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 following reactions proceed at low pressure, we need to analyze the change in the number of moles of gas on both the reactant and product sides of each reaction. The principle is that at low pressure, a reaction will favor the side with more moles of gas.

### Step-by-Step Solution:

1. **Understand the Concept of Pressure and Moles**:
- At low pressure, the reaction will shift towards the side with a greater number of moles of gas. This is based on Le Chatelier's principle, which states that a system at equilibrium will shift in response to changes in pressure.

2. **Identify the Reactions**:
- We need to analyze each reaction provided in the options to determine the number of moles of reactants and products.

3. **Calculate Change in Moles (Δn)**:
- Δn = (Number of moles of products) - (Number of moles of reactants)
- If Δn > 0, the reaction will proceed in the forward direction at low pressure.
- If Δn < 0, the reaction will favor the reverse direction.
- If Δn = 0, the reaction is at equilibrium and does not favor either direction.

4. **Analyze Each Option**:
- **Option 1**:
- Reaction: A → B + C
- Moles: Reactants = 1, Products = 2 (Δn = 2 - 1 = 1)
- Since Δn > 0, this reaction proceeds forward at low pressure.

- **Option 2**:
- Reaction: A + B ⇌ C + D
- Moles: Reactants = 2, Products = 2 (Δn = 2 - 2 = 0)
- Since Δn = 0, this reaction is at equilibrium and does not favor either direction.

- **Option 3**:
- Reaction: PCl5 ⇌ PCl3 + Cl2
- Moles: Reactants = 1, Products = 2 (Δn = 2 - 1 = 1)
- Since Δn > 0, this reaction proceeds forward at low pressure.

- **Option 4**:
- Reaction: N2 + O2 ⇌ 2NO
- Moles: Reactants = 2, Products = 2 (Δn = 2 - 2 = 0)
- Since Δn = 0, this reaction is at equilibrium and does not favor either direction.

5. **Conclusion**:
- The reactions that proceed at low pressure are those where Δn > 0. From our analysis, **Option 1** and **Option 3** are the reactions that proceed at low pressure.

### Final Answer:
- The reactions that proceed at low pressure are **Option 1** and **Option 3**.