CBSE Class 12 Chemistry Chapter 5: Surface Chemistry NCERT Solutions

The primary distinction between adsorption and absorption lies in where the substance accumulates:

Adsorption: This is a surface phenomenon. It involves the accumulation of molecules of a substance (the adsorbate) only on the surface of another substance (the adsorbent). The concentration of the adsorbate increases at the surface but not within the bulk of the adsorbent.

Example: When a chalk stick is dipped in ink, the surface of the chalk stick gets colored, but the inside remains white. This demonstrates that the ink molecules have adsorbed onto the chalk's surface.

Absorption: This is a bulk phenomenon. It involves the distribution of molecules of a substance uniformly throughout the entire bulk of another substance.

Example: When anhydrous calcium chloride is placed in water, it absorbs the water molecules, and the water gets distributed throughout the solid calcium chloride.

Physisorption and chemisorption are two types of adsorption, differing mainly in the nature of the forces involved and their characteristics:

Physisorption (Physical Adsorption):

• Involves weak van der Waals forces of attraction between the adsorbate and the adsorbent surface.
• No new chemical compounds are formed.
• It is generally reversible in nature.
• The enthalpy of adsorption is low, typically in the range of 20-40 kJ mol^-1, due to weak forces.
• It is favored by low temperatures.
• It can result in the formation of a multi-layer of adsorbate on the adsorbent surface.

Chemisorption (Chemical Adsorption):

• 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 in nature.
• The enthalpy of adsorption is high, typically in the range of 40-400 kJ mol^-1, due to the formation of strong chemical bonds.
• It is favored by high temperatures (often requiring initial heating to provide activation energy).
• It typically results in the formation of a mono-layer of adsorbate on the adsorbent surface.

Adsorption is fundamentally a surface phenomenon. The extent of adsorption is directly proportional to the surface area available on the adsorbent. A finely divided substance, such as charcoal powder or silica gel, has a very large surface area relative to its mass compared to a solid in a bulk form. This significantly increased surface area provides a greater number of active sites where adsorbate molecules can accumulate. Therefore, both physisorption and chemisorption are enhanced, making finely divided substances more effective as adsorbents.

Several factors influence the extent to which a gas adsorbs onto a solid surface:

1. Nature of the gas: Gases that are easily liquefiable are adsorbed more readily. This is because easily liquefiable gases have stronger intermolecular van der Waals forces. For example, gases like ammonia (NH₃) and hydrogen chloride (HCl) are adsorbed more strongly than gases like hydrogen (H₂) or oxygen (O₂).

2. Surface area of the solid adsorbent: Adsorption is directly proportional to the surface area of the adsorbent. A larger surface area provides more active sites for the gas molecules to adhere to, leading to greater adsorption. This is why finely divided solids are excellent adsorbents.

3. Effect of pressure: At a constant temperature, the extent of adsorption generally increases as the pressure of the gas increases. This is because higher pressure forces more gas molecules into contact with the adsorbent surface. However, at very high pressures, the surface becomes saturated with adsorbate molecules, and the rate of adsorption levels off, becoming independent of further pressure increases.