Question

Adsorption is accompanied by the evolution of heat, so according to the Le-Chatelier principle, the amount of the substance adsorbed should:
a.) increase with decrease in T
b.) increase with increase in T
c.) decrease with decrease in T
d.) decrease with increase in T

Answer 1

Hint: Attempt this question by evaluating the direction of the equilibrium shift. Also remember that the adsorption process is exothermic in nature.

Complete answer: Adsorption process is the adhesion of ions, atoms, or molecules from liquid, dissolved solid, or gas over the surface of the adsorbent. A film of adsorbate (liquid, dissolved solid, or gas) is produced over the adsorbent surface. The ions, atoms, or molecules adsorbed are called adsorbates and the surface material to which the adsorbates are adsorbed is called adsorbent.
The Le-Chatelier principle, also known as equilibrium law, is a principle used to predict the influence of alteration in the conditions on chemical equilibrium. According to this principle, if any system initially present at equilibrium from a long duration of time is exposed to an alteration in temperature, concentration, pressure or volume, then the system shifts its equilibrium to a new equilibrium, in order to counteract the applied changes.
As stated in question, adsorption is accompanied by the evolution of heat, therefore, the adsorption process is exothermic in nature. In an exothermic process, heat is removed from the system and released into the surrounding, resulting in a decrease in the temperature of the system.
Reactants $\rightleftharpoons $ products + heat
As the temperature is lowered due to the heat removal from the system, the equilibrium will tend to shift in the direction where more heat is produced, that is, in the forward direction, in order to nullify the heat loss. Therefore, more substrate will be absorbed according to Le-Chatelier's principle.
Now, considering the two conditions of increase and decrease in temperature:
When temperature is increased- As the adsorption process is exothermic in nature and exothermic reactions occur readily at low temperature. Therefore, when temperature is increased then the adsorption process will slow down resulting into a decrease in the amount of substrate adsorbed.
When temperature is decreased- When the temperature is decreased, then the adsorption process will increase, resulting in an increase in adsorption of the substrate.
So, the correct answer is “Option A”.

Additional Information: Adsorption process is of two types, chemical adsorption (chemisorption) and physical adsorption (physisorption). Chemical adsorption is a process in which adsorbates are bound to the surface via chemical bonding whereas physical adsorption is the one in which adsorbates are bound by the sum of repulsive or attractive forces.

Note: The adsorption process differs from the absorption process, as in absorption the fluid is dissolved or permeates a liquid or solid. Adsorption is a surface process whereas absorption includes the whole material volume.