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Unformatted text preview: Lecture 39 1 Solids and Surfaces Adsorption and Reactivity (31.6-8) As we know intuitively, molecules stick to surfaces, ads H<0. This is an exothermic process. Molecules can physically adsorb physisorption, where molecules (adsorbates) bind to the surface (substrate) via weak van der Waals or hydrogen-bonding interactions. No significant alteration of the adsorbate on the surface. Typical energies 20kJ/mol (according to the text) < % Molecules can also chemically adsorb chemisorption, where there is a covalent or ionic interaction between adsorbate and substrate. Typical energies 250-500kJ/mol. But for only the first layer (monolayer) of molecules on the surface. Note substrate has almost the opposite meaning from biological/biochemistry in discussing surfaces. Also note there is a gap of 20-250kJ/mol between text definitions of physisorption and chemisorption. Lecture 39 2 Fig 31.20 Qualitative comparison between physi- and chemisorption. Note in this case there is no barrier as physisorption moves to chemisorption. Molecule can desorb as atoms, since curve crossing is below V(z)=0. How do molecules adsorb on to a surface and how does temperature and pressure play a role? There are many models, but one of the simplest (and usable) was developed by Langmuir.simplest (and usable) was developed by Langmuir....
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