Which one of the following reactions does not involve either oxidation ore reduction ?

Options

A. `MnO_4^(-) rarrMnO_2`
B. `NararrNa^(+)`
C. `CrO_4^(2-) rarrCr_2O_7^(2-)`
D. `Zn^(2+) rarrZn`

Answer

To determine which of the given reactions does not involve oxidation or reduction, we need to analyze the oxidation states of the elements involved in each reaction. Let's go through the options step by step.

### Step-by-Step Solution:

1. **Option A: MnO4⁻ → MnO2**
- Determine the oxidation state of Mn in MnO4⁻:
- Let the oxidation state of Mn be \( x \).
- The equation becomes: \( x - 8 = -1 \) (since \( 4 \times (-2) = -8 \)).
- Solving for \( x \): \( x = +7 \).
- Determine the oxidation state of Mn in MnO2:
- The equation becomes: \( x - 4 = 0 \) (since \( 2 \times (-2) = -4 \)).
- Solving for \( x \): \( x = +4 \).
- Change in oxidation state: \( +7 \) to \( +4 \) indicates a reduction (gain of electrons).
- **Conclusion**: This reaction involves reduction.

2. **Option B: Na → Na⁺**
- Determine the oxidation state of Na:
- In elemental state, Na has an oxidation state of \( 0 \).
- In Na⁺, the oxidation state is \( +1 \).
- Change in oxidation state: \( 0 \) to \( +1 \) indicates oxidation (loss of electrons).
- **Conclusion**: This reaction involves oxidation.

3. **Option C: CrO4²⁻ → Cr2O7²⁻**
- Determine the oxidation state of Cr in CrO4²⁻:
- Let the oxidation state of Cr be \( x \).
- The equation becomes: \( x - 8 = -2 \) (since \( 4 \times (-2) = -8 \)).
- Solving for \( x \): \( x = +6 \).
- Determine the oxidation state of Cr in Cr2O7²⁻:
- The equation becomes: \( 2x - 14 = -2 \) (since \( 7 \times (-2) = -14 \)).
- Solving for \( 2x \): \( 2x = 12 \) → \( x = +6 \).
- Change in oxidation state: \( +6 \) to \( +6 \) indicates no change.
- **Conclusion**: This reaction does not involve oxidation or reduction.

4. **Option D: Mn²⁺ → Mn**
- Determine the oxidation state of Mn in Mn²⁺:
- The oxidation state is \( +2 \).
- In elemental state, Mn has an oxidation state of \( 0 \).
- Change in oxidation state: \( +2 \) to \( 0 \) indicates reduction (gain of electrons).
- **Conclusion**: This reaction involves reduction.

### Final Answer:
The reaction that does not involve either oxidation or reduction is **Option C: CrO4²⁻ → Cr2O7²⁻**.