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NEET-XII-Chemistry

05: Surface Chemistry

 
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    5 - Surface Chemistry
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    Section : I
    SECTION I Page No 127:
  • Qstn #1
    Write any two characteristics of Chemisorption.
    Ans : 1. Chemisorption is highly specific in nature. It occurs only if there is a possibility of chemical bonding between the adsorbent and the adsorbate.

    2. Like physisorption, chemisorption also increases with an increase in the surface area of the adsorbent.
  • Qstn #2
    Why does physisorption decrease with the increase of temperature?
    Ans : Physisorption is exothermic in nature. Therefore, in accordance with Le-Chateliere’s principle, it decreases with an increase in temperature. This means that physisorption occurs more readily at a lower temperature.
  • Qstn #3
    Why are powdered substances more effective adsorbents than their crystalline forms?
    Ans : Powdered substances are more effective adsorbents than their crystalline forms because when a substance is powdered, its surface area increases and physisorption is directly proportional to the surface area of the adsorbent.

    SECTION I SECTION I Page No 133:
  • Qstn #4
    Why is it necessary to remove CO when ammonia is obtained by Haber’s process?
    Ans : It is important to remove CO in the synthesis of ammonia as CO adversely affects the activity of the iron catalyst, used in Haber’s process.
  • Qstn #5
    Why is the ester hydrolysis slow in the beginning and becomes faster after sometime?
    Ans : Ester hydrolysis can be represented as:



    The acid produced in the reaction acts as a catalyst and makes the reaction faster. Substances that act as catalysts in the same reaction in which they are obtained as products are known as autocatalysts.
  • Qstn #6
    What is the role of desorption in the process of catalysis?
    Ans : The role of desorption in the process of catalysis is to make the surface of the solid catalyst free for the fresh adsorption of the reactants on the surface.
  • Qstn #7
    What modification can you suggest in the Hardy-Schulze law?
    Ans : Hardy-Schulze law states that ‘the greater the valence of the flocculating ion added, the greater is its power to cause precipitation.’

    This law takes into consideration only the charge carried by an ion, not its size. The smaller the size of an ion, the more will be its polarising power. Thus, Hardy-Schulze law can be modified in terms of the polarising power of the flocculating ion. Thus, the modified Hardy-Schulze law can be stated as ‘the greater the polarising power of the flocculating ion added, the greater is its power to cause precipitation.’
  • Qstn #8
    Why is it essential to wash the precipitate with water before estimating it quantitatively?
    Ans : When a substance gets precipitated, some ions that combine to form the precipitate get adsorbed on the surface of the precipitate. Therefore, it becomes important to wash the precipitate before estimating it quantitatively in order to remove these adsorbed ions or other such impurities.
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    Section : II
    SECTION I Page No 146:
  • Qstn #1
    Distinguish between the meaning of the terms adsorption and absorption.

    Give one example of each.
    Ans : Adsorption is a surface phenomenon of accumulation of molecules of a substance at the surface rather than in the bulk of a solid or liquid. The substance that gets adsorbed is called the ‘adsorbate’ and the substance on whose surface the adsorption takes place is called the ‘adsorbent’. Here, the concentration of the adsorbate on the surface of the adsorbent increases. In adsorption, the substance gets concentrated at the surface only. It does not penetrate through the surface to the bulk of the solid or liquid. For example, when we dip a chalk stick into an ink solution, only its surface becomes coloured. If we break the chalk stick, it will be found to be white from inside.

    On the other hand, the process of absorption is a bulk phenomenon. In absorption, the substance gets uniformly distributed throughout the bulk of the solid or liquid.
  • Qstn #2
    What is the difference between physisorption and chemisorption?
    Ans :



    Physisorption
    		Chemisorption
    1.
    		 In this type of adsorption, the adsorbate is attached to the surface of the adsorbent with weak van der Waal’s forces of attraction.
    		 In this type of adsorption, strong chemical bonds are formed between the adsorbate and the surface of the adsorbent.
    2.
    		 No new compound is formed in the process.
    		 New compounds are formed at the surface of the adsorbent.
    3.
    		 It is generally found to be reversible in nature.
    		 It is usually irreversible in nature.
    4.
    		 Enthalpy of adsorption is low as weak van der Waal’s forces of attraction are involved. The values lie in the range of 20-40 kJ mol-1.
    		 Enthalpy of adsorption is high as chemical bonds are formed. The values lie in the range of 40-400 kJ mol-1.
    5.
    		 It is favoured by low temperature conditions.
    		 It is favoured by high temperature conditions.
    6.
    		 It is an example of multi-layer adsorption
    		 It is an example of mono-layer adsorption.


  • Qstn #3
    Give reason why a finely divided substance is more effective as an adsorbent.
    Ans : Adsorption is a surface phenomenon. Therefore, adsorption is directly proportional to the surface area. A finely divided substance has a large surface area. Both physisorption and chemisorption increase with an increase in the surface area. Hence, a finely divided substance behaves as a good adsorbent.
  • Qstn #4
    What are the factors which influence the adsorption of a gas on a solid?
    Ans : There are various factors that affect the rate of adsorption of a gas on a solid surface.

    (1) Nature of the gas:

    Easily liquefiable gases such as NH3, HCl etc. are adsorbed to a great extent in comparison to gases such as H2, O2 etc. This is because Van der Waal’s forces are stronger in easily liquefiable gases.

    (2) Surface area of the solid

    The greater the surface area of the adsorbent, the greater is the adsorption of a gas on the solid surface.

    (3) Effect of pressure

    Adsorption is a reversible process and is accompanied by a decrease in pressure. Therefore, adsorption increases with an increase in pressure.

    (4) Effect of temperature

    Adsorption is an exothermic process. Thus, in accordance with Le-Chatelier’s principle, the magnitude of adsorption decreases with an increase in temperature.
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