Which anion can undergo both oxidation and reduction?

Options

A. `Cr_(2)O_(7)^(2-)`
B. `NO_(3)^(-)`
C. `OC l^(-)`
D. `S^(2-)`

Answer

To determine which anion can undergo both oxidation and reduction, we need to analyze the oxidation states of the given anions and their ability to change those states. Let's go through the options step by step.

### Step 1: Understand Oxidation and Reduction
- **Oxidation** is the process of losing electrons, which results in an increase in oxidation state.
- **Reduction** is the process of gaining electrons, which results in a decrease in oxidation state.

### Step 2: Analyze Each Anion

#### Option 1: Cr2O7^2-
1. **Calculate the oxidation state of chromium (Cr)**:
- Let the oxidation state of Cr be \( x \).
- The equation for the oxidation state is:
\[ 2x + 7(-2) = -2 \]
- Simplifying gives:
\[ 2x - 14 = -2 \implies 2x = 12 \implies x = +6 \]
2. **Conclusion**: Chromium in Cr2O7^2- has a maximum oxidation state of +6. It can only lower its oxidation state, acting solely as an oxidizing agent.

#### Option 2: NO3^-
1. **Calculate the oxidation state of nitrogen (N)**:
- Let the oxidation state of N be \( x \).
- The equation for the oxidation state is:
\[ x + 3(-2) = -1 \]
- Simplifying gives:
\[ x - 6 = -1 \implies x = +5 \]
2. **Conclusion**: Nitrogen in NO3^- has a maximum oxidation state of +5. It can only lower its oxidation state, acting solely as an oxidizing agent.

#### Option 3: OCl^-
1. **Calculate the oxidation state of chlorine (Cl)**:
- Let the oxidation state of Cl be \( x \).
- The equation for the oxidation state is:
\[ x + (-2) = -1 \]
- Simplifying gives:
\[ x - 2 = -1 \implies x = +1 \]
2. **Conclusion**: Chlorine in OCl^- has an oxidation state of +1. Chlorine can exhibit oxidation states ranging from -1 to +7, meaning it can both gain and lose electrons. Therefore, it can undergo both oxidation and reduction.

#### Option 4: S^2-
1. **Calculate the oxidation state of sulfur (S)**:
- The oxidation state of sulfur in S^2- is -2, which is the minimum oxidation state.
2. **Conclusion**: Sulfur can only increase its oxidation state, acting solely as a reducing agent.

### Final Conclusion
Among the given anions, only **OCl^-** can undergo both oxidation and reduction. Therefore, the correct answer is:

**OCl^- (Option 3)**