Relation between Kp and Kc - Practice Questions & MCQ

Relation between K_p and K_c

The reaction we have:

The equilibrium constant K_c for this reaction is given as:

For the reaction:

The equilibrium constant K_p is given as:

As we know:

PV = nRT

P = (n/V)RT

Also, n/V = C

Thus, P = CRT

Therefore, ?n = (n₃ + n₄) - (n₁ + n₂)]

Hence, K_p = K_c(RT)?n

Expressions of K_c and K_p for some reactions

• Evaluation of K_c and K_p for HI formation

For the reaction:

Initially:              a mole b mole   0 mole

At equil:             (a - X) (b - X)       2X moles

Thus, according to the equilibrium constant equation we have:

Similarly, for Kp we have;

Partial pressure of individual species are given as follows:

On substituting these values in the above equation, we get:

• Evaluation of K_c and K_p for PCl₅ dissociation

Similarly for PCl₅ dissociation reaction we have:

The equilibrium constant has the following characteristics:

• The value of the equilibrium constant for a particular reaction is always constant depending only upon the temperature of the reaction and is independent of the concentrations of the reactants with which we start of the direction from which the equilibrium is approached.

• If the reaction is reversed. The value of the equilibrium constant is inversed.

• If the equation is divided by 2, the equilibrium constant for the new equation is the square root of K, i.e .

• If the equation is multiplied by 2, the equilibrium constant for the new equation is the square of K, i.e K².

• If the equation is written in two steps, then K = K₁ x K₂.

• The magnitude of the equilibrium constant gives an idea of the relative amounts of reactants and the products.

• The value of the equilibrium constant is not affected by the addition of a catalyst to the reaction.