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12th Standard English Medium Chemistry Subject Surface Chemistry Book Back 2 Mark Questions with Solution Part - II updated Book back Questions

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Std: 12th Standard - Chemistry

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Questions:

Generic 2

4 x 2 = 8

Heat of adsorption is greater for chemisorptions than physisorption. Why?

Which will be adsorbed more readily on the surface of charcoal and why? NH₃ or CO₂?

In case of chemisorption, why adsorption first increases and then decreases with temperature?

Give two important characteristics of physisorption.

Generic 5

1 x 5 = 5

Differentiate physisorption and chemisorption.

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Answers Key:

Generic 2

4 x 2 = 8

In Chemisorptions the molecules are held on the surface by attractive forces by formation of chemical bonds. As strong bond is formed nearly 400 kJ / mol is given out as heat of adsorption chemical bonds are much stronger, whereas the Vander Waal's forces in physisorption is weaker.

The critical temperature of NH₃ is 406 K and that of CO₂ is 304 K. So, NH₃ has higher critical temperature and greater VanderWaal's forces of attraction than CO₂. So NH₃ will be more adsorbed than CO₂.

In chemical adsorption, \(\frac { x }{ m } \) increases with rise in temperature and then decreases. The increase illustrates the requirement of activation of the surface for adsorption is due to fact that formation of activated complex requires certain energy. The decrease at high temperature is due to desorption, as the kinetic energy of the adsorbate increases.

(i) It is instantaneous.
(ii) It is non-specific.
(iii) No transfer of electrons.
(iv) Multilayer of adsorbate is formula.

Generic 5

1 x 5 = 5

S.No	Physical adsorption or vander waals adsorption or Physisorption	Chemical adsorption or Chemisorption or Activated adsorption
1.	It is instantaneous	It is very slow
2.	It is non-specific	It is very specific depends on nature of adsorbent and adsorbate
3.	In Physisorption, when pressure increases the amount of adsorption increases	Chemical adsorption is fast with increase pressure, it can not alter the amount.
4.	Physisorption decreases with increase in temerature	When temperature is raised chemisorption first increases and then decreases
5.	No transfer of electrons	Chemisorption involves transfer of electrons between the adsorbent and adsorbate
6.	Heat of adsorption is low in the order of 40kJ/ mole.	Heat of adsorption is high i.e., from 40- 400kJ/mole.
7.	Multilayer of the adsorbate is formed on the adsorbent.	Monolayer of the adsorbate is formed
8.	It occurs on all sides	Adsorption occurs at fixed sites called active centres. It depends on surface area
9	Activation energy is insignificant.	Chemisorption involves the formation of activated complex with appreciable activation energy.