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Kc and Kp and Their Relationship

↪ Kc is the equilibrium  constant of reaction when active masses of all the components of the reaction is expressed in molar concentration terms.
↪ Kp is the equilibrium constant of the gaseous reaction when the active masses are expressed in partial pressure terms.

↪ For the general gaseous reaction

↪ when the active masses are in molar concentration, the equilibrium constant Kc is given as
 where [A], [B], [C] and [D] are their molar concentrations at equilibrium.

↪ Similarly, when the active masses are in partial pressures, the equilibrium constant Kp is given as
where PA, PB, PC and PD are their partial pressures at equilibrium.

↪ To relate Kc and Kp, consider ideal gas equation

Pi Vi = ni RT where Pi, Vi, and ni are partial pressure, volume and no. of mole of gaseous component, say "i"

↪ Now, substituting in the expression of Kp as

↪ where ∆n = no. of moles of gaseous products - no. of moles of gaseous reactants

↪ when no. of moles of reactants and products are same then ∆n = 0 and at that condition Kp = Kc.