₂O).
• It can also undergo oxygen production (2O₂ → O₂ + 2H⁺ + 2e⁻).

These reactions highlight H₂O₂'s dual role, but in the context of potassium dichromate, it primarily acts as a reducing agent.

Potassium Dichromate Chemistry

Potassium dichromate (0_{46}) is a powerful oxidizing agent widely used in various chemical processes. In oxidation-reduction reactions, dichromate ions (2O_7^{2-}) are key players.
When acidified, potassium dichromate releases 2O_7^{2-} ions in solution, which can oxidize other substances. For instance, they can convert reducing agents like hydrogen peroxide into their oxidized forms while themselves being reduced to chromium ions (Cr³⁺).

• The conversion is represented by: 2O_7^{2-} + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O.
• This transition from dichromate to chromium ions involves a visible color change, making it useful in laboratory demonstrations.

These properties make potassium dichromate valuable in reactions where oxidation is necessary.

Color Change in Chemical Reactions

Color change in chemical reactions is a fascinating aspect of chemistry that offers visual cues to the flow of a reaction. Such changes occur due to alterations in the oxidation states of participating atoms.
In the exercise involving hydrogen peroxide and potassium dichromate, the initial orange color of the 2O_7^{2-} ions reveals the presence of dichromate in the solution. Upon reaction, these ions are reduced to Cr³⁺ ions, which exhibit a green color.

• Initial state: Orange color due to 2O_7^{2-} ions.
• After reaction: Green color due to Cr³⁺ ions.

Understanding color changes in reactions can aid in determining the completion and progress of chemical processes, as evidenced by the color transitions in oxidation-reduction scenarios.