Which of the following reactions will not be affected by increasing the pressure?

Options

A. `PCl_(5)(g) hArr PCl_(3)(g)+Cl_(2)(g)`
B. `N_(2)(g)+O_(2)(g) hArr 2NO(g)`
C. `CaCO_(3)(s) hArr CaO(s)+CO_(2)(g)`
D. `CO(g)+H_(2)O(g) hArr CO_(2)(g)+H_(2)(g)`

Explanation

To determine which reaction will not be affected by an increase in pressure, we need to 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 the Effect of Pressure on Equilibrium:**
- According to Le Chatelier's principle, increasing the pressure in a gaseous reaction will shift the equilibrium towards the side with fewer moles of gas. This is because a higher pressure favors the formation of products or reactants that occupy less volume.

2. **Calculate the Change in Moles of Gas (Δng):**
- For each reaction, we need to identify the number of moles of gaseous reactants and products. The change in moles of gas (Δng) is calculated as:
\[ \Delta n_g = \text{(moles of gaseous products)} - \text{(moles of gaseous reactants)} \]
- If Δng = 0, it means that the number of moles of gas is the same on both sides of the reaction, and thus the reaction will not be affected by changes in pressure.

3. **Analyze Each Reaction:**
- **Option 1:**
- Reactants: 1 mole gas
- Products: 1 mole gas
- Δng = 1 - 1 = 0 (This reaction is not affected by pressure.)

- **Option 2:**
- Reactants: 1 mole gas
- Products: 2 moles gas
- Δng = 2 - 1 = 1 (This reaction is affected by pressure.)

- **Option 3:**
- Reactants: 1 mole gas
- Products: 1 mole gas
- Δng = 1 - 1 = 0 (This reaction is not affected by pressure.)

- **Option 4:**
- Reactants: 2 moles gas
- Products: 2 moles gas
- Δng = 2 - 2 = 0 (This reaction is not affected by pressure.)

4. **Conclusion:**
- The reactions that will not be affected by increasing the pressure are those where Δng = 0. From the analysis:
- Option 1, Option 3, and Option 4 have Δng = 0.
- Option 2 has Δng ≠ 0 and will be affected by pressure.

### Final Answer:
The reactions that will not be affected by increasing the pressure are **Option 1, Option 3, and Option 4**.