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8.CHEMICAL EQUILIBRIUM eLearn.Punjab

If the reactants A, B, and the products C, D of the reaction under consideration are ideal
gases, then molar concentration of each gas is proportional to its partial pressure . When the
concentrations are expressed in terms of partial pressures, the expression of Kp is,

Kp= pCc p d
D

pA a p b
B

Here PA, PB, PC and PD are partial pressures of A, B, C, D respectively at equilibrium position. As
long as the number of moles of products and reactants, which are in the gaseous state, are equal,

the values of KC and KP remain the same. Otherwise, the following relationship between KP and KC
can be derived by using Dalton’s law of partial pressures.

Kp =Kc (RT)Dn

Where ‘Dn’ is the difference between number of moles of the gaseous products and the
number of moles of gaseous reactants.
Dn = no. of moles of products - no. of moles of reactants
‘R’ is the general gas constant and ‘T’ is absolute temperature at which the reaction is being
carried out
Where, Dn = 0, then all the equilibrium constants have the same values.

Example 2:

N2 (g) and H2 (g) combine to give NH3 (g). The value of Kc in this reaction at 500 °C is 6.0 x 10-2.
Calculate the value of Kp for this reaction.

Solution:

The reaction for the synthesis of NH3 is

N2 (g) + 3H2 (g)  2NH3(g)

This reaction takes place with decrease in the number of moles. The relationship of Kpand Kc is

Kp =Kc (RT)Dn

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