211. What is the adsorption isotherm ? Distinguish between Freundlich adsorption isotherm and Langmuir adsorption isotherm with the help of appropriate graphs.

Answer:

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.

Freundlich adsorption isotherm: Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. The
relationship can be expressed by the following equation:

x/m = k.p1/n (n > 1) 

where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, k and n are constants which depend on the nature of the adsorbent and the gas at a particular
temperature.


LANGMUIR ADSORPTION:
It tells about the number of active site of the surface undergoing adsorption and pressure.

$\mathrm{\Theta } = \frac{\mathrm{kP}}{1+\mathrm{KP}}$

Where,

θ= the number of sites of the surface which are covered with gaseous molecule,

P= pressure 

K =is the equilibrium constant for distribution of adsorbate between the surface and the gas phase .

The basic limitation of Langmuir adsorption equation is that it is valid at low pressure only.

At lower pressure, KP is so small, that factor (1+KP) in denominator can almost be ignored. So Langmuir equation reduces to

$\theta =KP$
 

At high pressure KP is so large, that factor (1+KP) in denominator is nearly equal to KP. So Langmuir equation reduces to

$\theta =\frac{KP}{KP}=1$