JEE Advanced 2025 Surface Chemistry Revision Notes

Surface is an outermost or uppermost layer of a physical object or space.

Absorption:

• The phenomenon by which the molecules of a substance are attracted and uniformly distributed into the bulk of another solid or liquid substance is called absorption.

Example: Ink gets absorbed into the bulk of a piece of chalk. Water molecules are absorbed into anhydrous calcium chloride.

• The molecules present on the surface of a substance are of greater energy than the molecules present in the bulk of the substance.

Adsorption:

• The phenomenon of attracting and subsequently accumulating the molecules of a substance by a liquid or solid on its surface is called Adsorption.

• The surface molecules show a greater tendency to attract the molecules of other substances, allowing them to settle on their surface and overcome unbalanced forces.

• The substance getting adsorbed on the surface of other substances is called adsorbate.

• Adsorbate can be solid or liquid, or gas.

• The substance providing its surface for the adsorption phenomenon is called adsorbent. 

• Adsorbent can be solid or liquid but not gas.

Example: Important absorbents are activated charcoal, silica gel, alumina gel, clay, and colloidal particles.

• During adsorption, there is a decrease in the forces on the surface. As a result, heat is liberated.

• $\Delta {\rm H}$ is negative, and $\Delta S$ is negative for the adsorption process.

Free Energy Change, Entropy Change, And Enthalpy Change During Adsorption:

• As adsorption is a spontaneous process, change in free energy $\left( {\Delta G} \right)$ for the process is negative.

$\Delta S$ is -ve because adhering gas molecules to the surface lowers the randomness.

Further, $\Delta G = \Delta H - T\Delta S$

As $\Delta S$ is -ve, $\left( {\Delta G} \right)$ can be -ve only if $\Delta {\rm H}$ is -ve. Hence, adsorption is always exothermic.

• Adsorption is accompanied by a decrease in the system's free energy. When ${\Delta G}$ becomes zero, adsorption equilibrium is established, i.e., Rate of adsorption = Rate of desorption.

Positive and Negative Adsorption:

• When the concentration of the adsorbate is more on the surface of the absorbent than in bulk, it is called positive adsorption.

• If the adsorbate concentration is less on surface relative to its concentration in bulk, it is called Negative adsorption.

Difference Between Physisorption and Chemisorption:

Catalyst:

• A catalyst is a substance that increases the rate of a chemical reaction to which it is added without itself being consumed in the reaction.

• The substance which destroys or reduces the activity of the catalyst is known as catalytic poison. The phenomenon is called catalytic poisoning.

• $A{S_2}{O_3}$ acts as a poison for platinum in the Contact process of manufacture of ${H_2}SO_{4}$. ${H_2}S$ acts as a poison for Fe catalyst in Haber's process. CO poisons platinum in the oxidation reaction of ${H_2}$ to ${H_2}O$

• A substance that promotes the activity of the catalyst, to which it is added in small amounts is called a promoter or activator. The process is known as activation.

• Molybdenum (MO) is a promoter to the catalyst iron in the Haber process.

• The presence of a foreign substance which retards the rate of a reaction is called a negative catalyst or inhibitor. The phenomenon is negative catalysis.

• In homogeneous catalysis, the catalyst and the reactants are in the same phase. It is not possible in solid-state.

• In heterogeneous catalysis, the catalyst is in a different phase from the reactants.

• The action of a catalyst is explained by two different theories: Intermediate compound formation theory and the adsorption theory.

• A positive catalyst lowers the activation energy of the reaction by providing a new pathway.

• When one of the intermediates formed in a reaction itself acts as a catalyst for the reaction, the catalysis is called autocatalysis.

• $M{n^{2 + }}$ in the oxidation of oxalic acid by acidified $KMn{O_4}$ and As in the decomposition of arsenic are examples.

• The nature of a catalyst to speed up a reaction is called activity.  The nature of a catalyst to lead the reaction towards a specific product is called selectivity.
  
  

Enzymes:

• Enzymes are biological catalysts produced by living plants and animals which catalyze the biochemical reaction in living organisms. The activity of enzymes can be increased in the presence of co-enzymes and activators.

• Enzyme catalyzed reaction proceeds in two steps:

E + S →ES and ES →E+P

Aggregation Methods:

• These involve the joining together of a large number of smaller particles to form colloidal particles. This is done by the following methods

• By double decomposition:

         \[A{s_2}{O_3}_{\left( {aq} \right)} + 3{H_2}{S_{(aq)}} \to A{s_2}{S_3} + 3{H_2}O\]

• By reduction:

            $2AuC{l_{3(aq)}} + 3SnC{l_{2(aq)}} \to 2Au + 3SnC{l_4}$

• By oxidation:

            $B{r_{2\left( {aq} \right)}} + {H_2}{S_{\left( {aq} \right)}} \to S + 2HBr$

• By hydrolysis:

           $FeC{l_3} + 3{H_2}O \to Fe{(OH)_3} + 3HCl$

• Ultrasonic dispersion: In this method, ultrasonic vibrations having a frequency more than audible range can change the suspension or liquids like oil, mercury, etc., colloidal range.

• If a colloidal sol contains all colloidal particles of nearly the same size, it is called a monodisperse colloid. However, if it contains colloidal particles of different sizes, it is called   polydisperse colloid.

• The presence of trace amounts of the electrolyte is essential for the stability of the colloidal sol, whereas the presence of large amounts results in coagulation.

The dialysis process finds application in the purification of blood by the artificial kidney. This method introduces impure blood into the artificial kidney apparatus when the waste materials (electrolytes)diffuse through the dialyzing membrane. At the same time, colloidal blood particles are retained, and blood is purified. 

Tyndall Effect:

Tyndall effect is observed only when the following two conditions are satisfied:

1. The diameter of the dispersed particles is not much smaller than the wavelength of light used.

2. There is a large difference in the refraction index of the dispersed phase and the dispersion medium.

Coagulation: 

• The  stability of lyophobic sols is due to the presence of charge on colloidal particles.

• If the charge is removed, the particles will come near each other to form aggregates and settle down under the force of gravity.

• The process of settling colloidal particles is called coagulation.

• This is also called Flocculation or precipitation of the sol. 

The minimum amount of the electrolyte in millimoles must be added to one litre of the colloidal sol to bring about complete coagulation or precipitation. 

• The smaller the flocculation value of an electrolyte, the greater its coagulating or precipitating power. 

• As coagulating power is Inversely proportional to coagulation or flocculation value, to compare the relative coagulating powers of two electrolytes for the same colloidal sol we have the relation

$\dfrac{{Coagulating{\text{ }}power{\text{ }}of{\text{ }}electrolyte{\text{ 1}}}}{{Coagulating{\text{ }}power{\text{ }}of{\text{ }}electrolyte{\text{ 2}}}} \equiv \dfrac{{Coagulating{\text{ value }}of{\text{ }}electrolyte{\text{ 2}}}}{{Coagulating{\text{ value }}of{\text{ }}electrolyte{\text{ 1}}}}$

Coagulating value of electrolyte I Coagulation can also be caused by electrophoresis, mutual precipitation (mixing colloidal sols of opposite charge), and prolonged dialysis or by heating or cooling the sol.

The Isoelectric Point of a Colloid:

• In the case of certain lyophilic sols, particularly proteins, the charge sign depends upon the pH. The particles are negatively charged, while they have a positive charge below this pH. 

• At one particular pH, the particles are uncharged and, therefore, do not migrate under the influence of an electric field. This pH is called the isoelectric point of the. For example, for gelatine, the pH is 4.7; for casein, it is 4.1 — 4.7,and for haemoglobin, it is 4.3 — 5.3.

Sedimentation Potential and Dorn Effect: 

• The reverse of electrophoresis. The sedimentation potential is set up when a particle is forced to move in a resting liquid.

• The phenomenon discovered by Dorn is also known as the drone effect.

Hardy-Schulze laws:

The ion with a charge opposite to the charge of the colloidal particle is very effective. The greater the charge of the ion greater the ability for coagulation.

• Positive colloids are coagulated by negative ions and decreasing order of effectiveness is: ${\left[ {Fe{{\left( {CN} \right)}_6}} \right]^{4 - }} > PO_4^{3 - } > SO_4^{2 - } > C{l^ - }$

• Positive colloids are coagulated by positive ions and decreasing order of effectiveness is: $A{l^{3 + }} > B{a^{2 + }} > {K^ + }$.

Emulsion:

• An emulsion is a colloidal system in which both the dispersed phase and the dispersion medium are liquids.

• Emulsion is classified into two classes:

1. Oil in water (O/W) emulsion.

2. Water in oil (W/O) emulsion.

• The emulsifying agent is usually a soap or a sulfate detergent (or) a hydrophilic colloid (like gelatine, egg albumin, carbon powder, or graphite powder).

• Soap emulsifies kerosene in water emulsion. Egg albumin emulsifies an olive oil in water emulsion. Solid mercuric iodide emulsifies water in benzene emulsion. Casein emulsifies oil in water (milk) emulsion.

• Emulsions are used in the washing process of clothes and crockery, in the digestion of in intestines, in metallurgy, like lotions, creams, and ointments in pharmaceuticals and cosmetics, as drugs of oily type in the form of emulsions to facilitate their easy absorption.

• Emulsion can be broken into constituent liquid by heating, freezing, etc. This is called demulsification. 

Example: The churning of curd

• Emulsions are used to convert cream into butter by churning. This is the breaking of an emulsion of fats in water. In natural oil wells, oils and water form emulsions that require oil separation from water.

Solved Questions:

(1) One gram of charcoal adsorbs 100 mL of 0.5M $C{H_3}COOH$ to form a monolayer. Thereby the molarity of acidic acid is reduced to 0.49M. Calculate the charcoal surface area adsorbed by each acetic acid molecule.

(Surface area of charcoal is $(3.01 \times {10^2}{m^2}/g)$.

Answer:

Number of moles of acetic acid before the adsorption=$\dfrac{Molarity\times Volume~of~solution}{1000}$= $0.5 \times \dfrac{{100}}{{1000}} = 0.05$

number of moles of acetic acid after the adsorption= $0.49 \times \dfrac{{100}}{{1000}} = 0.049.$

Number of molecules of acetic acid adsorbed= $0.001 \times 6.023 \times {10^{23}} = 6.023 \times {10^{20}}$

Surface area of the charcoal occupied by each acetic acid molecule= $\dfrac{{3.01 \times {{10}^2}}}{{6.023 \times {{10}^{20}}}} = 5 \times {10^{ - 19}}{m^2}.$

(2) Is it possible to know the size and shape of colloidal particles by using ultramicroscope

Answer:

With the ultramicroscope, we can see only the light scattered by the colloidal particles, but not an actual colloidal particle.

Thus, an ultramicroscope does not provide any information about the size and shape of the colloidal particle.

Importance of Physical Chemistry Surface Chemistry

This chapter explains how the surface of different states of matter behaves. It describes how the two phases are separated by an interface. Most of the chemical and physical reactions take place in the interfaces. For instance, crystallization, dissolution, corrosion, electrolysis, catalysis, etc occur in an interface.

Surface Chemistry also explains how various phenomena such as absorption, occlusion, adsorption, etc occur. Every scientific term will be explained in this chapter. The importance of this chapter is that it explains how other physical phenomena happen in different cases and in different states of matter.

There are multiple factors that influence the occurrence and rate of physical phenomena mentioned in this chapter. This chapter describes to students how these physical reactions occur along with the explanation of various laws. All the laws and principles will be explained using mathematical derivations. All the terms derived from the laws and principles will be defined along with the symbols used.

By studying this chapter, students will learn how such physical changes occur in an interface between two or more phases. They will learn how chemical reactions are also governed by these physical reactions. Refer to the JEE Advanced Surface Chemistry PDF revision notes for faster comprehension of the mathematical concepts and scientific principles of this chapter.

Benefits of Vedantu’s JEE Advanced Surface Chemistry PDF Revision Notes

• These revision notes are designed by the experts with the sole aim to provide an easier explanation of all the concepts mentioned in the chapter. You will find the easy description of the scientific principles helpful to answer the fundamental questions.

• Use the notes to revise the concepts within a shorter period. The Surface Chemistry IIT JEE notes will also help you recall all the scientific principles you have studied. It will act as a cursor to remember the problems you have solved in the exercises and worksheets.

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