Which change requires an oxidising agent?

Options

A. `2S_(2)O_(3)^(2-) iff S_(4)O_(6)^(2-)`
B. `Zn^(2+) iff Zn`
C. `ClO^(-) iff Cl^(-)`
D. `SO_(3) iff SO_(4)^(2-)`

Answer

To determine which change requires an oxidizing agent, we need to analyze the oxidation states of the elements involved in each option and identify where oxidation occurs. An oxidizing agent is a substance that causes oxidation in another species while being reduced itself.

### Step-by-Step Solution:

1. **Understanding Oxidation and Reducing Agents**:
- Oxidation is defined as the loss of electrons or an increase in oxidation state.
- An oxidizing agent facilitates oxidation by accepting electrons and itself gets reduced.

2. **Analyzing the First Option: \( S_2O_3^{2-} \) to \( S_4O_6^{2-} \)**:
- Let the oxidation state of sulfur in \( S_2O_3^{2-} \) be \( x \).
- The equation for the charge balance is:
\[ 2x + 3(-2) = -2 \implies 2x - 6 = -2 \implies 2x = 4 \implies x = +2 \]
- For \( S_4O_6^{2-} \):
\[ 4x + 6(-2) = -2 \implies 4x - 12 = -2 \implies 4x = 10 \implies x = +2.5 \]
- The oxidation state of sulfur increases from +2 to +2.5, indicating oxidation occurs. Therefore, an oxidizing agent is required.

3. **Analyzing the Second Option: \( Zn^{2+} \) to \( Zn \)**:
- Here, zinc is gaining electrons (going from +2 to 0), which indicates reduction.
- Since this is a reduction process, an oxidizing agent is not required.

4. **Analyzing the Third Option: \( ClO^{-} \) to \( Cl^{-} \)**:
- Let the oxidation state of chlorine in \( ClO^{-} \) be \( x \).
- The equation for the charge balance is:
\[ x + (-2) = -1 \implies x - 2 = -1 \implies x = +1 \]
- For \( Cl^{-} \), the oxidation state is -1.
- The oxidation state of chlorine decreases from +1 to -1, indicating reduction. Thus, an oxidizing agent is not required.

5. **Analyzing the Fourth Option: \( SO_3 \) to \( SO_4^{2-} \)**:
- For \( SO_3 \):
\[ x + 3(-2) = 0 \implies x - 6 = 0 \implies x = +6 \]
- For \( SO_4^{2-} \):
\[ x + 4(-2) = -2 \implies x - 8 = -2 \implies x = +6 \]
- The oxidation state of sulfur remains +6, indicating no change. Therefore, neither oxidation nor reduction occurs, and no oxidizing agent is required.

6. **Conclusion**:
- The only option where oxidation occurs and requires an oxidizing agent is the first option: \( S_2O_3^{2-} \) to \( S_4O_6^{2-} \).

### Final Answer:
The change that requires an oxidizing agent is the first option: \( S_2O_3^{2-} \) to \( S_4O_6^{2-} \).

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