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Surface Chemistry Notes

Introduction

Surface Chemistry deals with the study of physical and chemical phenomena occurring at the boundary (interface) separating two bulk phases.

• The bulk phase can be a pure compound or a solution.
• The bulk phases may be solid – liquid, solid – gas, solid – vacuum, liquid – gas etc.

Let us consider a simple example of a dirty shirt. The dirt stays in the surface of the fabric and the study of phenomena occurring at the interface between the fabric and the dirt is surface chemistry, in simple words.

Adsorption and Absorption

The dirt staying on the surface of the skin as a layer is termed as adsorbtion.

Now consider applying soap solution to the shirt containing dirt.  The soap solution is absorbed by the fabric and does not stay on the fabric as a layer. This is called absorption.

Adsorption

Adsorption is the phenomenon of attracting and retaining molecules of a substance on the surface of a solid (or liquid) resulting as a higher concentration of molecules only on the surface.

• Adsorbent- the surface on which adsorption takes place
• Adsorbate- the substance which is adsorbed

Example- Water vapour adsorbed by silica gel

• Adsorbent- Silica gel
• Adsorbate- Water vapour

Differences between Adsorption and Absorption

ADSORPTION	ABSORPTION
Surface phenomenon- concentration of the adsorbate increases only on the surface	Bulk phenomenon- concentration is uniform throughout the solid
Exothermic process- heat is released	Endothermic process- heat is absorbed
It is favoured by low temperature	It is not affected by temperature
Eg- Chalk stick dipped in ink adsorbs the colour of the ink but when you break the piece of chalk, its core still remains white	Eg- When anhydrous CaCl2 absorbs water vapour, it becomes wet and pasty

Mechanism of Adsorption

• Adsorption occurs because the particle on the surface and the particle in the bulk of the adsorbent are not in the same environment. That is, the net force acting on them is not the same.
• The particle on the surface has unbalanced forces acting on it which are also called residual attractive forces
• Due to these forces, the surface particles of the adsorbent attract the adsorbate particles

• During adsorption, there is always a decrease in the residual attractive forces of the surface. That is, the energy of the surface decreases and this appears as heat. This is called the heat of adsorption
• The amount of heat evolved when one mole of adsorbate is adsorbed on the adsorbent surface is called enthalpy of adsorption
• Adsorption is always exothermic and the enthalpy change, ΔH is always negative
• When the adsorbate molecules are adsorbed on the surface of the adsorbent, their freedom of movement becomes restricted and hence ΔS the entropy decreases
• We know that Gibbs free energy,ΔG= ΔH –TΔS.

For adsorption to be spontaneous, ΔG must be negative. This can happen if ΔH has a significantly high negative value as –TΔS is positive.

• As the adsorption continues, ΔH becomes less and less negative till it becomes equal to TΔS and ΔG becomes zero. At this point, equilibrium is attained.

Factors affecting adsorption of Gases by Solids

• Nature and Surface area of adsorbent:
  • The same gas is adsorbed by different solids at different extents even at the same temperature.
  • Greater the surface area, greater is the volume of gases adsorbed.
•  Nature of the gas being adsorbed:
• Different gases are adsorbed are adsorbed to different extents even by the same solid.
• As the critical temperature of a gas increases, it is easier to liquefy and it is also more readily adsorbed.
• Reason- Higher the critical temperature, the easier it is to liquefy the gas as greater are the intermolecular forces to attraction between the molecules of the gas. For such a gas, the intermolecular forces of attraction are greater on the surface of the adsorbent and thus, the adsorption will be more.
• Temperature: As temperature increases, adsorption decreases
• Pressure: At constant temperature, the adsorption of a gas increases with increase in pressure.
• Activation of the solid adsorbent: It means increasing the adsorbing power of the adsorbent.

It can be done by-

1. Making the surface of the adsorbent rough- It can be done by rubbing the surface or chemical action or by depositing fine metal particles on the surface by electroplating
2. By dividing the adsorbent into small pieces or grains- It increases the surface area but this method has a practical limitation. If the particles are too fine like powder, the adsorption of the gas will become difficult.
3. By removing the already adsorbed gases

Types of Adsorption

Physisorption	Chemisorption
Occurs due van der Waals forces	Caused by chemical bond formation
Not specific in nature that is all gases are adsorbed on the surface to an extent	Highly specific in nature
Reversible	Irreversible
More easily liquefiable gases are adsorbed more readily	Gases which can react with the adsorbent show chemisorption
Enthalpy of adsorption is low(20-40 kJ)	Enthalpy of adsorption is high (80-240 kJ)
It decreases with increase in temperature. Favours low temperature	It increases with increase in temperature. Favours high temperature
It does not need any activation energy	It does require activation energy.
It results into multimolecular layers on adsorbent surface under high pressure	It results into a unimolecular layer.

Adsorption isotherms

The variation in the amount of gas adsorbed by the adsorbent at constant temperature with change in pressure is shown by a curve called adsorption isotherm.

Freundlich adsorption Isotherm:

• Freundlich proposed this relation to show a relation between the extent of adsorption and pressure.

If there are solutions involved, the above equations become

Where x -> amount of adsorbate

m-> mass of adsorbent

p-> pressure

C-> concentration of adsorbate

K and n->constants , n>1 always

Looking at equation 2, as we plot log x/m versus log p on a graph,  we get a straight line with slope=1/n and y- intercept= log k.

Applications of adsorption

1. Production of high vacuum– Remaining traces of air in a vessel already evacuated by vacuum pump can be adsorbed by charcoal to create high vacuum
2. Control humidity – Silica and aluminium gels can adsorb moisture and remove humidity
3. Gas masks- Gas masks consist of activated charcoal or mixture of adsorbents and are used to breathe in coal mines
4. Removing coloured substances from solutions– This is used in chromatographic analysis.
5. Separation of inert gases– Different inert gases are adsorbed to different extents on coconut charcoal
6. Heterogeneous catalysis– When gaseous reactants are adsorbed on the surface of a solid catalyst, the concentration of the reactants on the surface increases and thus, the rate of reaction also increases( adsorption theory). Eg- Using finely divided Nickel in the hydrogenation of vegetable oils
7. Adsorption indicators– Many dyes have become useful due to adsorption. These dyes have been introduced as indicators especially in precipitation titrations. Eg- KBr is easily titrated with AgNO₃ using eosin as the indicator

8. Froth floatation process– When sulphide ore is shaken with pine oil and water, the ore particles are adsorbed on the froth that floats and the gangue prticles ( like silica, mud) settle down in the tank. This process is used in the concentration of sulphide ores
9. Chromatographic analysis– The selective adsorption of some substances by a solution helps us separate components of a mixture. Example- All the dyes in ink
10. Curing Diseases- Some drugs can adsorb the germs on them and hence, kill them saving us from diseases

Catalysis

Substances which alter the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction are called catalysts.

• Promoters– Substances which enhance the activity of the catalyst
• Poisons- Substances which decreasethe activity of the catalyst

Example- In Haber’s process for manufacturing ammonia, molybdenum acts as promoter for the catalyst- iron

N₂ (g) + 3H₂ (g) –> 2NH₃ (g)

Homogenous and Heterogeneous Catalysis

Catalysis can be classified into 2 types.

1. Homogenous catalysis:

The reactants and the catalyst are in the same phase (that is liquid or gas).

Example- In Oxidation of sulphur dioxide,

 Sulphur dioxide, oxygen and the catalyst nitrogen oxide are all gases

2SO₂(g) + O₂(g) à 2SO₃ (g)

2. Heterogeneous catalysis:

The reactants and the catalyst are in different phases

Example- In Oxidation of ammonia,

Ammonia and oxygen are gases but the catalyst Platinum is solid

4NH₃(g) +5O₂(g) –> 4NO(g) + 6H₂O(g)

Adsorption theory of Heterogeneous catalysis:

• This theory explains the mechanism of heterogeneous catalysis.
• It is combination of the intermediate compound formation theory and the old adsorption theory. The Intermediate Compound Formation Theory is in the chapter of the Chemical Kinetics
• Old Adsorption theory- The reactants (gases or in solutions) are adsorbed on the surface of the solid catalyst. The increase in concentration of the reactant on the surface also increases the rate of reaction. The heat of adsorption( adsorption is exothermic) also increases the rate of reaction.

Mechanism of Catalytic Activity

1. Diffusion of reactants towards the surface of the catalyst
2. Adsorption of reactant molecules on the surface of the catalyst
3. Occurrence of a chemical reaction between the reactants and the catalyst which forms an intermediate
4. Desorption of product molecule from the surface because of the lack of its affinity to the catalyst’s surface by which the surface is now available for the adsorption of new reactant molecules
5. Diffusion of product molecules away from the surface of the catalyst

Advantages of This Theory:

• Only a small quantity of the catalyst is required as it is regenerated again and again
• The catalyst does take part in the reaction but its mass and composition remain unchanged at the end of the reaction
• If catalytic poisons are present they preferentially get adsorbed onto the surface of the catalyst. This hinders the adsorption of the reactant molecules

Note- This theory does not satisfactorily explain the action of catalytic promoters.

Important Features of Solid Catalysts

Activity

• Activity of a catalyst refers to the capacity of the catalyst to increase the speed of the chemical reaction. The activity depends upon the extent of chemisorption.
• The adsorption should be reasonably strong but not so strong that the adsorbed molecules become immobile and there is no place for other reactants to get adsorbed. Example- Combination of H₂ and O₂ in the presence of Platinum(catalyst) to form water is an explosive reaction.
• But in the absence of catalyst, H₂ and O₂ do not react and can be stored like that for a very long period of time.

2H₂(g) + O₂(g) –> 2H₂O(l)

Selectivity

• Selectivity of a catalyst refers to its ability to direct the reaction to form particular products excluding others.
• Example- CO and H2 form different products in the presence of different catalysts

Note- Catalysts are very specific and selective in their action. So a substance which acts as a catalyst in one reaction may fail to catalyse another reaction

Shape selective catalysis-Zeolites

• The reactions taking place in zeolites depend on-
• They are good shape selective catalysts due to their honey-comb like structures
• They are three dimensional network of silicates where some silicon atoms are replaced by aluminium atoms forming a Al-O-Si network.
• Zeolites are microporousaluminosilicates.
• The catalytic reaction which depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis
  • The size and shape of the reactants and product molecules
  • The pores and cavities of the zeolites
• Widely used as catalysts in the petrochemical industry for cracking of hydrocarbons and isomerisation
• Eg: ZSM-5 is used in converting alcohols directly to gasoline(petrol) by dehydrating them

Zeolite structure

Honeycomb Structure

Enzymes

• Enzymes are complex nitrogenous compounds which are produced by plants and animals
• They are protein molecules of high molecular mass and form colloidal solutions with water
• They are very effective catalysts
• They catalyse many reactions which occur in the bodies of animals and plants to maintain life processes
• Thus, enzymes are called biochemical catalysts
• The phenomenon is called biochemical catalysis

Enzyme catalysed Reactions:

1. Inversion of cane sugar

Invertase enzyme converts glucose into glucose and fructose

2. Conversion of glucose into ethyl alcohol

Zymase enzyme converts glucose into ethyl alcohol and carbon dioxide

3. Conversion of starch into maltose

Diastese enzyme converts starch to maltose

4. Conversion of maltose into glucose

Maltase enzyme converts maltose into glucose

5. Decompostion of Urea into Ammonia and Carbon Dioxide

Urease enzyme helps decompose ammonia and carbon dioxide

6. In stomach and intestine :

In stomach– Pepsin enzyme converts Proteins to Peptides

In the intestine– Pancreatic Trypsin converts Proteins to Amino acids by hydrolysis

7. Conversion of milk into curd : The Lacto bacilli enzyme present in curd helps convert  Milk to Curd

Characteristics of Enzyme catalysis

Highly Efficient

• One molecule of an enzyme may transform one million reactant molecules per minute

Highly Specific in Nature

• Catalysts are highly specific

Example- Urease enzyme catalyses the hydrolysis of urea only and not the hydrolysis of any other amide

Highly active at Optimum temperature

• The temperature at which the rate of an enzyme catalysed reaction is maximum is called Optimum temperature. On either side this temperature, enzyme activity decreases

Example- Human body temperature(310K) is suitable for enzyme-catalysed reactions

Highly active at Optimum pH

• The rate of an enzyme-catalysed reaction is maximumat a particular pH called the Optimum pH

Activity Increases in presence of Activators and Co-enzymes-

• The enzymatic activity increases considerably in the presence of certain other substances called co-enzymes.

Example- When a certain non-protein(vitamin) is present along with the enzyme, catalytic activity is enhanced

Influence of inhibitors and Poisons-

• Like ordinary catalysts, enzymes are also inhibited by the presence of certain substances (inhibitors or poisons).
• These substances interact with the active functional groups on the surface of the enzyme and usually reduce or completely destroy the catalytic activity of the enzymes

Example- Many of the drugs we use is related to their action as enzyme inhibitors in our body

Mechanism of enzyme catalysis

Enzyme-catalysed reactions usually proceed in 2 steps-

1. Binding of the enzyme to the substrate to form an activated complex

E + S –>  ES*

2. Decomposition of activated complex

ES*  –>  E +P

where E- Enzyme,   S- Substrate ,    ES*- Activated complex ,     P- Product

Lock and Key Mechanism- There are many cavities present on the surface of the enzyme. These cavities have a characteristic shape and active groups like  –NH2,  COOH, -SH, -OH etc. These are actually the active centres on the surface of the enzyme. The reactant molecules which have a complementary shape can fit into these cavities a lot like a key fitting into a lock. Due to the active centres (groups) present an activated complex also called enzyme-substrate complex is formed.

This complex decomposes to give products and the enzyme.

Catalysts in the industry-

1. Haber’s process to manufacture ammonia

2. Ostwald’s process to manufacture Nitric acid

3. Contact process to manufacture Sulphuric acid

Colloids:

Colloidal solutions are mixtures in which the particle size is between 1-1000nm such that they can pass through filter paper but not animal or plant membranes.  These particles also don’t settle upon standing for sometime.

They involve 2 substances called the dispersed phase and the dispersed medium

The dispersed phase- The substance that is dispersed in another medium to form a colloid. Usually lesser in quantity

Dispersed medium- The substance inside which the dispersed phase is dispersed

Classification of colloids

Colloids are classified in 2 different ways

Based on the physical state of the dispersed phase and dispersed medium:

S.No	Dispersed Phase	Dispersed Medium	Name of Colloid	Examples
1.	Solid	Solid	Solid sol	Gem stones
2.	Solid	Liquid	Sol	Muddy water, Paint, cell fluids
3.	Solid	Gas	Aerosol	Smoke,dust
4.	Liquid	Solid	Gel	Cheese, butter, jelly
5.	Liquid	Liquid	Emulsion	Milk, Hair cream
6.	Liquid	Gas	Aerosol	Fog, mist, cloud
7.	Gas	Solid	Solid Foam	Pumice Stone
8.	Gas	Liquid	Foam	Froth, soap lather

Based on nature of Interaction Between Dispersed Phase and Dispersed Medium

Lyophilic Colloids(liquid loving)–

• Some substances which can from colloids directly on mixing them with a suitable liquid(dispersion medium). These colloids are called lyophilic colloids. Examples of these substances are gum, geltine, starch, rubber.
• They are also called Reversible sols as in these sols(colloids) when the dispersion phase is separated from the dispersion medium (by say evaporation) , the sol can be formed again by just mixing the dispersion phase and medium again.
• They are also very stable and cannot be coagulated

Lyophobic colloids(liquid hating)

• Some substances cannot form colloid just by directly mixing them with a liquid. Their colloidal sols are prepared by special methods and are called lyophobic colloids. Examples of these substances are metals, metal sulphides.
• They are also called Irreversible colloids as on precipitation, they don’t give back the colloid on simply mixing the dispersed phase and the dispersed medium.
• They are unstable and coagulate easily by heating shaking or adding electrolytes. Stabilising agents are used to preserve them

Based on the Types of Particles of the Dispersed Phase-

• Multimolecular Colloids: Many particles(atoms or small molecules) of the dispersed phase aggregate together to form species having the size of a colloidal particle( 1-1000nm). These colloids are called multimolecular colloids. Example-gold sol, Sulphur sol

• Macromolecular colloids: Substances with large molecules (macromolecules) in suitable solvents form solutions but these macromolecules might be in the colloidal range. These solutions are called macromolecular colloids and resemble true solutions in many ways. Example- Starch, Cellulose, Proteins are natural macromolecules.       Nylon, polythene, polystyrene are man-made macromolecules

• Associated Colloids: Some substances at high concentrations act as colloids due to the formation of aggregates. But at low concentrations they behave like normal strong electrolytes. These aggregates formed are called micelles. Such colloids are called associated colloids.

Kraft Temperature- The formation of micelles takes place only above a particular temperature called Kraft’s temperature

Critical Micelle Concentration(CMC) – The concentration above which micelle formation takes place

Example- Soaps, synthetic detergents

Mechanism Of Micelle Formation-

• Soap is the sodium or potassium salt of fatty acid and may be represented as RCOO^– Na (e.g. sodium stearate, (CH₃(CH₂)16COO^–Na⁺]) .  Whendissolved into water , it dissociates into  RCOO^– and  Na⁺ ions
• The RCOO^–ion consists of two parts –  long hydrocarbon chain (also called  non – polar tail)  which is hydrophobic (water repelling ) and a polar group  COO^– (polar head) which is hydrophilic (water loving )

• At higher concentrations(CMC) RCOO- ions form an aggregate of spherical shape with the hydrocarbon chains pointing towards the centre and the COO- part facing outward on the surface of the sphere. This aggregate is called ionic micelle. It may have as many as 100 ions

Cleansing Action of Soaps

The cleansing action of soap is due to the formation of micelle by the soap molecules in such a way that the hydrophobic part is in the oil droplet(dirt) and the hydrolphilic part projects out. Since the polar groups interact with watersurrounded by soap ions is pulled from the surface and pulled into water.

Preparation of Colloids

A few ways to prepare colloids are given below

•  Chemical method : Colloids can be prepared by chemical reactions leading to formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate to form sols 

• Electrical Disintegration/ Bredig’s arc Method: This is applied to obtain colloidal sols of metals like gold, silver and platinum.  An electric arc is stuck between the electrodes of the metal immersed in the dispersion medium. The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size.

• Peptization- It is the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this is called peptizing agent. During peptization the precipitate absorbs the one of the ions of the electrolyte on its surface. This causes +ve or –ve charge to develop on the precipitate, which ultimately break up into small particles of the size of a colloid.

Purification of Colloids

The process used for reducing the amount of impurities to a required minimum is called purification of colloids. 

Some of processes used to do so are-

Dialaysis- The process of sepeartaing the particles of a colloid by diffusion through a suitable membrane .

Process – An apparatus called dialyser is used. A bag with a suitable membrane containing the colloid is suspended in a vessel through which fresh water flows continuously. The impurities diffuse through the membrane into the water leaving behind the colloid

Here Crystalloid-Impurities

Electro-dialysis- Dialysis is a slow method. It is made faster by this method. In this method, an electric filed is applied using metal electrodes. These ions present in the colloidal solution migrate out to the oppositely charge to electrodes. It is possible only if the dissolved substance in the impure colloid is an electrolyte.  

Ultrafiltration-

This is the process of separating colloidal particles from the soluble solutes(impurities) using specially prepared filters, which are permeable to all substances except the colloid.

Colloidal particles can usually pass through filter papers as the pores are too large.  An ultrafilter paper can be made by soaking the filter paper in a colloidal solution, hardening by formaldehyde and then finally drying it. As this is slow process, pressure or suction is applied to speed it up. The colloidal particles left on the ultra-filter paper are stirred with fresh dispersion medium(solvent) to form a pure colloid.

Properties of Colloids

Colligative Properties-

The particles in colloids are bigger aggregates than those in a true solution. So, the number of particles in a colloid is lesser than a true solution of the same concentration. The values of colligative properties (osmotic pressure, lowering of vapour pressure, depression in freezing point, elevation in boiling point) are of small order as compared to values shown by true solution at same concentration.

Tyndall effect-

Tyndall effect is the scattering of the light by the particles present in the colloidal solution when viewed at right angles to the passage of light.

It is observed only when-

• The diameter of the dispersed particles is not much smaller than the wavelength of light used
• The refractive indices of the dispersed phase and dispersed medium have a large difference
• This effect was used to make an ultramicroscope and differentiate between true solution and colloids.

Colour-

The colour of the colloidal solution depends on the wavelength of the light scatter by the dispersed particles, size and nature of the dispersed particles and the manner in which it is viewed.  Example- Finest gold sol is red in colour and as the size of the particle keeps increasing its colour changes to blue, then purple and finally gold.

Brownian Movement-

Brownian movement may be defined as continuous zigzag movement of the colloidal particles in a colloidal solution. It depends on the size of the particles and the viscosity of the colloid. Smaller the size of the particle and lesser its viscosity, faster is its motion. This movement is responsible for the stability of sols

Charge on Colloids-

Colloidal particles always have an electric charge. The nature of this charge is the same on all the particles in a given colloidal solution and maybe either +ve  or –ve.

Positively charged sols-

• Haemoglobin
• Oxides-TiO₂ solution
• Hydrated Metal oxides like Al₂O₃.xH₂O , Fe₂O₃.xH₂O
• Dyes-methylene blue sols

Negatively charged sols-

• Metals- Cu, Ag, Au sols
• Metallic sulphides-As₂S₃, CdS
• Acid dye stuffs- Eosin
• Sols of starch, gelatin

Electrophoresis-

The movement of colloidal particles under the influence of an electric field is called electrophoresis. Negatively charged particles move towards the cathode and Positively charged particles moves towards anode.

When the movement of particles is prevented, it is observed that the dispersion medium starts to move in the electric field. This is called electroosmosis.

Coagulation-

 It is process of settling of colloidal particles. Also called precipitation of sol

COAGULATION OF LYOPHOBIC SOLS- 

Coagulation of lyophobic sols can be done by the following methods:

• By electrophoresis – The colloidal particles move towards oppositely changed electrodes get discharged and precipitate.
• By mixing two oppositely charged sols – Oppositely charged sols when mixed together in almost equal proportion, neutralise their charges and get partially or completely precipitated.
• By Boiling- When a sol is boiled the adsorbed layer is disturbed due to increased number of collisions with the molecules of the dispersion medium. This reduces the charge on the particles and they ultimately settle down in the form of a precipitate.

• By Persistent dialysis – On prolonged dialysis, traces of the electrolyte present in the sol are removed almost completely. Colloids become unstable and coagulate.
• By addition of electrolyte – When excess of electrolyte is added, colloidal particles precipitate as colloids interact with ions carrying charge opposite to that present on themselves. This causes neutralisation leading to their coagulation.

Example- A negatively charged ion when added to a positively charged sol causescoagulation. The negatively charged ion is called coagulating ion/flocculative ion as it neutralises the colloid to cause coagulation.

COAGULATION OF LYOPHILLIC SOLS-

Lyophilic sols are stable because of charge and solvation of colloidal particles. So we remove these two factors to coagulate them. This is done by

• Addition of an electrolyte
• Addition of a suitable solvent

PROTECTION OF COLLOIDS –

• Lyophilic sols are more stable than lyophobic sols
• Lyophilic colloids have a unique ability to protect lyophobic colloids from electrolytes
• When a lyophilic sol is added to lyophobic sol, the lyophilic particles (colloids) form a layer around the particles of lyophobic sol
• Lyophilic colloids are also called protective colloids

HARDY-SCHULZE RULE-

• The greater the valency of the flocculating ion added, the greater is its precipitation.
• For negative sols, when positive ions are added

           Al³⁺> Ba²⁺>Na⁺ is the order in terms of flocculating power

• For positive sols, when negative ions are added

           [Fe(CN)₆] ^4-> PO₄^3->SO₄^2->Cl^– is the order in terms of flocculating power

Emulsions

• Emulsions are colloids where both the dispersion phase and dispersion medium are liquids. These two liquids are immiscible or partially miscible. Generally one of the liquids is water.
• There are two types of emulsions

1. Oil dispersed in water (o/w type)- Water acts as dispersion medium

Example –Milk, vanishing cream

2. Water dispersed in oil (w/o type)- Oil acts as dispersion medium

Example- Butter, Cream

• Emulsions like water and oil separate into two layers and make the emulsion unstable. So emulsions are stabilised by stabilising agents.

• o/w emulsions are stabilised by proteins, gum, natural and synthetic soaps
• w/o emulsions are stabilised by metal salts of fatty acids, long chain alcohols

Applications of Colloids

1. Electro precipitation of smoke – The smoke is led through a chamber containing plates having a charged opposite to that carried by smoke particles. The particles on coming in contact with these plates lose their charge and get precipitated.  The particles settle down on the floor of the chamber. The precipitator is called Cottrell precipitator.

2. Purification drinking water – Alum is added to impure water to coagulate the suspended impuritiesand  make water fit for drinking.
3. Medicines – Most of the medicines are colloidal in nature. Colloidal medicines are bmore effective because they have a larger surface area and are more easily absorbed by the body. Eg- Argyrol is a silver sol used as an eye lotion, milk of magnesia is used to cure stomach disorders
4. Tanning –  Animal hides are colloidal in nature. When a hide that has positively charged particles is soaked in tannin/chromium salts, which contains negatively charged particles , mutual coagulation takes place. This results in the hardening of leather. This process is termed as tanning.

5. Cleansing action of soaps- already explained
6. Photographic plates and films –  Photographic plates and films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films.
7. Rubber industry-  Latex is a colloidal solution of rubber particles which are negatively charged. Rubber is obtained by coagulation of latex.
8. Industrial products-  Paints inks, synthetic plastics, rubber, cement, graphite lubricants are all colloids

Chapter-5-Surface-Chemistry

NCERT Solution of Surface Chemistry

Chemistry NCERT Solutions for Class 12  Chapter 5 INTEXT Questions

Question 1. ( Surface Chemistry )
Why are substances like platinum and palladium often used for carrying out electrolysis of aqueous solutions?
Solution:
Due to their inert nature, these metals do not affect the products of electrolysis. They have good adsorbing capacity for hydrogen.

Question 2.( Surface Chemistry )
Why does physisorption decrease with the increase of temperature?
Solution:
Physical adsorption of a gas by a solid is generally reversible. Thus,
Solid + Gas  Gas / Solid + Heat
Since the adsorption process is exothermic, the physical adsorption occurs readily at low temperature and decreases with increasing temperature (Le Chatelier’s principle).

Question 3. ( Surface Chemistry )
Why are powdered substances more effective adsorbents than their crystalline forms?
Solution:
The extent of adsorption increases with increase in surface area of the adsorbent. Finely powdered substances have large, porous areas and act as good adsorbents.

Question 4.( Surface Chemistry )
Why is it necessary to remove CO when ammonia is obtained by Haber’s process?
Solution:
CO is a catalytic poison. It reacts with iron to form iron carbonyl thus inhibiting the activity of catalyst.

Question 5.( Surface Chemistry )
Why is the ester hydrolysis slow in the beginning and becomes faster after sometime?
Solution:
The acid formed during the reaction provides hydrogen ions which act as catalyst for the reaction and it becomes faster.

Question 6.( Surface Chemistry )
What is the role of desorption in the process of catalysis?
Solution:
In catalysis the products formed are desorbed and detached from the surface so that, more reactants can get adsorbed on the surface of catalyst.

Question 7.( Surface Chemistry )
What modification can you suggest in the Hardy Schulze law?
Solution:
The Hardy Schulze law considers the coagulation of sols because of neutralisation of their charges. Since coagulation can also occur by mixing two oppositely charged sols, it should also include “when oppositely charged sols are mixed in proper proportions to neutralize the charges of each other, the coagulation of both the sols occurs.”

Question 8.( Surface Chemistry )
Why is it essential to wash the precipitate with water before estimating it quantitatively?
Solution:
Few impurities which are soluble in water and are adsorbed on the surface of the precipitate are removed by washing them with water.

Chemistry NCERT Solutions for Class 12  Chapter 5 NCERT Exercises

Question 1.
Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.
Solution:
The phenomenon of accumulation of the molecules of a substance on a solid or liquid surface resulting in the increased concentration of the molecules on the surface is called adsorption. In absorption, the substance is uniformly distributed throughout the bulk of the solution. A distinction can be made by taking an example of water vapours. Water vapours are absorbed by anhydrous calcium chloride but adsorbed by silica gel.

	Adsorption	Absorption
1.	It is a surface phenomenon, i.e., it occurs only on the adsorbent surface.	It occurs throughout the body of the material. It is called bulk phenomenon.
2.	The concentration on the adsorbent’s surface is different from that in the bulk.	The concentration is same throughout.
3.	The rate varies throughout the process.	The rate remains the same.

Question 2.( Surface Chemistry )
What is the difference between physisorption and chemisorption?
Solution:

	Physisorption
1.	The adsorbate and adsorbent are held by weak van der Waals forces.	Chemisorption The adsorbate and adsorbent are held by forces similar to a chemical bond.
2.	Heat of adsorption is of the order of 20 kj/mol.	Heat of adsorption is of the order of 200 kj/mol.
3.	It is reversible.	It is irreversible.
4.	It decreases with increase in temperature and occurs at lower temperatures.	It increases with temperature and occurs at high temperature.
5.	It is not specific in nature, i.e., all gases are adsorbed on all solids to some extent.	It is specific in nature and occurs only when a chemical bond is formed between the adsorbate and adsorbent.
6.	Multimolecular layers may be formed on the adsorbent.	Usually unimolecular layer is formed on the adsorbent.

Question 3.( Surface Chemistry )
Give reason why a finely divided substance is more effective as an adsorbent.
Solution:
The extent of adsorption increases with increase in surface area of the adsorbent. Thus, finely divided metals and porous substances having large surface areas are good adsorbents.

Question 4.( Surface Chemistry )
What are the factors which influence the adsorption of a gas on a solid?
Solution:
Factors affecting adsorption of a gas on solids are :

Nature of the adsorbent : The same gas is adsorbed to different extents by different solids at the same temperature. Also, greater the surface area of the adsorbent, more is the gas adsorbed.

Nature of the adsorbate : Different gases are adsorbed to different extents by different solids at the same temperature. Higher the critical temperature of the gas, greater is its amount adsorbed.

Temperature : Since adsorption is an exothermic process, applying Le Chatelier’s principle, we can find out that adsorption decreases with an increase in temperature.

Specific area of the adsorbent : Surface area available for adsorption per gram of the adsorbent increases the extent of adsorption. Greater the surface area, higher would be the adsorption therefore, porous or powdered adsorbents are used.

Pressure : At constant temperature, the adsorption of gas increases with pressure.

Activation of adsorbent : It means increasing the adsorbing power of an adsorbent by increasing its surface area. It is done by :

1. making the adsorbent’s surface rough
2. removing gases already adsorbed
3. subdividing the adsorbent into smaller pieces.

Question 5.( Surface Chemistry )
What is an adsorption isotherm? Describe Freundlich adsorption isotherm.
Solution:
Adsorption isotherm is a graph between the amount of the gas adsorbed by an adsorbent and equilibrium pressure of the adsorbate at constant temperature. Freundlich obtained an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. It is mathematically represented in the following way :

Question 6.( Surface Chemistry )
What do you understand by activation of adsorbent? How is it achieved?
Solution:
Activation of an adsorbent means increasing its adsorbing power by increasing the surface area of the adsorbent by making its surface rough, and removing adsorbed gases from it. With an increase in surface area the adsorption increases.

Question 7.( Surface Chemistry )
What role does adsorption play in heterogenous catalysis?
Solution:
Adsorption of reactants on solid surface of the catalysts increases the rate of reaction. There are many gaseous reactions of industrial importance involving solid catalysts. Manufacture of ammonia using iron as a catalyst, manufacture of H₂SO₄ by Contact process and use of finely divided nickel in the hydrogenation of oils are excellent examples of heterogeneous catalysis.

Question 8.( Surface Chemistry )
Why is adsorption always exothermic?
Solution:
During adsorption, there is always a decrease in residual forces of the surface, i.e., there is decrease in surface energy which appears as heat. Adsorption therefore, is invariably an exothermic process. In other words, ∆H of adsorption is always negative to keep the value of ∆G negative for the reaction to be spontaneous as ∆S decreases during adsorption.

Question 9.( Surface Chemistry )
How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?
Solution:
Colloids can be classified into eight types depending upon the physical state of the dispersed phase and the dispersion medium.

Question 10.( Surface Chemistry )
Discuss the effect of pressure and temperature on the adsorption of gases on solids.
Solution:

1. Adsorption decreases with an increase in, temperature because it is an exothermic process and according to Le Chatelier’s principle the reaction will proceed in backward direction with increase in temperature.
2. At a constant temperature, adsorption increases with pressure.

Question 11.( Surface Chemistry )
What are lyophilic and lyophobic sols? Give one example of each type. Why are hydrophobic sols easily coagulated?
Solution:
There are two types of colloidal sols :

(i) Lyophilic sols : The word lyophilic means solvent loving. They are obtained by directly mixing the dispersed phase and the dispersion medium, e.g., sols of gum, gelatin, starch, etc. They are solvent attracting hence quite stable and cannot be coagulated easily.

(ii) Lyophobic sols : They cannot be prepared by directly mixing the dispersed phase and dispersion medium but are prepared by special methods, e.g., sols of metals. They are solvent repelling. Hydrophobic sols are easily coagulated due to repulsion between water and dispersed phase.

Question 12.( Surface Chemistry )
What is the difference between multimolecular and macromoiecular colloids? Give one example of each. How are associated colloids different from these two types of colloids?
Solution:
Depending upon the type of particles of the dispersed phase, colloids are classified as : multimolecular, macromoiecular and associated colloids.

(i) Multimolecular colloids : On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size in the colloidal range (diameter < 1 nm). The species thus formed are called multimolecular colloids. For example, a gold sol may contain particles of various sizes having many atoms. Sulphur sol consists of particles containing a thousand or more of S8 sulphur molecules.

(ii) Macromoiecular colloids : Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromoiecular colloids. These colloids are quite stable and resemble true solutions in many respects. Examples of naturally occurring macromolecules are starch, cellulose, proteins and enzymes; and those of man-made macromolecules are polythene, nylon, polystyrene, synthetic rubber, etc.

(iii) Associated colloids (Micelles) : There are some substances which at low concentrations behave as normal strong electrolytes but, at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are called micelles. These are also known as associated 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). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is 10^-4 to 10^-3 mol L^-1. These colloids have both lyophobic and lyophilic parts. Micelles may contain as many as 100 molecules or more.

Question 13.( Surface Chemistry )
What are enzymes? Write in brief the mechanism of enzyme catalysis.
Solution:
Enzymes are complex nitrogenous organic compounds which act as biological catalysts and increase the rate of cellular processes. According to the lock and key model, like every lock has a specific key, similarly every enzyme acts at a specific substrate.

When the substrate fits the active site (lock) of the enzyme, the chemical change begins. But it has also been noticed that enzyme changes shape, when substrate lands at the active site. This induced-fit model of enzyme action pictures the substrate inducing the 1 active site to adopt a perfect fit, rather than a rigid shaped lock and key. Therefore, the new model for enzyme action is called induced fit model.

Question 14.( Surface Chemistry )
How are colloids classified on the basis of

1. physical states of components
2. nature of dispersion medium and
3. interaction between dispersed phase and dispersion medium?

Solution:

1. Refer answer number 9.
2. Depending upon the nature of dispersion medium colloids can be classified as sol if the dispersion medium is liquid, gel if the dispersion medium is solid. If the dispersion medium is water, the sol is called hydrosol and if the dispersion medium is alcohol, it is called alcosol. A colloid in which the dispersion medium as well as dispersed phase are liquids, is called emulsion.
3. Refer answer number 11.

Question 15.( Surface Chemistry )
Explain what is observed

1. when a beam of light is passed through a colloidal sol.
2. an electrolyte, NaCI is added to hydrated ferric oxide sol.
3. electric current is passed through a colloidal sol?

Solution:
(i) When a beam of light is passed through colloidal particles, its path becomes clearly visible and is known as Tyndall effect. It is due to scattering of light by colloidal particles. The bright cone of the light is called Tyndall cone.

(ii) When NaCI is added to hydrated ferric oxide sol coagulation takes place. Since ferric oxide is a positive sol, it is coagulated by the negative chloride ions.

(iii) When electric potential is applied across two platinum electrodes dipped in a colloidal solution, the colloidal particles move towards one or the other electrode. The movement of colloidal particles under an applied electric potential is called electrophoresis.

Positively charged particles move towards the cathode while negatively charged particles move towards the anode. Since all the colloidal particles in a given colloidal solution carry the same charge, the particles move to one or the other electrode depending on the charge.

Question 16.( Surface Chemistry )
What are emulsions? What are their different types? Give example of each type.
Solution:
These are liquid-liquid colloidal systems, i.e., the dispersion of finely divided droplets in another liquid. If a mixture of two immiscible or partially miscible liquids is shaken, a coarse dispersion of one liquid in the other is obtained which is called emulsion. Generally, one of the two liquids is water. There are two types of emulsions :

1. Oil dispersed in water (O/W type) and
2. Water dispersed in oil (W/O type).

In the first system, water acts as dispersion medium. Examples of this type of emulsion are milk and vanishing cream. In milk, liquid fat is dispersed in water. In the second system, oil acts as dispersion medium. Common examples of this type are butter and cream.

Question 17.( Surface Chemistry )
What is demulsification? Name two demulsifiers.
Solution:
The process of converting the emulsion back into two distinct components, oil and water is called demulsification. This can be done by

1. boiling
2. freezing
3. changing pH
4. electrostatic precipitation.

Question 18. ( Surface Chemistry )

Action of soap is due to emulsification and micelle formation. Comment.
Solution:
Soap is sodium or potassium salt of a higher fatty acid and may be represented as RCOO^– Na⁺ (e.g., sodium stearate CH₃(CH₂)]₁₆ COO^– Na⁺, which is a major component of many bar soaps). When dissolved in water, it dissociates into RCOO^– and Na+ ions. The RCOO^– ions, however, consist of two parts – a long hydrocarbon chain R (also called non-polar ‘tail’) which is hydrophobic (water repelling), and a polar group COO^– (also called polar-ionic ‘head’), which is hydrophilic (water loving).

The RCOO^– ions are, therefore, present on the surface with their COO^– groups in water and the hydrocarbon chains R staying away from it and remain at the surface. But at critical micelle concentration, the anions are pulled into the bulk of the solution and aggregate to form a spherical shape with their hydrocarbon chains pointing towards the centre of the sphere with COO^– part remaining outward on the surface of the sphere. An aggregate thus formed is known as ‘ionic micelle’.

The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles

(a) Grease on cloth
(b) Stearate ions (from soap) arranging around the grease droplets
(c) Micelle formed

Since the polar groups can interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus soap helps in emulsification and washing away of oils and fats. Tne negatively charged sheath around the globules prevents them from coming together and forming aggregates.

Question 19. ( Surface Chemistry )
Give four examples of heterogeneous catalysis.
Solution:

Question 20. ( Surface Chemistry )
What do you mean by activity and selectivity of catalysts?
Solution:
(a) Activity : The activity of a catalyst depends upon the strength of chemisorption to a large extent. The reactants must get adsorbed reasonably strongly on to the catalyst to become active. But adsorption must not be so strong that they are immobilised. It is observed that maximum activity is shown by elements of groups 7 – 9 of the periodic table
2H₂ + O₂  2H₂O

(b) Selectivity : The selectivity of a calatyst is its ability to yield a particular product in the reaction e.g.,

Thus, a selective catalyst can act as a catalyst in one reaction and may fail to catalyse another reaction.

Question 21. ( Surface Chemistry )
Describe some features of catalysis by zeolites.
Solution:
(a) Zeolites are hydrated aluminosilicates which have a three dimensional network structure containing water molecules in their pores.
(b) The pores are made vacant by heating before catalysis.
(c) The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules and also on the pores and cavities in them, e.g., ZSM-5 converts alcohols to hydrocarbons by dehydrating them.

Alcohols  Hydrocarbons

Question 22. ( Surface Chemistry )
What is shape selective catalysis?
Solution:
The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis. Zeolites are good shape-selective catalysts because of their honeycomb-like structures. They are microporous aluminosilicates with three dimensional network of silicates in which some silicon atoms are replaced by aluminium atoms giving Al-O-Si framework. The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules as well as upon the pores and cavities of the zeolites. They are found in nature as well as synthesised for catalytic selectivity.

Question 23. ( Surface Chemistry )
Explain the following terms :

1. Electrophoresis
2. Coagulation
3. Dialysis
4. Tyndall effect

Solution:
(i) Electrophoresis : Refer answer number 15 (iii)

(ii) Coagulation or precipitation : The stability of the lyophobic sols is due to the presence of charge on colloidal particles. If somehow, the charge is removed, the particles will come nearer to each other to form aggregates (or coagulate) and settle down under the force of gravity. The process of settling down of colloidal particles is called coagulation.

(iii) Dialysis : It is the process of removing dissolved substances from a colloidal solution by means of diffusion through a suitable membrane. Since particles (ions or smaller molecules) in a true solution can pass through animal membrane (bladder) or parchment paper or cellophane sheet but not the colloidal particles, the membrane can be used for dialysis. The apparatus used for this purpose is called dialyser. A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flowing. The molecules and ions diffuse through membrane into the outer water and pure colloidal solution is left behind.

(iv) Tyndall effect : Refer answer number 15 (i)

Question 24. ( Surface Chemistry )
Give four uses of emulsions.
Solution:

1. Some of the medicines are effective as emulsions.
2. Paints are emulsions which are used in our daily life.
3. Soaps and detergents act as cleansing agents, action of which is based on emulsification.
4. Photographic films are coated with emulsion of AgBr on its surface.

Question 25.( Surface Chemistry )
What are micelles? Give an example of a micelle system.
Solution:
Micelles are substances that behave as normal strong electrolytes at low concentration but at high concentrations behave as colloids due to formation of aggregates. They are also called associated colloids, e.g., soaps and detergents. They can form ions and may contain 100 or more molecules to form a micelle.

Question 26. ( Surface Chemistry )
Explain the terms with suitable examples :

1. Alcosol
2. Aerosol
3. Hydrosol

Solution:
(i) Alcosol : The sol in which alcohol is used as dispersion medium is called alcosol e.g., sol of cellulose nitrate in ethyl alcohol.

(ii) Aerosol : The sol in which dispersion medium is gas and dispersed phase is either solid or liquid, the colloidal system is called aerosol e.g., fog, insecticides, sprays, etc.

(iii) Hydrosol : The sol in which dispersion medium is water is called hydrosol e.g., starch sol.

Question 27. ( Surface Chemistry )
Comment on the statement that’colloid is not a substance but a state of substance’.
Solution:
Colloid is not a substance, but a state of substance because the same substance may exist as a colloid or crystalloid under different conditions e.g., sulphur. Colloidal solution of sulphur consists of sulphur molecules dispersed in water. In this state, sulphur atoms combine to form multimolecules whose size lies between 1 nm to 1000 nm and form colloidal state. Sulphur forms true solution in carbon disulphide. Similarly soap is a solution at low concentration but a colloid at higher concentration.
Now that you are provided all the necessary information regarding NCERT Solutions for Class 12 Chemistry Chapter 5 Surface Chemistry and we hope this detailed NCERT Solutions are helpful.

Previous Years’ Questions of Surface Chemistry

Very Short Answer Type Questions [1 Mark]

1. Out of AICI₃ and NaCl, which is more effective in causing coagulation of a negative sol and why?
Answer: AlCL₃, because Al³⁺ has higher charge than Na⁺ ion. Higher the charge, more effective it will be for coagulation.

2. What is the type of charge on Agl colloidal sol formed when AgNO₃ solution is added to KI solution?
Answer: Negative charge because negatively charged I^– ions will get adsorbed on Agl.

3. What is the effect of temperature on adsorption?
Answer: Adsorption decreases with increase in temperature because adsorption is exothermic process.

4. Write the dispersed phase and dispersion medium of paints.
Answer: Solid is dispersed phase and liquid is dispersion medium.

5. Write the dispersed phase and dispersion medium of butter.
Answer: Liquid is dispersed in solid in solidified butter. So, liquid is dispersion medium and solid is dispersed phase.

6. Out of BaCl₂ and KC1, which one is more effective in causing coagulation of a negatively charged colloidal Sol? Give reason.
Answer: BaCl₂ will be more effective because Ba²⁺ has higher charge than K⁺.

7. A delta is formed at the meeting point of sea water and river water. Why?
Answer: Muddy river water is a colloidal solution, which gets coagulated by electrolytes present in sea water.

8. Physisorption is reversible while chemisorption is irreversible. Why?
Answer: In physisorption, forces of attraction are weak and no new substance is formed, therefore, it is reversible, whereas in chemisorption, new substances are formed, therefore, it is irreversible.

Short Answer Type Questions [II] [3 Marks]

9. Define adsorption with an example. Why is adsorption exothermic in nature?
Write the types of adsorption based on the nature of forces between adsorbate and adsorbent.
Answer: The accumulation of molecular species on the surface rather than bulk of solid or liquid, e.g. 0₂, H₂, CO, NH₃ , Cl2, SO₃, taking in a close vessel on charcoal. Adsorption is exothermic because of force of attraction between adsorbate and adsorbent.
Types of adsorption:
(i) Physical adsorption (Weak van der Waals’ forces)
(ii) Chemical adsorption (Due to formation of covalent bonds)

10. Differentiate between the following:
(i) Solution and colloid
(ii) Homogeneous catalysis and Heterogeneous catalysis
(iii) O/W emulsion and W/O emulsion.
Answer: (i) Solution is homogeneous, whereas colloid is heterogeneous.
(ii) In homogeneous catalysis, catalyst and reactants are in the same physical states, whereas in heterogeneous catalysis, reactants and catalyst are in different physical states.
(iii) Oil is dispersed phase and water is dispersion medium in O/W emulsion, whereas water is dispersed phase and oil is dispersion medium in W/O emulsion.

11. Give reasons of the following observations:
(i) Physisorption decreases with increase in temperature.
(ii) Addition of alum purifies the water.
(iii) Brownian movement provides stability to the colloidal solution.
Answer: (i) It is because of weak van der Waals’ forces of attraction which decrease with increase in temperature.
(ii) Alum coagulates mud particles and purify water.
(iii) Brownian movement does not allow colloidal solution to settle down due to constant zig-zag motion.

12.Give reasons for the following observations:
(i) Leather gets hardened after tanning.
(ii) Lyophilic sol is more stable than lyophobic sol.
(iii) It is necessary to remove CO when ammonia is prepared by Haber’s process.
Answer: (i) It is due to coagulation. When animals hide, which is positively charged is soaked in tannin, which contains negatively charged particles, mutual coagulation takes place.
(ii) It is due to more force of attraction between dispersed phase and dispersion medium in Lyophilic sol than Lyophobic sols.
(Hi) CO will react with Fe to form Fe(CO)₅ and acts as catalytic poison (inhibitor), therefore, it must be removed.

13. Write any three differences between Physisorption and Chemisorption.
Answer:

14. Give reasons for the following observations:
(i) A delta is formed at the meeting point of sea water and river water.
(ii) NH₃ gas adsorbs more readily than N₂ gas on the surface of charcoal.
(iii) Powdered substances are more effective adsorbents.
Answer: (i) Muddy river water is a colloidal solution, gets coagulated by electrolytes present in sea water and delta is formed.
(ii) NH₃ has more van der Waals’ forces of attraction than N₂,, as NH₃ is polar and can form H-bonds. Therefore, it can get adsorbed to more extent than N₂.
(iii) Powdered substance has more surface area, therefore, it gets adsorbed to more extent. Greater the surface area, more will be the extent of adsorption.

2014 

Very Short Answer Type Questions [1 Mark]

15. Give one example each of ‘oil in water’ and ‘water in oil’ emulsion.
Answer: Milk is an example of ‘oil in water’, whereas butter (liquid) is an example of ‘water in oil’ emulsion.

16. Give one example each of ‘sol’ and ‘gel’.
Answer: starch in water is an example of ‘sol’, whereas cheese is an example of ‘gel’.
17. Give one example each of lyophobic sol and lyophilic sol.
Answer: Starch in water is lyophilic sol, whereas As2S3 in water is lyophobic sol.

18.Why is adsorption always exothermic?
Answer:  Δ G = Δ H – TΔ S
Δ S = -ve only if Δ H is -ve,
i.e. adsorption is always exothermic process.

19. What are the dispersed phase and dispersion medium in milk?
Answer: Liquid (protein, lactose, fat) is dispersed phase and liquid (water) is dispersion medium.

20. What is the effect of temperature on chemisorption?
Answer: Chemisorption first increases and then decreases with increase in temperature.

21.What type of forces are responsible for the occurrence of physisorption?
Answer: Weak van der Waals’ forces of attraction are responsible for the occurrence of physisorption.

22. Name the temperature above which the formation of micelles takes place.
Answer: Kraft temperature.

23. Based on the type of dispersed phase, what type of colloid is micelles?
Answer: Associated colloids.

24. Define the term ‘Tyndall effect’.
Answer: When light passes through a colloidal solution, its path becomes clearly visible due to scattering of light by colloidal particles. This is called ‘Tyndall effect’.

25. What is meant by ‘shape-selective catalysis’ of reactions?
Answer: Shape-selective catalysis: The catalysis which depends upon the pore structure of the catalyst and molecular size of reactant and product molecules is called shape-selective catalysis, e.g. Zeolites are shape-selective catalysts due to their honeycomb structure. ZSM-5 (Zeolite Sieve of Molecular porosity 5) is used to convert methanol into gasoline (petrol).

26. What is meant by dialysis.
Answer: It is a process of removing dissolved substance from a colloidal solution by means of diffusion through suitable membranes.

Short Answer Type Question [II] [3 Marks]

27. What are emulsions? What are their different types? Give one example of each type.
Answer: Liquid in liquid colloidal dispersions are called emulsions.
Types of emulsions: (i) oil in water emulsions (it) water in oil emulsions.
Milk is an example of oil in water, whereas liquefied butter is an example of water in oil emulsion.

28. (a) In reference to Freundlich adsorption isotherm write the expression for adsorption of gases on solids in the form of an equation.
(b) Write an important characteristic of lyophilic sols.
(c) Based on type of particles of dispersed phase, give one example each of associated colloid and multimolecular colloid.
Answer: (a) log x/m = log k + 1/n log P
where x = mass of the gas adsorbed, m = mass of adsorbent at equilibrium pressure P, k and n are constants.
(b) Lyophilic sols are stable and reversible.
(c) Soaps and detergents form associated colloids, whereas As2S3 and gold sol form multimolecular colloids.

29. (a) Write the expression for the Freundlich adsorption isotherm for the adsorption of gases on solids, in the form of an equation.
(b) What are the dispersed phase and dispersion medium of butter?
(c) A delta is formed at the meeting place of sea and river water. Why?
Answer: (a) Refer Ans. to Q.28 (a).
(b) Refer Ans. to Q.5.
(c) Refer Ans. to Q.14 (i).

30. Define the following terms:
(i) Adsorption (ii) Peptization (iii) Sol.
Answer: (i) Adsorption: When concentration of solute is different at surface than bulk, it is called adsorption.
(ii) Peptization: It is a process of converting freshly prepared precipitate into colloidal solution in the presence of stabilizing agent.
(iii) Sol: When a solid is dispersed in liquid, it is called sol.

31. Giving appropriate examples, explain how the two types of processes of adsorption (physisorption and chemisorption) are influenced by the prevailing temperature, the surface area of the adsorbent and the activation energy of the process?
Answer: Effect of temperature: Physisorption decreases with increase in temperature, where chemisorption first increases, then decreases with increase in temperature. Surface area: Greater the surface area, greater is the physisorption and chemisorption.
Activation energy: In physisorption, no appreciable activation energy is needed, whereas in chemisorption, sometimes high activation energy is needed.

2013

Very Short Answer Type Questions [1 Marks]

32. Of physisorption or chemisorption, which has a higher enthalpy of adsorption?
Answer: Chemisorption because it has high enthalpy of adsorption.

33. Which aerosol depletes ozone layer?
Answer: Chlorofluorcarbons (CFC)

34. What is especially observed when a beam of light is passed through a colloidal solution?
Answer: Its path becomes clearly visible due to scattering of light. It is called ‘Tyndall effect’.

Short Answer Type Questions [I] [2 Marks]

35. Write the dispersed phase and dispersion medium of the following colloidal system:
(i) Smoke (ii) Milk
Answer:

36. What are lyophilic and lyophobic colloids? Which of these sols can be easily coagulated on the addition of small amounts of electrolytes?
Answer: Difference between lyophilic sols and lyophobic sols:

Lyophobic sols can be easily coagulated on addition of small amount of electrolytes.

37. What is the difference between oil/water (O/W) type and water/oil (W/O) type emulsions? Give an example of each type.
Answer: When oil is dispersed in water, it is called oil in water emulsion, e.g. milk. When water is dispersed in oil, it is called water in oil emulsion, e.g. butter.

38. What is the difference between multimolecular and macromolecular colloids? Give one example of each.
Answer: Differences between Multimolecular and Macromolecular colloids.

39.(a) What happens when a freshly precipitated Fe(OH)₃ is shaken with water containing a small quantity of FeCl₃?
(b) Why is a finely divided substance more effective as an adsorbent?

Answer: (a) Colloidal solution of Fe(OH)₃ is formed. FeCl₃acts as peptizing agent.
(b) It is due to greater surface area and it has more active sites. Therefore, extent of adsorption is more.

40. Write two differences between lyophobic and lyophilic sols. Give one example of each type of sol.
Answer: Refer Ans. to Q.36.

41. What is special about the following terms:
(i) Kraft Temperature (ii) Sorption
Answer: (i) Kraft Temperature (Tk): Micelles form ionic surfactants (surface active agents like soaps and detergents) only above a certain temperature called Kraft temperature.
(ii) Sorption: When adsorption and absorption take place simultaneously, it is called sorption, e.g. dyeing of cotton fabrics. The dye is adsorbed on the surface of cotton fibre but after it is dyed, the fibre has dye uniformly throughout, i.e. absorbed.

42. Write the dispersed phase and dispersion medium of the following colloids:
(i) Cheese (ii) Fog
Answer: (i) Dispersed phase is liquid (water), dispersion medium is solid (cheese).
(ii) Dispersed phase is liquid (water vapours), dispersion medium is gas(air).

43. Write the differences between physisorption and chemisorption with respect to the following:
(i) Specificity (ii) Temperature dependence
(iii) Reversibility and (iv) Enthalpy change
Answer: Differences between Physical and Chemical adsorptions.

44. Describe a chemical method each for the preparation of the sols of sulphur and platinum in water.
Answer:Sulphur sol can be prepared by oxidation of H2S with the help of cone. HNOg.

Bredig’s Arc Method:
Platinum (Pt) sol can be prepared by Bredig’s arc method. Two Pt metal electrodes are takeri through which high voltage is passed so that metal gets vaporised. When vapours are condensed, colloidal solution of Pt in water is obtained.

Short Answer Type Question [II] [3 Marks] 

45. What are the characteristics of the following colloids? Give one example of each.

(i) Multimolecular colloids
(ii) Lyophobic sols
(iii) Emulsions
Answer: (i) Multimolecular colloids: Colloidal particles consist of aggregates of small atoms or molecules. For example, As₂S₃ sol.
(ii) Lyophobic sols: There is no affinity between dispersed phase and dispersion medium. They are prepared by indirect methods. For example, Fe(OH)₃sol and gold sol.
(iii) Emulsions: When liquid is dispersed in liquid, it is called emulsion, e.g. milk and butter (liquid).

46. Define the following terms giving an example of each:
(i) Associated colloids (ii) Lyophilic sol (iii) Adsorption
Answer: (i) Associated colloids: These colloids behave like electrolytes in low
concentration but at high concentrations, due to aggregated particles, form colloidal solution.
They form ions in low’ concentration but ions get aggregated in high concentration, e.g. soaps and detergents (surface active agents).
(ii) Lyophilic sol: There is a force of attraction between dispersed phase and dispersion medium. They are easily prepared by shaking dispersed phase with dispersion medium. For example, gum and starch in water form lyophilic sols.
(iii) Adsorption: The existence of a substance at a surface in different concentration than in the adjoining bulk is called adsorption, e.g. 0₂, H₂, CO, Cl₂, NH₃ or S0₂ get adsorbed on the surface of charcoal.

47. Define the following terms writh an example in each case:
(i) Macromolecular sol (ii) Peptization (iii) Emulsion
Answer: (i) Macromolecular sol: Dispersed particles are large molecules (usually polymers). They have force of attraction between dispersion medium and dispersed phase, e.g. albumin in water, proteins and stajrch in water, cellulose nitrate in alcohol (collodion).
(ii) Peptization: The conversion of precipitate into colloidal solution in presence of peptizing agent is called peptization. Peptizing agent is generally an electrolyte. In this process, dispersion medium (liquid) breaks up the precipitate into colloidal state.
(iii) Emulsion: Emulsion is liquid dispersed in liquid, e.g. milk.

48. (a) Which will adsorb more gas, a lump of charcoal or its powder and why?
(b) Describe the preparation of the following colloidal solutions. Name the method used in each case:
(i) Silver sol (ii) Sulphur sol
Answer: (a) Charcoal powder will adsorb more gas because it has more surface area.
(b) (i) Bredig’s Arc method: Take silver electrodes in the form of electric arc in water and pass high voltage through it. A lot of heat will be generated which will vaporise silver metal into vapours which on condensing form silver sol.
(ii) Oxidation: Pass H₂S gas through aqueous solution of nitric acid. Colloidal solution of sulphur will be obtained.

49. (a) What are the two types of emulsions and how do they differ from one another? Give one example of each.
(b) Which one of the following electrolytes is most effective for the coagulation of Fe(OH)3 sol and why?
NaCl, Na₂SO₄, Na₃P0₄
Answer: (a) (i) Oil in water: When oil is dispersed in water, it is called oil in water emulsion, e.g. milk.
(ii) Water in oil: When water is dispersed in oil, it is called water in oil emulsion, e.g. butter in liquid state.
(b) Fe(OH)₃ is positively charged colloid, therefore, Na₃P0₄ will be most effective for coagulation. It is because P0₄ ions are negatively charged and have highest charge among these electrolytes.

2012

Very Short Answer Type Questions [1 Mark]

50. Define peptization?
Answer: Refer Ans. to Q.47 (ii).

51. What is meant by ‘shape-selective catalysis’?
Answer: Refer Ans. to Q.25.

52. How is sol different from an emulsion?
Answer: Sol: When solid is dispersed in liquid, it is called sol.
Emulsion: When liquid is dispersed in liquid, it is called emulsion.

53. Why is adsorption phenomenon always exothermic?
Answer: Refer Ans. to Q.18.

Short Answer Type Questions [i] [2 Marks]

54. Explain the following terms giving one example for each:
(i) Micelles (ii) Aerosol
Answer: (i) Refer Ans. to Q.58 (iii).
(ii) Refer Ans. to Q.58 (i).

55. Name the two groups into which phenomenon of catalysis can be divided. Give an example of each group with the chemical equation involved.
Answer: (i) Homogeneous catalysis
(ii) Heterogeneous catalysis
(i) Homogeneous catalysis:

(ii) Heterogeneous catalysis:

56. What is meant by coagulation of a colloidal solution? Describe any three methods by which coagulation of lyophobic sols can be carried out.
Answer: Coagulation: The process of converting a colloidal solution into precipitate is called coagulation.
Coagulation of lyophobic sols can be carried out by
(i) Electrophoresis: The colloidal particles get attracted towards oppositely charged electrodes, get discharged and coagulated.
(ii) Mutual coagulation: When negatively charged colloid is mixed with positively charged colloid, mutual coagulation takes place.
(iii) Boiling: The charge on colloidal particles is reduced due to collision of colloidal particles with particles of dispersion medium which leads to coagulation.

Short Answer Type Question [II] [3 Marks]

57. Explain what is observed when
(i) an electric current is passed through a sol.
(ii) a beam of light is passed through a sol.
(iii) an electrolyte (say NaCl) is added to ferric hydroxide sol.
Answer: (i) The colloidal particles get attracted towards one of the electrodes depending upon charge on colloidal particles.
(ii) The path of light becomes clearly visible due to scattering of light by colloidal particles.
(iii) The sol gets coagulated.
58. Explain the following terms giving a suitable example for each:
(i) Aerosol (ii) Emulsion
(iii) Micelles
Answer:

(ii) Refer Ans. to Q.47 (iii).
(iii) Micelles: Soaps and detergents consist of lyophilic and lyophobic parts which associate together to form micelles.

59. Differentiate among a homogeneous solution, suspension and a colloidal
solution, giving a suitable example of each.
Answer: Homogeneous solution: The particle size is less than 1 nm. Particles cannot be seen even with powerful microscope. It is transparent, e.g. salt solution. Suspension: It is heterogeneous, particle size is greater than 1000 nm. Particles can be seen with naked eye. It is opaque, e.g. chalk powder in water. Colloidal solution: It appears to homogeneous but actually heterogenous. Particles can be seen with powerful microscope. It is translucent, e.g. milk.

60. Present a classification of colloids where dispersion medium is water. State the characteristics and one example of each of these classes.
Answer: (i) Sol: When solid is dispersed in water, it is called sol, e.g. starch sol and gold sol.
(ii) Emulsion: When liquid is dispersed in water, it is called emulsion, e.g. milk.
(iii) Foam: When gas is dispersed in water, it is called foam or froth, e.g. lemonade froth.

61. Define each of the following terms:
(i) Micelles (ii) Peptization
(iii) Electrophoresis
Answer: (i) Refer Ans. to Q.58 (iii).
(ii) Refer Ans. to Q.47 (ii).
(iii) Electrophoresis: The movement of colloidal particles towards positive or negative electrode in an electric field is called electrophoresis. It occurs due to the presence of positive or negative charge on colloids.

62. Write three distinct features of chemisorptions which are not found in
physisorptions.
Answer: Refer Ans. to Q.13.

63. How are the two types of emulsions different from one another? Give suitable examples to justify the difference. State two applications of emulsions.
Answer: When oil is dispersed in water, it is oil in water emulsion, e.g. milk. It w-e add water, it mixes up showing that it is oil in water emulsion. When water is dispersed in oil, it is called water in oil emulsion, e.g. in butter, if we add oil, it will get mixed up”, so, it is water in oil emulsion.
Applications:
(i) Emulsifying properties of soaps and detergents are used in washing clothes, crockery, etc.
(ii) Digestion of fats in the intestine is facilitated by emulsification.

2011 

Very Short Answer Type Questions [1 Mark]

64. What is meant by ‘shape-selective catalysis’?
Answer: Refer Ans. to Q.25.

65. Define electrophoresis.
Answer: Refer Ans. to Q.61 (iii).

66. What are lyophobic colloids? Give one example for them.
Answer: The colloids in which there is least force of attraction between dispersed phase and dispersion medium e.g. As2S3 sol.

67. Why is a finely divided substance more effective as an adsorbent?
Answer: Refer Ans. to Q.39 (b).

68. What is meant by chemisorption?
Answer: Chemisorption: If the forces holding the adsorbate are as strong as in chemical bonds, the adsorption process is known as chemical adsorption or chemisorption.

69. What is the basic difference between adsorption and absorption?
Answer: Adsorption is a selective absorption when cone, of solute is more at the surface and less in the bulk. Absorption is a process in which cone, of solute is uniform throughout.

Short Answer Type Questions [I] [2 Marks]

70. Define the following terms giving an example of each:
(i) Emulsion
(if) Hydrosol
Answer: (i) Refer Ans. to Q.45 (iii).
(ii) When solid is dispersed in water, it is called hydrosol, e.g. starch dispersed in water.

71. Write four distinguishing features of operative between chemisorption and physisorption.
Answer: Refer Ans. to Q.43.

72. Define the following terms:
(i) Aerosol (ii) Coagulation of colloids
Answer: (i) Refer Ans. to Q.58 (i).
(ii) The process of converting colloidal solution into precipitate is called coagulation of colloids.

Short Answer Type Questions [II] [3 Marks]

73. Define each of the following terms:
(i) Micelles (ii) Peptization
(iii) Electrophoresis
Answer: Refer Ans. to Q.61.

74. Explain how the phenomenon of adsorption finds application in each of the following processes:
(i) Production of vacuum (ii) Heterogeneous catalysis (iii) Froth Floatation process
Answer: (i) Production of vacuum: The high vacuum can be created by adsorbing remaining gases by charcoal after removing air by vacuum pump.
(ii) Heterogeneous catalysis: When the catalyst forms a separate phase (usually a solid phase) from reactants, it is said to be heterogeneous and the catalysis is heterogeneous catalysis. The example of heterogeneous catalysis is manufacture of H₂S0₄ in contact process using V₂O₂ as catalyst. Usually, in a heterogeneous catalysis, the reactants are gases and reaction starts from the surface of the solid catalyst. This is the reason why heterogeneous catalysis is also called ‘surface catalysis’.

(iii) Froth Floatation Process: The principle of froth floatation process is that sulphide ores are preferentially wetted by (adsorbed on) the pine oil and frothing agent, whereas the gangue particles are wetted by the water.

75. Classify colloids where the dispersion medium is water. State their characteristics and. write an example of each of these classes.
Answer: Refer Ans. to Q.60.

76. Explain the following terms:
(i) Electrophoresis
(ii) Dialysis
(iii) Tyndall effect
Answer: (i) Electrophoresis: The movement of colloidal particles towards positive or negative electrode in an electric field is called electrophoresis. It occurs due to the presence of positive or negative charge on colloids.
(ii) Dialysis: It is a process of purification of sols containing electrolyte by keeping the sol in a bag made up of parchment or cellophane and suspending the bag in pure water. Small molecules or ions can pass through the membrane, but the sol is retained.
(iii) Refer Ans. to Q.24.

77. Explain what is observed when
(i) an electric current is passed through a sol.
(ii) a beam of light is passed through a sol.
(iii) an electrolyte (say NaCl) is added to ferric hydroxide sol.
Answer: (i) The colloidal particles get attracted towards one of the electrodes depending upon the charge on colloidal particles.
(ii) The path of light becomes clearly visible due to scattering of light by colloidal particles.
(iii) The sol gets coagulated.

2010 

Very Short Answer Type Questions [1 Mark]

78. Name the two types of adsorption phenomenon.
Answer: (i) Physical adsorption (physisorption)
(ii) Chemical adsorption (Chemisorption)

79. What is the basic difference between adsorption and absorption?
Answer: Refer Ans. to Q.69.

80. What is meant by Reversible sols.
Answer: Reversible sols: Those sols in which dispersion medium can be separated from the dispersed phase (e.g. by evaporation), the sol can be made again by remixing with dispersion medium, e.g. lyophilic sols.

81. What is an emulsion?
Answer: Refer Ans. to Q.45 (iii).

Short Answer Type Questions [I] [2 Marks]

82. Describe the following:
(i) Tyndall effect (ii) Shape-selective catalysis.
Answer: (i) Refer Ans. to Q.24.
(ii) Refer Ans. to Q.25.

83. What is meant by coagulation of a colloidal solution? Name any method by which coagulation of lyophobic sols can be carried out.
Answer: Refer Ans. to Q.56.

84. What are emulsions? State one application of emulsification.
Answer: Liquids dispersed in liquids are called emulsions. Cleansing action of soaps and detergents is based on emulsification.

85. What is the difference between a colloidal solution and emulsion? What is the role of emulsifier in forming emulsion?
Answer: Colloidal solution is a solution in which either solid is dispersed in solid, liquid or gases or liquid is dispersed in solid, liquid or gases or gases are dispersed in solids or liquids.
Emulsions are colloidal solution in which liquid is dispersed in liquid. Emulsifier stabilizes the emulsion.

86. Define the following:
(i) Peptization (ii) Reversible sols.
Answer: (i) Refer Ans. to Q.30 (ii).
(ii) Refer Ans. to Q.80.

Short Answer Type Questions [II] [3 Marks]

87. State what is observed when
(i) an electrolyte, NaCl, is added to hydrated ferric oxide sol.
(ii) an electric current is passed through a colloidal solution.
(iii) a beam of light is passed through a colloidal solution
Answer: (i) The sol gets coagulated.
(ii) The colloidal particles get attracted towards one of the electrodes depending upon charge on colloidal particles.
(iii) The path of light becomes clearly visible due to scattering of light by colloidal particles.

88. What is the difference between multimolecular and macromolecular colloids? Give one example of each type. How are associated colloids different from these two types of colloids?
Answer: (i) Multimolecular colloids: Those colloids in which small atoms or molecules combine together to form the particles of colloidal size are called multimolecular colloids, e.g. gold sol, sulphur sol, As₂S₃ sol, etc.
(ii) Macromolecular colloids: Those colloids in which dispersed particles are macromolecules (bigger molecules), i.e. polymers are called macromolecular colloids, e.g. starch in water, gum in water.
(iii) Associated colloids: Those colloids which behave as strong electrolyte at low concentrations, but show colloidal properties at higher concentration due to the formation of aggregated particles of colloidal dimensions. Such substances are called associated colloids, e.g. soaps and synthetic detergents belong to this class. Micelles are the aggregated particles or associated colloids in solution.

89. Discuss the effect of pressure and temperature on the adsorption of gases on solids. Describe the application of adsorption in controlling humidity.
Answer: Adsorption is a process in which the concentration of solute is more at the surface and less in the bulk. It is called positive adsorption.
Physical adsorption of a gas on solid decreases with increase in temperature and increases with increase in pressure.
Chemical adsorption first increases and then decreases with increase in temperature.
Chemical adsorption first increases and then becomes independent of pressure with increase in pressure. x/m represents the extent of adsorption where x is the mass of adsorbate and m is mass of adsorbent.

90. How are the following colloids different from each other in respect of their dispersion medium and dispersed phase? Give one example of each.
(i) Aerosol
(ii) Emulsion
(iii) Hydrosol
Answer:

91. Write three distinct features of chemisorptions which are not found in physisorptions.
Answer: Refer Ans. to Q. 13.

92. Write four features of chemisorptions which are not found in physisorptions. Illustrate your answer with suitable examples.
Answer: Refer Ans. to Q.43.

2009

Very Short Answer Type Questions [1 Mark]

93. What is an emulsion?
Answer: Refer Ans. to Q.45 {iii).

94. What is the ‘coagulation’ process?
Answer: The conversion of a colloidal solution into precipitate is called coagulation. In other words, the process of settling of colloidal particles is called coagulation.

95. Define the term Tyndall effect.
Answer: Refer Ans. to Q.24.

Short Answer Type Questions [II] [3 Marks]

96. State what is observed when
(i) an electrolyte, NaCl, is added to hydrated ferric oxide sol.
(ii) an electric current is passed through a colloidal solution.
{iii) a beam of light is passed through a colloidal solution
Answer: Refer Ans. to Q.87.

97. What is the difference between multimolecular and macromolecular colloids? Give one example of each type. How are associated colloids different from these two types of colloids?
Answer: Refer Ans. to Q.88.

98. How are the following colloids different from each other in respect of their dispersion medium and dispersed phase? Give one example of each.
(i) Aerosol {ii) Emulsion (iii) Hydrosol
Answer: Refer Ans. to Q.90.

( Surface Chemistry )

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