Pure ammonia is placed in a vessel at a temperature where its dissociation constant `(alpha)` is appreciable. At equilibrium,

Options

A. `K_(p)` does not change significantly with pressure
B. `alpha` does not change with pressure
C. The concentration of `NH_(3)` does not change with pressure.
D. The concentration of hydrogen is less than that of nitrogen.

Explanation

To solve the problem regarding the dissociation of pure ammonia (NH₃) and its equilibrium state, we will analyze the equilibrium reaction and the implications of pressure changes on the dissociation constant (Kp) and the degree of dissociation (α).

### Step-by-Step Solution:

1. **Write the Equilibrium Reaction:**
The dissociation of ammonia can be represented as:
\[ 2 \text{NH}_3(g) \rightleftharpoons \text{N}_2(g) + 3 \text{H}_2(g) \]

2. **Define Initial and Equilibrium Conditions:**
- Initially, we have 1 mole of NH₃ and no products (N₂ and H₂).
- At equilibrium, let α be the degree of dissociation of NH₃. Thus:
- Moles of NH₃ at equilibrium = \(1 - \alpha\)
- Moles of N₂ at equilibrium = \(\frac{\alpha}{2}\)
- Moles of H₂ at equilibrium = \(\frac{3\alpha}{2}\)

3. **Write the Expression for Kp:**
The equilibrium constant Kp for the reaction can be expressed in terms of partial pressures:
\[ K_p = \frac{P_{N_2} \cdot P_{H_2}^3}{P_{NH_3}^2} \]
Where:
- \(P_{N_2} = \frac{\alpha}{2} \cdot P_{total}\)
- \(P_{H_2} = \frac{3\alpha}{2} \cdot P_{total}\)
- \(P_{NH_3} = (1 - \alpha) \cdot P_{total}\)

4. **Analyze the Effect of Pressure on Kp:**
Kp is only a function of temperature. Although the degree of dissociation (α) changes with pressure, Kp remains constant at a given temperature. Thus:
- **Statement A:** "Kp does not change significantly with pressure" is **correct**.

5. **Evaluate the Other Statements:**
- **Statement B:** "α does not change with pressure" is **incorrect**. α does change with pressure to maintain the value of Kp.
- **Statement C:** "The concentration of NH₃ does not change with pressure" is **incorrect**. The concentration of NH₃ changes with α, which is affected by pressure.
- **Statement D:** "The concentration of hydrogen is less than that of nitrogen" is **incorrect**. From the stoichiometry of the reaction, the concentration of H₂ is greater than that of N₂.

### Conclusion:
The only correct statement is A.