CBSE 12th Chemistry Board Exam 2020-21: Important Questions & Answers from Chapter 5 - Surface Chemistry
Ques. Define adsorption.
Ans. The accumulation of molecular species at surface rather than in the bulk of a solid or liquid is termed as adsorption. It is a surface adsorption.
Note: The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.
Example of adsorption:
I. If a gas like O2, H2, CO, Cl2, NH3 or SO2 is taken in a closed vessel containing powdered charcoal, it is observed that the pressure of the gas in the enclosed vessel decreases. The gas molecules concentrate at the surface of the charcoal, i.e. gases are adsorbed at the surface.
II. In a solution of an organic dye, say methylene blue, when animal charcoal is added and the solution is well shaken, it is observed that the filtrate turns colourless. The molecules of the dye, thus, accumulate on the surface of charcoal i.e. are adsorbed.
III. Aqueous solution of raw sugar, when passed over beds of animal charcoal, becomes colourless as the colouring substances are adsorbed by the charcoal.
IV. The air becomes dry in the presence of silica gel because the water molecules get adsorbed on the surface of the gel.
Ques. Define desorption.
Ans. The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.
Ans.
ABSORPTION
1. It is a bulk phenomena.
2. The phenomenon in which the accumulation of species occurs inside the bulk of a substance is called absorption.
3. Example- anhydrous CaCl2 absorbs moisture.
ADSORPTION
1. It is a surface phenomena.
2. The phenomena in which the accumulation of species occurs on the surface is called adsorption.
3. Example- silica gel adsorbs
Note: water vapours are adsorbed by anhydrous calcium chloride but adsorbed by silica gel.
Ques. Define sorption.
Ans. When adsorption and absorption can take place simultaneously, then it is called sorption. For eg- when a chalk stick is dipped in ink, dye gets adsorbed and water gets absorbed.
Ques. What is the mechanism of adsorption? Or why does adsorption occurs?
Ans. 1. Adsorption arises due to that facet that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk.
2. Inside the adsorbent all the forces acting between the particles are mutually balanced but on the surface the particles are not surrounded by atoms or molecules of their kind on all sides.
3. Hence, they possess unbalanced or residual attractive forces which attract the adsorbate particles
on its surface.
4. The extent of adsorption increases with the increase of surface area per unit mass of the
adsorbent at a given temperature and pressure.
Ques. Why is it important to have clean surface in the surface studies?
Ans. It facilitates the adsorption of species on the adsorbent.
Ques.5.8 Why adsorption is exothermic? Or What is the value of ∆G, ∆H, ∆S in the adsorption?
Ans. 1. Adsorption is invariably an exothermic gas process.
2. In other words, ∆H of adsorption is always negative.
3. When a gas is adsorbed, the freedom of movement of its molecules becomes restricted. This amounts to decrease in the entropy of the gas after adsorption, i.e. ∆S is negative.
4. Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system.
5. For a process to be spontaneous, the thermodynamic requirement is that, at constant temperature and pressure, ∆G must be negative.
6. On the basis of equation, ∆G= ∆H - T∆S, ∆G can be negative if ∆H has sufficiently high negative value as T∆S is positive, at this state equilibrium is attained.
Ques. How does the liquefaction of gas depends on critical temperature?
OR
Out of SO and methane, which gas easily adsorbed on the surface on 1g activated charcoal and why?
Ans. The amount of gas adsorbed by a solid depends on the nature of gas. In general, easily liquefiable gases (i.e. with higher critical temperature) are readily adsorbed as vander Waal’s forces are stronger near critical temperatures. Thus, 1g of activated charcoal adsorbs more sulphur dioxide (critical temperature 630K), than methane (critical temp 190 K).
Ques. Which will be adsorbed more readily on the surface of charcoal and why NH3 or CO2?
Ans. NH3 has higher critical temperature than carbon dioxide i.e. NH3 gas is more easily liquefiable than CO2 . Hence, NH3 has greater inter molecular forces of attraction and hence will be adsorbed more readily.
Ques. Which will absorb more gas: a lump of charcoal or its powder and why?
Ans. Powdered charcoal will absorb more gas because it has more surface area than a lump of charcoal per unit mass or volume at constant pressure and temperature.
Types of Adsorption
Physisorption
1. It arises because of vander Waal’s forces.
2. It is not specific in nature.
3. It is reversible in nature.
4. It depends on the nature of gas. More easily liquefiable gases are adsorbed readily.
5. Enthalpy of adsorption is low. (20-40 Kj/mol)
6. Low temp is favourable.
7. No appreciable activation energy is needed.
8. It depends on the surface area. It increases with an increase of surface area.
9. It results into multimolecular layers on adsorbent surface under high pressure kJ/mol)
Chemisorption
1. It caused by chemical bond formation.
2. It is highly specific in nature.
3. It is irreversible.
4. It also depends on the nature of gas. Gases which can react with the adsorbent show chemisorption.
5. Enthalpy of adsorption is high. (80-240 Kj/mol)
6. High temperature is favourable for adsorption.
It increases with increase of temperature.
7. High activation energy is sometimes needed.
8. It also depends on the surface area. It too
increases with an increase of surface area.
9. It results into unimolecular layer.
Ques. How does physisorption and chemisorption gets affected by temperature and pressure?
Ans.
PHYSISORPTION
A) Effect of temperature:
Since there are weak vander Waal forces of attraction between adsorbate and adsorbent therefore, with increase in temperature, the molecules of adsorbate gain kinetic energy. The forces gets weakened. Hence, with the increase in temperature, physisorption decreases.
B) Effect of pressure:
With increase in pressure, the extent of physisorption increases and it is multi-layered. But at very high pressure, it is independent of pressure.
CHEMISORPTION
A) Effect of temperature:
Since there are chemical forces between adsorbate and adsorbent, therefore some activation energy is required for chemisorption.
B) Effect of pressure:
It increases with increase in pressure, but at very high pressure, chemisorption decreases.
Ques. 5.3Why are powdered substances more effective adsorbents than their crystalline solid?
Ans. Powdered substances has more surface area per unit mass/volume at constant temperature and pressure.
Ques. (NCERT)Why physisorption decreases with increase in temperature?
Ans. With increase in temperature, the adsorbate particles gain kinetic energy, hence they overcome the vander Waal forces of attraction. Hence q, extent of physisorption decreases. Physisorption is an exothermic process and is reversible.
Solid + Gas ↔ Adsorption + heat
Physisorption occurs at low temperature and increasing the temperature will decrease the adsorption in keeping with le-Chatelier’s principle.
Ques. How does a solid catalyst enhance the rate of combination of gaseous molecules?
Ans. When gaseous molecules come in contact with the surface of a solid catalyst, a weak chemical combination between the surface of the catalyst and gaseous molecules takes place.
This increases the concentration of the reactants on the surface. Acc to the law of mass action, rate of a reaction is proportional to the concentration of the reactants. With increased concentration of the reactants, the reaction takes place faster. Also adsorption process is exothermic, it releases energy which helps in further increasing the rate of reaction.
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