Question

Discuss the effect of pressure and temperature on the adsorption of gases on solids.

Answer 1

Effect of pressure on adsorption At constant temperature the extent of adsorption of a gas (x/m) in solid increases with pressure. A graph between x/m and the pressure p of the gas at constant temperature is called adsorption isotherm.

Effect of pressure can be explained by Freundlich adsorption isotherm.

(i) At lower range of pressure, (x/m) is directly proportional to the applied pressure.

$\frac{x}{m}\propto p^{\prime }$

(ii) At higher range of pressure, (x/m) is independent of applied pressure.

$\frac{x}{m}\propto p^{\circ }$

(iii) At intermediate pressure range, $\frac{x}{m}$ is proportional to a fractional power of pressure, i.e.,

$\frac{x}{m}\propto p^{\frac{1}{n}}$

where, $\frac{1}{n}$ is a fraction. Its value may be 0 and 1

$\frac{x}{m}=k{p}^{\frac{1}{n}}$

$log\left(\frac{x}{m}\right)=logk+\frac{1}{n}logp$

Effect of temperature on adsorption Adsorption is generally temperature dependent. Being exothermic, adsorption decreasing with increasing temperature. However for an endothermic adsorption process, adsorption increases with increase in temperature.

Adsorption increases with increase in pressure. Effect of temperature depends on the nature of adsorption. In the case of physical adsorption, it decreases with increase in temperature because it is an exothermic process. In the case of chemical adsorption, it first increases and then starts decreasing with increase in temperature.