Introduction to Surface Chemistry: Types, Mechanism, Comparison

Adsorption

When a solid surface is exposed to a gas or liquid, molecules from the gas or solution phase collect or concentrate on the surface.

Adsorption is the process of a gas or liquid’s molecules concentrating on a solid surface. Adsorbate refers to the substance that deposits at the surface, whereas adsorbent refers to the solid on whose surface the deposition occurs.

Examples of Adsorption

(1) Adsorption of a dye by charcoal: When finely divided charcoal is added to a dilute solution of methylene blue (an organic dye), the solution’s colour depth is significantly reduced. Charcoal particles have adsorbed the dye molecules.

(2) Adsorption of gas by charcoal: The gas pressure is found to decrease when a gas \(\left( {{\rm{S}}{{\rm{O}}_{\rm{2}}}{\rm{,C}}{{\rm{l}}_{\rm{2}}}{\rm{,N}}{{\rm{H}}_{\rm{3}}}} \right)\) is treated with powdered charcoal in a closed vessel. The gas molecules bind to the surface of the charcoal and are said to be adsorbed.

Adsorption Versus Absorption

The phrase ‘adsorption’ must be separated from another term with a similar sounding name, ‘absorption.’ Adsorption refers to deposition just on the surface of a solid, whereas absorption refers to penetration into the solid’s body. For example, a chalk crayon dipped in ink absorbs the latter and, when broken, reveals itself to be white from within. Water, on the other hand, is absorbed by a sponge and uniformly dispersed throughout the sponge. Adsorption and absorption frequently occur simultaneously. As a result, experimentally distinguishing between the two processes is difficult. Mc Bain coined the term “sorption,” which encompasses both adsorption and absorption.

Mechanism of Adsorption

A solid surface’s atoms or molecules act similarly to the surface molecules of a liquid. These aren’t surrounded by similar atoms or molecules. As a result, they have imbalanced or residual attractive forces on the surface, allowing adsorbate particles to stick to it.

Physical van der Waals force or chemical forces due to residual valence bonds can hold adsorbed atoms or molecules on the surface of metal like platinum (Pt). As a result, hydrogen adsorption on platinum can occur in two ways (molecularly or atomically, as shown above).

Types of Adsorption

The adsorption of a gas into a solid surface is mainly of two types:

(a) Physical Adsorption
This is due to van der Waals attraction forces holding the gas molecules to the solid surface. Van der Waals Adsorption is another name for it. For example, the adsorption of hydrogen or oxygen on charcoal is Physical Adsorption.

(b) Chemical Adsorption or Chemisorption
Chemical bonds hold gas molecules or atoms to the solid surface in this type of adsorption. These bonds might be either covalent or ionic. Hydrogen, for example, is chemisorbed on nickel. Van der Waals forces adsorb hydrogen molecules, which then dissociate. As a result, the hydrogen atoms are chemisorbed on nickel.

Adsorption is frequently a mixture of the two types of adsorption mentioned above.

Adsorption of Gases by Solids

Gas adsorption by solid adsorbents has a few distinguishing characteristics. In many ways, physical adsorption and chemisorption are found to be different.

(1) Adsorption and Surface area
Because adsorption is a surface phenomenon, the amount of adsorption is proportional to the area of the surface. The total amount of gas adsorbed increases as the surface area of the adsorbent grows. Finely split metals (nickel, platinum) and porous substances (charcoal, silica gel) are the best solid adsorbents because they have a huge surface area.

(2) Nature of Gas
The amount of gas absorbed by a material is determined by the gas’s composition. In general, the easier a gas is to liquefy (i.e., the higher its critical temperature), the more quickly it can be adsorbed. \({\rm{1\, g}}\) of activated charcoal adsorbs \(380{\rm{ ml}}\) sulphur dioxide (critical temperature \(157\,^\circ {\rm{C}}\)), \({\rm{16\, ml}}\) methane (critical temperature \(83\,^\circ {\rm{C}}\)), and \({\rm{4}}{\rm{.5 ml}}\) hydrogen (critical temperature \({\rm{ – 20}}\,{\rm{^\circ C}}\)). In contrast to physical adsorption, chemisorption is significantly more specific. However, when there is a chance of chemical action between the gas adsorbed and the solid, it will not happen.

(3) Heats of Adsorption
The energy released when \({\rm{1\, gm}}\) mole of a gas is adsorbed on a solid surface is known as the heat of adsorption. Gas molecules concentrate on the solid surface during physical adsorption. As a result, it resembles the condensation of a gas into a liquid. As a result, like condensation, adsorption is an exothermic process. Adsorption temperatures are minimal (about \({\rm{5], kcal], mo}}{{\rm{l}}^{{\rm{–1}}}}\)) because the interactions between gas molecules and solid surfaces are due to comparatively weak van der Waals forces. The attractive forces in chemisorption are caused by the creation of real chemical bonds. As a result, adsorption temperatures range from \(20\) to \({\rm{100}}\,{\rm{kcal}}\,{\rm{mo}}{{\rm{l}}^{{\rm{–1}}}}\).

(4) Reversible Character
The process of physical adsorption is reversible. Under the right temperature and pressure, the gas adsorbed on a solid can be removed (desorbed). Thus,

Chemisorption, on the other hand, is irreversible due to the formation of a surface compound.

(5) Effect of Temperature
Physical adsorption happens quickly at low temperatures and slows down as the temperature rises (Le Chatelier’s Principle). Chemisorption, like other chemical changes, increases as the temperature rises. As a result, a change in temperature might induce physical adsorption to become chemisorption. At \(190\,^\circ {\rm{C}}\), nitrogen is physically adsorbed on iron, but at \(500\,^\circ {\rm{C}}\), it is chemisorbed to create a nitride.

(6) Effect of Pressure Le Chatelier’s Principle is used because a dynamic equilibrium exists between the adsorbed gas and the gas in contact with the solid, as indicated in \(\left( 4 \right)\). Actually, it has been discovered that as pressure rises, adsorption rises, and as pressure falls, desorption rises.

(7) Thickness of Adsorbed Layer of Gas
Langmuir discovered that at low pressure, the physically adsorbed gas forms only one molecular thick layer based on isotherms linking the amount of gas adsorbed to the equilibrium pressure. Above a certain pressure, however, a multimolecular thick layer forms.

Comparison of Physical Adsorption and Chemisorption

Summary

Adsorption is the process of a gas or liquid’s molecules concentrating on a solid surface. Adsorbate refers to the substance that deposits at the surface, whereas adsorbent refers to the solid on whose surface the deposition occurs. Physical adsorption is due to van der Waals attraction forces holding the gas molecules to the solid surface. Chemical bonds hold gas molecules or atoms to the solid surface in chemical adsorption. In many ways, physical adsorption and chemisorption are found to be different.

FAQs on Introduction to Surface Chemistry

Q.1. What is Adsorption?
Ans: When a solid surface is exposed to a gas or liquid, molecules from the gas or solution phase collect or concentrate on the surface. Adsorption is the process of a gas or liquid’s molecules concentrating on a solid surface. Adsorbate refers to the substance that deposits at the surface, whereas adsorbent refers to the solid on whose surface the deposition occurs.

Q.2. What is the difference between Adsorption and Absorption?
Ans: Adsorption refers to deposition just on the surface of a solid, whereas absorption refers to penetration into the solid’s body. For example, a chalk crayon dipped in ink absorbs the latter and, when broken, reveals itself to be white from within. Water, on the other hand, is absorbed by a sponge and uniformly dispersed throughout the sponge. Adsorption and absorption frequently occur simultaneously. As a result, experimentally distinguishing between the two processes is difficult. Mc Bain coined the term “sorption,” which encompasses both adsorption and absorption.

Q.3. Give the names of Types of Adsorption.
Ans: Physical adsorption and Chemisorption

Q.4. What is the effect of temperature on physical adsorption and chemisorption?
Ans: Physical adsorption happens quickly at low temperatures and slows down as the temperature rises (Le Chatelier’s Principle). Chemisorption, like other chemical changes, increases as the temperature rises. As a result, a change in temperature might induce physical adsorption to become chemisorption. At \({\rm{190}}\,{\rm{^\circ C}}\), nitrogen is physically adsorbed on iron, but at \({\rm{190}}\,{\rm{^\circ C}}\), it is chemisorbed to create a nitride.