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 can use Le Chatelier's principle. This principle states that if an external change is applied to a system at equilibrium, the system will adjust itself to counteract that change and restore a new equilibrium.

### Step-by-Step Solution:

1. **Identify the Equilibrium Reaction**:
We need to analyze the provided options to identify the number of moles of reactants and products in each equilibrium reaction.

2. **Apply Le Chatelier's Principle**:
According to Le Chatelier's principle, if pressure is applied to a gaseous equilibrium, the equilibrium will shift towards the side with fewer moles of gas. If the reaction involves solids or liquids, the effect of pressure is negligible.

3. **Evaluate Each Option**:
- **Option 1**: If the reaction has 4 moles of gas on the reactant side and 2 moles on the product side, applying pressure will shift the equilibrium to the right (forward direction) since it moves from higher moles to lower moles. **(Incorrect)**

- **Option 2**: If there are 2 moles of gas on both sides (reactants and products), applying pressure will have no effect on the equilibrium position. **(Incorrect)**

- **Option 3**: This option involves water and ice. Ice (solid) has a greater volume than water (liquid). When pressure is applied, the equilibrium will shift from the higher volume (ice) to the lower volume (water), thus moving backward. **(Correct)**

- **Option 4**: If one side of the reaction is a gas and the other is a solid, applying pressure will shift the equilibrium forward (to the side with fewer moles of gas). **(Incorrect)**

4. **Conclusion**:
The correct answer is **Option 3**, where the equilibrium shifts backward when pressure is applied.