Which of the following reactions proceed at low pressure?

Options

A. `N_(2)(g)+3H_(2)(g)hArr 2NH_(3)(g)`
B. `H_(2)(g)+I_(2)(g)hArr2HI(g)`
C. `PCl_(5)(g)hArrPCl_(3)(g)+Cl_(g)`
D. `N_(2)(g)+O_(2)(g)hArr2NO(g)`

Answer

To determine which of the given reactions proceeds at low pressure, we can apply Le Chatelier's principle. This principle states that if an external change is applied to a system at equilibrium, the system will adjust to counteract that change and restore a new equilibrium.

### Step-by-Step Solution:

1. **Understand Le Chatelier's Principle**:
- When pressure is applied to a system, the equilibrium shifts towards the side with fewer moles of gas. Conversely, when low pressure is applied, the equilibrium shifts towards the side with more moles of gas.

2. **Analyze Each Reaction**:
- For each reaction, count the number of moles of gas on both the reactant and product sides.

3. **Evaluate the Options**:
- **Option 1**: \( N_2 + 3H_2 \rightleftharpoons 2NH_3 \)
- Reactants: 4 moles (1 mole of \( N_2 \) + 3 moles of \( H_2 \))
- Products: 2 moles (2 moles of \( NH_3 \))
- Since there are more moles on the reactant side, applying low pressure would shift the equilibrium backward, making this option incorrect.

- **Option 2**: \( A + B \rightleftharpoons C + D \)
- Reactants: 2 moles (1 mole of \( A \) + 1 mole of \( B \))
- Products: 2 moles (1 mole of \( C \) + 1 mole of \( D \))
- Since there are equal moles on both sides, applying low pressure would not favor either direction, making this option incorrect.

- **Option 3**: \( C \rightleftharpoons D + E \)
- Reactants: 1 mole (1 mole of \( C \))
- Products: 2 moles (1 mole of \( D \) + 1 mole of \( E \))
- Since there are more moles on the product side, applying low pressure would shift the equilibrium forward, making this option correct.

- **Option 4**: \( N_2 + O_2 \rightleftharpoons 2NO \)
- Reactants: 2 moles (1 mole of \( N_2 \) + 1 mole of \( O_2 \))
- Products: 2 moles (2 moles of \( NO \))
- Since there are equal moles on both sides, applying low pressure would not favor either direction, making this option incorrect.

4. **Conclusion**:
- The only reaction that proceeds in the forward direction at low pressure is **Option 3**: \( C \rightleftharpoons D + E \).

### Final Answer:
The reaction that proceeds at low pressure is **Option 3**.