Adsorption, Physisorption and Chemisorption

When you buy a pair of new leather shoes, have you ever wondered how a small sachet of silica gel, placed discreetly in the box, can keep them safe from moisture? Or how does water get purified when you place alum stone in it?

This is due to the phenomenon called adsorption.

Simply put, adsorption is the capability of substances to attract molecules of gases and liquids which are in contact with them to their surface. In this case, the substances which attract the molecules to their surface are called adsorbates, while the liquids or gases whose molecules get attracted are referred to as adsorbents.

Defining Adsorption

Adsorption is the deposition of atoms or molecules from a gas, liquid or dissolved solid onto a surface. A layer of the adsorbate gets created on the surface of the adsorbent. Silica gel and charcoal are well-known examples of adsorbents. 

This phenomenon is normally reversible. When reversed, the process is called desorption.

How Is Absorption Different from Adsorption?

Being a surface phenomenon, adsorption takes place at the surface of the adsorbent only. On the other hand, absorption means that the substance gets uniformly dissolved and distributed throughout the bulk of the material.

The term sorption is normally used to refer to both these processes. The opposite of these processes is called desorption.

The main differences between the two are:

What Causes Adsorption?

Adsorption occurs because the environment of the surface particles of the adsorbent is different from the environment of particles in the interior of the material.

Inside the bulk, the atoms or molecules are surrounded by their kind on all sides. Hence, the forces between them are mutually balanced. However, the atoms or molecules are not surrounded by similar particles from above on the surface. This creates an unbalance, and hence the surface particles have residual attractive energy on them. These unbalanced residual forces attract the particles of the adsorbate to the surface.

Adsorption results in a decrease in surface energy. This, in turn, gets converted to heat energy. That’s why adsorption is an exothermic process, and the change in enthalpy is negative. 

When molecules of the adsorbate are adsorbed on the surface, their freedom of movement gets restricted. This leads to a decrease in entropy.

Types of Adsorption

We classify this process into two types of adsorption based on the nature of the forces of interaction. These are physisorption and chemisorption.

Physical Adsorption or Physisorption

Physical Adsorption refers to the phenomenon when the adsorption of gases on a solid surface occurs due to weak van der Waals forces.

Chemical Adsorption or Chemisorption

When chemical bonds hold the gas molecules onto a solid surface, the process is referred to as Chemical Adsorption.

Characteristics of Physisorption and Chemisorption

A few key characteristics of both physisorption and chemisorption are listed below.

Characteristics of Physisorption

• Physisorption is caused by physical forces that are inherently weak.
• It is not specific, meaning that every gas is adsorbed and that adsorption occurs across the entire surface of the solid.
• Gases that are easily liquefiable are more strongly adsorbed physically.
• It is an exothermic process.
• Physical adsorption is a reversible process.
  • When we increase the pressure, the gas decreases in volume, so more gas is adsorbed. Conversely, by reducing the pressure, we can remove the gas from the surface of the solid.
  • Low temperature supports physisorption. High temperatures, on the other hand, decrease the rate of adsorption.
• The rate of adsorption increases with an increase in surface area. That is why finely divided metals and porous substances (e.g. activated charcoal) are good adsorbents.
• It requires low activation energy. 
• It is a multi-layered process.

Characteristics of Chemisorption

• Chemisorption is caused by relatively stronger chemical bonds.
• It is specific, meaning it occurs only if chemical bonds form between the adsorbent and adsorbate.
• Chemical adsorption is irreversible. 
• High pressure is conducive for chemisorption.
• Chemisorption is an exothermic process. However, at low temperatures, the process occurs at a slower pace.
• Greater surface area increases chemisorption.
• The enthalpy of chemisorption is high because of the formation of chemical bonds.
• High activation energy is needed.
• It results in a unimolecular layer.

What Factors affect Adsorption?

• Nature of Adsorbate – Easily liquefiable Gases is more easily adsorbed.
• Temperature – Low temperature is suitable for adsorption since it is an exothermic process.
• Surface Area – Adsorption being a surface phenomenon, increases as surface area increases.
• Pressure – Adsorption increases with an increase in pressure until saturation is reached. After achieving saturation, further adsorption will stop irrespective of the magnitude of pressure applied. The relationship between the temperature and the quantum of adsorption at constant pressure is called Adsorption Isobar.

Applications of Adsorption

• Soaps and detergents can remove dirt and clean due to the process of adsorption.
• The air pollution masks and the gas masks used in mining adsorb the smoke, dust and poisonous gases. When the impure air passes through the mask, the impurities get removed, and we can breathe in pure air.
• Charcoal is often used in separating the Noble gases.
• The chromatographic analysis is based on this phenomenon.
• Charcoal powder is used to decolourise sugar.
• Adsorption plays a critical role in the paint industry too. If the paint contains dissolved gases, its coating will not stick well to the surface being painted. This will result in poor covering power.

Conclusion

In this article, you learned about adsorption, why it occurs, types of adsorption, and their practical use in our daily lives.Adsorption has various day to day examples like using silica sachet in shoes or using silica gel to adsorb moisture etc.