Some oxidation reactions of methane are given below. Which of them is/are controlled oxidation reactions?

Options

A. `CH_(4)(g) + 2O_(2)(g) rarr CO_(2)(g)+2H_(2)O(l)`
B. `CH_(4)(g)+O_(2)(g)rarr C(s)+2H_(2)O(l)`
C. `CH_(4)(g)+O_(2)(g)overset(Mo_(2)O_(3))rarrHCHO+H_(2)O``2CH_(4)(g)+O_(2)(g)overset("Cu//523//100atm")rarr2CH_(3)OH`
D. `2CH_(4)(g)+O_(2)(g)overset("Cu//523//100atm")rarr2CH_(3)OH`

Explanation

To determine which oxidation reactions of methane are controlled oxidation reactions, we will analyze each reaction provided in the question step by step.

### Step 1: Understanding Controlled Oxidation Reactions
Controlled oxidation reactions are those where the oxidation process is intentionally limited to prevent the reactant from reaching its maximum oxidation state. This can be achieved by using a catalyst, changing temperature, pressure, or adding a poison that slows down the reaction.

### Step 2: Analyzing Each Reaction

**Reaction A:**
\[ \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \]

- In this reaction, methane is fully oxidized to carbon dioxide and water.
- There are no catalysts or changes in conditions mentioned.
- **Conclusion:** This is not a controlled oxidation reaction.

**Reaction B:**
\[ \text{CH}_4 + \text{O}_2 \rightarrow \text{C} + 2\text{H}_2 \]

- Here, methane is oxidized to carbon (solid) and hydrogen gas.
- Again, there are no catalysts or controlled conditions mentioned.
- **Conclusion:** This is also not a controlled oxidation reaction.

**Reaction C:**
\[ \text{CH}_4 + \text{O}_2 \rightarrow \text{HCHO} + \text{H}_2\text{O} \]

- In this reaction, methane is oxidized to formaldehyde (HCHO) and water.
- The presence of \( \text{Mo}_2\text{O}_3 \) as a poison indicates that the reaction is controlled to stop at the formaldehyde stage instead of proceeding to formic acid (HCOOH).
- **Conclusion:** This is a controlled oxidation reaction.

**Reaction D:**
\[ \text{CH}_4 + \text{O}_2 \text{ (in the presence of copper at 523 K and 100 atm)} \rightarrow \text{CH}_3\text{OH} \]

- Here, methane is oxidized to methanol (CH3OH).
- The specific conditions of high pressure (100 atm) and temperature (523 K) indicate that the reaction is controlled to stop at the alcohol stage, preventing further oxidation to aldehydes or acids.
- **Conclusion:** This is also a controlled oxidation reaction.

### Final Conclusion
From the analysis, we find that:
- **Controlled oxidation reactions:** Reaction C and Reaction D.
- **Not controlled oxidation reactions:** Reaction A and Reaction B.

### Summary of Controlled Reactions
- **C:** CH4 + O2 → HCHO + H2O (controlled by Mo2O3)
- **D:** CH4 + O2 (in presence of Cu, 523 K, 100 atm) → CH3OH (controlled by pressure and temperature)