question_answer 1)

At 473 K, the equilibrium constant \[{{K}_{c}}\] for the decomposition of phosphorus pentachloride \[(PC{{l}_{5}})\]is\[8.3\times {{10}^{-3}}\]. If decomposition proceeds as:\[PC{{l}_{5}}(g)\rightleftharpoons PC{{l}_{3}}(g)+C{{l}_{2}}(g);\] \[\Delta H=+124.0kJ\,mo{{l}^{-1}}\] (a) Write an expression for \[{{K}_{c}}\] for the reaction. (b) What is the value of \[{{K}_{c}}\] for the reverse reaction at the same temperature? (c) What would be the effect on \[{{K}_{c}}\]if (i) More of\[PC{{l}_{5}}\] is added (ii) Temperature is increased?

Answer:

(a) \[{{K}_{c}}\frac{[PC{{l}_{3}}][C{{l}_{2}}]}{[PC{{l}_{5}}]}=8.3\times {{10}^{-3}}mol\,{{L}^{-1}}\] (b)\[PC{{l}_{3}}(g)+C{{l}_{2}}(g)\rightleftharpoons PC{{l}_{5}}(g)\] \[{{K}_{c}}=\frac{[PC{{l}_{5}}]}{[PC{{l}_{3}}][C{{l}_{2}}]}=\frac{1}{8.3\times {{10}^{-3}}}\] \[=120.48L\,mo{{l}^{-1}}\] (c) (i) \[{{K}_{c}}\] is unaffected by addition of \[PC{{l}_{5}}\]. (ii) The reaction is endothermic, hence the equilibrium constant\[{{K}_{c}}\] will increase with increase in temperature.