2024 EquilibriumRelationship between Kp and Kc Medium Question 10 of 104 in Equilibrium

In which of the following equilibria, $K_p$ and $K_c$ are NOT equal?

A $CO(g) + H_2O(g) \rightleftharpoons CO_2(g) + H_2(g)$
B $2BrCl(g) \rightleftharpoons Br_2(g) + Cl_2(g)$
C $PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g)$
D $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$

The answer is C.

Solution

$K_p = K_c(RT)^{\Delta n_g}$; if $\Delta n_g = 0$, then $K_p = K_c$
$CO + H_2O \rightleftharpoons CO_2 + H_2$: $\Delta n_g = 2 - 2 = 0$
$2BrCl \rightleftharpoons Br_2 + Cl_2$: $\Delta n_g = 2 - 2 = 0$
$PCl_5 \rightleftharpoons PCl_3 + Cl_2$: $\Delta n_g = 2 - 1 = 1$
$H_2 + I_2 \rightleftharpoons 2HI$: $\Delta n_g = 2 - 2 = 0$
So $K_p$ and $K_c$ are not equal for $PCl_{5(g)} \rightleftharpoons PCl_{3(g)} + Cl_{2(g)}$.

Concept Condition for Kp = Kc based on change in moles of gas

Equilibrium in past papers

104 questions from this chapter have appeared across 27 exam years.