Question 5.4:

What are the factors which influence the adsorption of a gas on a solid?

Answer:

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, O2etc. This is because Van der Waals 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.

How does adsorption increase with an increase in pressure? The previous sentence is contrasting.

The last sentence is correct and  is in agreement with second phrase. 
Please read it carefully. 
In first sentence, it has been written that Adsorption is a reversible reaction which is accompanied by a decrease in pressure. 
Consider a closed chamber containing a solid charcoal. Initially there won't be any adsorption and gas present in that system will exert some pressure on the walls of container. The internal pressure will be equal to the external pressure. 
when adsorption occur, some molecules of gases get accumulated on the surface of charcoal. This will lead to the decreases in the amount of gas present freely in the system. Hence, the internal pressure of the system decreases. 
Now, if you increases the external pressure, i.e. if we push the wall of container from out side then volume of container will decreases. The decrease in volume will allow the molecules of gases to come closer to the adsorbent (charcoal) and hence the extend of adsorption will also increase. 
I think you were getting confused in internal and external pressure. In case you still have any doubt then do get back to us. 

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