Adsorption Isotherms

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chemistry 12th class cbse notes

Adsorption Isotherms

The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as adsorption isotherm.

A mathematical equation which describes the relationship between pressure (p) of the gaseous adsorbate and the extent of adsorption at any fixed temperature is called adsorption isotherms.

Thus, if x g of an adsorbate is adsorbed on m g of the adsorbent, then

Extent of adsorption = x/m

Freundlich adsorption isotherm: Freundlich adsorption isotherm is obeyed by the adsorptions where the adsorbate forms a monomolecular layer on the surface of the adsorbent.

Adsorption Isotherms

Adsorption Isotherms

where, x is the weight of the gas adsorbed by m gm of the adsorbent at a pressure P, thus x/m represents the amount of gas adsorbed by the adsorbent per gm (unit mass), k and n are constant at a particular temperature and for a particular adsorbent and adsorbate (gas), n is always greater than one, indicating that the amount of the gas adsorbed does not increase as rapidly as the pressure.

Adsorption Isotherms

At low pressure, the extent of adsorption varies linearly with pressure

Adsorption Isotherms

At high pressure, it becomes independent of pressure

Adsorption Isotherms

At moderate pressure x/m depends upon pressure raised to powers

Adsorption Isotherms

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