Question

Which of the following is/are properties of chemisorptions?
A. Easily liquefied gases are more adsorbed.
B. Weak van der Waals forces cause the adsorption process.
C. Unimolecular layer formation.
D. Adsorption increases with rise in temperature.

Answer 1

Hint: It is a chemical process, all the properties, promoters and conditions unfavorable for a chemical reaction will help in obtaining the correct options. The concept of activation energy, Le Chatelier’s principle and critical temperatures also.

Complete step by step answer:
Chemisorption is a process of adherence of the adsorbate and the surface using chemical reactions. It is a kind of adsorption. Adsorption is a process of bond formation of any kind of matter to a surface which results in formation of a thin film around the surface.
Option A stands true. This is because easily liquefied gases have higher critical temperature which allows them to be adsorbed so easily. This is because the presence of strong van der Waals forces leads to quicker and stronger adherence. Critical temperature of a gas is a temperature above or at which the gas cannot be liquefied, that is, it cannot exist as a liquid.
Option B is fallacious. This is because the chemical process of adsorption requires strong ionic and covalent bonds.
Option C is also correct. Chemisorption occurs only at high temperature and pressure. As we know that at higher pressure ranges, the activation energy is also very high. That is why the adsorbent forms a unimolecular layer on the surface the adsorbent adheres to. Activation energy is the energy required for a chemical reaction to initiate.
Option D is true. Chemisorption is a chemical process involving chemical reactions. Reactions are favorable in higher temperature conditions. This is because the energy is used to raise the activation energy for the reaction and promotes bond formation. But after a certain point or temperature, the reaction starts to follow Le Chatelier’s principle and the rate of adsorption will decrease. According to it, an exothermic reaction, such as adsorption will shift backwards with rise in temperature.

So, the correct options are A, C and D.

Note:
 Liquefied gases like water, toluene and benzene have very high critical temperature. Their critical temperatures are as high as \[{\text{374}}.{\text{2}}\], \[{\text{318}}.{\text{6}}\] and 289 degree Celsius, respectively. Le Chatelier’s principle studies the rate of reaction and how different factors affect it. The principle states that the shift of the equilibrium of a system is changed with a change in any variable that determines the equilibrium state in such a direction which tries to counteract the changes caused to the variable in the first place. The increase in temperature will tend to backward shift the exothermic reaction to produce more products based on Le Chatelier’s principle. This is because with increase in temperature which means to heat the system, the reaction will counteract the change and as a result shift the reaction in a direction which makes the system cooler. In an exothermic reaction, the products are hotter than the reactants and so they will form less likely and will try to reverse in the direction to form reactants.