how does enthalpy change during adsorption
Answers
Answer:
Important factor featuring adsorption is heat of adsorption. It is an exothermic process and ΔH adsorption is always negative as there is a decrease in residual forces on the surface. As the molecule adheres to the surface, the residual force decreases and as the movement is restricted so entropy ΔS also decreases.
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Answer:
Explanation:
The integral adsorption enthalpies reported below, which are in agreement with compiled data, are taken from immersion results obtained in the laboratory for well-defined members of each family, saturated where possible with Na+. The adsorption enthalpies at zero coverage have been deduced from these results taking the molecular area of the adsorbing water as 18 Å2 which seems a reasonable value at low coverage. With this value, the results obtained are in agreement with average extrapolated values at zero coverage generally reported in the literature for a family.
The film pressures have been obtained by integration of adsorption isotherms obtained by volumetric methods in the laboratory. The film pressures are in agreement with classical literature data. The number of layers is estimated in agreement with the usual multilayer models. The surface tension versus vacuum is obtained by using an empirical model [45]. It is clear, looking at the result obtained for illite, that this estimation is approximate. However the results deduced from this surface tension, namely the contact angle assuming that γSL is nil (following classical literature) are surprisingly good in the case of talc [52,105], and correct for kaolinite (see below).
The enthalpies of adsorption obtained have to be compared to the surface energies reported above. There is a strong link between these two types of results, indicating that an estimation of the adsorption effect using the crystallographic data is possible (see Figure 17 for the correlation). Linearity is not expected, even though we believe that surface energy is the major influence on the adsorption heat. At a primary level of interpretation, the surface tension is 40% of the surface energy.