What is an adsorption isotherm? Describe Freundlich adsorption isotherm.

Explanation

Adsorption isotherm The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve known as adsorption isotherm. Adsorption isotherms are of two types : (a) Freundlich adsorption isotherm and (b) Langmuir adsorption isotherm.
Freundlich adsorption isotherm Freundich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. The relationship is expressed by the equation : `x/m=k^1//p^n` ...........(i)
where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure p. k and n are the constants which depend on the nature of the adsorbent and the gas at a particular temperature.

These curves indicate that at a fixed pressure there is a decrease in physical adsorption with increase in temperature. Taking logarithm of equation (i)
`log""x/m log` ............(ii)
When `log""x/m` is plotted on Y-axis and log p on X-axis, we get a straight line. It indicates the validity of Freundlich isotherm. Slope = `1/n` and Intercept on Y-axis = log k.
Factor `1/n` can have values 0 to 1. Equation (ii) holds good over a limited range of pressure :
(i) When `1/n=x/m in o n` the adsorption is independent of pressure.
(ii) When `1/n=x/m =kp` i.e, `x/m?p` the adsorption varies directly with pressure.