Surface Chemistry | Chapter 5 | Chemistry | Class 12 | NCERT |

Surface Chemistry | Chapter 5 | Chemistry | Class XII | NCERT |

 

Surface Chemistry 

Surface Chemistry is the branch of chemistry which deals with phenomenon that occur at surfaces or interfaces i.e at the boundary separating two bulk phases .

Surface 

When particles are arranged then the outer covering that is in contact with surrounding is called as surface .     

Surface or interface is present only in solid and liquids . Due to complete miscibility between the gases , there is no interface between the gases .                                     

 Adsorption 

The phenomenon of attraction or accumulation of molecular species of a substance on the surface of solid or liquid resulting into higher concentration of molecules on surface is called adsorption.                    Adsorbate 

The substance adsorbed on the surface is called the adsorbate.                                   

Adsorbent 

The substance on which adsorbate is adsorbed is called adsorbent.               

 Desorption 

The reverse process i.e. removal of the adsorbed substance from surface is called desorption ( which can be brought by heating or reducing the pressure).                                                                                                              Occlusion 

The adsorption of gases on surface of metal is called occlusion.                                                             

Sorption 

Both adsorption as well as absorption takes place simultaneously is called sorption.

Distinction between Adsorption and Absorption

Adsorption Absorption
It is a surface phenomenon. It is a bulk phenomenon .
The concentration of adsorbate is high at surface . The concentration of adsorbate is same throughout the material .
Rate of adsorption is high at beginning and then decreases till equilibrium is attained . Rate of adsorption remains same throughout the process.
Ex : silica gel adsorb water vapour on its surface . Ex : Anhydrous CaCl2 absorb water vapour and form CaCl2.2H2O

Mechanism of Adsorption

 Adsorption arises due to the fact that molecules on the surface are not in the same environment as particles inside the bulk . A molecule in the interior is completely surrounded by other molecules on all sides . However , a molecule on the surface are not surrounded by atoms or molecule of their kind on all sides and possess unbalanced or residual attractive force . These forces are responsible for adsorption .

Thermodynamics of Adsorption

  • When adsorbate is attracted toward adsorbent then energy is released due to attraction between them and this energy is called released surface energy or enthalpy of adsorption . Thus , adsorption is exothermic .
  • When a gas is adsorbed , the freedom of movement of its molecules become restricted . This results in decrease in entropy of gas . Thus is negative for adsorption . 

We know ,ΔG = ΔH – TΔS ; for the adsorption process to occur ΔG must be negative i.e ΔH should have high negative value than – TΔS. As adsorption proceeds , ΔH keeps on decreasing and  TΔS keeps on increasing till ΔG = 0 i.e equilibrium is attained . This state is called adsorption equilibrium.

You May Also Read The Solid State, Solutions, Electrochemistry, Chemical Kinetics, General Principles and Processes of Isolation of Elements, The P Block Elements Group 15 Part 1, The P Block Elements Group 16 Part 2, The P Block Element Group 17 and 18 Part 3, The d-And F-Block Elements, Coordination Compounds, Haloalkanes and Haloarenes, Alcohols, Phenols and Ethers, Aldehydes, Ketones & Carboxylic Acids, Amines, Biomolecules, Polymers, Chemistry in Everyday Life for acquiring more knowledge about Chemistry and its chapters.

Factors affecting Adsorption

1) Nature and surface area of adsorbent : Same gas is adsorbed to different extent by different solids at same temperature . Further , greater the surface area of adsorbent greater is the volume of gas adsorbed               

Thus , substances like charcoal and silica gel are excellent adsorbent because they have highly porous structure and large surface area .                                                                      

2) Nature of adsorbate : Different gases are adsorbed to different extent by same adsorbent at same temperature . Higher the critical temperature , greater is the gas adsorbed                                                           

 3) Temperature : The adsorption decreases with increase in temperature                                             

4) Pressure : Adsorption increases with increase in pressure                                                                        

 5) Activation of solid adsorbent : It means increasing the adsorbing power of adsorbent . This is usually done by increasing the surface area of adsorbent which is achieved by following ways : 

  • By making the surface of adsorbent rough
  • By subdividing the particle into smaller pieces
  • By removing the gases already adsorbed

Types of Adsorption 

Properties Physisorption Chemisorption
Nature of force Weak vanderwaal force Formation of chemical bond
Heat of adsorption Less value of heat of adsorption ( 20 – 40 KJ/mol) High value of heat of adsorption ( 80 – 240 KJ/mol)
Activation Energy Ea is insignificant . Ea is significant .
Specificity It is not specific in nature It is highly specific in nature
Structure of absorbed layer Formation of multimolecular layer of absorbate on surface Formation of unimolecular layer of absorbate on surface
Reversibility Reversible Irreversible
Speed or rate of reaction Very fast speed i.e instantaneous speed equilibrium is readily reached Slow rate and rate also depend on nature of adsorbate and adsorbent

                                                 

Adsorption Isotherm 

A graph between the amount of gas adsorbed per gram of the adsorbent ( x / m) and the equilibrium pressure of the adsorbate at constant temperature is called the adsorption isotherm .

                       

 

At low pressure

x / m is directly proportional to temperature .

x / m ∝   P1 

x / m = constant x P 

At high pressure :

x / m is independent of pressure. 

x / m ∝   P0  

x / m = constant

At moderate pressure :  

x / m will depend on P raised to power between 1 and 0 i.e fraction

x / m ∝ P 1/n   

x / m = k  P 1/n   

taking logarithm both sides

log ( x /m ) = log k + (1/n) log P    

Freundlich Equation for Solution 

Freundlich equation describes the behaviour of adsorption from solution . The concentration term is taken instead of pressure .

x / m ∝ c 1/n   

x / m = k  c 1/n   

taking logarithm both sides

log ( x /m ) = log k + (1/n) log c

Adsorption isobar 

A graph between the amount of gas adsorbed per unit of adsorbent (x/m) and temperature at a constant equilibrium pressure of adsorbate gas is known as adsorption isobar .                                                        

 For physisorption 

As temperature increases , extent of physisorption decreases at constant pressure .

                                                           

For chemisorption 

As temperature increases extent of chemisorption increases initially but after sometime it start to decrease . 

 

Application of Adsorption 

  • Production of high vacuum : Remaining traces of air can be adsorbed by charcoal from a vessel .
  • In gas masks : Gas mask ( a device which consist of activated charcoal ) is used for breathing in coal mines to adsorb poisonous gases .
  • In separation of inert gases : Different inert gases are adsorbed to different extent at same temperature on coconut charcoal .
  • In curing disease : Some drugs can adsorb the germs on them and hence kill them and save them from disease .
  • In clarification of sugar : Sugar is decolourised by treating sugar solution with charcoal powder .
  • In removing moisture from air in storage of delicate instrument : Such instrument which may be harmed by contact with moist air are kept out of contact with moisture using silica gel.

Catalysis 

Catalyst is a substance which can change the rate of chemical reaction without itself undergoing any change in mass and chemical composition at the end of the reaction and the phenomenon is known as catalysis .  .                                                  Positive catalysis

If a catalyst increases(accelerates) the speed of reaction , it is called a positive catalyst and the phenomenon is called positive catalysis .                                                        

Negative catalysis 

If a catalyst decreases(retards ) the speed of reaction , it is called a negative catalyst and the phenomenon is called negative catalysis .                                

Promoters 

A substance if present along with the catalyst enhances the activity of the catalyst , it is called a catalytic promoter , e.g., molybdenum acts as a promoter for iron catalyst in the manufacture of ammonia by Haber’s process.                           

Poisons 

Certain substances like arsenic , CO if present lower the activity of the catalyst . These substances are called catalytic poisons . 

Types of Catalysis

 1) Homogenous catalysis 

 The catalysis in which catalyst and reactant are present in same phase is called Homogenous catalysis .

Ex : 2 SO 2 + O 2 → 2 SO

Sulphur dioxide and oxygen are reactant and nitric oxide is catalyst 

C12H22O11 + H2O → C6H12O6 (glucose) + C6H12O6 (fructose )

Mechanism 

Homogenous catalysis is based on intermediate compound theory . In this a catalyst enters in chemical combination with one or more reactant forming an intermediate compound which decomposes or combine with remaining reactant to product .  

2) Heterogenous Catalysis 

If a catalyst and reactant are present in different phase then it is called heterogenous catalyst and phenomenon is called hetrogenous catalysis .

Haber’s Process :   

    

Mechanism

Heterogenous catalysis depend on modern adsorption theory                                               

  • Diffusion of reactant molecule on surface of catalyst.
  • Adsorption of reactant molecule on surface catalyst.
  • Occurrence of chemical reaction takes place between reactant and catalyst.
  • Desorption of product molecule from catalyst surface.
  • Diffusion of product molecule away from catalyst.

Important Features of Solid Catalyst

1) Activity  By activity of catalyst we mean its capacity to increase the speed of chemical reaction . A catalyst can increase the speed of reaction by 1010 times . The activity depends upon the extent of chemisorptions . The adsorption should be reasonably strong but not so strong that adsorbed molecules become immobile and no space is available for other reactant to get adsorbed .                                            

 2) Selectivity : By selectivity of catalyst we mean its ability to direct reaction to form particular product excluding others                                                                                                         

  CO and H2 react to form different product in presence of different catalyst . 

       

Shape Selective Catalysis 

  • The catalysis which takes place with the help of zeolite is called shape selective catalysis .
  • Zeolites are aluminosilicates with general formula 

         

  • They are microporous three dimensional network silicates in which silicon atoms are replaced by aluminum atoms .
  • They have honey comb like structure.
  • Zeolites to be used as catalyst are heated ion vacuum so that water of hydration is lost .
  • The size of the pores generally varies between 260pm and 740pm . Thus , only those molecules can be adsorbed in these pores whose size is small enough to enter theses cavities and also leave easily .
  • Consequently , reaction taking place in zeolites depends upon the size and shape of reactant and product molecules as compared to those of pores and cavities of zeolite . Thus , reactions are called shape – selective catalysis
  • Zeolite used in petroleum industry is ZSM-5 ( zeolite sieve of molecular Porosity 5) .

Enzymes 

All biochemical reaction are catalysed by special catalysts called enzymes .Enzymes are defined as bio-chemical catalyst. Chemically all enzymes are globular proteins (which are complex organic compound ) with higher molar mass ranging from 15,000 to 1,000,000 g mol-1 and form colloidal solution in water . Enzymes are produced by living plants and animals .

Inversion of cane sugar 

Characteristics of Enzymes 

1) Specificity : Enzymes are highly specific in nature i.e each enzyme catalyses only one reaction  NH2CONH2 + 2 H2O → 2NH3 + CO2                                                     

2) Efficiency : Enzymes are very efficient catalysts . They speed up the reaction by factors upto 1020. One molecule of enzyme may transform one million molecules of the reactant per minute .                                                                                                     

3) Small quantity : Only small quantity of enzymes can be highly efficient .                                       

4) Highly active under optimum pH : The rate of enzyme catalysed reaction is maximum at particular pH called optimum pH i.e around 7.4                                                                   

5) Increasing activity in presence of activators and co-enzymes : The activity of certain enzymes is increased in presence of certain substances called co-enzymes . If a protein contain small amount of vitamins as the non protein part , its activity is enhanced considerably . 

Mechanism of Enzyme Catalysis 

The specificity of enzyme is due to presence of some specific region called the active sites on their surface . These active sites have functional groups such as –NH2 , -COOH which form weak bond with reactant . The shape of active site of any given enzyme is like a cavity such that

only specific substrate can fit into a particular lock . This leads to formation of enzyme substrate complex . This complex breaks to give product and release enzyme .

Colloids 

A colloid is a heterogenous system in which one substance is dispersed as very fine particles in another substance .

Dispersed phase  It is the component present in small proportion .                                                   

 Dispersion medium It is the component present in excess                                                             

Classification of Colloids

Classification of Colloid Based on Physical State of Dispersed Phase and Dispersion Medium

Dispersed Phase Dispersion Medium Type of Colloid Examples
Solid Solid Solid Sol Some coloured glasses
Solid Liquid Sol Paints
Solid Gas Aerosol Smoke
Liquid Solid Gel Cheese , butter
Liquid Liquid Emulsion Milk , hair cream
Liquid Gas Aerosol Fog , mist
Gas Solid Solid sol Foam rubber
Gas Liquid Foam Whipped cream

Classification Based on Nature of Interaction Between Dispersed Phase and Dispersion Medium 

Property Lyophilic sols Lyophobic sols
Ease of Preparation Prepared easily by direct mixing with liquid Cannot be prepared directly , prepared by some special method
Stability They are quite stable and not easily precipitated They are less stable since they are easily precipitated by addition of suitable electrolyte .
Hydration Highly hydrated Less hydrated
Reversibility They are reversible in nature They are irreversible in nature
Visibility Particles cannot be seen under microscope Particles can be seen under microscope only .
Viscosity Their viscosity is much higher than dispersion medium Their viscosity is same as dispersion medium
Surface tension Their surface tension is usually lower than that of dispersion medium Their surface tension is nearly same as that of dispersion medium .

 Classification Based on Type of Particle of the Dispersed Phase                                                1)Multimolecular Colloids 

A large number of atom or molecules of a substance ( size less than 1nm) aggregate in a suitable dispersion medium and form colloidal range of solution are called multimolecular colloids . Ex : sulphur sol                                    

2)Macromolecular Colloids 

When large size molecules or higher mass molecules starts to decompose or dispersed in a suitable dispersion medium and form colloidal range of solution . Ex : starch , Cellulose                                                                                     

3)Associated Colloids

The substances which when dissolve in a medium at low concentration behave as normal , strong electrolyte but at higher concentration exhibit colloidal state properties due to formation of aggregated particles are called as associated colloids . These aggregated particles are called micelles .                                   

Kraft Temperature 

The temperature above which micelle formation takes place is called kraft temperature .                                                                                                                  

Critical Micelle concentration (CMC) 

The concentration above which micelle formation takes place is called critical micelle concentration

Mechanism of Micelle Formation 

Let us take the example of soap solution . Soap in solution  behave as associated colloid . Soap is sodium or potassium salt of higher fatty acid and represented as RCOONa+ or RCOOK+ .                                                                     

When soap is dissolved in water then it dissociates into RCOOand Na+ ions . The RCOO ions have two parts – a long hydrocarbon chain R (hydrophobic) and a polar group COO (hydrophilic).                                                                                    The RCOO ions are therefore present on surface with COO group in water and hydrocarbon chain R staying away from it and remain at surface . But at CMC ions anions are pulled into bulk of the solution and aggregate to form a spherical shape with hydrocarbon chain pointed towards the centre of sphere . An aggregate thus formed is called ionic micelle .

surface chemistry

Cleansing Action of Soap 

It is due to the fact that soap molecules form micelle around the oil droplet present in cloth . The hydrocarbon part of stearate ion enter in oil droplet and COO part goes away and interact with water molecule . The oil droplet surrounded by stearate ion is now pulled in water and hence removed from dirty cloth . The oil droplet thus break up and form small globules . 

surface chemistry

Preparation of Colloids    

                                          Chemical Method                                                                 

1) By double decomposition : As2O3 + 3H2S → As2S 3 (colloidal solution ) + 3 H2S                                       

2) By hydrolysis : FeCl 3 + 3 H2O  → Fe(OH) 3 (colloidal solution) + 3HCl                                                                                 Electrical Disintegration or Bredig’s Arc Method 

This method is used to prepare colloidal solution of metal such as platinum , silver , copper and gold .            Two electrodes of metal whose colloidal solution is to be prepared are immersed in dispersion medium such as water. This medium is kept cooled by surrounding it with ice . An electric arc is struck between the electrodes . The intense heat of arc first changes the metal into vapour and get condensed immediately in liquid to colloidal solution .

                                                         surface chemistry 

Peptization 

The process of converting a freshly prepared precipitate into colloidal solution by shaking it with dispersion medium in presence of small amount of electrolyte is called Peptization .

Purification of Colloids 

Colloidal solution when prepared generally contain excess amount of electrolyte and some other impurities . The process used for reducing the amount of impurities to a required or desired minimum quantity is called purification of colloids .  1) Dialysis 

The process of separating dissolved excess quantity of electrolyte from colloidal solution by diffusion through suitable membrane .                                                           

Principle : The electrolyte particles easily pass through animal membrane or parchment paper or cellophane sheet but not colloid particles .                                                    

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

surface chemistry

2) Electro-Dialysis 

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

surface chemistry

3) Ultrafiltration 

 This is the process of separating colloidal particles from impurities using specially prepared filter paper which are permeable to all substances except colloid . Colloidal particles can usually pass through normal filter paper as the pores are two large .                                                                                                                                      

 Preparation of ultra filter paper : An ultra filter paper can be made by soaking normal filter paper in collodion  solution ( colloidal solution is a 4% solution of cellulose in a mixture of alcohol and ether ) , hardening by formaldehyde and then finally drying it .                                                                                       

 As this is a slow process , pressure or suction is applied to speed it up . The colloidal particles left on filter paper are stirred with fresh dispersion medium (solvent) to form a pure colloid .

Properties of Colloid 

1) Heterogenity 

Colloidal solution are heterogenous in nature i.e its components are present in different phase .These consist of two phases : Dispersed phase and dispersion medium .

2) Visibility of Dispersed Particles 

Although , colloidal particles are heterogenous in nature , yet the dispersed phase particles present in them are not visible to the naked eye and they appear homogenous . This is because colloidal particles are too small to be visible through the naked eye .                                                                   

3) Filterability

Due to very small size , colloidal particles pass through an ordinary filter paper . However , they can be retained by animal membrane , cellophane membrane and Ultrafilter paper                                     

 4) Colour

The colour of colloidal solution depend on wavelength of scattered light by colloidal particles .

The wavelength of scattered radiation depend on following factors:

  • Size of colloidal particles :                                                                                                              

Larger particles adsorb light of longer wavelength and therefore transmit light of shorter wavelength              Ex : a silver sol having particles of size 150nm appears violet whereas that having particles  of size 60nm appears golden yellow.                                                                                   

Similarily , colour of gold sol with finest particles is red . As the size increases colour changes to purple , blue and finally it becomes golden yellow.                                                               

  • Colour also depend on the way by which observer receives the light                                                

Ex : Milk appears blue if viewed by scattered light and appears red if viewed by transmitted radiation .

5) Colligative Property 

Colloidal particles being bigger aggregates. The number of particles in a colloidal solution is comparatively small as compared to the solution. Hence the value of colligative properties are of small order as compared to values shown by true solution at same concentration . 

6) Optical Properties of Colloids (Tyndall effect )

When an intense beam of light is passed through colloidal solution kept in dark , the path of beam gets illuminated with a bluish light . This phenomenon is called Tyndall effect  and illuminated path is known as Tyndall cone.

Cause  

The tyndall effect is due to scattering by colloidal particles.                                                                     

Conditions

Tyndall effect is noticed under following conditions                                                                 

The diameter of dispersed phase particles should not be very small as compared to wavelength of light since they will not be in position to scatter light and no tyndall effect will not be seen .

The refractive index of dispersed phase and dispersion medium vary in magnitude on large scale .

Importance of Tyndall Effect 

Tyndall effect can be used to distinguish a colloidal solution from true solution . It explains heterogenous nature of colloidal solution . 

Examples 

  • Milk looks cloudy due to scattering of light by fat particles i.e dispersed phase particles
  • In picture hall , the picture of light can be seen during projection due to scattering of light by dust and smoke particles present in air .

7) Mechanical Properties (Brownian movement ) 

The continuous zig – zag movement of the colloidal particles / dispersed phase particles in a colloidal solution is called Brownian movement .                                                                                                      

Cause

 Brownian movement is due to unequal bombardment of moving molecule of dispersion medium on colloidal particles . The moving molecule of dispersion medium continuously attack on colloidal particles from all sides and impart momentum to them . Since the chances of collision are unequal , the net driving force on colloidal particle forces it to move in a particular direction . As particles moves in that direction , other molecules of medium again collide with it and particle changes its direction . The process continues and this results in random zig-zag movement of colloidal particles .

Importance of Brownian Movement 

 Brownian motion plays an important role in imparting stability to the sol . This is because Brownian movement opposes the gravitational forces acting on them and prevent them from settle down .

8) Electrical Properties 

  • In a particular colloidal solution , all the colloidal particles carry same charge while dispersion medium has equal but opposite charge . Thus the charge on colloidal particle is balanced by that of dispersion medium and colloidal particles are electrically neutral .
  • The stability of colloidal solution is mainly due to presence of particular type of charge on colloidal particles . Due to presence of similar and equal charges , colloidal particle repel one another and thus unable to combine to settle down .

Origin of Charge on Colloidal Particle :

i)Due to dissociation of adsorbed molecular electrolyte : Colloidal particles have a strong tendency to adsorb reactant or product molecule . The molecule thus formed on the surface of colloidal particles may themselves undergo dissociation and may impart charge to them .

ii)Due to selective adsorption of ions : The colloidal particle have a tendency to preferentially adsorb a particular type of ions . Colloidal particle usually adsorb those ion which are in excess and are common to its own lattice .                                                     

Example : When a small amount of silver nitrate is added to large amount of potassium iodide , Colloidal particles of silver iodide adsorbs I from the solution to become negatively charged .                                                                                             

iii)Due to fractional distillation : It is supposed fractional electrification is due to rubbing of the dispersed phase particles with that of dispersion medium resulting in some charge on colloidal particles .

9) Electrophoresis 

It is defined as due to presence of particular type of electrical charge , colloidal particles present in a colloidal solution move towards particular electrode under the influence of electric field .

surface chemistry

10) Electro- Osmosis 

It is defined as movement of dispersion medium under the influence of electric field in the situation when movement of colloidal particle is prevented with the help of suitable membrane .

11) Electrical Double layer 

Having acquired a positive or negative charge by selective adsorption on surface of colloidal particles , this layer attract counter ions from medium . The combination of two layers of opposite charge around the colloidal particle is called Helmholtz electrical double layer .

The first layer is fixed layer while second layer is diffused layer . The potential difference between fixed layer and diffused layer of opposite charges is called electrokinetic potential or Zeta potential .

 Coagulation of Colloidal Sol

It may be defined as phenomenon involving precipitation of colloidal solution on addition of an electrolyte . 

Mechanism  

When an electrolyte is added in excess , opposite charges ions in electrolyte neutralize the charge on colloidal particles and compel the sol to get coagulated .

Coagulation Techniques

  • By Electrophoresis
  • By mixing two oppositely charged sols
  • By boiling
  • By persistent dialysis

Hardy – Shulze Law 

  • The ion responsible for causing coagulation is the one which carries charge opposite to that present on colloidal particles
  • The coagulating capacity of an electrolyte depends upon valency of ion responsible for causing coagulation.

Protection of Colloids 

The process of protecting a lyophobic sol from being coagulated on addition of an electrolyte by the use of lyophilic sol is called protection and lyophilic colloid used for this purpose is called protective colloid .

Mechanism  

 Lyophilic colloid particles gets adsorbed on the surface of the colloid particles present in lyophobic sols . The adsorbed lyophilic particle thus formed an envelope around the lyophobic sol and protect them from action of electrolyte .

Gold Number

The minimum amount of protective colloids in milligram required to just prevent the coagulation of 10ml of given gold sol when 1 ml of 10% solution of sodium chloride is added to it .

Emulsion 

Emulsion can be defined as colloidal solution in which both the dispersed phase and dispersion medium are liquids and immiscible .

Properties of Emulsion

  • Emulsion contain a boundary that exist between dispersed phase and dispersion medium .
  • Emulsion are highly unstable system and require an Emulsifying agent or emulsifier .
  • The colour of emulsion depend on its concentration : In case of dilute emulsion, a low wavelength will be scattered and emulsion will be blue in colour . In case of concentrated solution , colour will be more yellow . 

Emulsifier or Emulsifying Agent 

 Substances which are added to emulsion for stabilization process .

Types of Emulsion

1)Oil in water : In this , oil will be dispersed phase and water will be dispersion medium . Ex : Milk , Vanishing cream                                                                                                             

2) Water in oil : water will be dispersed phase and oil will be dispersion medium . Ex : Butter , Cold cream

Demulsification 

The process of separation of constituents of an emulsion . This can be done by Boiling , Freezing , by adding excess electrolyte etc . 

 

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