Which one of the following equilibrium moves backward when pressure is applied ?

Options

A. `N_2(g) +3H_2(g) hArr 2NH_3(g)`
B. `N_2 (g) +O_2 (g) hArr 2NO (g)`
C. `"water" hArr "Ice"`
D. `I_2 (g) hArr I_2 (s)`

Answer

To determine which equilibrium moves backward when pressure is applied, we need to analyze the changes in the number of moles of gas on both sides of the equilibrium reaction. The principle that governs this behavior is Le Chatelier's principle, which states that if a system at equilibrium is subjected to a change in pressure, the equilibrium will shift in the direction that opposes the change.

### Step-by-Step Solution:

1. **Identify the Equilibrium Reactions**:
We need to consider the given equilibrium reactions and their respective changes in the number of moles of gas (ΔNG) when pressure is applied.

2. **Calculate ΔNG for Each Reaction**:
- For each reaction, calculate ΔNG using the formula:
\[ \Delta NG = \text{(moles of gaseous products)} - \text{(moles of gaseous reactants)} \]
- If ΔNG is positive, it means there are more moles of gas on the product side. If ΔNG is negative, it means there are fewer moles of gas on the product side.

3. **Analyze the Effect of Pressure**:
- According to Le Chatelier's principle:
- If ΔNG < 0 (more moles of gas on the reactant side), increasing pressure will shift the equilibrium to the left (backward).
- If ΔNG > 0 (more moles of gas on the product side), increasing pressure will shift the equilibrium to the right (forward).
- If ΔNG = 0, pressure changes will have no effect on the position of equilibrium.

4. **Determine the Reaction that Moves Backward**:
- From the analysis, identify which reaction has a negative ΔNG. This reaction will shift backward when pressure is applied.

5. **Conclusion**:
- Based on the calculations and analysis, the equilibrium that has a negative ΔNG will move backward when pressure is applied.