Adsorption Questions

Adsorption is the process by which atoms, ions, or molecules attach to a surface from a gas, liquid, or dissolved solids. It’s important to note that absorption occurs across the entire volume, whereas adsorption appears on the surface. So, adsorption is a surface phenomenon. Adsorption, like surface tension, results from the presence of surface energy. Attracting and keeping particles on the liquid or solid surface causes the molecules to become more concentrated on that surface.

Adsorption

Adsorption is the phenomenon through which atoms, molecules, or ionic constituents of one substance aggregate on the surface. Heinrich Kayser, a German scientist, coined the term adsorption in 1881. When it comes to surface phenomena, adsorption is the adhesion of particles to the surface. It is typically composed of dissolved gas, liquids, solid molecules, atoms, or ions.

Surface chemistry is concerned with the events at or near surfaces or interfaces.

Adsorption Components

This surface phenomenon requires two components – adsorbate and adsorbent.

• Adsorbate: Adsorbate is the term used to describe the adsorbed substance on a surface. For example, if gas is adsorbed on the solid surface, it is referred to as the adsorbate.
• Adsorbent: When adsorption occurs, the substance that forms a surface on which it can take place is referred to as adsorbent. Adsorbents can be solid or liquid. It includes metal powders, charcoal powder, and silica powder. Due to their high surface area, activated charcoal, silica, and alumina, among others, are extremely effective adsorbents.

Different Types of Adsorption

Adsorption is classified into two types – physical adsorption, also known as physisorption, and chemical adsorption, also known as chemisorption.

Physical Adsorption

Van der Waals forces play a role in the adsorption of gases onto a solid surface.

Properties of Physical Adsorption

• In the event of physical adsorption, there is no specialisation. Each gas is adsorbed onto the solid’s surface.
• Easily liquefiable gases are physically adsorbed to a great extent.
• In nature, physical adsorption is a reversible process. The volume of gas drops due to a rise in pressure, and more gas is adsorbable. So, lowering the pressure removes gas from a solid surface. Physical adsorption is facilitated by low temperatures, while high temperatures slow the adsorption process.
• Adsorption rate increases as the surface area increases. Porous materials and finely split metals act as excellent adsorbents.
• Physical adsorption is exothermic.
• There is no requirement for activation energy.

Chemical Adsorption

Chemical adsorption is also known as chemisorption. Chemical adsorption occurs when gas molecules or atoms are bound to a solid surface by chemical bonding.

Properties of Chemical Adsorption

• In comparison to physical adsorption, this type of adsorption is more specialised. Adsorption happens only when the adsorbate and adsorbent form chemical bonds.
• Chemical adsorption is an irreversible process. Although this is an exothermic process, it is slow at low temperatures. Chemisorption occurs in the presence of an increase in temperature. Chemisorption is facilitated by high pressure.
• Chemisorption increases as the surface area increases.
• Chemisorption has a high enthalpy due to the formation of chemical bonds.
• There is a requirement for activation energy.
• The unimolecular layer is the final result.

Adsorption Mechanism

• Adsorption happens when the surface and mass of the adsorbent’s net force is not identical.
• Unbalanced or residual attractive forces act on the particles on the adsorbate.
• Charge transfer happens between both the adsorbent and the adsorbate.
• This leads to the formation of a dipole moment.
• Ions, atoms, and molecules from liquids, gases, and dissolved solids adhere to a surface.
• Adsorbate film forms on the adsorption surface.
• Adsorption always reduces the surface’s residual attractive forces.
• An exothermic reaction occurs, meaning it releases energy.
• The surface’s energy decreases, resulting in heat. This is referred to as the adsorption heat.
• Adsorption occurs naturally at room temperature and pressure.

Factors Affecting the Adsorption Extent and Rate

Adsorption occurs on practically all solid surfaces. The extent and rate of adsorption on an adsorbent surface depend on the following factors:

• Pressure Effect
• Temperature Effect
• Adsorbate’s Nature
• Adsorbent’s Nature
• Adsorbent’s Surface Area

Applications of Adsorption

The following are the applications of adsorption:

• The adsorption technique can be used to create a high vacuum. Activated charcoal is needed to make the vacuum.
• Adsorption is applied in coal mine gas masks. Gas masks adsorb harmful gases. This purifies the air for breathing.
• Moisture is adsorbed by silica and aluminium gels.
• Charcoal can be used to separate noble gases.
• Drug adsorption is a method of killing microorganisms.
• Adsorption is the basis of chromatographic analysis.
• Charcoal powder is used to decolourise sugar solutions. 
• Adsorption is important in the paint industry. Paints with dissolved gases do not adhere well to surfaces and, hence, have poor covering power.
• This technique is also used to create stable emulsions for cosmetics and syrups.
• Adsorption also plays a role in the cleaning activities of soaps and detergents.

Conclusion

An adsorption reaction occurs when molecules of materials attract and bind to the liquid surface or solid, increasing their concentration. Due to the exothermic nature of adsorption, the resulting change in enthalpy is always negative. When adsorbate molecules adhere to a surface, their freedom of movement is limited, reducing entropy. Gibbs free energy is reduced due to spontaneous adsorption under constant pressure and temperature.