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Surface Chemistry

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Class 10 Class 12
Is adsorption exothermic or endothermic?

Adsorption is always exothermic. This statement can be explained in two ways.

(i)Adsorption leads to a decrease in the residual forces on the surface of the adsorbent. This causes a decrease in the surface energy of the adsorbent. Therefore, adsorption is always exothermic.

(ii)ΔH of adsorption is always negative. When a gas is adsorbed on a solid surface, its movement is restricted leading to a decrease in the entropy of the gas i.e., ΔS is negative. Now for a process to be spontaneous, ΔG should be negative.

ΔG= ΔH-TΔS
Since ΔSis negative, ΔH has to be negative to make ΔG negative. Hence, adsorption is always exothermic.

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Why does physisorption decrease with the increase of temperature?

It is because physical adsorption involves weak forces of attraction between adsorbent and adsorbent molecules, generally of Vander Waal's type. These forces decrease rapidly with increase in temperature.
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Why are powdered substances more effective adsorbents than their crystalline forms?

Powderded substance are more effective adsorbent than their crystalline forms because when a substance is powdered its surface area increase and physisorption is directly proportional to the surface area of adsorbent,
1109 Views

Why is the ester hydrolysis slow in the beginning and becomes faster after sometime?

Ester + water ---> Acid + Alcohal
The acid produced in the reaction acts as a catalysis and makes the reaction faster.

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Why are substances like platinum and palladium often used for carrying out electrolysis of aqueous solutions?

The Plantium and palladium are used for electrolysis of aqueous solution due to the chemically inert in nature and good adsorptive capacity for hydrogen.
Because Gases like hydrogen are absorbed on the surface of platinum and palladium (adsorption) and then passes into the bulk of the adsorbent (platinum and palladium).

1699 Views

In Haber's process, hydrogen is obtained by reacting methane with steam in presence of NiO as catalyst. The process is known as steam reforming. Why is it necessary to remove CO when ammonia is obtained by Haber’s process?