Physisorption and Chemisorption — Revision Notes

⚡ 30-Second Revision

Physisorption: — Weak Van der Waals forces, low $\Delta H_{ads}$ ($20-40 \text{ kJ/mol}$), non-specific, reversible, multilayer, decreases with T, negligible activation energy.
Chemisorption: — Strong chemical bonds, high $\Delta H_{ads}$ ($80-240 \text{ kJ/mol}$), highly specific, irreversible, monolayer, increases then decreases with T (often needs activation energy).
Both are exothermic processes.

2-Minute Revision

Physisorption and chemisorption are two distinct types of adsorption. Physisorption, or physical adsorption, is characterized by weak Van der Waals forces between the adsorbate and adsorbent. It's a non-specific, reversible process with a low enthalpy of adsorption ($20-40 \text{ kJ/mol}$) and forms multilayers.

Its extent decreases as temperature increases. Chemisorption, or chemical adsorption, involves the formation of strong chemical bonds, making it highly specific and generally irreversible. It has a high enthalpy of adsorption ($80-240 \text{ kJ/mol}$) and forms a monolayer.

Chemisorption often requires an activation energy, so its extent typically increases with temperature initially, then decreases at very high temperatures. Both processes are exothermic.

5-Minute Revision

Adsorption is a surface phenomenon where molecules accumulate on a surface. It's categorized into physisorption and chemisorption based on the nature of forces. Physisorption involves weak Van der Waals forces.

Imagine $N_2$ gas adsorbing on charcoal at low temperatures. This process is non-specific, meaning any gas can adsorb on any solid, though easily liquefiable gases (higher critical temperature) adsorb more readily.

It's reversible; increasing temperature or decreasing pressure desorbs the gas. It forms multilayers and has a low enthalpy of adsorption ($20-40 \text{ kJ/mol}$). Its extent always decreases with increasing temperature.

Chemisorption involves strong chemical bonds between adsorbate and adsorbent. For example, $H_2$ gas adsorbing on nickel. This process is highly specific, requiring chemical affinity. It's generally irreversible and forms only a monolayer.

The enthalpy of adsorption is high ($80-240 \text{ kJ/mol}$). Chemisorption often requires an activation energy, so its extent typically increases with temperature up to a certain point, then decreases.

Both are exothermic processes. Remember these key differences for NEET success.

Prelims Revision Notes

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Adsorption: — Surface phenomenon, accumulation of adsorbate on adsorbent surface.
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Physisorption (Physical Adsorption):

* Forces: Weak Van der Waals forces (London dispersion, dipole-dipole, dipole-induced dipole). * **Enthalpy of Adsorption ($\Delta H_{ads}$):** Low, $20-40 \text{ kJ/mol}$. * Specificity: Non-specific.

Any gas on any solid. * Reversibility: Reversible. Easily desorbed by heating or reducing pressure. * Layer Formation: Multilayer (can form multiple layers). * Effect of Temperature: Decreases with increasing temperature (exothermic, weak forces overcome).

* Effect of Pressure: Increases with increasing pressure. * Activation Energy: Negligible. * Critical Temperature: Gases with higher critical temperatures are more readily physisorbed. * Example: Adsorption of $N_2$ on mica at $77\text{ K}$.

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Chemisorption (Chemical Adsorption):

* Forces: Strong chemical bonds (covalent or ionic). * **Enthalpy of Adsorption ($\Delta H_{ads}$):** High, $80-240 \text{ kJ/mol}$. * Specificity: Highly specific. Requires chemical affinity.

* Reversibility: Irreversible. Difficult to desorb. * Layer Formation: Monolayer (forms only a single layer). * Effect of Temperature: Increases initially, then decreases at higher temperatures (requires activation energy).

* Effect of Pressure: Increases with increasing pressure (up to saturation). * Activation Energy: Often significant. * Example: Adsorption of $H_2$ on nickel at $200\text{ K}$, formation of metal oxides.

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Common Point: — Both physisorption and chemisorption are exothermic processes ($\Delta H_{ads}$ is negative).

Vyyuha Quick Recall

To remember the key differences: Physical is Peaceful, Chemical is Committed.

Physical (Physisorption):

Peaceful forces (Weak Van der Waals)
Paltry energy (Low $\Delta H_{ads}$)
Plethora of layers (Multilayer)
Passive to temperature (Decreases with T)
Partial commitment (Reversible)
Particularity not needed (Non-specific)

Chemical (Chemisorption):

Committed bonds (Strong chemical bonds)
Considerable energy (High $\Delta H_{ads}$)
Coverage is one (Monolayer)
Challenged by temperature (Increases then decreases with T, needs activation)
Complete commitment (Irreversible)
Choosy (Highly specific)