The Langmuir model of chemisorption equilibrium

The Langmuir model is the basic model for chemisorption equilibrium. It rests on a certain number of assumptions that are thus modified to take more complex assumptions into account. We start from the following assumptions  

Under these conditions, we can write adsorption reaction in the form  

Indicate by the total number of sites per unit area before adsorption and by S the number of free sites at a given time, the amount of G - s is then S -S and the fraction of covering as  

At the adsorption equilibrium, the degree of covering is 0, and equation [6.1] becomes  

The coefficients of activity are not independent at fixed total pressure and temperature. They are associated with the Gibbs-Duhem equation [SOU 90, p. 69]  

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