Which is not a cause of acid rain?

Options

A. `NO_(2)`
B. `CH_(4)`
C. `SO_(2)`
D. `CO_(2)`

Answer

To determine which option is not a cause of acid rain, we need to analyze each of the given compounds: NO2, CH4, SO2, and CO2.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- The options given are:
a. NO2 (Nitrogen Dioxide)
b. CH4 (Methane)
c. SO2 (Sulfur Dioxide)
d. CO2 (Carbon Dioxide)

2. **Analyze NO2**:
- NO2 reacts with water to form nitric acid (HNO3).
- Reaction:
\[ \text{NO}_2 + \text{H}_2\text{O} \rightarrow \text{HNO}_3 \]
- Conclusion: NO2 is a cause of acid rain.

3. **Analyze CH4**:
- CH4 is methane, a greenhouse gas with a very high pKa (approximately 56), indicating it is a very weak acid.
- Conclusion: CH4 does not contribute to acid rain.

4. **Analyze SO2**:
- SO2 reacts with water to form sulfuric acid (H2SO4).
- Reaction:
\[ \text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3 \]
\[ \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \]
- Conclusion: SO2 is a cause of acid rain.

5. **Analyze CO2**:
- CO2 reacts with water to form carbonic acid (H2CO3).
- Reaction:
\[ \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \]
- Conclusion: CO2 can contribute to acid rain, albeit to a lesser extent.

6. **Final Conclusion**:
- Among the options, CH4 (methane) is the only compound that does not cause acid rain.
- Therefore, the answer is **b. CH4**.