Adsorption is a surface phenomenon in which adsorbate molecules are attracted to the adsorbent. Adsorption reaction is an exothermic process because energy (heat) is always released when attractive interactions occur. The enthalpy of adsorption is the quantity of heat emitted when one mole of adsorbate is adsorbed mainly on the surface of an adsorbent.

Adsorption Enthalpy

Adsorption enthalpy is a thermodynamic term referring to the amount of heat emitted or utilised during the adsorption process. Adsorption is a thermodynamically advantageous mechanism in the vast majority of cases. The Van’t Hoff equation will be used to mathematically express the adsorption enthalpy notes.

The heat of adsorption is the quantity of heat generated when one mole of gas is adsorbed on the adsorbent. Adsorption is the primary cause of London Dispersion Forces, a Van der Waals force between molecules. The force acts similarly to magnetic forces between planets. Chemisorption has a higher enthalpy of adsorption than physical adsorption because it necessitates the formation of chemical bonds.

Mechanism of Adsorption

Adsorption is the reaction in which ions, atoms, or molecules from the liquid, gas, or dissolved solids adhere to a surface. This process results in forming an adsorbate layer on the surface of the adsorbent.

Adsorption happens because the surface particles of the adsorbent aren’t even in the environment as the bulk particles. Throughout the adsorbent, forces between the particles are balanced, but because their kind does not accompany them, they exhibit the unequal or residual force of attraction. Adsorbate particles are drawn to surfaces of adsorbent particles by adsorbent forces.

Adsorption increases when the adsorbent’s surface area for each unit mass increases at a specific temperature and pressure. Another critical feature of adsorption is the heat of adsorption. During adsorption, residual surface forces or surface energy constantly decrease, resulting in heat.

The adsorption enthalpy example is an exothermic reaction with a constant negative enthalpy change. When adsorbate molecules adhere to a surface, their freedom of movement is limited, decreasing entropy. Because adsorption occurs spontaneously at constant temperature and pressure, Gibbs free energy similarly decreases.

Types Of Adsorption

Adsorption is categorised on the basis of the nature of the forces that keep the molecules of the adsorbate to the surface of the adsorbent. There are two different types of adsorption, physical and chemical adsorption.

Physical Adsorption (Physisorption)

Physical adsorption occurs when adsorbate particles are adhered to the surface of the adsorbent by physical processes like Van Der Waals forces. Because attractive forces are weak, they can be easily countered by increased temperature or decreasing pressure. Physical adsorption is reversible or decreased. The adsorption enthalpy is low because the attractive interactions between the adsorbent and adsorbate particles are weak (Van Der Waals forces).

Features of Physisorption

• There is no restriction on the type of gas that can be adsorbed into the surface.
• It has been shown that gases with high liquifiability are more strongly adsorbed physically.
• It is reversible and reliant on pressure and temperature. Increased pressure reduces gas volume and thus enhances molecule adsorption. Gas molecules on the solid surface will be removed when the pressure decreases.
• Due to the exothermic nature of the adsorption process, physical adsorption occurs rapidly at low temperatures and declines with rising temperatures (Le-Chatelier’s principle).
• Porous materials are superior adsorbents because their greater surface area facilitates adsorption.
• Activation does not require any energy.

Chemical Adsorption (Chemisorption)

Chemical adsorption or chemisorption occurs when the adsorbate molecules are bound to the surface of an adsorbent by chemical forces or chemical bonds. A chemical reaction occurs on the surface between the adsorbate molecules and the adsorbent. This is an irreversible type of adsorption. The attractive forces between the adsorbent and adsorbate particles are based on strong chemical bonds, which results in a high molar heat of adsorption.

Features of chemisorption

• The fact that the process is specific in nature means that it will only take place if a chemical bond is formed between both the adsorbent and the adsorbate is present.
• The procedure is irreversible.
• It is an exothermic reaction that increases the temperature.
• At low temperatures, it occurs more slowly, while at higher pressures, it occurs more quickly.
• Surface area is directly related to the rate at which chemisorption occurs, and this rate increases as the surface area grows.
• The adsorption enthalpy is high because of the chemical bond formation.
• It necessitates a specific amount of activation energy.

Conclusion

The adsorption enthalpy is a critical metric for determining the energetic variability of the surface of the adsorbent. The isosteric adsorption is an essential parameter in providing insight into the adsorption mechanism of an adsorption process on an adsorbent. It is defined as the amount of heat released when an adsorptive adheres or detaches from an adsorbent’s solid surface. Adsorption enthalpy is either estimated directly using calorimetry or computed using the thermodynamics Clausius–Clapeyron equation from adsorption mechanism recorded at various temperatures.