Surface Chemistry

Introduction to Surface Chemistry

“The branch of physical chemistry which deals with the nature of surfaces and the chemical and physical processes which take place on the surfaces.”

Studies adsorption, catalysis, and colloidal properties.

Adsorption

Phenomenon of attracting and retaining molecules of a substance on the surface of a liquid or solid, leading to higher concentration on the surface.

Basic Terms

Term	Description
Interface	Plane separating two phases.
Adsorbate	Substance adsorbed (e.g., gas on solid).
Adsorbent	Surface where adsorption occurs.
Desorption	Removal of adsorbed substance.
Absorption	Uniform distribution throughout solid/liquid.
Sorption	Adsorption + Absorption simultaneously.
Occlusion	Adsorption of gases on metals.

Adsorption vs Absorption

Adsorption	Absorption
Surface phenomenon	Bulk phenomenon
Concentration higher on surface	Uniform distribution
Rapid initially, slows down	Uniform rate
e.g., NH₃ on charcoal	e.g., NH₃ in water

Causes of Adsorption

Unbalanced forces (van der Waals, chemical bonds) at surface attract and retain molecules.

Example: NH₃ adsorbed on charcoal, absorbed in water.

Reversible vs Irreversible

Reversible

Easily removed by physical methods (e.g., gas from solid in vacuum)

Irreversible

Not easily removed (e.g., O₂ on tungsten)

Characteristics

Higher Concentration

At surface

Exothermic

ΔH negative

Decreases Entropy

ΔS negative

Spontaneous

ΔG negative

Classification of Adsorption

By Concentration

Positive Adsorption

More on surface than bulk (e.g., conc. KCl on blood charcoal)

Negative Adsorption

Less on surface than bulk (e.g., dil. KCl on blood charcoal)

By Nature

Physical (Physisorption)

Van der Waals forces, reversible, low ΔH (20-40 kJ/mol)

Chemical (Chemisorption)

Chemical bonds, irreversible, high ΔH (50-400 kJ/mol), specific

Physisorption	Chemisorption
Low heat (20-40 kJ/mol)	High heat (50-400 kJ/mol)
Van der Waals forces	Chemical bonds
Reversible	Irreversible
At low temp, decreases with temp	At high temp
Related to gas liquefaction	Not related
Multimolecular layers	Monomolecular layers
No activation energy	High activation energy
High pressure favorable	High pressure favorable, no desorption on decrease
Not specific	Highly specific

Factors Affecting Adsorption

Nature of Adsorbate/Adsorbent

Easily liquefiable gases adsorb more (CO₂ > H₂)

Porous solids adsorb more

Surface Area

Larger area, more adsorption

Smaller particles, larger area

Pressure

Increases adsorption

At high pressure, reaches limit

Temperature

Decreases with rise (exothermic)

Chemisorption increases initially

Adsorption Isotherms

Relation between pressure (p) and extent of adsorption (x/m) at fixed temperature.

Freundlich Isotherm

x/m = k p^{1/n} (n > 1)
log(x/m) = log k + (1/n) log p

At low p: x/m ∝ p

At high p: x/m constant

At intermediate p: x/m ∝ p^{1/n}

Langmuir Isotherm

Assumes unimolecular layer, no interaction between molecules.

x/m = (a p) / (1 + b p)
1/(x/m) = (1/a) + (b/a) (1/p)

At low p: x/m ∝ p

At high p: x/m constant (saturation)

Adsorption Isobars

Extent vs temperature at constant pressure.

Physisorption: Decreases with temperature.

Chemisorption: Increases initially, then decreases.

Applications of Adsorption

Gas Masks

Activated charcoal adsorbs poisonous gases

Catalysis

Reactants adsorb on catalyst surface

Water Softening

Ion exchange resins

Chromatography

Separation based on adsorption

Medicine

Activated charcoal removes toxins

Humidity Control

Silica gel adsorbs moisture

Important Points to Remember

Key JEE Points

Adsorption: Surface, exothermic, ΔG negative
Physisorption: Weak, reversible, multimolecular
Chemisorption: Strong, irreversible, monomolecular
Freundlich: x/m = k p^{1/n}
Langmuir: Assumes monolayer, saturation
Increases with pressure, decreases with temperature (physisorption)
Larger surface area, more adsorption
Easily liquefiable gases adsorb more