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  • At 473 K, equilibrium constant K_ c for decomposition of phosphorus pentachloride PCl _5 is 8.3 X 10^-3.If decomposition is depicted as PCl_5 (g) PCl _3 (g)+ Cl_2 (g) (a) write an

7.30     At 473 K, equilibrium constant Kc for decomposition of phosphorus pentachloride, PCl5 is 8.3 ×10-3. If decomposition is depicted as,

                PCl_{5}(g)\rightleftharpoons PCl_{3}(g)+Cl_{2}(g), \Delta _{r}H^{+}=124.0kJ mol^{-1}

    (a)       write an expression for 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

                (i) more PCl5 is added
                (ii)   pressure is increased
                (iii)  the temperature is increased ?  

Answers (1)

best_answer

We have,
PCl_{5}(g)\rightleftharpoons PCl_{3}(g)+Cl_{2}(g), \Delta _{r}H^{+}=124.0kJ mol^{-1}
Equilibrium constant for the above reaction = 8.3\times 10^{-3}

(a) Expression of K_c for this reaction-
K_c=\frac{[PCl_3][Cl_2]}{PCl_5}

(b) The value of reverse equilibrium constant can be calculated as;

                  K'_c = \frac{1}{K_c} 
                           =\frac{1}{8.3\times 10^{-3}}=1.20\times 10^2

(c).i   K_c would remain the same because the temperature is constant in this case.

(c). ii  If we increase the pressure,  there is no change in K_c because the temperature is constant in this case also.

(c). iii  In an endothermic reaction, the value of   K_c increases with increase in temperature.

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manish

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