This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Surface Chemistry – Adsorption”.
1. A finely divided substance is more effective as an adsorbent.
Explanation: Adsorption is a surface phenomenon. It is dependent on the amount of the adsorbent exposed to the adsorbate. A finely divided substance provides a large surface area and hence, provides more sites where adsorption can take place.
2. What is the process called when the molecules of a substance are retained at the surface of a solid or a liquid?
Explanation: Adsorption is the process which involves the accumulation of the molecules of a substance in higher concentration on the surface of a solid or a liquid. For example, gasses are adsorbed on the surface of charcoal.
3. Which of the following forces is involved in physical adsorption?
a) Gravitational force
b) Magnetic force
c) Van der Waals force
d) Electromagnetic force
Explanation: In physical adsorption, the molecules of the adsorbate stick to the surface of the adsorbent due to very weak forces which is known as Van der Waals force. Van der Waals force is similar to the forces that cause condensation of gas into liquid.
4. In physisorption, the adsorbent does not show specificity towards a particular gas.
Explanation: In physisorption, there is no chemical bond between the adsorbent and adsorbate. They are held together by Van der Waals forces. Since Van der Waals forces are universal and almost same for all gases, the adsorbent does not show specificity to a particular gas.
5. Which of the following statements is true with respect to the extent of physisorption?
a) Increases with increase in temperature
b) Decreases with increase in surface area
c) Decreases with increase in the strength of Van der Waals forces
d) Decreases with increase in temperature
Explanation: Physisorption is an exothermic process. According to Le-Chatelier’s principle, an exothermic reaction is favoured by a decrease in temperature. Therefore, the extent of physisorption decreases on increasing temperature.
6. Which of the following can result in a transition from physisorption to chemisorption?
a) Decrease in temperature
b) Increase in temperature
c) Decrease in pressure
d) Increase in surface area
Explanation: Physisorption occurs due to Van der Waals forces. On increasing the temperature, the adsorbate can split into atoms and form chemical bonds with the adsorbent and hence, cause chemisorption.
7. Which of the following statements is incorrect with respect to physisorption?
a) It is reversible
b) It is spontaneous
c) ΔH < 0
d) ΔS > 0
Explanation: Physisorption is reversible as the molecules of the adsorbate are held to the adsorbent by weak Van der Waals forces which can be broken easily. Physisorption is exothermic and so, ΔH is negative. Adsorption is a spontaneous process. In adsorption, the movement of the adsorbed molecule is restricted. As a result the entropy change (ΔS) is negative.
8. Which of the following statements is true with respect to the types of adsorption?
a) Chemisorption is stronger than physisorption
b) Physisorption is stronger than chemisorption
c) They are both equal
d) They cannot be compared
Explanation: Chemisorption is stronger than physisorption. Chemisorption involves the forming of a chemical bond between the adsorbent and the adsorbate whereas in physisorption, the molecules are held together by weak Van der Waal’s forces. Therefore, chemisorption is stronger.
9. Which of the following is an example of sorption?
a) Sponge in water
b) Cotton dipped in ink
c) Water on silica gel
d) Oxygen on metal surface
Explanation: Sorption refers to the process where adsorption and absorption occur at the same time. Cotton dipped in ink is one of the cases where both adsorption and absorption, that is, sorption occurs.
10. Which of the following statements is not true with respect to chemisorption?
a) Depends on nature of adsorbate and adsorbent
b) Has a large heat of adsorption
c) Forms a unimolecular layer
d) Occurs at low temperature
Explanation: Chemisorption is a kind of adsorption that involves a chemical reaction between the adsorbent and adsorbate, resulting in the formation of a chemical bond between the two. Since a bond is to be formed, chemisorption is highly dependent on the reactants(adsorbate and adsorbent) and has a large heat of adsorption. Also, since it involves bond formation, it cannot be multi-layered. Chemisorption is favoured to occur at high temperature.
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