Adsorption is a surface process where a fluid molecule becomes stuck on the solid’s surface. Adsorption occurs due to the creation of chemical bonds or the attraction of physical forces. Adsorption can be irreversible or reversible based on the interaction between the adsorbent and adsorbate molecules. Adsorption is usually reversible and is referred to as desorption. Silica and aluminium gel, charcoal gas masks, and alum filtration of water are adsorption examples. Adsorption of gaseous adsorbate molecules on the surface of a solid is a spontaneous exothermic reaction. The amount of heat released out when a unit mass of a gas is adsorbed on a surface is known as the heat of adsorption. Physisorption and chemisorption are the two forms of adsorption.

What is chemisorption?

Chemisorption is defined as the transfer, exchange, or sharing of electrons between adsorbates and adsorbents (atoms or molecules), with adsorbates adhering to adsorbents via chemical bonds. Chemisorption or chemical adsorption occurs mainly due to the formation of chemical bonds between the adsorbent and adsorbate molecules. The bonds can be ionic or covalent. Chemical adsorption may not be completely reversible, and regeneration may require a lot of energy to dissociate the adsorbent from the adsorbate molecules. Chemisorption occurs when a single layer of adsorbate forms on the adsorbent. It has a high adsorption enthalpy.

Factors like temperature, pressure and surface area affect the rate of chemisorption.

Chemisorption depends on the nature of the gas that is being adsorbed. When the gases react with the adsorbent, then the rate of adsorption increases.

Dissociation Chemisorption

One type of gas-surface chemisorption is the dissociation of diatomic gas molecules like hydrogen, oxygen, and nitrogen. This process can be explained using the Precursor-mediation model. The molecule is absorbed and adsorbed onto a surface, forming a precursor state. The chemical subsequently diffuses to the chemisorption sites over the surface. They displace the molecular link in favour of new surface bonds. Translational and vibrational energy is commonly used to overcome the activation potential of dissociation.

Characteristics of chemisorption

• The formation of chemical bonds aids chemisorption.
• Chemisorption is an irreversible process.
• Chemisorption requires a higher energy of activation(80kJ – 240kJ)
• It is highly selective in nature and occurs only by forming  ionic or covalent bonds.
• Chemical adsorption for a unimolecular layer.
• High heat of adsorption process
• High temperature favours the process of chemisorption.

Factors affecting chemisorption

• Temperature: The extent of chemisorption of gas first increases with an increase in temperature and then decreases as the temperature increases further.
•  Surface Area: The rate of chemisorption increases with surface area. The surface area is directly proportional to the extent of chemical adsorption. When the surface is more, more particles will undergo adsorption. Porous materials and finely divided metals work well as adsorbents.
•  Pressure: The volume of gas decreases as pressure rises, and more gas is adsorbed. As a result, gas can be released from the solid surface by lowering the pressure. As the pressure increases, the adsorption increases with it.
• The energy of Activation: Chemisorption is a high energy process, and hence it requires an appreciable amount of activation energy.
• Nature of Adsorbate: The more liquefiable gases, or readily soluble in water, are absorbed more quickly than others because intermolecular forces are more potent in these gases. As a result, they are absorbed more quickly.
• Nature of Adsorbent: Even for the same adsorbate, the adsorption rate varies under similar conditions. Adsorbent molecules that are porous and finely split adsorb faster. Compared to a smooth surface, the adsorbent with a rough surface adsorbs more.

Uses of chemisorption

Chemisorption is a technique for evaluating active surface sites on a substrate and is used to develop, monitor and measure corrosion inhibitors.

Chemisorption is when a solid metal substrate and an adsorbent, such as a corrosion inhibitor molecule, share or transfer a charge. Metal dissolving in an electrolyte fluid is inhibited by chemically adsorbed inhibitors on the metal surface.

Examples of chemisorption

Heterogeneous catalysis involves molecules reacting with each other via the creation of chemisorbed intermediates. Heterogeneous catalysis is an excellent example of chemical adsorption. 

Some common examples where chemisorption is employed are Haber’s process and the hydrogenation of fats. Chemisorption is also responsible for the rusting and corrosion of metal surfaces.

Conclusion

Adsorption is the process of a material accumulating in molecular forms at increasing concentrations on the surface. Chemisorption is a type of adsorption where chemical bonds are formed. Because the adsorbed molecules are connected to the surface by valence bonds, they typically occupy specific adsorption sites, resulting in only one layer of chemisorbed molecules. Chemisorption is a high energy process due to bond formation. Chemisorption rate depends on the nature of adsorbate, surface area, temperature and pressure. The principle of chemisorption is used in heterogeneous catalysis. Some examples of chemisorption employed are hydrogenation of oils and fats and Haber’s process.