Physisorption, Chemisorption and their Characteristics

Introduction

This article will elucidate the critical concepts related to physisorption and its characteristics. We have introduced physisorption and its attributes. Physical bonding of Gas molecules with the solid and liquid surface is known as Physical absorption or physisorption.

This article will also help you understand the basics of physisorption and its characteristics, the difference between the physisorption and chemisorption of nitrogen, and the physisorption of nitrogen to help you solve all your textbook questions. 

Physisorption

Physisorption is a physical bond of gas molecules with the solid and liquid surface at a low temperature. Physisorption is a type of adsorption. Adsorption is when solid surfaces hold the gas and liquid molecules as a thin film. The first is physical, and the second is chemical. If they are attached by physical force, the bond is called physisorption, and if they are attached by chemical force, the bond is called chemisorption. Physisorption is a weaker bond in comparison to chemisorption. 

Let us understand physisorption by an example. Suppose you have a piece of wood and calcium carbonate, and you put calcium carbonate powder on the wood. Here, the wood and calcium carbonate bonding will be called physisorption.

Characteristics of Physisorption

We have discussed the meaning of physisorption. It has many characteristics. Some important characteristics are given below:-

• In physisorption, the force of attraction is weak
• The surface energy of physisorption is less; this is why it makes a weak bond
• In physisorption, there are multiple layers because gas molecules are spread unevenly on the liquid or solid surface and make multiple layers
• Removal of bonding is easy in physisorption because it is a weak bond
• When we increase the heat, the strength of physisorption will decrease
• This is because the surface and gas molecules start vibrating due to heat, and the bond gets weaker

Chemisorption

The chemical reaction amidst the adsorbent and the adsorbate that takes place is known as Chemisorption. In the process of chemisorption, sharing and transfer of the electrons take place between the both (adsorbent and adsorbate). This process occurs when there is a high or increase of temperature and is also called an exothermic process.

The nature of the chemisorption process is specific. In the chemisorption process, the absorbents and absorbates get linked through chemical bonds, and the nature of these bonds are ionic or covalent. In the chemisorption process, the activation energy is high, and the temperature is also high. One of the interesting facts about this process is that it is irreversible. But it is not affected by the slight or less change in pressure. Hence this process occurs at high temperatures.

An example of chemisorption is corrosion which is a macroscopic phenomena. 

Chemisorption of Nitrogen – The chemisorption of nitrogen and nitrogen oxide on a polycrystalline tungsten filament was studied by using flash desorption mass spectroscopy. The saturation coverage for nitrogen and nitrogen oxide on tungsten is 1.9 x 1014 molecules/cm2 and 8.8 x 1014 molecules/cm2, respectively. The saturation coverage on tungsten is almost 1.7 times more when compared to the saturation coverage on rhenium. The sticking probability of nitrogen oxide is higher compared to other gases.

Difference between Physisorption and Chemisorption

1. Force of attraction:- In physisorption, the force of attraction is weak and in chemisorption, the force of attraction is strong.
2. Surface Energy:- The surface energy of physisorption is less than the surface energy of chemisorption is more.
3. Layer:- In physisorption, there are multiple layers of molecules and in chemisorption, there is a single layer of molecule.
4. Removal:- Removal of bonding is easy in physisorption, while removal of bonding is hard in chemisorption.
5. Effect of heat:- When we increase heat, the strength of physisorption decreases while the strength of chemisorption increases.
6. Nature:- The nature of chemisorption is specific and irreversible while the nature of physisorption is reversible and non-specific.

Factors that affect the Extent of Absorption

• Nature of the absorbent and adsorbate

Usually, gases such as CO2, NH3, Cl2 and at are easily liquefiable, are absorbed to a much larger extent than the gases such as H2, O2, N2, and He, which are elemental gases.

Finely powdered and porous solids such as fuller’s earth and charcoal absorb more than non-porous hard materials. Powdered charcoal is used in gas masks because of this property.

• The Surface area of the Adsorbent

The surface area of the adsorbent is directly related to the extent of adsorption. The greater the surface area of the adsorbent, the larger is the extent of adsorption. The particle size determines the surface area of a powdered solid adsorbent. The surface area is greater when the particle size is smaller.

• Effect of Pressure on the Adsorbate Gas

A rise in the pressure of the adsorbate gas raises the extent of adsorption. At low temperatures, the extent of adsorption rises fastly with pressure.

• Effect of Temperature

The magnitude of adsorption falls with the increase in temperature, according to the Le-Chatelier’s principle. 

Conclusion

In this article, we have studied adsorption, which can be defined as a mass transfer process that is a phenomenon of sorption of gases or solutes by solid or liquid surfaces. There are two types of adsorption. One is physisorption, where a physical bond of gaseous molecules was made with the solid and liquid surface at a low temperature. And the other is chemisorption, where the chemical bond was made or formed between the adsorbent and the adsorbate at a high temperature. Both bonds are contrasting in nature and formation but are significant in chemistry.