Question

According to Langmuir adsorption isotherm, the amount of gas adsorbed at very high pressure :
(A) reaches a constant limiting value
(B) goes on increasing with pressure
(C) goes on decreasing with pressure
(D) increases first and decreases later with pressure

Answer 1

Understand the term adsorption given in the chapter surface chemistry. Isotherm represents a graph where temperature factor is taken to be constant throughout the experiment. Langmuir's adsorption isotherm talks about the relation between partial pressure of the adsorbate particles and the total volume of substance.

Complete step by step answer:
- Physisorption, also called physical adsorption, is a process in which the electronic structure of the molecule/atom is attached upon adsorption.
- The main interacting force in physical adsorption is the Van der Waals force. Although the interacting force is very weak, it plays a crucial role in nature.
- Due to weak forces being present, it is observed that easily liquefiable gases are readily adsorbed on the surface of adsorbent.
- When sufficient pressure is applied, it is seen that the layer of adsorbent adsorbs multiple layers of attacking particles called adsorbate.
- The enthalpy of adsorption is low and negative in polarity. The value varies between -20 to -40 kJ/mol.
- An adsorption isotherm is a graph that depicts the variation in the amount of adsorbate(x) adsorbed on the surface called adsorbent along with the change in pressure assuming the temperature to be constant.
- Langmuir adsorption isotherm is used to deal with chemical adsorption in particular which fails at high pressure. This is because the mass adsorbed reaches a constant value when the adsorbed surface gets completely covered by a unimolecular layer of different gases.
According to this, the amount of gas adsorbed at very high pressure reaches a constant limiting value. So, the correct answer is “Option A”.

Note: It is important to know that Langmuir's adsorption isotherm is defined for chemical adsorption and for ideal gases only. For real gases, the above statements do not hold true.