The exchange equilibrium corresponds to what occurs in the competitive adsorption process between two adsorbate molecules of a mixture. Adsorption sites are either occupied by adsorbate A or adsorbate B, leaving the other in the gas phase. We model this equilibrium by the equation A (ads) + B (gas) = A (gas) + B (ads) , and the equilibrium constant corresponds to the selectivity 𝑠 A/B = (𝑞 A 𝑦 B )/(𝑞 B 𝑦 A ). The exchange Gibbs free energy then simply derives from the selectivity: