Which of the following reactions involves neither oxidation nor reduction ?

Options

A. `CrO_(4)^(2-)rarrCr_(2)O_(7)^(2-)`
B. `Cr rarr CrCl_(3)`
C. `Na rarrNa^(+)`
D. `2S_(2)O_(3)^(2-)rarrS_(4)O_(6)^(2-)`

Answer

To determine which of the given reactions involves neither oxidation nor reduction, we will analyze each reaction by examining the changes in oxidation states of the elements involved.

### Step-by-Step Solution:

1. **Identify the Reactions:**
- Reaction 1: CrO4^2- → Cr2O7^2-
- Reaction 2: Cr → CrCl3
- Reaction 3: Na → Na+
- Reaction 4: 2S2O3^2- → S4O6^2-

2. **Analyze Reaction 1: CrO4^2- → Cr2O7^2-**
- In CrO4^2- (chromate), the oxidation state of chromium (Cr) is +6.
- In Cr2O7^2- (dichromate), the oxidation state of chromium (Cr) is also +6.
- Since the oxidation state of chromium does not change, there is neither oxidation nor reduction in this reaction.

3. **Analyze Reaction 2: Cr → CrCl3**
- In elemental chromium (Cr), the oxidation state is 0.
- In CrCl3, the oxidation state of chromium is +3.
- The oxidation state increases from 0 to +3, indicating that oxidation occurs.

4. **Analyze Reaction 3: Na → Na+**
- In elemental sodium (Na), the oxidation state is 0.
- In Na+, the oxidation state of sodium is +1.
- The oxidation state increases from 0 to +1, indicating that oxidation occurs.

5. **Analyze Reaction 4: 2S2O3^2- → S4O6^2-**
- In S2O3^2- (thiosulfate), the oxidation state of sulfur (S) is +2.
- In S4O6^2- (tetrathionate), the oxidation state of sulfur is +2.5.
- The oxidation state increases from +2 to +2.5, indicating that oxidation occurs.

6. **Conclusion:**
- Among the four reactions analyzed, only Reaction 1 (CrO4^2- → Cr2O7^2-) shows no change in oxidation state, indicating that it involves neither oxidation nor reduction.
- Therefore, the correct answer is **Reaction 1: CrO4^2- → Cr2O7^2-**.