Physisorption and Chemisorption Adsorption Characteristics

The concept of adsorption is divided into 2 different categories i.e. Physisorption and Chemisorption. Before diving into the detailed characteristics of both these types, let us know what Physisorption and Chemisorption stand for. 

1: Physisorption: Physisorption can be termed as the condition when the adsorbate is held onto the surface of the adsorbent by weak physical forces i.e. Van Der Waals’ force. This is why physisorption is also known as van der Waals’ adsorption. 

For Example Adsorption of a layer of water vapour on the upper surface of silica gel. Another example that represents Physisorption maybe: 

• Adsorption of various gases on the upper layer of animal charcoal

Characteristics of Physisorption: 

As mentioned, in physisorption the upper layer of the adsorbent attracts the adsorbate with a very weak physical force. Now, let us know what all are the effects on physisorption when temperature, pressure, etc. comes into play. 

1: Physical change: As we all know that in physisorption there is no formation of chemical bond on the upper surface of the adsorbent. Thus, it simply goes through a physical change. Therefore, it is called physical adsorption or physisorption.

2: Non-specific nature: Another characteristic of physisorption is that as Van Der Waals’ force is universal and not associated with a particular type of substance, physisorption becomes non-specific in nature. 

3: Reversible nature: A very important characteristic of physisorption is that it is reversible in nature. With the increase or decrease of external pressure or temperature, physisorption can be easily reversed. 

4: Effect of temperature: When we talk about the effect of temperature on adsorption, physisorption generally decreases with an increase in temperature. 

This is because the energy of activation of physisorption is almost equal to zero. Therefore, even if the temperature is very low, the rate of adsorption remains unchanged.

5: Effect of pressure: Physisorption increases with an increase in pressure. This is because when on a surface the pressure increases the molecules start to hit the surface with more intensity increasing the effect of physical forces on the upper surface of the adsorbent. 

6: Multi Layered nature: Physisorption is multilayered in nature due to the weak forces by which the adsorbent is helped by the adsorbate. The weak forces hold the adsorbate in such a way that they form a layer over the adsorbent, giving them a multi-layered view. 

7: Nature of adsorbate: For physisorption, we generally use easily liquefiable gases as they have stronger van der Waals forces of attraction. Gases with stronger van der Waals forces are very much suitable for physisorption. 

8: Enthalpy of Physisorption: Physisorption is an exothermic process, but, during the process of physisorption, the heat of adsorption involved in it is quite low. 

2: Chemisorption: 

Moving forward to the next type of adsorption, we will study chemisorption. Chemisorption can be defined as when the molecules, ions, or atoms of gases, solids, or liquids are held on the upper surface of the absorbent by a strong chemical bond. Chemisorption is better and long-lasting than physisorption

Here are a few examples of chemisorption: 

Example 1: Rusting of iron – The rust holds the upper surface of the iron because of a strong chemical reaction. 

Example 2: Haber Process – In this process, nitrogen combines from the air with hydrogen derived mainly from methane (natural gas) into ammonia.

Characteristics of Chemisorption: 

We now have a basic knowledge about what chemisorption is and some of its examples. Now, moving further, let us know the characteristics of chemisorption. : 

1: Chemical change: As the name suggests, chemisorption involves the formation of a chemical bond between the adsorbate and the adsorbent; thus, during the process of chemisorption, a chemical change occurs. 

2: Nature of adsorbate: Another important characteristic of chemisorption is that while chemisorption, the adsorbate should have the ability to form a chemical bond with the adsorbent.

3: Irreversible nature: Unlike physisorption, chemisorption is irreversible in nature due to irreversible chemical change and the formation of chemical bonds between the surface and the molecules. 

4: Effect of temperature: The process of chemisorption is exothermic in nature. Thus, it is favourable to low temperatures. As the temperature during chemisorption rises, The extent of adsorption first increases and then decreases.

5: Effect of pressure: Chemisorption increases with an increase in pressure. 

During the process of chemisorption, at higher pressure, the gas molecules get attracted towards the surface of the adsorbent and leverage the formation of chemical bonds.

6: Unimolecular layers formation in Chemisorption: Chemisorption, as we all know forms chemical bonds between adsorbate and adsorbent. Thus, in Chemisorption a unimolecular layer is formed because the chemical bond formed only stays with the layer of molecules coming in direct contact with the surface of the adsorbent. 

7: Enthalpy of Chemisorption: Chemisorption is an exothermic process. But, during the process of chemisorption, the heat of adsorption involved in it is higher than that of physisorption. 

8: Highly specific nature: Chemisorption is highly specific in nature because it occurs only if chemical bond formation is possible between the adsorbate and the adsorbent. Thus, we cannot execute the process of chemisorption over every combination of adsorbate and adsorbent. 

Conclusion: 

With this, now we know all the characteristics of both, physisorption and chemisorption. Other than this, you should also focus on topics like Adsorption isotherms, Theories of adsorption etc. which fall under Adsorption. These are some of the most important topics when someone is preparing for not only IIT-JEE examinations but also for elementary examinations.