Question

Which of the following(s) is/are the drawbacks of Freundlich adsorption isotherm $(\frac{x}{m}=k{p}^{\frac{1}{n}})$ ?

• A. Isotherm deviates from straight line at high pressure.
• B. Isotherm deviates from straight line at low temperature
• C. Isotherm is applicable only when adsorbate forms unimolecular layer on adsorbent surface
• D. Both (a) and (c)

Answer 1

The correct option is D Both (a) and (c)

Expression of Freundlich adsorption isotherm is
$\frac{x}{m}=k{p}^{\frac{1}{n}}....\left(1\right)$
$\frac{x}{m}$ is the extent of adsorption which means the amount of gas adsorbed by the adsorbent with pressure at constant temperature.
k and n are constants that depends on the nature of gas and adsorbent.

Taking log of equation (1) gives,

log $\left(\frac{x}{m}\right)=logk+\frac{1}{n}logP$
It is in a form of straight line equation.
Thus, plot of $log\frac{x}{m}vslogP$ should give a straight line.

Freundlich isotherm is valid only if the plot is a straight line.
But it holds good over a limited range of pressure. At intermediate and high pressure the isotherm deviates from the straight line.


Drawback of Freundlich adsorption isotherm:
1. It fails at high pressure range.
2. The isotherm is applicable only when adsorbate forms unimolecular layer on adsorbent surface.

Hence, statements (a) and (c) are correct.