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Home » Chemistry Homework Help » Physical Chemistry » Adsorption of Gases
Adsorption of Gases
Gases are adsorbed onto solid surfaces.

It is convenient in the study of adsorption to recognize that adsorptions can be placed in one of the two categories. These categories are suggested by the possibilities of having essentially physical factors holding the gas molecules to the solid or of having chemical bonds serve that function. The categories of physical adsorption and chemical adsorption, or more commonly, chemisorption, thus arise. The observed characteristics of any adsorption process are commonly allowing it to place in on e category to the other. The experimental features that adsorption should be recognized as haveng the features of a physical process, such as condensation, o r those of a chemical reaction. It should be monolayer is being formed is not directly observable but, as we shall see, can often be deduced from experimental data.

Most interest in adsorption and in closely related field of heterogeneous catalysis is chemisorption. Two of the items mentioned in table of this graph, therefore are dealt with more detail and with particular emphasis on chemisorption.

Physical adsorption: enthalpy of adsorption is less than about 40 K J mol -1

Adsorption appreciably only at temperatures below the boiling point of the adsorbate

Incremental increase in amount adsorbed in pressure of adsorbate

Amount of adsorption on surface a function more of adsorbate than adsorbent;

No appreciable activation energy involved in adsorption process

Multilayer adsorption occurs.

Chemisorption: enthalpy of adsorption greater than about 80 K J mol -1;

Adsorption greater than at high temperatures decreases with high temperatures.

Amount of adsorption characteristic of both adsorbate and adsorbant;

Activation energy may be involved in both adsorption processes;

Adsorption leads to a monolayer, at most.

Enthalpy of adsorption: in all adsorption of the type:

Gas adsorbed layer

Energy is given to the thermal surrounds and, therefore, ∆H is negative. (the entropy of the gas decreases as it is converted to the more restricted, and perhaps more ordered, adsorbed layer. For the adsorption to be spontaneous, energy must be supplied to the thermal surroundings.)

For physical adsorption the energy involved is of the order of enthalpies of vaporization, i.e. generally less than 40 KJ mol-1. In keeping with the idea that physical adsorption may be leading to the formation of multi layers, these enthalpies are more dependent on the nature of the gas than they are that of the solid adsorbent. Adsorptions classed as chemisorptions, however, have enthalpies of adsorption that compare with those of ordinary chemical reactions. Enthalpies of up to about 630 kJ mol-1 have been measured.

The appropriate values mentioned for physical and chemical adsorption are not intended to imply constancy for these enthalpies as a function of the amount of gas adsorbed. Some of the variations in differential enthalpies of adsorption in the chemisorption region are the frequency observed curve over the region of adsorption from low coverages to multilayer formation has a high initial enthalpy that falls off at large amounts of adsorption. This behavior is taken as indicative of an initial chemisorption, to form something like a monolayer, followed by the formation of multiple layers that are bound by physical forces.

Even within a range attributed to chemisorption, however, the enthalpy of adsorption is usually found to be a function of the amount adsorbed. A number of molecular explanations have been offered for this variation.

(1) Active sites can be assumed to exist on the adsorbent, and since there are occupied by the first additions of gas, the binding of later additions must occur on less active sites and strength of binding falls off.

(2) The binding of some gas to the solid can occur with electrons being given up to the solid by the adsorbed molecules or with electrons being withdrawn from the solid, and as such processes continue, the solid becomes more and more reluctant to gain or lose more electrons. Such an explanation is particularly appropriate to semiconducting and conducting adsorption.

(3) The mutual repulsion and conducting molecules, especially if they acquire a net charge when they are adsorbed, operates to oppose the addition of further molecules.

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