Which adsorption points are asked most in NEET previous-year papers?

NEET questions on adsorption usually test the signs of enthalpy, entropy and Gibbs energy, the differences between physisorption and chemisorption, the effect of critical temperature on extent of adsorption, the Freundlich equation and its log plot, and common uses such as silica gel and gas masks.

Adsorption

Q: What is adsorption in chemistry?

Adsorption is the accumulation of gas or solute molecules on the surface of a solid or liquid rather than in its bulk. The substance that collects is the adsorbate and the surface is the adsorbent. Examples are water vapour on silica gel, dyes on charcoal and gases on finely divided metals.

Q: Why is adsorption always exothermic?

Surface particles have unbalanced residual forces. When adsorbate binds, these forces are satisfied and surface energy falls, and this energy is released as heat. Also, adsorption lowers entropy, so Gibbs energy can fall only if the enthalpy change is negative. Hence enthalpy of adsorption is always negative.

Q: What is the difference between physisorption and chemisorption?

Physisorption is due to weak van der Waals forces, is reversible, not specific, releases 20 to 40 kJ per mol and can form multilayers. Chemisorption involves chemical bonds, is specific and usually irreversible, releases 80 to 240 kJ per mol, needs activation energy and forms a monolayer.

Q: Why does chemisorption first increase and then decrease with temperature?

Chemisorption needs activation energy to form bonds with the surface. At low temperature few molecules have this energy, so raising the temperature increases adsorption. Once bonding is fast, the exothermic nature takes over and further heating decreases adsorption, giving a hump-shaped isobar.

Q: What does the Freundlich adsorption isotherm state?

It states that the mass of gas adsorbed per gram of adsorbent, x/m, equals k times p raised to 1/n, with n greater than 1, at constant temperature. A plot of log x/m against log p is a straight line with slope 1/n and intercept log k. It fails at high pressure.

Q: Why are easily liquefiable gases adsorbed more readily?

Easily liquefiable gases have higher critical temperatures and stronger van der Waals forces between molecules, and the same forces hold them on the adsorbent surface. So sulphur dioxide and ammonia are physisorbed on charcoal far more than hydrogen or nitrogen.

Q: What is asked from adsorption in JEE Advanced?

The JEE Advanced syllabus lists physisorption, chemisorption and the Freundlich adsorption isotherm. Questions test isobars for both types, slope and intercept of the log plot, limits of 1/n, thermodynamic signs and, in multi-correct questions, comparisons with the Langmuir monolayer model.

ChemistrySurface ChemistryFor JEE aspirants

Adsorption is the accumulation of molecules of a gas or a solute on the surface of a solid or liquid, not inside its bulk. It happens because surface particles have unbalanced (residual) forces, which is why adsorption always releases heat. This page covers adsorption vs absorption, physisorption vs chemisorption, the factors that control adsorption of gases on solids, the Freundlich isotherm and its log plot, adsorption from solution and the uses examiners ask about. Adsorption is now a JEE Advanced-only topic: Surface Chemistry has been removed from the JEE Main and NEET syllabi.

On this page1Adsorption vs absorption2Why surfaces adsorb3Physi vs chemi4Factors5Freundlich isotherm6From solution7Applications

Key Formulas - Quick Reference

★ Must learn Adsorption is spontaneous with $Δ H < 0$ and $Δ S < 0$ , so $Δ G = Δ H - T Δ S < 0$ only while $∣Δ H ∣ > T ∣Δ S ∣$ ; equilibrium when $Δ H = T Δ S$ .
Enthalpy of adsorption: physisorption $20$ - $40$ kJ mol $^{- 1}$ ; chemisorption $80$ - $240$ kJ mol $^{- 1}$ .
★ Must learn Extent of physisorption follows critical temperature: $S O_{2}$ $>$ $N H_{3}$ $>$ $HCl$ $>$ $C O_{2}$ $>$ $C H_{4}$ $>$ $N_{2}$ $>$ $H_{2}$ .
★ Must learn Isobars: physisorption falls continuously with $T$ ; chemisorption first rises (activation energy) and then falls.
★ Must learn Freundlich isotherm: $\frac{x}{m} = k p^{1/ n}$ with $n > 1$ ; from solution $\frac{x}{m} = k C^{1/ n}$ .
★ Must learn Log form: $lo g \frac{x}{m} = lo g k + \frac{1}{n} lo g p$ . Slope $= \frac{1}{n}$ , intercept $= lo g k$ .
Limits: $\frac{1}{n} = 1 \Rightarrow \frac{x}{m} \propto p$ (low $p$ ); $\frac{1}{n} = 0 \Rightarrow \frac{x}{m}$ constant (high $p$ ); usually $\frac{1}{n} = 0.1$ - $0.5$ .
Mass adsorbed from solution: $x = (C_{i} - C_{f}) \times V \times M$ (molar mass of the solute).

1. What Adsorption Is

Adsorption: the accumulation of molecular species at the surface rather than in the bulk of a solid or liquid. The substance that collects is the adsorbate; the surface it collects on is the adsorbent. Removing adsorbate from the surface is desorption.

Good adsorbents have a large surface area per gram: activated charcoal, silica gel, alumina gel, clay, colloids and finely divided metals. Four observations show adsorption at work:

Powdered charcoal in a closed vessel of $O_{2}$ , $H_{2}$ , CO, $C l_{2}$ , $N H_{3}$ or $S O_{2}$ lowers the gas pressure: gas molecules collect on the charcoal.
Methylene blue solution shaken with animal charcoal gives a colourless filtrate: the dye sits on the charcoal surface.
Raw sugar solution passed over animal charcoal comes out colourless: the colouring matter is adsorbed.
Silica gel keeps air dry: water molecules are adsorbed on the gel.

1.1 Adsorption vs absorption

In adsorption the substance stays on the surface; in absorption it spreads uniformly through the bulk. A chalk stick dipped in ink shows both: the coloured dye is adsorbed on the surface, while the solvent is absorbed deep inside, so the broken stick is white within. When both happen together, the process is called sorption.

Figure 1: Adsorption is a surface effect (concentration high only at the surface); absorption is a bulk effect (uniform concentration). Silica gel adsorbs water vapour, anhydrous

CaC l_{2}

absorbs it.

Adsorption (surface)

Concentration high only at the surface. Fast at first, then slows as the surface fills. Water vapour on silica gel.

Absorption (bulk)

Concentration the same throughout. Proceeds at a uniform rate. Water vapour in anhydrous $CaC l_{2}$ .

Exam Trick AD sits at the Door, AB goes into the Body. Silica gel adsorbs water (Door); anhydrous

CaC l_{2}

absorbs it (Body).

2. Why Surfaces Adsorb

A particle inside a solid is pulled equally from all sides, so the forces on it cancel. A particle in the surface has no neighbours on the outside, so part of its attraction is unused. These residual (unbalanced) forces pull adsorbate molecules onto the surface. The more surface per gram, the more residual force is available, so extent of adsorption rises with surface area.

Figure 2: Surface particles have unbalanced (residual) forces; satisfying them lowers the surface energy, so adsorption always releases heat (

Δ H < 0

). More surface per gram means more residual force to use.

2.1 The thermodynamics in three signs

$Δ H < 0$ : when adsorbate binds, residual forces are satisfied and surface energy falls. The energy lost appears as heat, so adsorption is always exothermic.
$Δ S < 0$ : a gas molecule held on a surface loses freedom of movement.
$Δ G < 0$ : with $Δ S$ negative, $- T Δ S$ is positive. So $Δ G = Δ H - T Δ S$ is negative only because $Δ H$ is negative enough.
Equilibrium: as the surface fills, $Δ H$ becomes less negative. When $Δ H = T Δ S$ , $Δ G = 0$ and adsorption stops (adsorption equilibrium).

Figure 3: Both

Δ H

and

Δ S

are negative, so adsorption is spontaneous only while

∣Δ H ∣ > T ∣Δ S ∣

. As the surface fills,

∣Δ H ∣

falls; at

Δ H = T Δ S

Δ G = 0

and adsorption equilibrium is reached.

Exam Trick Three minus signs. For adsorption,

Δ H

Δ S

and

Δ G

are all negative. Any option with a positive

Δ H

Δ S

for adsorption is wrong.

Key idea

Unbalanced surface forces make adsorption exothermic; entropy loss makes it stop when

Δ H = T Δ S

Quick Recall: tap to check

Water vapour and silica gel: adsorption or absorption?

Adsorption. Anhydrous

CaC l_{2}

absorbs water vapour.

Why is

Δ S

negative for adsorption of a gas?

The gas molecules lose freedom of movement when held on the surface.

Why does a finely divided solid adsorb more?

More surface area per gram, so more residual surface force to hold adsorbate.

3. Physisorption and Chemisorption

If the adsorbate is held by weak van der Waals forces, the process is physical adsorption (physisorption). If it is held by chemical bonds (covalent or ionic), it is chemical adsorption (chemisorption). Chemisorption needs a high activation energy, so it is also called activated adsorption.

The two can occur together, and one can turn into the other. $H_{2}$ is first physisorbed on nickel at low temperature; on heating, the molecules dissociate and the H atoms are held by chemisorption.

Property	Physisorption	Chemisorption
Force	van der Waals	chemical bond
Specificity	not specific (vdW forces are universal)	highly specific: needs a possible bond (O on metals as oxide, H on transition metals as hydride)
Reversibility	reversible	usually irreversible
Enthalpy	low, $20$ - $40$ kJ mol $^{- 1}$	high, $80$ - $240$ kJ mol $^{- 1}$
Activation energy	negligible	high (activated adsorption)
Effect of temperature	favoured by low $T$ , falls as $T$ rises	rises with $T$ at first, then falls
Effect of pressure	rises with pressure	high pressure also favourable
Layer	multimolecular (at high pressure)	unimolecular (monolayer)
Nature of gas	easily liquefiable gases (high $T_{c}$ ) adsorb more	gases that can react with the surface
Surface area	increases with surface area	increases with surface area

The energy picture explains the table. A molecule falls into a shallow physisorption well without any barrier; to reach the deep chemisorption well it must first climb an activation barrier.

Figure 4: Physisorption is a shallow well far from the surface with no barrier; chemisorption is a deep well close to the surface behind an activation barrier (here about 31 kJ/mol above the gas). That barrier is why chemisorption is called activated adsorption.

Key idea

Physisorption: weak, multilayer, reversible, no barrier. Chemisorption: a bond, monolayer, specific, activated.

4. Factors Affecting Adsorption of Gases on Solids

4.1 Nature of the gas

For physisorption, gases that liquefy easily (higher critical temperature $T_{c}$ ) have stronger van der Waals forces and are adsorbed more. So 1 g of activated charcoal adsorbs more $S O_{2}$ than $C H_{4}$ , and far more $C H_{4}$ than $H_{2}$ .

Figure 5: Order of physisorption on charcoal at the same

T

and

p

follows

T_{c}

S O_{2} > N H_{3} > HCl > C O_{2} > C H_{4} > N_{2} > H_{2}

. Hydrogen (

T_{c} = 33

K) is adsorbed least.

4.2 Surface area and activation of the adsorbent

Extent of adsorption rises with surface area per gram, which is why finely divided metals and porous solids are good adsorbents. Activating an adsorbent means raising its adsorbing power by creating more free surface:

breaking it into fine particles (more area per gram);
making the surface rough or porous;
heating it in vacuum or superheated steam (charcoal at about 623-1273 K) to drive out gases already adsorbed in its pores.

4.3 Temperature: adsorption isobars

An adsorption isobar plots $x / m$ against temperature at constant pressure. Physisorption is an exothermic equilibrium, Solid + Gas ⇌ Gas/Solid + heat, so raising $T$ shifts it back (Le Chatelier) and $x / m$ falls all the way. Chemisorption first rises with $T$ , because heat supplies the activation energy for bond formation, and only then falls.

Figure 6: Adsorption isobars. Physisorption falls steadily with

T

; chemisorption first rises (activation energy supplied) and then falls (exothermic). A rising part on a graph is the signature of chemisorption.

4.4 Pressure

At constant temperature, raising the pressure pushes the equilibrium towards the adsorbed state, so $x / m$ increases, first sharply and then more slowly until the surface is saturated. Lowering the pressure removes physisorbed gas. This dependence is the adsorption isotherm (Section 5).

Exam Trick Physi slides, chemi climbs a hill. On an

x / m

T

graph, a curve that only falls is physisorption; a curve with a hump is chemisorption.

Key idea

More adsorption with: higher

T_{c}

of the gas, more surface area, higher pressure, and lower temperature (for physisorption).

Quick Recall: tap to check

Arrange by extent of adsorption on charcoal:

C H_{4}

H_{2}

S O_{2}

N_{2}

S O_{2}

>

C H_{4}

>

N_{2}

>

H_{2}

(order of critical temperature).

Why does chemisorption increase with temperature at first?

Heat supplies the activation energy needed to form the surface bond.

Enthalpy ranges of the two types?

Physisorption

20

40

kJ mol

^{- 1}

; chemisorption

80

240

kJ mol

^{- 1}

5. Freundlich Adsorption Isotherm

An adsorption isotherm is the curve of mass of gas adsorbed per gram of adsorbent ( $x / m$ ) against pressure at a constant temperature. Real isotherms rise steeply at low pressure, bend, and level off at high pressure. At any fixed pressure, less gas is physisorbed at a higher temperature.

Figure 7: Isotherms rise linearly at low

p

, bend in the middle (Freundlich region) and level off at high

p

. At a fixed pressure, less gas is physisorbed at higher

T

(195 K > 244 K > 273 K).

In 1909 Freundlich gave an empirical equation for the middle part of the curve:

\frac{x}{m} = k p^{1/ n} (n > 1)

Here $x$ is the mass of gas adsorbed on mass $m$ of adsorbent at pressure $p$ ; $k$ and $n$ are constants for a given gas, adsorbent and temperature. Taking logarithms gives a straight line:

lo g \frac{x}{m} = lo g k + \frac{1}{n} lo g p

So a plot of $lo g (x / m)$ against $lo g p$ is linear if the isotherm holds. Its slope is $1/ n$ and its intercept is $lo g k$ .

Figure 8:

lo g \frac{x}{m} = lo g k + \frac{1}{n} lo g p

is a straight line. Here slope

= 0.5

and intercept

= 0.301

, so

k = 2

n = 2

and

\frac{x}{m} = 4

p = 4

atm.

5.1 The two limits of $1/ n$

$1/ n = 1$ : $x / m = k p$ , adsorption directly proportional to pressure (true at low pressure).
$1/ n = 0$ : $x / m = k$ , adsorption independent of pressure (the surface is saturated at high pressure).
Between them, $0 < 1/ n < 1$ , usually $0.1$ to $0.5$ . The equation therefore holds only over a limited pressure range.

Figure 9:

1/ n = 1

gives

x / m \propto p

(low pressure) and

1/ n = 0

gives

x / m

independent of

p

(saturation). Freundlich works between these limits, so it fails at high pressure where real isotherms flatten.

Freundlich isotherm fails at high pressure: real isotherms level off (saturation), but

k p^{1/ n}

keeps rising. It is empirical, with no theoretical basis.

JEE Advanced Langmuir isotherm (monolayer model). Langmuir assumed a uniform surface of equivalent sites, one molecule per site (monolayer) and no interaction between adsorbed molecules. Balancing the rates of adsorption and desorption gives the fraction of surface covered

θ = \frac{K p}{1 + K p}

, or

\frac{x}{m} = \frac{a p}{1 + b p}

. At low

p

(

b p ≪ 1

x / m \approx a p

(first order in

p

); at high

p

(

b p ≫ 1

x / m \approx a / b

(saturation, zero order). Its linear form

\frac{p}{x / m} = \frac{1}{a} + \frac{b}{a} p

gives slope

b / a

. Unlike Freundlich, it explains saturation, which is why the curves in the isotherm figure above level off.

5.2 Solving any adsorption question

Figure 10: One route for every adsorption question: decide the type first, then apply the temperature rule, then the Freundlich equation for pressure.

Key idea

Freundlich:

lo g (x / m)

lo g p

is a line with slope $1/ n$ and intercept $lo g k$ ; valid between

1/ n = 0

and

1

, fails at high

p

6. Adsorption from Solution

Solids adsorb solutes from solution too. Acetic acid shaken with charcoal loses part of its acid to the charcoal. Litmus solution shaken with charcoal turns colourless. $Mg (OH)_{2}$ precipitated in the presence of magneson reagent turns blue because it adsorbs the dye. The same rules apply:

Extent of adsorption decreases as temperature rises.
It increases with the surface area of the adsorbent.
It depends on the concentration of the solute and on the nature of both adsorbent and adsorbate.

The mechanism is not fully known, but Freundlich's equation works with concentration in place of pressure ( $C$ = equilibrium concentration):

\frac{x}{m} = k C^{1/ n}, lo g \frac{x}{m} = lo g k + \frac{1}{n} lo g C

To test it, equal volumes of acetic acid of different concentrations are shaken with equal masses of charcoal. Titrating each flask before and after gives $x$ from the fall in concentration; a straight line of $lo g (x / m)$ against $lo g C$ confirms the isotherm.

7. Applications of Adsorption

Application	Adsorbent	What is adsorbed / how it works
Very high vacuum	charcoal	last traces of air in a pumped vessel
Gas masks (coal mines)	activated charcoal or mixed adsorbents	poisonous gases
Humidity control	silica gel, alumina gel	moisture
Decolourising solutions (sugar)	animal charcoal	coloured impurities
Heterogeneous catalysis	Fe (Haber), Pt or $V_{2} O_{5}$ (contact process), Ni (hydrogenation of oils)	reactants concentrate on the catalyst surface
Separation of noble gases	coconut charcoal	gases adsorb to different extents at different temperatures
Curing diseases	drugs	adsorb on germs and kill them
Froth flotation	pine oil + frothing agent	sulphide ore particles are wetted by oil and float with the froth; silica and earthy matter sink
Adsorption indicators	AgX precipitate	dyes such as eosin and fluorescein adsorb at the end point and give a colour
Chromatography	stationary phase (silica, alumina, paper)	components adsorb to different extents and separate

Quick Recall: tap to check

Slope and intercept of the Freundlich log plot?

Slope

= 1/ n

; intercept

= lo g k

Why does Freundlich fail at high pressure?

Real isotherms reach saturation (

x / m

constant), but

k p^{1/ n}

keeps increasing.

Which adsorbent separates noble gases?

Coconut charcoal, at different temperatures.

7.1 The whole concept at a glance

Figure 11: The whole concept on one page. Revise from the centre outwards.

8. Solved Examples

Solved Example 1

Why does physisorption decrease with an increase of temperature? (NCERT Intext 5.2)

Solution:

Physisorption is an exothermic equilibrium: Solid + Gas ⇌ Gas/Solid + heat. By Le Chatelier's principle, raising the temperature shifts it backwards, so gas is desorbed. Also, the weak van der Waals forces cannot hold molecules that have more kinetic energy.

Solved Example 2

Why are powdered substances more effective adsorbents than their crystalline forms? (NCERT Intext 5.3)

Solution:

Adsorption is a surface effect, and powdering increases the surface area per gram. A 1 cm cube has 6 cm $^{2}$ of surface. Cut into 1 mm cubes, it gives $1000$ cubes of $6$ mm $^{2}$ each $= 6000$ mm $^{2} = 60$ cm $^{2}$ , ten times more area for the same mass.

Solved Example 3

A graph of

lo g (x / m)

against

lo g p

for adsorption of a gas on a solid is a straight line with slope

0.5

and intercept

0.3010

. The value of

x / m

p = 4

atm is
(A) 2
(B) 4
(C) 8
(D) 1

Solution:

Answer: (B). Intercept $lo g k = 0.3010$ , so $k = 2$ ; slope $1/ n = 0.5$ .

\frac{x}{m} = k p^{1/ n} = 2 \times 4^{0.5} = 2 \times 2 = 4

Solved Example 4

For a gas obeying the Freundlich isotherm,

x / m

doubles when the pressure is increased 16 times at constant temperature. Find

1/ n

and

n

Solution:

\frac{( x / m ) _{2}}{( x / m ) _{1}} = (\frac{p _{2}}{p _{1}})^{1/ n} \Rightarrow 2 = 1 6^{1/ n} = 2^{4/ n}

So $4/ n = 1$ , giving $n = 4$ and $1/ n = 0.25$ .

Solved Example 5

100 mL of 0.10 M acetic acid is shaken with 2.0 g of activated charcoal. After adsorption the concentration of the acid is 0.08 M. Find the mass of acetic acid adsorbed per gram of charcoal.

Solution:

Moles adsorbed $= (0.10 - 0.08) \times 0.100 = 2.0 \times 1 0^{- 3}$ mol.

Molar mass of $C H_{3} COOH$ is 60 g mol $^{- 1}$ , so mass adsorbed $x = 2.0 \times 1 0^{- 3} \times 60 = 0.12$ g.

$x / m = \frac{0.12}{2.0} = 0.060$ g of acid per g of charcoal.

Solved Example 6

Under the same conditions, which gas is adsorbed to the greatest extent by 1 g of activated charcoal?
(A)

H_{2}

(B)

C H_{4}

(C)

N_{2}

(D)

S O_{2}

Solution:

Answer: (D). Physisorption increases with the ease of liquefaction. $S O_{2}$ has the highest critical temperature (431 K) of the four, $H_{2}$ the lowest (33 K).

Solved Example 7

Which statement about chemisorption is incorrect?
(A) It is highly specific
(B) It forms a unimolecular layer
(C) Its enthalpy is about 80-240 kJ mol

^{- 1}

(D) It decreases continuously as temperature rises

Solution:

Answer: (D). Chemisorption first increases with temperature, because heat supplies the activation energy for bond formation, and falls only at higher temperature. A continuous fall is the behaviour of physisorption.

Practice Questions

Write any two characteristics of chemisorption. (NCERT Intext 5.1)Answer: Any two: highly specific; usually irreversible; high enthalpy (80-240 kJ mol $^{- 1}$ ); needs activation energy; forms a monolayer.
Distinguish between adsorption and absorption, with one example each.Answer: Adsorption: on the surface only (water vapour on silica gel). Absorption: through the bulk (water vapour in anhydrous $CaC l_{2}$ ).
What is the difference between physisorption and chemisorption?Answer: Physisorption: van der Waals forces, reversible, not specific, 20-40 kJ mol $^{- 1}$ , multilayer. Chemisorption: chemical bonds, irreversible, specific, 80-240 kJ mol $^{- 1}$ , monolayer.
What are the factors that influence the adsorption of a gas on a solid?Answer: Nature of the gas (critical temperature), nature and surface area of the adsorbent, activation of the adsorbent, pressure and temperature.
What is an adsorption isotherm? Write the Freundlich isotherm.Answer: Plot of $x / m$ against $p$ at constant $T$ . Freundlich: $x / m = k p^{1/ n}$ ( $n > 1$ ), or $lo g (x / m) = lo g k + \frac{1}{n} lo g p$ .
What is meant by activation of an adsorbent? How is it done?Answer: Increasing its adsorbing power by increasing free surface: fine division, roughening, or heating in vacuum or steam to clear the pores.
Why is adsorption always exothermic?Answer: Binding satisfies residual surface forces and lowers surface energy, released as heat. Also $Δ S < 0$ , so $Δ G < 0$ needs $Δ H < 0$ .
Discuss the effect of pressure and temperature on the adsorption of gases on solids.Answer: $x / m$ rises with $p$ until saturation. Physisorption falls as $T$ rises; chemisorption first rises, then falls.

Common Mistakes to Avoid

Watch out

Treating adsorption and absorption as the same. Silica gel adsorbs water vapour; anhydrous $CaC l_{2}$ absorbs it.
Calling adsorption endothermic because chemisorption rises with temperature at first. That rise is the activation energy being supplied; $Δ H$ is still negative.
Writing $Δ S > 0$ for adsorption. The adsorbed gas loses freedom, so $Δ S < 0$ .
Reversing the critical temperature rule. Gases with higher $T_{c}$ ( $S O_{2}$ , $N H_{3}$ ) are physisorbed more; $H_{2}$ least.
Reading the slope of the Freundlich log plot as $n$ , or the intercept as $k$ . Slope $= 1/ n$ ; intercept $= lo g k$ .
Writing $n < 1$ . In $x / m = k p^{1/ n}$ , $n > 1$ , so $0 < 1/ n < 1$ .
Saying Freundlich explains saturation. It fails at high pressure; the Langmuir isotherm explains saturation.
Swapping the layers: physisorption can be multilayer; chemisorption is a monolayer.

Frequently Asked Questions

What is adsorption in chemistry?

Adsorption is the accumulation of gas or solute molecules on the surface of a solid or liquid rather than in its bulk. The substance that collects is the adsorbate and the surface is the adsorbent. Examples are water vapour on silica gel, dyes on charcoal and gases on finely divided metals.

Why is adsorption always exothermic?

Surface particles have unbalanced residual forces. When adsorbate binds, these forces are satisfied and surface energy falls, and this energy is released as heat. Also, adsorption lowers entropy, so Gibbs energy can fall only if the enthalpy change is negative. Hence enthalpy of adsorption is always negative.

What is the difference between physisorption and chemisorption?

Physisorption is due to weak van der Waals forces, is reversible, not specific, releases 20 to 40 kJ per mol and can form multilayers. Chemisorption involves chemical bonds, is specific and usually irreversible, releases 80 to 240 kJ per mol, needs activation energy and forms a monolayer.

Why does chemisorption first increase and then decrease with temperature?

Chemisorption needs activation energy to form bonds with the surface. At low temperature few molecules have this energy, so raising the temperature increases adsorption. Once bonding is fast, the exothermic nature takes over and further heating decreases adsorption, giving a hump-shaped isobar.

What does the Freundlich adsorption isotherm state?

It states that the mass of gas adsorbed per gram of adsorbent, x/m, equals k times p raised to 1/n, with n greater than 1, at constant temperature. A plot of log x/m against log p is a straight line with slope 1/n and intercept log k. It fails at high pressure.

Why are easily liquefiable gases adsorbed more readily?

Easily liquefiable gases have higher critical temperatures and stronger van der Waals forces between molecules, and the same forces hold them on the adsorbent surface. So sulphur dioxide and ammonia are physisorbed on charcoal far more than hydrogen or nitrogen.

Is adsorption in the JEE Main and NEET syllabus?

No. Surface Chemistry, which includes adsorption, has been removed from the JEE Main and NEET syllabi after NCERT dropped the chapter from the rationalised Class 12 textbook. It remains in JEE Advanced, which lists physisorption, chemisorption and the Freundlich adsorption isotherm, so JEE Advanced aspirants still need this topic.

What is asked from adsorption in JEE Advanced?

The JEE Advanced syllabus lists physisorption, chemisorption and the Freundlich adsorption isotherm. Questions test isobars for both types, slope and intercept of the log plot, limits of 1/n, thermodynamic signs and, in multi-correct questions, comparisons with the Langmuir monolayer model.

Previous year questions on Adsorption

4 questions from past papers, each with a step-by-step solution.

Ready to master Surface Chemistry?

Take a full mock test, practice concept-by-concept, and get an AI-powered rank prediction — all on Fundamenthol.

Practice questions →Take a mock test

1. What Adsorption Is

1.1 Adsorption vs absorption

2. Why Surfaces Adsorb

2.1 The thermodynamics in three signs

3. Physisorption and Chemisorption

4. Factors Affecting Adsorption of Gases on Solids

4.1 Nature of the gas

4.2 Surface area and activation of the adsorbent

4.3 Temperature: adsorption isobars

4.4 Pressure

5. Freundlich Adsorption Isotherm

5.1 The two limits of 1/n

5.2 Solving any adsorption question

6. Adsorption from Solution

7. Applications of Adsorption

7.1 The whole concept at a glance

8. Solved Examples

Common Mistakes to Avoid

Frequently Asked Questions

What is adsorption in chemistry?

Why is adsorption always exothermic?

What is the difference between physisorption and chemisorption?

Why does chemisorption first increase and then decrease with temperature?

What does the Freundlich adsorption isotherm state?

Why are easily liquefiable gases adsorbed more readily?

Is adsorption in the JEE Main and NEET syllabus?

What is asked from adsorption in JEE Advanced?

Previous year questions on Adsorption

Ready to master Surface Chemistry?

5.1 The two limits of $1/ n$