Question

At 473 K, equilibrium constant K_c for decomposition of phosphorus pentachloride, PCl₅ is \(8.3 ×10^{-3}\). If decomposition is depicted as,
\(PCl_5 (g) ⇋ PCl_3 (g) + Cl_2 (g)\)   \(∆_r H^Θ = 124.0\  kJ mol^{–1}\)

1. write an expression for K_c for the reaction.
2. what is the value of K_c for the reverse reaction at the same temperature?

1. what would be the effect on K_c if
   1. more PCl₅ is added
   2. pressure is increased
   3. the temperature is increased ?

Answer 1

(a) \(K_c= \frac {[PCl_3(g)][Cl_2(g)]}{[PCl_5(g)]}\)

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(b) Value of K_c for the reverse reaction at the same temperature is:
\(K'c = \frac {1}{K_c}\)

\(K'c  = \frac {1}{8.3×10^{- 3}} \)
\(K'c  = 1.2048×10^2\)
\(K'c  = 120.48\)

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(c) (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.