Question

What is an adsorption isotherm? Describe Freundlich adsorption isotherm.

Answer 1

The plot between the extent of adsorption against the pressure of gas (P) at constant temperature (T) is called the adsorption isotherm.

Freundlich adsorption isotherm:

Freundlich adsorption isotherm gives an empirical relationship between the quantity of gas adsorbed by the unit mass of solid adsorbent and pressure at a specific temperature.

From the given plot it is clear that at pressure P_S, reaches the maximum valve. Ps is called the saturation pressure. Three cases arise from the graph now.

Case I- At low pressure:

The plot is straight and sloping, indicating that the pressure in directly proportional to i.e.,

Case II- At high pressure:

When pressure exceeds the saturated pressure, becomes independent of P values.

Case III- At intermediate pressure:

At intermediate pressure, depends on P raised to the powers between 0 and 1. This relationship is known as the Freundlich adsorption isotherm.

On plotting the graph between log and log P, a straight line is obtained with the slope equal to and the intercept equal to log k.