Question

At 473 K, equilibrium constant Kc for decomposition of phosphorus pentachloride, $PC{l}_{5}is8.3\times {10}^{-3}$. If decomposition is depicted as,
$PC{l}_5\left(g\right)\leftrightarrow PC{l}_3\left(g\right)+C{l}_2\left(g\right){\Delta }_r{H}^{\circ }=124.0kJmo{l}^{-1}$

Write an expression for $K_c$ for the reaction.

What is the value of $K_c$ for the reverse reaction at the same termperature?

What would be the effect on $K_c$ if
(i) more $PC{l}_5$ is added
(ii) pressure is increased?
(iii) The temperature is increased?

Answer 1

$K_c=\frac{\left[PC{l}_{3\left(g\right)}\right]\left[C{l}_{2\left(g\right)}\right]}{\left[PC{l}_{5\left(g\right)}\right]}$

Value of K_c for the revese reaction at the same temperature is:
$K_c^{\prime }={\frac{1}{K}}_c=\frac{1}{8.3\times {10}^{-3}}=1.2048\times {10}^2=120-48$

(i) $K_c$ would remain the same because in this case, the temperature remains the same.
(ii) $K_c$ is constant at constant temperature. Thus, in this case, $K_c$ would not change.
(iii) In an endothermic reaction, the value of $K_c$ increases with an increase in temperature.
Since the given reaction in an endothermic reaction, the value of $K_c$ will increase if the temperature is increased.