It is possible to predict the equilibrium constant of a reaction by combining tw

Question

It is possible to predict the equilibrium constant of a reaction by combining two or more reactions for which the value of K is known. When combining equilibrium constants, it is important to note the following: When reactions are added, their K values are multiplied. When a reaction is reversed, its K value is inverted; that is Krevese = 1/Kforward. When the coefficients of a reaction are multiplied by a factor, the K value is raised to the power of that factor. Determine the equilibrium constant, Kgoal, for the reaction 4PCl5(g) P4(s) + 10Cl2(g), Kgoal = ? by making use of the following information: P4(s) + 6Cl2(g) 4PCl3(g), K1 = 2.00 times 1019 PCl5(g) PCl3(g) + Cl2(g), K2 = 1.13 times 10-2

Explanation / Answer

We have eqn 1

P4 (s) + 6 Cl2 (g) ------> 4PCl3 , K1 = 2.00*1019

and eqn 2

PCl5 (g) -------> PCl3 (g) + Cl2 (g) , K2 = 1.13*10-2

Inverting eqn 1 we get eqn 3

4PCl3 (g) ---------> P4 (s) + 6 Cl2 (g) , K3 = 1/K1 = 5*10-20

Multiplying eqn 2 by 4 we get eqn 4

4PCl5 (g) --------> 4 PCl3 (g) + 4 Cl2 (g) , K4 = K24 = 1.63*10-8

Adding eqn 3 & 4

4 PCl5 (g) ------------> P4 (s) + 10 Cl2 (g) , Kgoal = K3*K4 = 8.15*10-28

So

  Kgoal = K3*K4 = 8.15*10-28

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