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 solve the question of which reaction does not involve either oxidation or reduction, we will analyze the oxidation states of the elements involved in each reaction. Oxidation involves an increase in oxidation state, while reduction involves a decrease in oxidation state.

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to analyze the given reactions to determine the oxidation states of the elements involved.

2. **Reaction 1: MnO4⁻ to MnO2**:
- In MnO4⁻, the oxidation state of Mn can be calculated as follows:
- Let the oxidation state of Mn be \( x \).
- The equation becomes: \( x + 4(-2) = -1 \) (since the overall charge is -1).
- This simplifies to \( x - 8 = -1 \) → \( x = +7 \).
- In MnO2, the oxidation state of Mn is:
- \( x + 2(-2) = 0 \) (since the overall charge is 0).
- This gives \( x - 4 = 0 \) → \( x = +4 \).
- **Change in oxidation state**: From +7 to +4 (reduction).

3. **Reaction 2: 2H₂ + O₂ → 2H₂O**:
- In H₂, the oxidation state of H is 0.
- In O₂, the oxidation state of O is also 0.
- In H₂O, the oxidation state of H is +1 and O is -2.
- **Change in oxidation state**: H goes from 0 to +1 (oxidation) and O goes from 0 to -2 (reduction).

4. **Reaction 3: 2Fe²⁺ + 2e⁻ → 2Fe**:
- In Fe²⁺, the oxidation state of Fe is +2.
- In elemental Fe, the oxidation state is 0.
- **Change in oxidation state**: From +2 to 0 (reduction).

5. **Reaction 4: 2Na + Cl₂ → 2NaCl**:
- In Na, the oxidation state is 0.
- In Cl₂, the oxidation state is also 0.
- In NaCl, the oxidation state of Na is +1 and Cl is -1.
- **Change in oxidation state**: Na goes from 0 to +1 (oxidation) and Cl goes from 0 to -1 (reduction).

6. **Conclusion**:
- The only reaction that does not involve a change in oxidation state is the one where the oxidation states remain the same.
- In the case of the second reaction (2H₂ + O₂ → 2H₂O), the oxidation states of the elements change, while in the first reaction, the oxidation states change, indicating reduction.
- Therefore, the reaction that does not involve oxidation or reduction is the one where the oxidation states do not change.

### Final Answer:
The reaction that does not involve either oxidation or reduction is **Reaction 2: 2H₂ + O₂ → 2H₂O**.