Which requires catalyst ?

Options

A. `S + O_2 rarr SO_2`
B. `2 SO_2 + O_2 rarr 2 SO_3`
C. `C + O_2 rarr CO_2`
D. All of these

Answer

To determine which reaction requires a catalyst, we will analyze the given options based on the oxidation states of the elements involved and the stability of those states.

### Step-by-Step Solution:

1. **Identify the Reactions:**
- Option 1: \( S + O_2 \rightarrow SO_2 \)
- Option 2: \( 2SO_2 + O_2 \rightarrow 2SO_3 \)
- Option 3: \( C + O_2 \rightarrow CO_2 \)
- Option 4: All of the above.

2. **Analyze Option 1: \( S + O_2 \rightarrow SO_2 \)**
- In this reaction, sulfur (S) goes from an oxidation state of 0 to +4 in \( SO_2 \).
- The oxidation state change is from 0 to +4, which is a relatively stable oxidation state for sulfur.
- This reaction does not require a catalyst.

3. **Analyze Option 2: \( 2SO_2 + O_2 \rightarrow 2SO_3 \)**
- Here, sulfur in \( SO_2 \) has an oxidation state of +4 and changes to +6 in \( SO_3 \).
- The change from +4 to +6 is less stable and typically requires a catalyst to proceed efficiently.
- Therefore, this reaction requires a catalyst.

4. **Analyze Option 3: \( C + O_2 \rightarrow CO_2 \)**
- In this reaction, carbon (C) goes from an oxidation state of 0 to +4 in \( CO_2 \).
- The change from 0 to +4 is stable and does not require a catalyst.

5. **Conclusion:**
- Based on the analysis, the only reaction that requires a catalyst is Option 2: \( 2SO_2 + O_2 \rightarrow 2SO_3 \).

### Final Answer:
**Option 2 requires a catalyst.**