Adsorption — Revision Notes

⚡ 30-Second Revision

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: — $\frac{x}{m} = kP^{1/n}$ or $\log\left(\frac{x}{m}\right) = \log k + \frac{1}{n}\log P$.
Langmuir Isotherm: — $\theta = \frac{bP}{1 + bP}$ (monolayer, specific sites).
Applications: — Gas masks, catalysis, dehumidification, chromatography.

2-Minute Revision

Adsorption is the accumulation of molecules (adsorbate) on a surface (adsorbent), distinct from absorption where molecules penetrate the bulk. It's driven by unbalanced surface forces and is typically exothermic.

There are two main types: physisorption and chemisorption. Physisorption involves weak van der Waals forces, is reversible, non-specific, forms multilayers, and is favored by low temperatures and high pressures.

Chemisorption involves strong chemical bonds, is irreversible, highly specific, forms a monolayer, and often requires activation energy. The extent of adsorption increases with surface area, pressure (for gases), and decreases with temperature.

Adsorption isotherms, like Freundlich's $\frac{x}{m} = kP^{1/n}$ and Langmuir's $\theta = \frac{bP}{1 + bP}$, describe the relationship between adsorbed amount and pressure at constant temperature. Key applications include gas masks (charcoal), dehumidifiers (silica gel), and heterogeneous catalysis.

5-Minute Revision

Adsorption is a crucial surface phenomenon where molecules of a substance (adsorbate) accumulate on the surface of another (adsorbent). This is different from absorption, which involves uniform penetration into the bulk.

Adsorption is driven by residual forces on the adsorbent's surface, making it an exothermic process (releases heat). This exothermic nature means that increasing temperature generally decreases the extent of adsorption, following Le Chatelier's principle.

Conversely, increasing the surface area of the adsorbent or the pressure (for gases) / concentration (for solutions) of the adsorbate typically increases adsorption.

We classify adsorption into two types:

1
Physisorption (Physical Adsorption): — Characterized by weak van der Waals forces. It's reversible, non-specific, forms multi-molecular layers, and has a low enthalpy of adsorption (20-40 kJ/mol). It's favored at low temperatures.
2
Chemisorption (Chemical Adsorption): — Involves strong chemical bonds. It's irreversible, highly specific, forms a monolayer, and has a high enthalpy of adsorption (80-240 kJ/mol). It often requires activation energy and is favored at moderate to high temperatures initially, then decreases.

Adsorption isotherms describe the relationship between the amount adsorbed and pressure/concentration at constant temperature. The Freundlich isotherm is empirical: $\frac{x}{m} = kP^{1/n}$. Its logarithmic form, $\log\left(\frac{x}{m}\right) = \log k + \frac{1}{n}\log P$, is useful for graphical analysis (slope $1/n$, intercept $\log k$). The Langmuir isotherm is theoretical, based on monolayer formation on specific sites: $\theta = \frac{bP}{1 + bP}$.

Worked Example: If 2g of activated charcoal adsorbs 0.5g of $\text{SO}_2$ gas at 1 atm and 273 K, and 0.8g at 2 atm and 273 K, calculate $1/n$ for Freundlich isotherm. For 1 atm: $\log(0.5/2) = \log k + (1/n)\log(1) \implies \log(0.

25) = \log k$. So,$\log k = -0.602$. For 2 atm:$\log(0.8/2) = \log k + (1/n)\log(2) \implies \log(0.4) = \log k + (1/n)(0.301)$. Substitute$\log k$:$\log(0.4) = -0.602 + (1/n)(0.301)$.$-0.398 = -0.

602 + (1/n)(0.301) \implies 0.204 = (1/n)(0.301)$.$1/n = 0.204 / 0.301 \approx 0.678$.

Applications are widespread: gas masks (charcoal adsorbs toxic gases), dehumidifiers (silica gel adsorbs moisture), heterogeneous catalysis (reactants adsorb on catalyst surface), decolorization (animal charcoal removes colors), and chromatography.

Prelims Revision Notes

Adsorption: NEET Quick Recall

1. Definition & Distinction:

Adsorption: — Surface phenomenon. Accumulation of adsorbate on adsorbent surface. Exothermic ($\Delta H < 0$).
Absorption: — Bulk phenomenon. Uniform penetration throughout the material.
Adsorbate: — Substance adsorbed. Adsorbent: Surface on which adsorption occurs.
Desorption: — Removal of adsorbate from adsorbent surface.

2. Types of Adsorption:

Feature	Physisorption (Physical Adsorption)	Chemisorption (Chemical Adsorption)
Forces	Weak van der Waals forces	Strong chemical bonds (covalent/ionic)
Enthalpy ($\Delta H_{ads}$)	Low (20-40 kJ/mol)	High (80-240 kJ/mol)
Reversibility	Reversible	Irreversible
Specificity	Non-specific	Highly specific
Layers	Multi-molecular layers	Monolayer
Temperature	Favored at low temperature, decreases with $\uparrow T$	Favored at moderate T, increases then decreases with $\uparrow T$ (requires activation energy)
Activation Energy	Low/Nil	High

3. Factors Affecting Adsorption:

Surface Area: — Directly proportional to adsorption. Porous/finely divided solids are good adsorbents.
Temperature: — Adsorption is exothermic. $\uparrow T \implies \downarrow$ adsorption (Le Chatelier's principle).
Pressure (for gases) / Concentration (for solutions): — $\uparrow P/C \implies \uparrow$ adsorption (up to saturation).
Nature of Adsorbate: — Easily liquefiable gases (higher critical temperature) are more readily adsorbed (e.g., $\text{NH}_3 > \text{CO}_2 > \text{CH}_4 > \text{N}_2 > \text{H}_2$).
Nature of Adsorbent: — Depends on active sites and surface characteristics.

4. Adsorption Isotherms (Constant Temperature):

Freundlich Adsorption Isotherm (Empirical):

* $\frac{x}{m} = kP^{1/n}$ (for gases) or $\frac{x}{m} = kC^{1/n}$ (for solutions) * $x$: mass of adsorbate, $m$: mass of adsorbent, $P$: pressure, $C$: concentration. * $k, n$: constants ($n > 1$). * Logarithmic form: $\log\left(\frac{x}{m}\right) = \log k + \frac{1}{n}\log P$. * Plot of $\log(x/m)$ vs. $\log P$ is a straight line with slope $1/n$ and y-intercept $\log k$.

Langmuir Adsorption Isotherm (Theoretical):

* Assumes monolayer formation, localized sites, no interaction between adsorbed molecules. * $\frac{x}{m} = \frac{aP}{1 + bP}$ or $\theta = \frac{bP}{1 + bP}$ (where $\theta$ is fraction of surface covered). * At low P, $\theta \propto P$. At high P, $\theta \approx 1$ (saturation).

5. Applications:

Gas masks: — Activated charcoal adsorbs poisonous gases.
Catalysis: — Heterogeneous catalysts (e.g., Haber process, hydrogenation).
Dehumidification: — Silica gel, alumina adsorb moisture.
Decolorization: — Animal charcoal removes colors from sugar solutions.
Chromatography: — Separation technique based on differential adsorption.
Vacuum production: — Charcoal adsorbs residual gases.

Vyyuha Quick Recall

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

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