Chemisorption equilibrium and kinetics

The measurement of the equilibrium between the gaseous and the chemisorbed state is frequently difficult because of the very low equilibrium pressures required to saturate the surface. Often, in the case of strong interactions the monolayer is completed at very low pressures, even less than 1 torr, as shown in Fig. 19.1. 

When an adsorbate molecule is strongly bound and localized to one adsorption site, as in chemisorption, the Langmuir equation is applicable  

This region of the isotherm is the Henry s law region the uptake is directly proportional to the pressure. At high pressures, KP 1, so that Langmuir s equation becomes IV = 

The transition from physical adsorption to chemisorption occurs at point A. The potential energy at A is in excess of that for the adsorbate and the adsorbent when separated and represents the activation energy required for chemisorption, A fl. If curve I resided more to the right or curve II more to the left, then the transition from physical to chemical adsorption would occur with no activation energy since the crossover point would reside beneath zero potential energy. 

The concept of activated adsorption explains why the heat of adsorption is often small at low temperatures and large at higher temperatures. This is because at low temperature the heat is due to physical adsorption 

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