Question

Iron (III) catalyses the reaction between iodide and persulphate ions, in which
A. \(\text{Fe}^{3+}\) oxidises the iodide ion
B. \(\text{Fe}^{3+}\) oxidises the persulphate ion
C. \(\text{Fe}^{2+}\) reduces the iodide ion
D. \(\text{Fe}^{2+}\) reduces the persulphate ion
Choose the most appropriate answer from the options given below:

• A. B and C only
• B. B only
• C. A only
• D. A and D only

Answer 1

The Correct Option is D

To understand the catalytic action of Iron (III) in the reaction between iodide and persulphate ions, let's analyze the roles of Iron (III) \((\text{Fe}^{3+})\) and Iron (II) \((\text{Fe}^{2+})\) in this process:

• The catalytic function of Iron (III) involves facilitating the following redox reactions:
• Iron (III) \([\text{Fe}^{3+}]\): Iron (III) acts as an oxidizing agent. It oxidizes the iodide ions \((\text{I}^-)\) to iodine \((\text{I}_2)\), thereby reducing itself to Iron (II) \((\text{Fe}^{2+})\).
• The reaction can be represented as: \(\text{I}^- + \text{Fe}^{3+} \rightarrow \text{I}_2 + \text{Fe}^{2+}\)
• Iron (II) \([\text{Fe}^{2+}]\): The reduced form, Iron (II), acts as a reducing agent and reduces the persulphate ions \((\text{S}_2\text{O}_8^{2-})\), thereby getting oxidized back to Iron (III).
• The reaction can be represented as: \(\text{Fe}^{2+} + \text{S}_2\text{O}_8^{2-} \rightarrow \text{Fe}^{3+} + 2\text{SO}_4^{2-}\)

Through these steps, Iron (III) catalyzes the reaction between iodide and persulphate ions by alternating between its oxidized and reduced states, thus:

1. Iron (III) \(\text{Fe}^{3+}\) oxidizes iodide ions \((\text{I}^-)\). This corresponds to Option A.
2. Iron (II) \(\text{Fe}^{2+}\) reduces the persulphate ions. This corresponds to Option D.

Therefore, the most appropriate options are "A and D only," as Iron (III) initially oxidizes iodide ions and then the Iron (II) formed reduces the persulphate ions.

Answer 2

The given question involves the reaction mechanisms of iodide (\( \text{I}^- \)) and persulphate (\( \text{S}_2\text{O}_8^{2-} \)) ions in the presence of the catalyst iron (III) ions (\( \text{Fe}^{3+} \)). Let's analyze this step by step:

1. The role of \(\text{Fe}^{3+}\) in this reaction is to facilitate the oxidation of iodide ions.
2. In the first step, \(\text{Fe}^{3+}\) oxidizes iodide ions to iodine, itself getting reduced to \(\text{Fe}^{2+}\):

\(2\text{Fe}^{3+} + 2\text{I}^- \rightarrow 2\text{Fe}^{2+} + \text{I}_2\)

1. Next, the reduced \(\text{Fe}^{2+}\) ions are re-oxidized back to \(\text{Fe}^{3+}\) by persulphate ions, which are reduced to sulfate ions:

\(2\text{Fe}^{2+} + \text{S}_2\text{O}_8^{2-} \rightarrow 2\text{Fe}^{3+} + 2\text{SO}_4^{2-}\)

• Explanation of the Correct Answer: The correct answer is option "A and D only".
• Option A (Correct): \(\text{Fe}^{3+}\) oxidizes iodide ions. This is described correctly by the reaction mechanism.
• Option D (Correct): \(\text{Fe}^{2+}\) reduces persulphate ions. In this step, \(\text{Fe}^{2+}\) serves as a reducing agent, oxidizing back to \(\text{Fe}^{3+}\).
• Options B and C (Incorrect): These options do not correctly describe the roles of \(\text{Fe}^{3+}\) and \(\text{Fe}^{2+}\) in the described catalytic process.

Thus, the catalytic cycle involves the continuous regeneration of \(\text{Fe}^{3+}\) ions, thereby accelerating the rate of reaction between iodide and persulphate ions.