Surface Chemistry | Unit 5 | Chemistry

Surface chemistry is the area of chemistry that examines the physical and chemical processes that take place on surfaces.


Adsorption is the term used to describe the process where a substance accumulates on the surface rather than in the inside of a solid or liquid.
The fact that unbalanced forces are present on the surfaces of solids and liquids causes this surface phenomena, which it is.
In the inside of solids and liquids, molecular forces are balanced, but they are not perfectly balanced on the surface.


Absorption is the process by which adsorbate diffuses throughout the lattice of the adsorbent and permeates the bulk of the adsorbent.

Difference between Adsorption and Absorption
How Adsorption Works

The surfaces of solids and liquids often attract the atoms, molecules, or ions of other substances with which they come into contact in order to balance out their uneven molecular forces.
These particles and the surface interact through some physical and chemical interaction.
As the particles build up on the surface, the surface has a higher concentration of them than the inside of the solid or liquid.
Adsorption is the term for the process whereby residual surface forces affect the concentration at the surface.
Adsorption happens on its own when temperature and pressure are constant.
As a spontaneous process, adsorption results in a negative change in free energy (G).

  • △G= △H – T△S

Adsorption is always exothermic.

Adsorption Types

Adsorption of gases on solids occurs in two ways:

  • Physisorption
  • Chemisorption

Adsorption is referred to as physical adsorption when the adsorbate is kept on a surface of an adsorbent by weak van der Waals forces.


Chemisorption Adsorption is known as chemisorption or chemical adsorption when the gas molecules or atoms are attached to the solid surface by chemical bonding.

Adsorption’s Physical Characteristics
  • A lack of particularity.
  • Nature is reversible.
  • The heat of adsorption is just 20–40 KJ/mol, which is very low. 
  • Adsorbate Nature

The quantity of gas a material can absorb varies on the type of gas.

Because van der Waals’ forces are strong close to the critical temperature, gases with high critical temperatures are easily adsorbed.

Difference between Physical Adsorption and Chemical Adsorption
Adsorption Isotherm

A curve known as the adsorption isotherm can be used to express the variation in the amount of gas absorbed by the adsorbent with pressure at constant temperature.
To describe the degree of adsorption, various forms of adsorption isotherms are used.
The Freundlich isotherm is a mathematical model used to describe the adsorption of solutes onto solid surfaces. It relates the equilibrium concentration of adsorbate to the adsorbent’s surface coverage.
Freundlich provided an empirical relationship between pressure at a specific temperature and the amount of gas a unit mass of adsorbent can absorb.
The connection is described as x=k.P 1/n (n>1) x= mass of gas adsorbed m= mass of adsorbent P= Pressure

The behavior of gas adsorption on an adsorbent can be described by the Freundlich isotherm. The constants k and n in the equation determine the nature of the gas and vary with temperature. The curves obtained from the isotherm demonstrate that, at a constant pressure, the amount of gas adsorbed per gram of the adsorbent decreases as the temperature increases. This suggests that higher temperatures reduce the affinity between the gas molecules and the adsorbent surface. Furthermore, as the pressure increases, the curve approaches a saturation point where further increases in pressure do not significantly increase the amount of gas adsorbed. This indicates that the adsorbent surface becomes saturated with the gas molecules at high pressures. Overall, the Freundlich isotherm provides insights into the temperature and pressure dependence of gas adsorption on an adsorbent.

Adsorption From the Phase of Solution

Solute from solution is absorbed by solids.

A portion of the acetic acid is absorbed by the charcoal when it is shaken with a solution of acetic acid in water.

In a solution, the acid’s concentration falls.

When the litmus solution is shaken with charcoal, the colour disappears.

When precipitated with Magneson Reagent present, the Mg(OH) 2 precipitate turns blue. The adsorption of Magneson is what gives the colour.

Observation of Adsorption from the Solution Phase

As temperature increases, so does the amount of adsorption.

As the adsorbent’s surface area grows, so does the extent of adsorption.

The concentration of the solute in solution determines the extent of adsorption.


A catalyst is a material that modifies the rate of a chemical reaction without being consumed by the reaction itself.
The event is referred to as catalysis.
By reducing the reactant’s activation energy, the catalyst speeds up the process.
Promoter the elements that make the catalyst more active. For instance, in the synthesis of ammonia, Mo is used as a promoter in addition to finely divided Fe as a catalyst. Poison 2N2+3H2Fe/MM 2NH3 the chemical that lowers a catalyst’s activity. Different Catalysts

  • Homogeneous catalysis
  • Heterogeneous catalysis
  • Homogeneous catalysis

catalysts and reactants are both present in the same phase during catalysis.

Theory of Heterogeneous catalysis
1. Shape Selective Catalysis by Zeolites

Zeolites exhibit shape-selective catalysis due to their unique crystalline structure and pore system. The precise arrangement of channels and cavities in zeolites allows for the preferential adsorption and reaction of molecules based on their size and shape, enabling highly selective catalytic transformations.

Aluminosilicates with micropores are zeolites. They could be manufactured or natural.

Their honey-comb-like structure makes them effective shape-selective catalysts.

They have a three-dimensional network of silicates called the Al-O-Si network, which is formed when some silicon atoms are swapped out for aluminium atoms.

The shape selectivity of the zeolite catalysis is its most significant characteristic.

The size of the pores and cavities of zeolite determines its catalytic activity.

Reactant molecules that are larger than the catalyst’s cavities and pores cannot penetrate the catalyst’s pores and are unable to be adsorbed as a result. Zeolite consequently functions as a selective adsorbent.

2. Catalysis by Enzymes

Enzyme catalysis relies on various factors for efficient and specific reactions. These factors include substrate binding, where enzymes recognize and bind specifically to their substrates.

Transition state stabilization lowers the activation energy by binding to the transition state, enabling the reaction to proceed. Proximity and orientation effects bring substrates together in the correct orientation, increasing the likelihood of successful collisions. Catalytic groups within the enzyme’s active site participate directly in the reaction by donating or accepting protons, electrons, or functional groups. Optimal pH and temperature maintain the enzyme’s highest catalytic activity.

Activators and coenzymes enhance enzyme activity and facilitate the catalytic process. Finally, the induced fit model describes the conformational changes in the enzyme’s active site upon substrate binding. These factors collectively enable enzymes to accelerate biochemical reactions, ensuring the efficient functioning of biological systems.

3. Industry Uses of Catalysts

The Haber process, which produces ammonia, uses finely divided iron as a catalyst.

N 2 + 3 H2 + 2 NH 3 (g)

Asbestos that has been platinumed serves as a catalyst in Ostwald’s method for producing nitric acid.

NH 3 + 5 O 2 2NO 2 (g) + 6 H 2 O (g) 4HNO 3 = 2NO(gg+(g)2NO 2 4NO 3 (g)+2H 2 O(l)+O 2 (aq)

Use of the contact process


Colloids In a heterogeneous system known as a colloid, one component is disseminated (or the distributed phase) in another substance, which is known as the dispersion medium.

Colloids are categorised according to:

  • Physical condition:- There are eight different types of colloidal systems that might exist depending on the physical condition of the dispersed phase and the dispersion medium.
Particle kinds

 Colloids can be categorised into three groups based on the sorts of particles they contain:

  • Multi-molecular Colloid:- These are collections of many atoms or smaller molecules. Its dimensions fall between 1 and 1000 nm.

The term “macromolecular colloids” refers to particles that are similar to colloidal particles that are created when macromolecules are dispersed in appropriate solvents.

Colloid(Micelle) Associated With They function as potent electrolytes at low concentrations, but at high quantities, they aggregate or form micelles.

Micelles form above a specific concentration known as the critical micelle concentration and at a specific temperature known as Kraft’s temperature. A soap solution example is used to describe the mechanism of micelle production.

Long chain fatty acid salts in the form of sodium and potassium make up soap.

The Cleaning Effect of Soap

Our clothing is covered in oily dirt.

A micelle is created by the addition of soap around the oil droplet.

Water and the polar groups in soap molecules interact. The oil droplet encircled by the stearate ions is drawn into the water when the cloth is stirred, leaving the cloth clean.

Colloids Preparation

There are three ways to make colloids:

  1. Chemical Approach
  2. Using Bredig’s Arc
  3. Peptization
Chemical Technique

It involves oxidation, reduction, hydrolysis, and double decomposition.

By using the twofold decomposition approach, the sols of insoluble inorganic salts like arsenious sulphide and silver halide can be created as

2O3 +3H 2s Double-decomposition

As 2S3 (sol)+3H2O

The Bredig’s Arc Method is used to create metal sols such as gold, silver, and platinum. It is sometimes referred to as the Electrical Disintegration Method.

In this technique, metal electrodes that are submerged in the dispersion media are connected by an electric arc.

High voltage current is passed, creating an electric arc.

The metal is vaporised by the tremendous heat generated, which causes the metal to condense into colloidal-sized particles.

Read Me : Chemistry Class 12 Chapter 4 – Chemical kinetics


Peptization is the process of transforming a precipitate into a colloidal sol by agitating it with a dispersion medium while an electrolyte solution is present in minute amounts.

Peptizing agent is the name given to the electrolyte utilised.

The precipitate adsorbs one of the electrolyte’s ions on its surface during peptization.

Electrolyte impurities in colloidal solutions, consisting of dissolved salts or other ions, can destabilize the system. Techniques like dialysis, ultrafiltration, or precipitation methods can remove these impurities, ensuring stability and preserving the desired properties of the colloidal solution.

The electrolyte stabilises the sol if it is present in tiny concentrations.

However, the sol becomes unstable if the electrolyte is present in high concentrations.

Three techniques can be used to purify colloids:

  1. Dialysis
  2. Electrodialysis
  3. Ultrafiltration
1. Dialysis

Dialysis is the technique of removing dissolved materials from colloidal solutions by diffusion across an appropriate membrane, such as parchment paper.

Its foundation is the fact that a membrane made of parchment paper or cellophane cannot let colloidal particles through.

Electrolyte ions can travel through a membrane made of parchment paper or cellophane.

Purified colloidal solution is left behind as the contaminants gradually diffuse out of the bag.

2. Electrodialysis

Applying an electric field helps speed up the laborious process of dialysis.

When the impurities are electrolytes, the method is effective.

3. Ultrafiltration

Using specialised filters known as ultra-filter sheets, this procedure enables the separation of colloidal particles from the solvent and the soluble contaminants.

The collodion solution is applied to filter paper to create them. Colloidal particles cannot pass through this filter paper. 

Colloids’ Properties
1. Collaborative Qualities

Because they are aggregates, the particles in a colloidal solution are less numerous than they would be in a pure solution.

Because of this, a colloidal solution’s collative properties are of lower order than those of real solutions.

2. Tyndall Effect

When a light beam passes through a colloidal particle, it scatters. The tyndall effect is the name of this phenomenon.

The Tyndall effect can only be observed under the following circumstances:

The dispersed particles’ diameter and the light’s wavelength are nearly equal in size.

The dispersion medium’s and the dispersed phase’s refractive indices are very different from one another.

3. Color

The wavelength of light scattered by the dispersed particles determines the colour of the colloidal solution.

The type and size of the particle affect the wavelength of the light.

Red is the colour of the purest gold sol. It first appears purple, then blue, and eventually golden as the particle size rises.

Brownian Movement

When examined via an ultramicroscope, the colloidal particles seem to be moving continuously in a zigzag pattern throughout the field of vision. Brownian movement is the name for this zigzag motion.

Brownian motion is based on the size of the particles and the solution’s viscosity rather than the type of colloid.

Sols are stable because of Brownian motion.

Colloidal particle charge

All of the particles in a given colloidal solution contain the same kind of electric charge, which is characteristic of colloidal solutions.

Zeta potential is another name for electrokinetic potential.

The Helmholtz Electrical Double Layer is the combination of the two layers of opposing charges surrounding the colloidal particle.

Colloids and Emulsions

Colloids called emulsions are those in which the dispersion medium and dispersion phase are both liquids.

These two liquids cannot mix or can mix only slightly.

They come in two varieties:

Oil-in-water emulsion
– Water acts as the dispersion medium.
– Stabilizers such as proteins, gums, and natural/manufactured soaps help maintain the stability of the emulsion.
– Examples include milk and vanishing cream.

Water-in-oil emulsion
– Oil acts as the dispersion medium.
– Stabilizers such as long chains of alcohols and metal salts of fatty acids contribute to emulsion stability.
– Examples include butter and cream.


Because the liquids used to make emulsions are wholly incommensurable, an emulsifier is necessary to stabilise the emulsion. They also go by the name “emulsifying agents.”

Soaps, detergents, proteins, gums, and agar are a few typical examples.

  • around us, colloids
  • Mist and fog

In one way or another, food items including milk, butter, cheese, halwa, ice cream, etc. are colloids.

A colloidal solution of an albuminoid material makes up blood.

The nature of fertile soils is colloidal.

The reason the sky is blue is because airborne colloidal particles scatter blue light.

The Uses of Colloids

Electrical smoke precipitation refers to the process of using electrostatic precipitators to remove particulate matter from industrial smokestacks.

Medicines for drinking water purification encompass various methods and chemicals used to purify water, including disinfection, filtration, and UV treatment.

Tanning of photographic plates and films refers to the process of developing images using chemicals and light exposure. These three topics involve different applications: environmental pollution control, ensuring safe drinking water, and image development in photography.

Products from the rubber industry include paints, inks, synthetic polymers, cement, graphite lubricants, and rubber.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top