What is the primary difference between adsorption and absorption?

The fundamental difference lies in where the substance accumulates. Adsorption is a surface phenomenon where molecules adhere only to the surface of another material (the adsorbent). Think of gas molecules sticking to a charcoal surface. Absorption, on the other hand, is a bulk phenomenon where molecules penetrate uniformly throughout the entire volume of the material. A classic example is a sponge soaking up water, where water molecules are distributed throughout the sponge's interior. Adsorption is selective and depends on surface area, while absorption is less selective and depends on the bulk volume.

Why is adsorption generally an exothermic process?

Adsorption is exothermic because it involves a decrease in the residual surface energy of the adsorbent. The unbalanced forces on the adsorbent's surface are satisfied when adsorbate molecules bind to them. This process of bond formation (even weak van der Waals forces in physisorption or stronger chemical bonds in chemisorption) releases energy, leading to a decrease in the system's enthalpy. According to thermodynamics, a decrease in energy makes the system more stable, and this energy is released as heat, hence exothermic.

How does temperature affect the extent of adsorption?

Generally, increasing the temperature decreases the extent of adsorption. Since adsorption is an exothermic process (releases heat), according to Le Chatelier's principle, raising the temperature shifts the equilibrium towards the endothermic direction, which is desorption. This means that at higher temperatures, adsorbed molecules gain enough kinetic energy to overcome the attractive forces of the adsorbent surface and escape, leading to a reduction in the amount of adsorbate on the surface. This effect is more pronounced in physisorption.

What is the significance of critical temperature in gas adsorption?

The critical temperature of a gas is the temperature above which it cannot be liquefied, no matter how much pressure is applied. Gases with higher critical temperatures are more easily liquefiable because their intermolecular forces are stronger. Stronger intermolecular forces mean these gases are more readily attracted to and held by the adsorbent surface. Therefore, gases with higher critical temperatures are generally adsorbed to a greater extent than gases with lower critical temperatures under similar conditions.

Can chemisorption occur at low temperatures?

While physisorption is favored at low temperatures, chemisorption often requires an activation energy, similar to a chemical reaction. This means that at very low temperatures, the molecules may not have sufficient energy to overcome this activation barrier and form chemical bonds with the surface. Therefore, chemisorption typically increases with temperature initially, as more molecules gain the necessary activation energy, before eventually decreasing at very high temperatures due to the exothermic nature of the process and increased desorption.

Adsorption

Chemistry

NEET UG

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Adsorption is a surface phenomenon where molecules of a substance (adsorbate) accumulate on the surface of another substance (adsorbent), forming a layer. This process is distinct from absorption, which involves uniform penetration throughout the bulk of the material. Adsorption is driven by unbalanced residual forces present on the surface of the adsorbent, leading to a decrease in surface energy…

Quick Summary

Adsorption is a surface phenomenon where molecules (adsorbate) accumulate on the surface of a solid or liquid (adsorbent), driven by unbalanced surface forces. It's distinct from absorption, which involves bulk penetration.

Adsorption is generally an exothermic process, meaning it releases heat. The extent of adsorption is influenced by the nature of the adsorbate and adsorbent, surface area, temperature, and pressure (for gases) or concentration (for solutions).

There are two main types: physisorption, involving weak van der Waals forces, being reversible, non-specific, and forming multilayers; and chemisorption, involving strong chemical bonds, being irreversible, highly specific, and forming a monolayer.

Adsorption isotherms, like Freundlich and Langmuir, describe the relationship between the amount adsorbed and pressure/concentration at constant temperature. Key applications include catalysis, gas masks, dehumidification, and chromatography.

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Key Concepts

Freundlich Adsorption Isotherm

The Freundlich isotherm is an empirical equation that describes the variation of the amount of gas adsorbed…

Langmuir Adsorption Isotherm

The Langmuir isotherm is a theoretical model based on specific assumptions: monolayer adsorption, localized…

Factors Affecting Adsorption

Several factors critically influence the extent of adsorption: 1. **Surface Area:** Larger surface area…

Adsorption: — Surface phenomenon, adsorbate on adsorbent.

Absorption: — Bulk phenomenon, uniform distribution.

Physisorption: — Weak van der Waals, reversible, non-specific, multilayer, low

\Delta H_{ads}

(20-40 kJ/mol), low temp favored.

Chemisorption: — Strong chemical bonds, irreversible, specific, monolayer, high

\Delta H_{ads}

(80-240 kJ/mol), requires activation energy.

Factors: — Surface area

\uparrow

adsorption

\uparrow

; Temperature

\uparrow

adsorption

\downarrow

(exothermic); Pressure

\uparrow

adsorption

\uparrow

Freundlich Isotherm: —

\frac{x}{m} = kP^{1/n}

\log\left(\frac{x}{m}\right) = \log k + \frac{1}{n}\log P

Langmuir Isotherm: —

\theta = \frac{bP}{1 + bP}

(monolayer, specific sites).

Applications: — Gas masks, catalysis, dehumidification, chromatography.

Physical Adsorption is Weak, Reversible, Non-specific, Multilayered, Low Heat, Low Temp.

Chemical Adsorption is Strong, Irreversible, Specific, Monolayered, High Heat, Activation Energy.