In reaction `N_(2)(g)+3H_(2)(g) hArr 2NH_(3)(g), DeltaH=-93.6kJ`/mole, the yield of ammonia does not increase when

Options

A. Pressure is increased
B. Temperature is lowered
C. Pressure is lowered
D. Volume of the reaction vessel is decreased

Answer

To solve the question regarding the reaction \( N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \) with \(\Delta H = -93.6 \, \text{kJ/mole}\), we need to analyze the conditions under which the yield of ammonia does not increase. We will apply Le Chatelier's principle to evaluate the effect of changes in pressure, temperature, and volume on the equilibrium.

### Step-by-step Solution:

1. **Identify the Reaction and its Characteristics**:
- The reaction is \( N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \).
- It is an exothermic reaction since \(\Delta H\) is negative (\(-93.6 \, \text{kJ/mole}\)).

2. **Analyze the Effect of Increasing Pressure**:
- According to Le Chatelier's principle, increasing the pressure shifts the equilibrium towards the side with fewer moles of gas.
- On the reactant side, there are 4 moles (1 mole of \(N_2\) and 3 moles of \(H_2\)), and on the product side, there are 2 moles of \(NH_3\).
- Therefore, increasing pressure will shift the equilibrium to the right, increasing the yield of ammonia.

3. **Analyze the Effect of Lowering Temperature**:
- For an exothermic reaction, lowering the temperature will favor the formation of products.
- This means that the equilibrium constant will increase, and the reaction will shift to the right, thus increasing the yield of ammonia.

4. **Analyze the Effect of Lowering Pressure**:
- Lowering the pressure will shift the equilibrium towards the side with more moles of gas.
- In this case, it will shift to the left (towards the reactants), which means the yield of ammonia will not increase and may even decrease.

5. **Analyze the Effect of Decreasing the Volume of the Reaction Vessel**:
- Decreasing the volume increases the pressure.
- As established, increasing the pressure shifts the equilibrium to the side with fewer moles (the product side), thus increasing the yield of ammonia.

### Conclusion:
The yield of ammonia does not increase when the pressure is lowered. Therefore, the correct answer is **option 3**.

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