Question

At 473 K, equilibrium constant, Kc, for the decomposition of phosphorus pentachloride PCl5 is 8.3 × $10^{-3}$: $PCl_{5}(g)$⇄$PCl_{3}(g)+Cl_{2}(g)$, ΔH=124 kJ $mol^{-1}$.
(a) Write an expression of Kc for the reaction.
(b) What is the value of Kc for the reverse reaction at the same temperature?
(c) What would be the effect on Kc if the temperature is increased?

Answer 1

a) ${ K }_{ c }\quad =\quad \frac { \left[ { PCl }_{ 3(g) } \right] \left[ { Cl }_{ 2(g) } \right]  }{ \left[ { PCl }_{ 5(g) } \right]  }$

b) At the same temperature, the value of $K_c$ for reverse reaction is ${ K' }_{ c }\quad =\quad \frac { 1 }{ { K }_{ c } }$

$=\quad \frac { 1 }{ 8.3\quad \times \quad { 10 }^{ -3 } } \quad =\quad 1.2048\quad \times \quad { 10 }^{ 2 }\quad =\quad 120.48$

c) $K_c$ is same for the reaction because the temperature remains same.

At constant temperature, $K_c$ is constant. In this condition $K_c$ will not change.

In case of endothermic reaction, $K_c$ value increases with increase in temperature. The given reaction is an endothermic reaction. Hence, as the temperature increases the value of $K_c$ is also increased.