CBSE Class 12 Chemistry Chapter 10: Surface Chemistry NCERT Solutions

Solution: Adsorption and absorption are often confused, but they describe different processes.

Adsorption is a surface phenomenon. It refers to the process where molecules of a substance (the adsorbate) accumulate or stick to the surface of another substance (the adsorbent). The concentration of the adsorbate increases only at the surface. For example, when a chalk stick is dipped in ink, the ink particles stick to the outer surface of the chalk. If the chalk stick is broken, the inside remains white, demonstrating that the ink did not penetrate the bulk.

Absorption is a bulk phenomenon. In this process, the substance is uniformly distributed throughout the entire volume or bulk of the other substance. For example, when a sponge is placed in water, the water is absorbed by the sponge and penetrates its entire structure, not just the surface.

Solution: Physisorption and chemisorption are two types of adsorption, differing primarily in the nature of the forces involved and the resulting characteristics:

Physisorption (Physical Adsorption):

• It involves weak van der Waal's forces of attraction between the adsorbate and the adsorbent surface.
• No new chemical compounds are formed.
• It is generally reversible; the adsorbed substance can be easily removed by changing conditions like temperature or pressure.
• The enthalpy of adsorption is low, typically in the range of 20-40 kJ mol^-1, because only weak forces are overcome.
• It is favored by low temperatures, as lower temperatures increase the van der Waal's forces.
• It often results in the formation of a multimolecular layer of adsorbate on the adsorbent surface.

Chemisorption (Chemical Adsorption):

• It involves the formation of strong chemical bonds (like covalent or ionic bonds) between the adsorbate and the adsorbent surface.
• New chemical compounds are formed at the surface.
• It is usually irreversible or difficult to reverse because breaking the chemical bonds requires significant energy.
• The enthalpy of adsorption is high, typically in the range of 40-400 kJ mol^-1, due to the energy released during chemical bond formation.
• It is favored by high temperatures, as higher temperatures provide the activation energy needed for bond formation.
• It typically results in the formation of a unimolecular (mono-layer) adsorption.

Solution: Adsorption is fundamentally a surface phenomenon. The extent of adsorption depends directly on the available surface area of the adsorbent. A finely divided substance, such as charcoal powder or platinum black, has a very large surface area relative to its mass compared to a solid in a bulk form. This significantly increased surface area provides many more active sites where gas molecules can attach. Therefore, a finely divided substance is much more effective as an adsorbent for both physisorption and chemisorption.

Solution: Several factors influence the extent to which a gas is adsorbed onto the surface of a solid:

1. Nature of the gas (Adsorbate): Gases that are easily liquefiable tend to be adsorbed more readily. This is because easily liquefiable gases have stronger intermolecular attractive forces (van der Waal's forces). For example, gases like ammonia (NH₃) and hydrogen chloride (HCl), which have higher critical temperatures and are easily liquefied, are adsorbed more strongly than gases like hydrogen (H₂) or oxygen (O₂), which are difficult to liquefy and have weaker van der Waal's forces.

2. Surface area of the solid (Adsorbent): Adsorption occurs at the surface. Therefore, the greater the surface area of the adsorbent per unit mass, the greater the extent of adsorption. This is why finely divided solids or porous materials with large surface areas are excellent adsorbents.

3. Effect of pressure: At a constant temperature, the extent of adsorption of a gas on a solid generally increases as the pressure of the gas increases. Initially, the rate of adsorption increases rapidly with pressure. However, as the pressure continues to rise, the surface of the adsorbent becomes progressively covered with gas molecules, and the rate of adsorption slows down. Eventually, at high pressures, the surface becomes saturated, and the extent of adsorption reaches a maximum, becoming independent of further increases in pressure.