[Chemistry Class Notes] on Surface Chemistry Pdf for Exam

Surface chemistry is the branch of chemistry in which we study those chemical reactions or chemical changes which are taking place at the interface of two phases which can be solid-gas, solid-liquid, liquid – gas etc.  

 

Surface Chemistry has various applications in analytical work, medicinal field, paint industry etc. It is the study of chemical reactions that take place at the interface of two surfaces, which can be solid-liquid, solid-gas, solid-vacuum, liquid-gas, and so on. Surface engineering refers to some of the applications of surface chemistry. There are a variety of phenomena that occur on the surface of substances, including

• Adsorption

• Heterogeneous Catalysis

• Corrosion

• Crystallization

What is Adsorption? 

Adsorption is a surface phenomenon in which atoms, ions or molecules of liquids, gases or solids (dissolved) get accumulated at the surface of another substance (which is an indifferent phase) through adhesive forces. 

 

The substance, which concentrates or accumulates at the surface of another substance is termed adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.

 

Mechanism of Adsorption 

In adsorption, molecules, atoms or ions of one substance gets stick loosely on the surface of another substance of a different kind. 

 

Difference Between Absorption and Adsorption 

 

Types of Adsorption

The understanding of the nature of forces existing between the adsorbate and the adsorbent molecules helps us to classify the process of adsorption in the following two types:

• Physical Adsorption 

• Chemical Adsorption

                           

 

Uses of Adsorption

1. The poisonous gases such as carbon monoxide, methane etc. can be adsorbed by activated charcoal which is widely used in gas masks.

2. Animal charcoal is used to adso
   rb the colouring matter from cane-sugar juice in the process of manufacture of sugar.

3. Ion exchange resin is used to remove the hardness of water since it can adsorb the salts of calcium and magnesium.

4. Chromatographic adsorption techniques are widely used to purify organic compounds.

5. To control humidity the mechanism of adsorption by silica gel is used.

6. The principle of adsorption is also used to explain the results of many catalysts, e.g.: the spongy iron in the making of ammonia and Ni, Pt/Pd in the lowering of unsaturated hydrocarbons.

Adsorption Isotherm 

The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed employing a curve termed as adsorption isotherm. 

 

Freundlich adsorption isotherm – It is an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. It can be expressed mathematically as follows –

 

[frac {x}{m}]=[k.p^{frac {1}{n}}](n > 1)    

 

x = mass of the gas adsorbed on the solid adsorbent 

 

m = mass of solid adsorbent 

 

p = pressure 

 

k and n = Constants which depend on the following –

Logarithmic Equation of Freundlich Adsorption Isotherm – log[frac {x}{m}] = log k + [frac {1}{n}]log p

 

If [frac {1}{n}] = 0,[frac {x}{m}] = constant then the adsorption is independent of pressure. 

 

If [frac {1}{n}] = 1, [frac {x}{m}] p, the adsorption varies directly with pressure. 

 

Catalysis 

Well, you will be amazed to know that long before chemists recognized catalysts and the process of catalysis, common people were already using them for several purposes such as fermenting wine to produce vinegar or raising bread. The uses of catalysts are countless. Catalysts are playing a vital role in many processes or manufacturing techniques on which industries are based. It was the late 18th century when chemists started to recognize the catalysis process. The concept of catalysis was invented by chemist Elizabeth Fulham and was reported in her book in 1794. Although the term catalysis was first used by Jons Jakob Berzelius in 1835. In the 1880s, Wilhelm Ostwald investigated reactions catalyzed by the presence of acids and bases, for this he was awarded the Nobel Prize in Chemistry in 1909. 

 

What is Catalysis?

The process of increasing the rate of a chemical reaction by adding a substance that does not take part in the reaction is called catalysis and the substance which is added and increases the rate of reaction is called a catalyst. A very small amount of catalyst is required to alter the rate of reaction. For example, in the reaction of converting hydrogen peroxide into water and oxygen gas, potassium permanganate is used as a catalyst which increases the rate of reaction. 

 

2H2O2 Potassium permanganate→ 2H2O + O2

 

Types of Catalysis 

Based on phases of catalysts and reactants, catalysis can be divided into the following two types –

What is Homogeneous Catalysis and Catalyst? 

The catalyst that is present in the same phase as the reactants in the reaction is called homogeneous catalyst and this type of catalysis process is called homogeneous catalysis. 

 

Examples of Homogeneous Catalysis and Catalysts – 

1. Hydrolysis of Sugar – In the hydrolysis of sugar reactants sugar (sucrose solution) and water are used in liquid states and the catalyst sulfuric acid is also used in the liquid state. The reaction is given below –

C12H22O11(l) + H2O(l) H2SO4(l)→ C6H12O6(l) + C6H12O6(l)

Sucrose                                            Glucose      Fructose

2. Hydrolysis of the Ester – In the hydrolysis of the ester, the ester is taken in a liquid state with water (liquid) for the reaction in presence of catalyst hydrochloric acid which is also taken in a liquid state. The reaction is given below –

CH3COOCH3(l) + H2O(l) HCl(l)→ CH3COOH(l) CH3OH(l)

 

What is Heterogeneous Catalysis and Catalysts? 

The catalyst whose phase differs from that of the reactants in the reaction is called heterogeneous catalyst and this type of catalysis process is called heterogeneous catalysis. 

 

Examples of Heterogeneous catalysis and catalysts – 

1. In Haber’s process of formation of ammonia, nitrogen and hydrogen are used in gaseous forms while catalyst iron is used in solid form. 

N2(g) + 3H2(g) Fe(s)→ 2NH3

2. Formation of Sulfuric Acid – In this process sulfur dioxide (gas) is oxidized to sulfur trioxide (gas) by heterogeneous catalysis in presence of a solid V2O5 catalyst. Then sulfur trioxide is hydrolyzed to sulfuric acid. 

SO2(g) + O2(g) V2O5(s)→ 2 SO3(g)

 

What is Adsorption Theory of Heterogeneous Catalysis? 

Modern Adsorption theory of heterogeneous catalysis is the mixture of moderate compound hypothesis and the old adsorption hypothesis or old adsorption theory. Old adsorption theory lacked specificity so there was a need for modern adsorption theory. 

 

According to the adsorption theory of heterogeneous cataly
sts, there are free valances in the catalyst on which reactant molecules get attached. The mechanism of the adsorption theory of heterogeneous catalysis involves the following steps –

• Step 1. Diffusion of reactant molecules 

• Step 2. Adsorption 

• Step 3. Intermediate complex formation 

• Step 4. Desorption 

• Step 5. Diffusion of product molecules 

Step 1. Diffusion of Reactant Molecules – In this step reactant molecules get diffused towards the surface of the catalyst. 

 

Step 2. Adsorption – In this step reactant molecules get adsorbed on the surface of the solid catalyst or form loose bonds with the free valencies of the catalyst. 

 

Step 3. Intermediate Complex Formation – In this step adsorbed reactant molecules on the surface of the catalyst react with each other and form new stronger bonds with each other which leads to the formation of an intermediate. 

 

Step 4. Desorption – In this step intermediate converts into the product as it loses its affinity towards the catalyst. The product molecule gets desorbed from the surface of the catalyst. 

 

Step 5. Diffusion of Product Molecules – In this step desorbed product molecules from the surface of the catalyst gets diffused away from the catalyst. 

 

()

 

What is Shape Selective Catalysis by Zeolites? 

Shape selective catalysis is that type of catalysis which depends upon the pore structure of the catalyst and the size of the reactant and product molecules. Zeolite is a porous solid made up of silicon, aluminium and oxygen and a good shape-selective catalyst. It has cavities in its structure where ions or atoms or small molecules can reside. 

 

What is Enzyme Catalysis? 

Many enzymes also act as catalysts for many reactions. As enzymes catalyze many reactions which occur in our body, plants and animals. So, they are known as biochemical catalysts and the phenomenon is called biochemical catalysis. 

 

Characteristics of Enzyme Catalysis 

• Highly efficient 

• Very specific nature 

• They perform best at optimum temperature and optimum pH

• The activity of enzymes is increased in the presence of activators and coenzymes

• Inhibitors can destroy or reduce the activity of enzymes 

What are Colloids? 

In chemistry, colloids are heterogeneous mixtures of two substances in which minute particles of one substance are dispersed in another substance. The substance whose minute particles are suspended in another substance is called the dispersed phase while the substance in which it is suspended is called dispersion medium. For example, in the fog dispersed phase is water (liquid) and dispersion medium is different gases. We can’t see particles of a dispersed phase in colloids by naked eyes as they are very small in size.  

 

Examples of Colloids 

We see many colloidal solutions around us. Many food items such as cake, milk, bread, butter, ice cream fruit juices, whipped cream etc. are examples of colloids. Apart from these fog, mist clay etc. are also examples of colloids. We are providing a list of examples of colloids with their dispersed phase and dispersing medium below –

 

Properties of Colloids 

Colloids show the following properties –

• It is a heterogeneous mixture. 

• The size of colloidal particles is very small. Their particle size ranges between 1-1000 nanometers. 

• It shows a Tyndall effect. It means it scatters the beam of light and shows its path through itself. 

• They don’t settle down when left undisturbed for some time. it means colloidal solutions are quite stable. 

• They cannot be separated by a filtration process. 

• They can be separated by centrifugation. 

• Colloidal particles show Brownian movement. 

Classification of Colloids 

Based on the physical state of the dispersed phase and dispersion medium – Colloids can be divided into 8 types based on the physical state of the dispersed phase and dispersion medium. All eight types of colloids are listed below –

 

Based on the nature of the interaction between the dispersed phase and dispersion medium – Colloids can be divided into the following two types based on the nature of the interaction between the dispersed phase and dispersion medium –

What are Lyophilic Sols?

Lyophilic means ‘liquid loving’. Those sols in which dispersed phase and dispersion medium (water) have strong attraction between them are called lyophilic sols. For example, a colloidal solution is formed by dissolving starch in water. In this colloidal solution dispersion medium is water and the dispersed phase is starch. This sol can be prepared by heating water at 100 ℃ and dissolving starch in it. It is a stable sol and cannot be separated easily due to the strong attraction between the dispersed phase and dispersion medium. Egg albumin sol is another example of lyophilic sol. 

 

What are Lyophobic Sols?

Lyophobic means ‘Liquid hating’. Those sols in which dispersed phase and dispersion medium (water) have very little attraction or no interaction between them are called lyophobic sols. In these sols, dispersed phase particles show very less or no affinity for dispersion medium. These are not stable sols and can be separated easily. If a small quantity of an electrolyte is added to these types of sols, then dispersed phase and dispersion medium are easily get separated. To make them stable sols, we need to add stabilizers while preparing them. These are also known as hydrophobic sols. An example of a lyophobic sol is ferric hydroxide sol. It is prepared by hydrolysis of ferric chloride. In this boiling water is used. In this process, hydrochloric acid is also produced which is removed from the sol as it makes the sol unstable. It is removed by dialysis of the ferric hydroxide sol (lyophobic sol).

 

Difference between Lyophilic and Lyophobic Sols 

 

Based on the type of particles of the dispersed phase – Colloids can be classified into the following three types based on the pe of particles of the dispersed phase –

Multimolecular Colloids – In these colloids, particles (atoms or small molecules) of dispersed phase aggregate together to form species that have a size of 1-1000nm range. These types of colloids are called multimolecular colloids. Example – In Sulphur sol, generally Sulphur particles consist of 8 Sulphur atoms. 

 

Macromolecular Colloids – Those colloidal solutions which are formed by the dispersion of macromolecules in the suitable dispersion medium are called macromolecular colloids. Example – starch sol. 

 

Associated Colloids – Some substances at higher concentrations show colloidal behaviour due to the formation of aggregates or micelles. These are also known as micelles colloids. The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). Micelles change into individual ions on dilution. Associated colloids exhibit both lyophilic and lyophobic parts. Example – soap solution. 

 

Preparation of Colloids 

Colloids can be prepared by the following methods –

• Chemical Methods – Double decomposition, hydrolysis, oxidation and reduction are the various chemical reactions that are used in the preparation of colloids through chemical methods. 

Example – SO2 + 2H2S Double decomposition→ 3S + 2H2O 

• Electrical Disintegration or Bredig’s Arc Method – In this method metal electrodes are immersed in the dispersion medium and intense heat is produced which converts the metal into vapour. The vapour of metal gets condensed into particles of colloidal size. Example – gold sol, silver sol etc. 

• Peptization – Peptization is the process of converting a precipitate into colloidal sol by shaking it with a dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose is called a peptizing agent.

• Purification of Colloidal Solution – Colloidal solutions generally contain some impurities like an extra amount of electrolytes etc. The process used for reducing the number of impurities to a requisite minimum is known as the purification of colloidal solution. Dialysis, electrodialysis, ultrafiltration are the main methods that are used in the purification of colloids. 

 

Applications of Colloids 

Colloids have various
applications in many fields. Some uses of colloids are listed below –

• Colloids are used in the foods and food industries at a large level. Many foods which we consume are colloidal. Such as milk, cheese etc. 

• Colloids have various applications in the medicinal field as well. Many medicines which we use are in the form of emulsions. Antibiotics such as penicillin and streptomycin are given in the form of colloidal solutions so that they can be absorbed by the human body easily. 

• Colloids are used in water purification. 

• Sewage water contains impurities like dirt, stool, urine etc. which are dispersed in water. Thus, forms a colloidal system. These can be removed by electrophoresis. 

• Smoke is also a colloidal system of carbon particles in the air. This can also be purified by electrophoresis. 

• These are used in artificial rain as well. 

• Rubber is obtained by a colloidal solution called latex through coagulation. 

• Treatment of the skin of animals to get leather is called tanning. In the process of tanning, colloids are used. 

Emulsions: What is Emulsion? 

A mixture of two or more liquids that are normally immiscible is called Emulsion. Generally, students get confused between the terms – emulsion and colloid. The term emulsion is specifically used for mixtures in which dispersed phase and dispersion medium both are liquids. Although emulsion is a type of colloid, it does not mean that all colloids are emulsions only. In colloidal solution, both dispersed phase and dispersion medium don’t need to always be liquids. For example, aerosol (Examples – Clouds, Fog etc.) is also a colloidal solution in which the dispersion medium is gas and the dispersed phase is liquid. The term emulsion comes from the Latin word ‘emulate’ which means “to milk out”. 

 

Examples of Emulsions – Milk, mayonnaise, hand creams (lotions), Latex, cutting fluid, vinaigrettes etc. are examples of emulsions. 

 

What is Emulsification?

The process of formation of emulsions is called emulsification. In this process, one immiscible liquid is dispersed in another immiscible liquid. Thus, we can say to emulsifying two immiscible liquids is called emulsification. For example, in oil-water cutting fluid emulsion used for metalworking is formed by emulsifying oil in the water medium. 

 

Mechanism of Emulsification

Many different chemicals and physical processes and mechanisms can be involved in the process of emulsification. The mechanism of emulsification can be based on the following three theories –

• Surface Tension Theory – According to surface tension theory, emulsification takes place by the reduction of interfacial tension between the dispersed phase and the dispersion medium. 

• Repulsion Theory – According to repulsion theory repulsion force between the particles of the dispersed phase cause them to remain dispersed in the dispersion medium. The emulsifying agent makes a film over one phase which makes globules of that phase and these globules repel each other. 

• Viscosity Modifications – Some emulsifying agents increase the viscosity of the medium. Due to an increase in the viscosity of the medium, globules of the dispersed phase remain dispersed in the dispersion medium. 

Properties of Emulsions 

Properties of emulsions are listed below –

• Emulsions contain both a dispersed phase and a dispersion medium.

• The boundary between the dispersion phase and dispersed medium is called “interface”. 

• They have a cloudy appearance.

• They show various colours depending on the dilution. Such as, the emulsion appears white if it scatters the light equally. If it is diluted it will appear blue while if it is concentrated, then it will appear yellow. 

• It shows the Tyndall effect. 

• The particle size of a dispersed phase in emulsions may vary.

• Generally, emulsions are inherently unstable, exposure to energy and power ultrasound is needed to form a stable emulsion.

• Emulsion particles form dynamic inhomogeneous structures on a small length scale. 

• Both the phases of emulsion may get separated If it is kept undisturbed for a longer period or in absence of an emulsifying agent. 

Types of Emulsions 

Broadly, Emulsions can be divided into two types –

• Simple Emulsions 

• Complex Emulsions 

Simple Emulsions – Simple emulsions are those emulsions that are formed by either dispersing oil in water or water in oil. Simple emulsions can be divided into the following two types –

• Water in oil emulsion 

• Oil in water emulsion 

Water in Oil Emulsion – If the dispersed phase is water and the dispersion medium is oil in the emulsion, then these types of emulsions are called water in oil emulsions. It is also called W/O types of emulsions. 

 

()

 

In these types of emulsions, water is an internal phase and oil is an external phase. Cold cream, butter etc. are examples of water in oil emulsions. 

 

Oil in Water Emulsion – If the dispersed phase is oil and the dispersion medium is water in the emulsion, then these types of emulsions are called oil in water emulsions. It is also called O/W types of emulsions.

 

()

 

Complex Emulsions – Complex emulsions are also called multiple emulsions. In these types of emulsions, a complex system exists in which both oils in water and water in oil emulsion exist together and are stabilized by surfactants. These can be divided into the following types –

Water – in – Oil – in – Water Emulsion – These are also called W/O/W emulsions. In these types of emulsions oil droplets enclosing water, droplets are dispersed in water. These are double emulsions of O/W emuls
ion and W/O emulsion. 

 

()

 

Oil – in – Water – in – Oil Emulsion – These are also called O/W/O emulsion. In these types of emulsions water droplets enclosing oil, droplets are dispersed in the oil phase. These are double emulsions of O/W emulsion and W/O emulsion. 

 

()

 

This ends our coverage on the summary of the unit “Surface Chemistry”. We hope you enjoyed learning and were able to grasp the concepts. You can get separate articles as well on various subtopics of this unit such as what is adsorption? Colloids, Emulsions etc. on the website. We hope after reading this article you will be able to solve problems based on the topic. We have already provided detailed study notes or revision notes for this unit, which you can easily download by registering yourself on the website. Here in this article, we have discussed the unit in a summarized way with an emphasis on important topics of the unit.  If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to the website or download Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.

 

Adsorption’s Function in Surface Chemistry

Adsorption is the accumulation of species at higher concentrations on the surface of a substance due to intermolecular forces. Gases like H2, O2, and N2 adsorb on the surface of activated charcoal, for example.

Also see: Adsorption and Absorption: What’s the Difference?

The quantity of heat energy released when one mole of gas is adsorbed on the unit surface area of an adsorbent is referred to as the enthalpy of adsorption.

What is the Definition of Catalysis?

To produce products, all reactants must overcome a specific amount of energy known as activation energy. The difference between the energy of the transition state and the energy of the reactant species is called the activation energy. Some reactant molecules have enough kinetic energy to break through this barrier, whereas others do not.

()

Catalysis comes in a variety of forms.

Chemical reactions are often split into two types by catalysis:

What Exactly is a Colloid?

A colloid is a heterogeneous mixture in which minute particles of one component are scattered in a dispersion medium of another substance.

The tiny particles in this combination range in size from 1 to 1000 nanometers in diameter, but they remain suspended and do not settle to the bottom. They can be seen with an optical or electron microscope (smaller particles).

Colloids are classified into Several Categories.

Colloids are categorised based on the features of the dispersed phase and the media in which they are suspended.

To begin, colloids are categorised according to the sorts of particles in the dispersed phase:

• Multimolecular colloids

• Macromolecular colloids

• Associated colloids

()

Surface Chemistry Emulsions

Emulsions are colloidal solutions in which both the dispersed phase and the dispersion medium are liquids.

Paints, colours, milk, and disappearing cream are among the examples. Emulsions in surface chemistry are categorised into two categories based on their dispersion medium:

Emulsions of oil and water

The oil or fat droplets are spread in water in this method. Consider the case of milk.

Oil Emulsions with Water

The water droplets are spread in oil in this method. Vanishing cream is a good example.

Emulsions are inherently unstable, hence emulsifiers are used to improve their stability. Milk, for instance, includes natural emulsion casein.