↪ 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.