Surface Chemistry Notes Class 12 Chemistry

 Surface Chemistry :
Surface chemistry deals with the phenomena that occur at the surfaces or interface. The interface or surface is represented by separating the bulk phases by hyphen or slash. Example: solid-solid, solid-liquid, etc.  The bulk phases may be pure compounds or solutions. Some important features of surface chemistry such as adsorption, catalysis and colloids including emulsion and gel have been discussed in this chapter.Adsorption:The phenomenon of accumulation of higher concentration of any molecular species at the surface rather than in the bulk of a solid (or liquid) is called adsorption. Mostly adsorption processes are exothermic, therefore, adsorption decreases with increasing temperature.The adsorbent is a surface on which adsorption takes place. Adsorbate is the species which is adsorbed.In adsorption, the concentration of the adsorbate increases only at the surface of the adsorbent, while in absorption the concentration is uniform throughout the bulk of the solid.  Adsorption is a surface phenomenon, while absorption is a bulk phenomenon.  Example: water vapours are absorbed by anhydrous CaCl2 but adsorbed by silica gel.Note: The term sorption is used to describe both the processes adsorption and absorption. Desorption is the process of removing an adsorbed substance from a surface on which it is absorbed.Types of adsorption:

There are mainly two types of adsorption of gases on solid

Physisorption:

In physical adsorption, the force of attraction between adsorbent and adsorbate is the Van der Waals’ type with low heat of adsorption. Easily liquefiable gases are readily absorbed.

Characteristics of physisorption are:

  • It lacks specificity.
  • The amount of gas absorbed by solid depends on the nature of gas.
  • It is a reversible process.
  • It is an exothermic process and has a low enthalpy of absorption (20-40 kJ/mol).
  •  No appreciable activation energy is required.
  • At a given temperature and pressure with the increase in the surface area of the abdomen, the extent of absorption increases too.

Chemisorption:

In chemical absorption, adsorbate is held to adsorbent by a strong chemical bond. Enthalpy of adsorption is high.

Characteristics of chemisorption are:

  •    It is highly specific in nature and forms a unimolecular layer.
  •    It is Irreversible in nature.
  •     With the increase in surface area, the rate of chemisorption rises.
  •    It has a high heat of adsorption.
  •     It first increases and then decreases at high temperature.
  •     Sometimes high activation energy is required.

 

Adsorption isotherms:

Adsorption isotherm is a curve representing the variation in the amount of gas absorbed by the adsorbent with pressure, keeping the temperature constant.

Freundlich’s adsorption isotherm:

In order to show a relation between the extent of absorption and pressure, Freundlich proposed the following relation

A plot of log x/m versus log p gives a straight line with slope= 1/n and y intercept= log k.

In case of a solution, the isotherm takes the form

Where x is the amount of adsorbate adsorbed on m gram of adsorbent at pressure p or concentration C of the exhibit, k and n are constant, n>1.

Application of adsorption:

The phenomenon of adsorption finds a number of applications. Important ones are listed here

Production of high vacuum:  The remaining traces of air can be adsorbed by charcoal from a vessel evacuated by a vacuum pump to give a very high vacuum.

Gas masks: Gas mask (a device which consists of activated charcoal or mixture of adsorbents) is usually used for breathing in coal mines to adsorb poisonous gas.

Separation of inert gases: Due to the difference in the degree of adsorption of gases by charcoal, a mixture of noble gases can be separated by adsorption on coconut charcoal at different temperatures.

In curing diseases: A number of drugs are used to kill germs by getting adsorbed on them.

Froth floatation process: A low-grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method using pine oil and frothing agent.

Chromatographic analysis: A chromatographic analysis based on the phenomenon of adsorption finds a number of applications in analytical and industrial fields.

Catalysis:

Substances which alter the rate of a chemical reaction and themselves, remain chemically and quantitatively unchanged, are known as catalysts and the phenomenon is known as catalysis.

Promoters are the substances which increase the activity of a catalyst while poisons are the substances which decrease the activity of a catalyst.

Homogeneous and heterogeneous catalysis:

In homogeneous catalysis, the reactants and the catalysts are in the same phase (i.e. liquid or gas) while in heterogeneous catalysis, the reactants and the catalysts are in different phases.

According to absorption theory of heterogeneous catalyst, the reactants in the gaseous state or in solutions, are adsorbed on the surface of the solid catalyst by physisorption or chemisorption. The increase in the concentration of the reactants on the surface increases the rate of reaction.

Important features of solid catalyst:

Activity: The activity of a catalyst depends on the strength of chemisorption to a larger extent.

Selectivity: For a particular reaction, the selectivity of a catalyst is its ability to direct a reaction to yield a particular product.

Shape-selective catalysis by Zeolites:

The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactants and product molecules is called shape selective catalysis. Zeolites are good shape-selective catalysts because of the honeycomb-like structure. An important catalyst used in the petroleum industry is ZSM-5 which converts alcohols directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons.

Enzyme Catalysis:

These are complex nitrogenous organic compounds. They are usually protein molecules of high molecular mass. These are biochemical catalysts as numerous reactions that occur in the bodies of animals and plants to maintain the life process are catalyzed by enzymes. They are most highly efficient, highly specific in nature and highly active under optimum temperature and optimum pH.

The enzymatic activity is increased in the presence of certain substances, known as coenzymes. Certain substances known as inhibitors or poisons reduce or completely destroy the catalytic activity of the enzymes.

Mechanism of Enzyme Catalysts:

The enzyme catalyzed reactions may be considered to proceed in 2 steps.

Step I: Binding of the enzyme to substrate to form an activated complex

E + S  <–> ES                  (Fast and reversible)

Step II: Decomposition of the activated complex to form the product.

ES  –> E + P                    (Slow and rate-determining)

where E is an enzyme, S is substrate and P is the product.

Catalysts in industry:

Some industrial catalytic processes are as follows:

 

Colloids:

A colloid is a heterogeneous system in which one substance is dispersed as very fine particles in another substance, called dispersion medium.

Finely divided particles of any substance with a diameter lying within 1 to 1000 nm range dispersed in any medium constitute the colloidal system. They are intermediate between true solutions and suspensions. The dispersion medium is continuous while the dispersed phase is discontinuous.

The state of a substance in which dispersed phase (solute) and the dispersion medium (solvent) floats with each other, is called colloidal state.

Classification of colloids:

Colloids are classified on the basis of different properties

Classification based on nature of the interaction between dispersed phase and dispersion medium:

Depending upon the nature of interaction between dispersed phase and the dispersion medium, these are of two types

Lyophilic colloids have a strong interaction with the dispersion medium. These are reversible and more stable.

Lyophobic colloids have very less interaction with the dispersion medium. These are irreversible and less stable. They need stabilizing agents for their preservation.

Classification based on physical state of the dispersed phase and dispersion medium:

Depending upon whether the dispersed phase and dispersion medium are solid, liquid or gas, 8 types of colloidal system are known.

Based on the type of particles of the dispersed phase:

Multimolecular colloids consist of aggregates of atoms or small molecules with a diameter of less than 1 nm. Example:  gold sol and sulphur sol.

In the macromolecular type of colloids, the dispersed particles are themselves large molecules of colloidal dimensions. Example: cellulose, starch, proteins, synthetic polymers, etc.

Micelles are associated colloids.  Surface active agents like soaps and synthetic detergents belong to this class.  CMC (critical micelle concentration) is the lowest concentration at which micelle formation appears. Micelle from the ionic surfactant can be formed only above a certain temperature, called the Kraft temperature.

The function of a surface active agent (soap, detergent) is to convert oily and greasy dirt to micelles.

Preparation of colloids:

Peptization:

The process of conversion of fresh precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte (peptizing agent) is known as peptization.

Purification of a colloidal solution:

The process used for reducing the amount of impurities to a requisite minimum is known as purification of a colloidal solution.

The purification is carried out by the following methods:

Dialysis:

Colloidal sols are purified by dialysis. Dialysis is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane.

Electro-dialysis:

When dialysis is taking place in the applied electrical field is known as electrodialysis.

Ultrafiltration:

Ultrafiltration is the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution.

Properties of a colloidal solution:

Colloids are heterogeneous in nature, have colligative properties, undergo Brownian motion, electroosmosis and electrophoresis, exhibit Tyndall effect (scattering of light).

Brownian movement:

The continuous rapid zig-zag motion of the colloidal particles in the dispersion medium is called Brownian movement. It is due to the unbalanced bombardment of the particles of the dispersed phase by the molecules of the dispersion medium. This stabilizes the sol.

Tyndall effect:

When a beam of light is passed through a colloidal solution and viewed perpendicular to the path of incident light, the path of the beam is illuminated by a bluish light.  This phenomenon is known as Tyndall effect and the bright cone of the light is called Tyndall cone.

 

Electrophoresis and electroosmosis:
 
When electrophoresis, i.e., movement of particles is prevented by some suitable means, it is observed that the dispersion medium begins to move in an electric field. This phenomenon is known as electroosmosis. The movement of colloidal particles under the influence of an electric field is called electrophoresis.
 
Charge on colloidal particles:
 
Charge on colloidal sol is due to the preferential adsorption of either positive ion or negative ion which is common and present in excess.

(i) AgI will absorb I of KI (Ibeing common) to make negatively charged sol [AgI]I.

(ii) AgI will absorb Ag+ of AgNO3 (Ag+ being common) to form positively charged colloidal sol [AgI]Ag+.

Helmholtz Electrical Double Layer Theory:

There is the formation of an electrical double layer of opposite charges on the surface of colloidal particles. This electrical double layer is called Helmholtz electrical double layer. The ions adsorbed by the colloidal particles form a fixed layer because these ions are held firmly, while the ions present in the bulk are weakly held form a diffused layer. The potential difference between these two layers of opposite charges is called the electrokinetic or zeta potential.

Coagulation:

The process of settling of colloidal particles is called coagulation or precipitation of the sol. Change of state from colloidal to suspended is called coagulation and flocculation. Greater the valency of the coagulating ion, greater is the coagulating power (Hardy Schulze rule).

Coagulating power of anions to coagulate a positively charged colloidal sol is

                               I < SO42- < PO43-

Coagulating power of cations to coagulate a negatively charged colloidal sol is

                            Ag+ < Pb2+ < Fe3+ < Si4+

The minimum concentration of an electrolyte required to cause coagulation or flocculation of a sol is its flocculation value      (mmolL-1).

Protection of colloid:

Coagulation can be prevented by the addition of suitable lyophilic colloidal sols.  Such sols are called protective colloids.

The minimum number of milligrams of a lyophilic sol needed to protect 10 ml of gold sol by the addition of 1 ml of 10% NaCl is called gold number.

Protective power is the reciprocal of the gold number.

Emulsions:

It is the dispersion of one liquid into another which are immiscible. It can be broken into constituent liquids by heating, freezing, centrifuging, etc.  The process of making emulsion is called emulsification.

Types of emulsions:

(i) Oil in water type (O/W) 

(ii) Water in oil type (W/O)

To obtain a stable emulsion, an emulsifier is added. Protein, gums, natural and synthetic soaps are emulsifiers for oil in water type emulsions. While heavy metal salts of fatty acids, long chain alcohols, and lamp black, are emulsifiers for water in oil type emulsions.

Colloids around us:

Blue Colour of the Sky: Dust particles and water vapours dispersed in air act as a colloid. They scatter the blue light which makes the sky looks blue. This effect is called the Tyndall effect.

Food Articles: We have colloids in our food items like ice cream, butter, milk.

Fog, Mist and Rain: Similar to dust particles, fog and mist are also colloids. Fog and mist have condensed moisture on the surface forming fine droplets which are floating in the air.

Clouds are aerosol in which small droplets of water are suspended in air.

It is possible to cause artificial rain by throwing electrified sand or spraying a sol carrying a charge opposite to the one on clouds from an airplane.

Soil: The reason why soil can absorb moisture is that it is also a colloid. Also, a delta is formed due to the coagulation action of electrolytes in the seawater.

Blood is albuminoid suspended in water, which implies blood also acts as a colloid. Alum and FeCl3 solution stop bleeding due to coagulation.

Applications of colloids:

  • Electrical precipitation of smoke
  • Purification of drinking water
  • Medicine
  • Tanning
  • Cleansing action of soaps and detergents, etc.

 

 

 

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