At 473 K, equilibrium constant Kcfor decomposition of phosphorus pentachloride, PCl5 is 8.3 ×10-3. If decomposition is depicted as,
PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)
= 124.0 kJ mol–1
A. write an expression for Kcfor the reaction.
B. what is the value of Kcfor the reverse reaction at the same temperature?
C. what would be the effect on Kc if (i) more PCl5 is added (ii) pressure is increased?
(iii) Is the temperature increased?
Equilibrium constant(Kc) is the product of the molar concentrations of the product, each raised to the power equal to its stoichiometric coefficient divided by the product of the molar concentrations of the reactants, each raised to the power equal to its stoichiometric coefficient is constant at constant temperature.
∴ Kc = 
(Law of Chemical Equilibrium)
For the given decomposition reaction,
PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)
Given: The equilibrium constant (Kc) is 8.3 × 10-3
a) From Law of Chemical Equilibrium, we know that
⇒Kc = 
Where X and Y are the products and A and B are the reactants
Hence, for the given decomposition reaction
PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)
⇒Kc = 
b) Reverse reaction: PCl3 (g) + Cl2 (g) ⇌ PCl5 (g)
Kc’ = 
Kc = 8.3 × 10-3 (given)
∴ Kc’ = 
⇒Kc’ = 120.48
c) (i) If more PCl5 is added in the reaction, no effect as KC is constant at constant temperature.
(ii) If the pressure is increased, no effect as KC is constant at constant temperature.
(iii) In an endothermic reaction, the value of Kc increases with an increase in temperature. Since the given reaction is an endothermic reaction, the value of Kc will increases with the increase of temperature.