# Free energy change during adsorption

by dikmikkel
Tags: adsorption, dissociative, entropy change, extrapolation
 P: 168 Hi, Suppose I have a dissociative adsorption reaction and want to find the free energy change ΔF, this can be written at constant temperature: $\Delta F = \Delta E -T\Delta S$ Now given the adsorption energy $\Delta E_{ads}$ I would like to extrapolate the Free energy assuming that the adsorbate is an ideal gas since I know the experimental entropy. I Assume that the vibrations from the adsorped moleule does not perturb the substrate vibrations and also that the configurational entropy is not changes(maybe rough): $\Delta S = S_{ads}-S_{noads}$ $S_{ads} = S_{subs+a} - S_{subs}-S_{a} = -\Delta S_{vib,gas} -S_{gas}$ The vibrations term does not need to be extrapolated but the gas term does so: $S_{gas} = S^{\theta}_{gas} - c_p\log(\dfrac{T}{T^{\theta}}) + k_b\log(\dfrac{p}{p^{\theta}})$ I feel like i am missing a term of the type $- \dfrac{7}{2}k$ since the translation of the gas is not possible when adsorbed. Does anyone know if I am right?

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