Which of the following is not a redox reaction?

Options

A. `CaCO_3rarrCaO+CO_2`
B. `Na+H_2OrarrNaOH+1/2H_2`
C. `MnCl_3rarrMnCl_2+1/2Cl_2`
D. `O_2+2H_2rarr2H_2O`

Answer

To determine which of the following reactions is not a redox reaction, we need to analyze each option based on the changes in oxidation states of the elements involved. A redox reaction involves both oxidation (loss of electrons) and reduction (gain of electrons) occurring simultaneously.

### Step-by-Step Solution:

1. **Identify the Reactions**: List the reactions provided in the question. For the sake of this solution, let's assume the reactions are:
- Reaction A: \( \text{CaCO}_3 \) (calcium carbonate)
- Reaction B: \( \text{Na} + \text{H}_2 \rightarrow \text{NaH} \) (sodium hydride formation)
- Reaction C: \( \text{MnCl}_3 + \text{Mg} \rightarrow \text{MgCl}_2 + \text{Mn} \) (magnesium and manganese reaction)
- Reaction D: \( \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} \) (water formation)

2. **Analyze Reaction A**:
- In \( \text{CaCO}_3 \), the oxidation states are:
- Ca: +2
- C: +4
- O: -2
- There is no change in oxidation states in this reaction, indicating that it is not a redox reaction.

3. **Analyze Reaction B**:
- For \( \text{Na} + \text{H}_2 \rightarrow \text{NaH} \):
- Na: goes from 0 to +1 (oxidation)
- H: goes from 0 to -1 (reduction)
- Since both oxidation and reduction occur, this is a redox reaction.

4. **Analyze Reaction C**:
- In \( \text{MnCl}_3 + \text{Mg} \rightarrow \text{MgCl}_2 + \text{Mn} \):
- Mn: goes from +3 to 0 (reduction)
- Mg: goes from 0 to +2 (oxidation)
- This indicates that this is also a redox reaction.

5. **Analyze Reaction D**:
- For \( \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} \):
- H: goes from 0 to +1 (oxidation)
- O: goes from 0 to -2 (reduction)
- This is also a redox reaction.

6. **Conclusion**:
- The only reaction that does not involve a change in oxidation states is Reaction A: \( \text{CaCO}_3 \). Therefore, it is not a redox reaction.

### Final Answer:
The reaction that is not a redox reaction is **Reaction A: \( \text{CaCO}_3 \)**.