Which of the following is not a redox reaction?

Options

A. `CaCO_(3)rarrCaO+CO_(2)`
B. `O_(2)+2H_(2)rarr2H_(2)O`
C. `Na+H_(2)OrarrNaOH+(1)/(2)H_(2)`
D. `MnCl_(3)rarrMnCl_(2)+(1//2)Cl_(2)`

Explanation

To determine which of the given reactions is not a redox reaction, we need to analyze each reaction by examining the oxidation states of the elements involved. A redox reaction involves a change in oxidation states, where one species is oxidized (loses electrons) and another is reduced (gains electrons).

### Step-by-Step Solution:

1. **Identify the Reactions**: Let's denote the reactions as follows:
- Reaction 1: \( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)
- Reaction 2: \( \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} \)
- Reaction 3: \( \text{Na} + \text{H}_2\text{O} \rightarrow \text{NaOH} + \text{H}_2 \)
- Reaction 4: \( \text{MnCl}_2 + \text{Cl}_2 \rightarrow \text{MnCl}_3 \)

2. **Analyze Reaction 1**:
- Calcium (Ca) in \( \text{CaCO}_3 \) has an oxidation state of +2 and remains +2 in \( \text{CaO} \).
- Carbon (C) in \( \text{CaCO}_3 \) has an oxidation state of +4 and remains +4 in \( \text{CO}_2 \).
- Oxygen (O) is -2 in both compounds.
- **Conclusion**: No change in oxidation states; therefore, **this is not a redox reaction**.

3. **Analyze Reaction 2**:
- In \( \text{H}_2 \), hydrogen has an oxidation state of 0 and in \( \text{H}_2\text{O} \), it is +1.
- Oxygen in \( \text{O}_2 \) is 0 and in \( \text{H}_2\text{O} \), it is -2.
- **Conclusion**: Oxidation (H) and reduction (O) are occurring; hence, **this is a redox reaction**.

4. **Analyze Reaction 3**:
- Sodium (Na) in elemental form has an oxidation state of 0 and in \( \text{NaOH} \), it is +1.
- Hydrogen in \( \text{H}_2\text{O} \) is +1 and remains +1 in \( \text{H}_2 \).
- Oxygen is -2 in both \( \text{H}_2\text{O} \) and \( \text{NaOH} \).
- **Conclusion**: Oxidation (Na) and no reduction; hence, **this is a redox reaction**.

5. **Analyze Reaction 4**:
- Manganese (Mn) in \( \text{MnCl}_2 \) has an oxidation state of +2 and in \( \text{MnCl}_3 \), it is +3.
- Chlorine (Cl) in \( \text{Cl}_2 \) has an oxidation state of 0 and in \( \text{MnCl}_3 \), it is -1.
- **Conclusion**: Oxidation (Cl) and reduction (Mn) are occurring; hence, **this is a redox reaction**.

### Final Conclusion:
The reaction that is **not a redox reaction** is **Reaction 1: \( \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \)**.

---