Equilibrium · Question 38

Consider the following chemical equilibrium of the gas phase reaction at a constant temperature : A(g) B(g) + C(g) If p is the total pressure, Kp is the pressure equilibrium constant and is the degree of dissociation, then which of the following is true at equilibrium?

1. If Kp value is extremely high compared to p, becomes much less than unity
2. When p increases, increases
3. If p value is extremely high compared to Kp, 1
4. When p increases, decreases

Answer

(4) When p increases, decreases

Solution

Take 1 mol of A initially.

At equilibrium: A = (1 - ), B = , C = ; total = (1 + ) mol

pA = 1 - 1 + p, pB = pC = 1 + p

Kp = pB pCpA = 2 p(1 + )(1 - ) = 2 p1 - 2

= KpKp + p

(1) Kp p 1, not much less than unity — incorrect.

(2), (4) At constant temperature Kp is constant; as p increases, KpKp + p decreases, so decreases (Le Chatelier: higher pressure favours the side with fewer gas moles, i.e. A). Hence (2) is incorrect and (4) is correct.

(3) p Kp Kpp 1, not 1 — incorrect.

Answer: (4)