SURFACE CHEMISTRY

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The branch of chemistry that deals with the study of surface phenomena like adsorption, catalysis etc. is called surface chemistry.

ADSORPTION

Itis a surface phenomenon which involves attracting and retaining the molecules of a substance on the surface of some other substance.E.g. Adsorption of ammonia gas on the surface of activated charcoal.

ADSORBENT

The substance on the surface of which adsorption takes place is called adsorbent E.g. activated charcoal, silica gel etc.

ADSORBATE

The substance whose particles get adsorbed on the surface of adsorbent is called adsorbate.E.g. NH_3, CO₂ etc.

DESORPTION

The process of removal of particles of adsorbate from the surface of adsorbent is known as desorption .It is reverse of adsorption.

ABSORPTION

It is a bulk phenomenon in which particles of one substance go deep into the body of some other substance.

E.g. ammonia dissolved in water, water present in CaCl etc.

SORPTION

The process in which absorption and adsorption occur together is known as sorption.E.g. Dye present is in the fabric is adsorbed as well as absorbed by the fabric.

OCCULUSION

The adsorption of gases on the surface of metals is called occulusion.E.g. adsorption of hydrogen gas on the surface of palladium.

TYPES:ADSORPTION

(1)PHYSICAL ADSORTION

When particles of adsorbate are held on the surface of an adsorbent by weak Van der Waal’s forces or physical forces then it is called physical adsorption.

(2)CHEMICAL ADSORPTION

When particles of adsorbate are held on the surface of an adsorbent by chemical bonds then it is called chemical adsorption.

DIFFERENCE BETWEEN PHYSICAL AND CHEMICAL ADSORPTION

PHYSISORPTION 1) It involves weak van der waal’s attractive forces. 2) It takes place at low temperature. 3) It is reversible in nature. 4) It is non-specific. 5) It is multi-layered 6) No need of high activation energy.

CHEMISORPTION 1) It involves strong chemical bonds 2) It takes place at high temperature. 3) It is irreversible in nature. 4) It is specific in nature. 5) It is mono-layered 6) High activation energy is required.

FACTORS AFFECTING ADSORPTION

1. NATURE OF GAS: Easily liquefiable gases like ammonia and carbon dioxide are adsorbed to a greater extent than the gases which are hard to liquefy like N_2,O₂ etc. REASON: Easily liquefiable gases have greater magnitude of attractive forces.
2. NATURE OF SOLID: The solids with porous nature have larger surface area and hence act as  good adsorbents.E.g. Charcoal, silica gel, alumina gel and clay.
SURFACE AREA: ADSORBENT:Larger the surface area of the adsorbent greater will be the rate of adsorption.
E.g. adsorption in solid adsorbents is enhanced when finely divided solid particles are used.

3. ACTIVATION OF ADSORBENT:It means increasing the adsorbing power of the adsorbent. It is done by heating or grinding the adsorbent. Heating is done to increase the number of active sites and grinding is done to increase the surface area of adsorbent.
EFFECT OF TEMPERATURE: Effect of temperature can be studied from adsorption isobar.


ADSORPTION ISOBAR
The graph between x/m and temperature at constant pressure is called adsorption isobar. There are two types of adsorption isobars.
1) ADSORPTION ISOBAR FOR PHYSICAL ADSORPTION

It is clear from the isobar that for physical adsorption the rate of adsorption decreases with the increase in temperature.
REASON:With the increase in temperature the already adsorbed gas molecules absorb heat energy and their kinetic energy is enhanced as result of which molecules start escaping from the surface of adsorbent hence the rate of adsorption decreases.
(ii)ASORPTION ISOBAR FOR CHEMICAL ADSORPTION
In this case value of x/m first increases and then decreases with further rise in temperature.

REASON: In the beginning rise in temperature helps to provide activation energy to the gaseous molecules so that chemical bond can be formed. Hence the rate of adsorption increases in the beginning. But with the further increase in temperature energy of the bonded molecules increases which results in the breaking of chemical bonds hence the rate of adsorption decreases.




EFFECT OF PRESSURE : In general the rate of adsorption increases with the increase in pressure. The effect of pressure on the rate of adsorption can be studied in detail from adsorption isotherm.



ADSORPTION ISOTHERM

The graph between x/m and pressure at constant temperature is called adsorption isotherm. In the beginning the rate of adsorption increases with the increase in pressure then it becomes constant at a particular pressure called saturated pressure.





TYPES OF ADSORPTION ISOTHERM

They are of two types.

(i) FREUNDLICH ADSORPTION ISOTHERM: - The variation of rate of adsorption with pressure at constant temperature can be studied by Freundlich adsorption isotherm as follow:

AT LOW PRESSURE: - At low pressure graph is nearly straight which shows that:

x/m α P

AT HIGHER PRESSURE: - The graph becomes almost constant which reveals that x/m becomes independent of the pressure.

x/m α P0

AT INTERMEDIATE RANGE OF PRESSURE

x/m depends upon P raised to the power between 1 and 0.

x/m α P1/n

x/m = k × P1/n………………(1)



To check the validity of this equation (1) take log on both sides.

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

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

If we draw a graph between log x/m and log P we get a straight line .This graph is called Freundlich adsorption isotherm



Freundlich adsorption isotherm





(ii) LANGMUIR ADSORPTION ISOTHERM

On the basis of theory of adsorption Langmuir gave a relationship between x/m and pressure at constant temperature.

x/m = aP/1+bP ………………………(1)

In this equation a and b are called Langmuir parameters.The graph obtained from this relation is called Langmuir adsorption isotherm.

Now divide equation (1) with P.



If we plot a graph between P/x/m and P, we get a straight line.







ENZYMES

Enzymes are the biocatalysts which speed up the biochemical reactions. They are made up of proteins. E.g. maltase, zymase, lactase.



CHARACTERISTICS:ENZYMES

1. SPECIFICITY:Enzymes are very specific in nature that is each enzyme catalyses only one specific reaction.
2. EFFICIENCY:Enzymes are very efficient biocatalysts as they can speed up the reaction upto 10⁸ to 10²⁰ as compared to uncatalyzed reactions.
3. QUANTITY: They are required in very very small quantities. As small as millionth of a mole
4. OPTIMUM TEMPERATURE: They work efficiently at an optimum temperature.
5. OPTIMUM pH: Each Enzyme exhibits highest activity at a particular pH.
6. ENZYMES ACTIVATORS OR COENZYMES:The activity of certain enzymes is enhanced in the presence of some substances called Coenzymes.E.g. Na⁺ , K⁺, Ca²⁺etc.
7. ENZYME INHIBITOR: The substances which are used to inhibit the activity of enzymes are  called enzyme inhibitors or enzyme poisons.




MECHANISM:ENZYME CATALYSIS

Two mechanisms have been proposed for  enzyme action.

1) LOCK AND KEY MODEL

This model involves following steps.

i) Formation of enzyme substrate complex. this step is fast and reversible.

ii) Formation of product in the enzyme substrate complex.This step is slow and reversible.

iii) Separation of  product and enzyme. In this step product gets  separated from enzyme.


 

2) INDUCED FIT MODEL

According  to this model the substrate induces the enzyme to change its shape and size so that enzyme substrate complex can be formed.In the enzyme substrate complex  product is formed which finally gets separated from the enzyme.


 






CATALYSIS

The phenomenon of increasing or decreasing the rate of a chemical reaction by the use of certain substances is called catalysis.

CATALYST

The substances which alter the rate of a reaction without being consumed are known as catalysts.

 

CHARACTERISTICS OF CATALYSTS

1) A catalyst does not initiate a chemical reaction.

2) It alters up the rate of a reaction.

3) It composition remains unchanged during the reaction.

4) They are highly specific in nature.

5) It does not affect equilibrium state of the reaction.

7) They are specific in nature.

8) They are highly efficient in their activity.

9) (promoters:- These are the substances that are added along with catalysts to enhance their activity.)

Their activity can be enhanced with catalytic promoters like Mo enhances the activity of Fe in Haber’s process for the synthesis of ammonia.

10) ) Catalytic poisons:- These are the substances that suppress  the activity of a catalyst.)Their activity can be suppressed with the use of catalytic poisons like hydrogen sulphide or carbon monoxide inhibits the activity of Fe in the Haber’s process for synthesis of ammonia. Similarly presence of traces of arsenious oxide (As₂O₃)  in the reacting gases reduces  the activity  of platinized asbestos catalyst in contact process of manufacturing sulphuric acid.

Q: How catalytic poisonous works ?

Ans:A catalytic poison blocks the activesites of catalyst thus inhibits its ability to adsorb the reactants molecules on its surface.

Q : -  What do you mean by the term activity.?

Ans :- The ability of a catalyst to speed up the rate of a chemical reaction is called  its activity.They are very efficient in their activity.E.g H₂  and O₂  do not react in the absence of  catalyst but in the presence of  platinum catalyst reaction occurs explosively.

2H₂   +    O₂    ^platinum→   2H₂O

This is due to greater extent of chemisorption.

Q :-What do you mean by the term  selectivity of the catalysts ?

Ans :- The ability of a catalyst to direct  a reaction to form a particular product is called its selectivity.E.g.

CO  +  3H₂  ^Ni→  CH₄ +  H₂O 

CO  +  H₂  ^Cu→  HCHO





TYPES OF CATALYSIS

Catalysis can be classified in two different ways. On the basis of action of catalysts and on the basis of phase of the catalysts.

1) ON THE BASIS OF ACTION OF THE CATALYSTS

i) POSITIVE CATALYSIS:- The phenomenon of increasing the rate of a reaction by the use of certain substances is called positive catalysis.The substances used to speed up the rate of a reaction are called  positive catalysts.E.g.

2SO₂  + O₂     ^P₂^O₅→ 2SO₃

 





ii) NEGATIVE CATALYSIS: - The phenomenon of decreasing the rate of a chemical reaction by using certain substances is called negative catalysis.The substances used to decrease the rate of a reaction are called negative catalysts.

H₂O₂  ^H3PO4→ H₂ + O₂ (The rate of decomposition gets reduced.)

 



2) ON THE BASIS OF PHASE OF THE CATALYSTS.

i) HOMOGENEOUS CATALYSIS: - The catalysis in which reactants and the catalyst have similar phase is called homogeneous catalysis.E.g.

 

2SO₂ (g) ₊ O₂ (g)   ^NO(g)→ 2SO₃  (g)

 

CH₃COOC₂H₅ (l)   +  H₂O (l)    ^{H+ (l)}→ CH₃COOH (l) + C₂H₅OH (l)

 

ii) HETEROGENEOUS CATALYSIS:-In this process  phase  of the catalyst is different from that of the reactants. This is also known  as surface catalysis. This is due to the reason that reaction takes place at the surface of solid catalyst.E.g . catalytic hydrogenation alkene.

CH₂=CH₂  (g) +  H₂ (g)    ^{Ni (s)}→ CH₃-CH₃ (g)

 

MECHANISM OF HOMOGENEOUS CATALYSIS 

 The mechanism can be explained on the basis of intermediate compound formation theory. According to which a catalyst combines with one of the reactants to form an intermediate compound which is unstable and decomposes or combine with another reactant to form the product.E.g.

 

2SO₂ (g) ₊ O₂ (g)   ^NO(g)→ 2SO₃  (g)

In this reaction NO combines with  oxygen to form an intermediate compound NO₂ which further combines with SO₂ to form SO₃ and the catalyst is recovered unchanged in compositon.The various steps are as follow :

 

NO  +  ½ O₂  → NO_{2        (Intermediate)}

SO₂  + NO₂  →   SO₃ + NO  (catalyst)

SO₂ +  ½ O₂  →  SO₃





MECHANISM OF HETEROGENEOUS CATALYSIS

In this case catalyst is generally solid and reactants are liquids or gases. Mechanism proceeds through the adsorption of reactant molecules on the surface of the catalyst where chemical bond is formed between the reactant molecules.E.g catalytic hydrogenation of ethene.

2 Ni + H₂ --->2 Ni...H₂----> 2 Ni-H
                    solid    gas     physisorption   chemisorption

 

CH₂=CH₂  + 2 H   →  CH₃-CH₃

                                                                  _{(taking place at the surface of catalyst nickel}

CATALYTIC CONVERTER

This is an antipollution device fitted to the exhaust system of vehicles  to convert harmful exhaust gases into non toxic gases. It works on the principle of heterogeneous catalysis.E.g.  Carbon monoxide and hydrocarbons from the exhaust are converted into carbon dioxide.

 

2CO + O₂  ^{catalytic converter}→ 2CO₂

HC      +     O₂  ^{catalytic converter}→        CO₂  + H₂O

                                                                                     _{( HYDROCARBONS)}



 

SHAPE SELECTIVE CATALYSIS

The catalysis which depends upon the pore structure of the catalyst and size of reactant and product molecules is called Shape Selective Catalysis.The catalysts which adsorb only those reactant molecules whose shape and size can easily fit in their pores are called shape selective catalysts. E.g. zeolites





ZEOLITES

Zzeolites are microporous aluminosilicates having three dimensional cage like structure in which some of the silicon atoms are replaced by aluminium atoms.e.g Sodium aluminium silicates (NaAlSi₂O₆H₂O).

The zeolites act as molecular sieves for the reactants. To achieve this, zeolites are heated in vacuum so that the water of hydration is lost. Once the water is removed, the zeolites structure becomes porous. The pore size in zeolites is generally in the range 260 - 740 pm. Only those reactant molecules are adsorbed which have size comparable to those of pores and cavities of the zeolites.That is why they are called shape selective catalysts. They are widely being used in petrochemical industry to prepare gasoline from alcohol. The zeolites used for this purpose is ZSM-5 (Zeolite Sieve of Molecular Porosity-5).

 

Alcohol  ^ZSM-5→ Gasoline

 

XCH₃OH  ^ZSM-5→ (CH₂)_x +   XH₂O

 



TYPES OF SOLUTIONS

On the basis of size of the solute  particles solutions are of three types.

i) TRUE SOLUTIONS: - size of solute particles is less than one nanometer.E.g sugar in water, salt in water etc.

ii) COLLOIDAL SILUTION:- Size of solute particles ranges from 1nm to 100 nm. E.g. milk, blood etc.

iii) SUSPENSION: - Particle size is more than 100 nm. E.g. muddy water, chalk in water etc.

 

COLLOIDAL SOLUTIONS OR COLLOIDS

A colloid is a solution in which the particle size ranges from 1 nm to 100 nm.For example, milk, blood, honey, smoke, ink, gum, starch solution etc. It consists of two components dispersed phase and dispersion medium.

Dispersed phase consists of colloidal particles which remain suspended in other substance known as dispersion medium. E.g. in muddy water the colloidal particles are tiny grains of sand, silt, and clay. The dispersing medium is the water in which these particles are suspended.

              

TYNDALL EFFECT

The process of scattering of light by colloidal particles is known as Tyndall effect.

BROWNIAN MOVEMENT

The zig-zag motion of colloidal particles is known as Brownian movement

 

CLASSIFICAITON OF COLLOIDAL SOLUTIONS

Colloids can be classified in three different ways.

(i) On the basis physical state of the dispersed phase and dispersion medium.

(2) On the basis of interactions between dispersed phase and dispersion medium.

(3)On the basis of types of colloidal particles.

 

I) ON THE BASIS PHYSICAL STATE OF DISPERSED PHASE AND DISPERSION MEDIUM.

 

(2) ON THE BASIS OF INTERACTIONS B/W DISPERSED PHASE &DISPERSION MED.

 

i) LYOPHILIC COLLOIDS: - The colloidal solutions in which particles of the dispersed phase have attraction for the dispersion medium. They are easily formed, very stable and reversible in nature.E.g. Colloidal solutions of gum, starch, protein and starch in water.

 

ii) LYOPHOBIC COLLOIDS:-The colloidal solutions in which particles of the dispersed phase have very little or no affinity for dispersion medium. They are formed with difficulty, unstable and irreversible in nature.E.g.colloidal solution of Au and Ag in H₂O.

 



(3)ON THE BASIS OF TYPES OF COLLOIDAL PARTICLES.

 

I) MULTIMOLECULAR COLLOIDS:-The colloids formed by the aggregation of large  no of tiny particles having size less than one nanometer.E.g gold sol ,sulphur sol etc.

 

II) MACROMOLECULAR COLLOIDS: - The colloidal solutions which consist of macromolecules having sizes in the range of 1nm to 100 nm are called macromolecular colloids.E.g colloidal solutions of starch and protein in water.

 

III) ASSOCIATED COLLOIDS OR MICELLES: - These colloids are formed by the association of particles of some substances above a particular temperature and particular concentration. That temperature is called Kraft’s temperature (T_k) and that concentration is known as critical micelle concentration (CMC).The associated particles are also known as  micelles.E.g aqueous solution of soap.

Q :- What are Kraft’s temp and CMC ?

Ans : - Kraft’s temp The  formation of micelles takes place above a particular temperature called Kraft’s temperature.

Ans: - CMC: - The formation of micelles takes place above a particular concentration known as Critical Micelle Concentration.

 



Q : Write a short note on cleansing action of soap.

Ans :-    CLEANSING ACTION OF SOAP

 

SOAPS ARE SODIUM SALTS OF LONG CHAIN FATTY ACIDS .THEY HAVE A POLAR HEAD AND A NON POLAR TAIL

For example sodium sterate consists of non polar tail of long hydrocarbon chain and a polar head of carboxylate ion and sodium Ions. The polar head is water soluble whereas the non polar tail is soluble in organic solvents. Due to the presence of grease or fats on the cloth ,the dust particles get stick to the cloth and the cloth becomes dirty. When the aqueous solution of soap is applied on the cloth the non polar end is directed towards the oil or grease present on the cloth and the polar head is directed towards the water, in this way each oil droplet is surrounded by  a large number of negatively charged carboxylate ions to form an aggregate called Micelle .A large numbers of micelles are formed during the process of washing. When cloth is rubbed with hands or agitated with washing Machine these micelles are dispersed in the soapy water and dirt gets detached from the surface of the cloth.

                METHODS OF PREPARTION OF COLLOIDAL SOLUTIONS

 

1) PREPARATION OF LYOPHILIC COLLOIDS: these are easily formed by direct mixing molecules of the dispersed phase and dispersion medium. e.g.  Colloidal sols of gum, starch, gelatin and egg albumin.

2) PREPARATION OF LYOPHOBIC COLLOIDS: can be prepared by the following four methods.

 

i) Chemical method

ii) Mechanical method

iii) Electro disintegration or Bredig's method

iv) Peptization

 



B) MECHANICAL METHOD:

In this method large sized particles are broken down in smaller particles of the colloidal size with help of colloidal mill.

Colloidal sol of black ink, paints etc are prepared by this method.

 

C) ELECTRICAL DISINTEGRATION OR BREDIG'S ARC METHOD:

In this method, an electric arc is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced by the arc vaporizes the metal, which then condenses to form particles of colloidal size. By this method, sols of metals such as gold, silver and platinum can be prepared.

D) PEPTISATION:

The process of converting a freshly prepared precipitate into a colloidal sol by shaking it with the dispersion medium, in the presence of a small amount of electrolyte having common ion is called peptization. The electrolyte used is called the peptizing agent.E.g.

 

Fe (OH)₃   +      Fe³⁺     →      Fe (OH)₃ Fe³⁺

                                                         ^{(ppt)             (electrolyte)                 (sol)}

 

PURIFICATION OF COLLOIDAL SOLUTION:

 

The colloidal solutions prepared by various methods usually contain electrolytes and other soluble substances as impurities. These impurities if not removed can destabilize the sols. Impurities are removed by the following methods:

1) DIALYSIS

2) ELECTRO DIALYSIS

3) ULTRA-FILTRATION

4) ULTRA-CENTRIFUGATION.

 

1) DIALYSIS: The process of removal of ions of an electrolyte (impurities) from the colloidal solution by the use of semipermeable membrane (such as parchment paper or cellophane) is known as dialysis. The dialysis membrane permits excess ions  to pass through and does not allow the colloidal particles  to pass through.

The most important application of dialysis is in the purification of blood with the aid of an artificial kidney machine. The dialysis membrane permits excess ions and waste products like urea molecules to pass through and does not allow the colloidal  particles of blood  to pass through.

2) ELECTRODIALYSIS: The movement of the ions across the semipermeable membrane can be enhanced by applying electric potential across two electrodes. This is known as electro dialysis.

3) ULTRA-FILTRATION: The process of removal of impurities from the colloidal solution by passing through ultra filter papers is called ultrafiltration.These filters allow the electrolytes to pass through but not the colloidal particles. Filtration is usually carried out by either by applying pressure or by the use of suction.

4) ULTRA-CENTRIFUGATION.: Ultra Centrifugation is used to separate colloidal particles from the impurities by centrifuging the colloidal sol. Centrifugation is carried out in a centrifuging machine where the tube containing the colloidal sol is rotated at very high speeds. The colloidal particles settle down at the bottom of the tube and the impurities remain in the solution. In this way colloidal solution gets purified.

ELECTRICAL PROPERTIES

The colloidal particles are electrically charged and they carry the same type of charge, that is, either the colloids are negatively charged or positively charged. E.g. colloidal solutions of gold and arsenious sulphide are negatively charged whereas colloidal solutions of ferric hydroxide and aluminium hydroxide are positively charged. The dispersion medium has an equal and opposite charge making the system neutral as a whole. Since the colloids carry the same type of charge, they repel each other and do not combine to form bigger aggregates. This is the reason why a sol is stable.

Q: - What is the cause of stability of colloidal solutions?

Ans:-The colloidal particles are electrically charged and they carry the same type of charge, that is, either the colloids are -vely charged or +vely charged. Since the colloids carry the same type of charge, they repel each other and do not combine to form bigger aggregates. This is the reason why a sol is stable.

 





Q: - How does the charge originate on the colloidal particles?

Ans:-ORIGIN OF CHARGE ON COLLOIDS :-

 i) Due to friction between the colloidal particles and molecules of dispersion medium, the colloidal particles become charged.

ii) Charges on colloids appear due to preferential adsorption of ions from solutions. An ionic colloid adsorbs ions common to its own lattice structure. For e.g., AgCl particles can adsorb Cl^- ions if excess of KCl solutions is used for its preparation where as the same colloid can adsorb Ag⁺ ion if AgNO₃ solution is used. Hence in the first case, AgCl colloid will be negatively charged while in the second case it will be positively charged.

iii) Colloids can acquire charge by dissociation of surface molecules. For e.g., soaps expel alkali ions to acquire a negative charge.

colloidal particles can be negatively charged or positively charged. Some common positively charged and negatively charged colloids are listed below

Positively charged colloids: Fe(OH)3, Cu(OH)₃, Al(OH)₃, Ca(OH)_2.

Negatively charged colloids: AS₂S₃, Sb₂S₃, metal sols of Cu, Au, Pt, Ag, starch, and clay.

Q:- How can you determine charge on  colloids?

Ans:- DETERMINATION OF CHARGE ON COLLOIDAL PARTICLES :-

The charge on a colloidal solution can be determined with the help of following two methods.

1) ELECTROPHORESIS: - The phenomenon of movement of colloidal particles towards the oppositely charged electrodes in the presence of an electric field is called electrophoresis. This is done in a U-shaped apparatus.E.g a colloidal sol of As₂S₃, which is negatively charged, will move towards the anode when placed in an electric field.

 

2) ELECTRO-OSMOSIS:- The process of movement of molecules of dispersion medium under the influence of electric field through semipermeable membrane  keeping  the colloidal particles unmoved is called electro-osmosis.

COAGULATION

The phenomenon of precipitation of colloidal solution by the addition of excess of an electrolyte is known as coagulation or flocculation.E.g when barium chloride is added to arsenious sulphide sol the barium ions get attracted towards the negatively charged sol particles as a result of which charge gets neutralized .This leads to coagulation.

REASON:- Small amount of an electrolyte is required for the stability of  colloids. The ions of the electrolytes are preferentially adsorbed on the surface of colloidal particles. In this way colloidal particles get charged. All the colloidal particles possess same charge. Hence they repel each other and their coagulation is prevented. However when excess of electrolyte is used, the charge on colloidal particles gets neutralized and they come closer which leads to precipitation or coagulation. 

 





COAGULATION OR FLOCCULATION VALUE

The minimum concentration of an electrolyte required to cause coagulation of colloidal solution is known as coagulation value. It is expressed in milimoles/Lt.

 

HARDY- SCHULZE’S RULE

According to this rule :- 1) Ions having charge opposite to that of colloidal particles can cause coagulation. Those ions are called active ions.

2) Greater the valency of the active ion, greater will be its coagulating power.E.g. Coagulating power of PO₄^3- is grater than SO₄^2-.Similarly coagulating power of Al³⁺

is grater than Na⁺.

PROTECTION OF COLLOIDS

The phenomenon of protecting a lyophobic colloid from precipitation by the addition of some lyophilic colloid is called protection. The lyophilic colloid used to protect is called protecting colloid.

MECHANISM OF PROTECTION

The particles of lyophilic colloid get adsorbed on the surface of lyophobic colloidal  and form a protective layer around the colloidal particles which prevents their precipitation by the electrolyte. The protecting power of a lyophilic colloid is expressed in gold number.

GOLD NUMBER

The minimum amount of a protective colloid in milligrams required to prevent the coagulation of 10 ml of a given gold sol when 1 ml of 10% solution of NaCl is added to it .E.g.  The gold number for gelatin is 0.005 to 0.01 milligrams. For starch is 20 to 25 milligrams.

 





APPLICATIONS OF COLLOIDS

1) Colloids are used to prepare paints on the paint industry.

2) Colloids are used to prepare medicines in Pharmaceutical industry.

3) Smoke is a colloidal solution of carbon particles and dust in air. Smoke from the chimneys of factories is passed through a Cottrell Smoke Precipitator to remove the smoke particles by precipitation.

4) The impure water containing suspended colloidal impurities is purified by precipitation of these suspended colloidal particles. This is done by adding small amount of alum to impure water.

5) In rubber industry rubber is obtained by the coagulation of latex which is a colloidal solution of rubber particles in water.

EMULSION

A colloidal solution in which both the dispersed phase and the dispersion medium are liquids is called emulsion. E.g. Milk, Hair cream etc.

EMULSIFIER OR EMULSIFYING AGENT

The substances which are added to stabilize an emulsion are called emulsifiers.E.g. soap, gelatin, gum etc.

EMULSIFICATION

 
     Emulsification is the process of making emulsions. Emulsions are made by shaking the dispersed phase and dispersion medium vigorously and then stabilizing the emulsion with an emulsifier. Most often soaps and detergents are added to stabilize emulsions. Stabilization is obtained by the coating of the drops of an emulsion by the stabilizer. This prevents the drops of the emulsion from combining together and separating out as a separate layer

TYPES OF EMULSION

1)OIL IN WATER EMULSION( O/W TYPE) :-  In this case oil acts as dispersed phase and water as dispersion medium. E.g. milk is an emulsion of soluble fats in water here casein acts as an emulsifier.

2) WATER IN OIL EMULSION ( W/O TYPE):-

In this case water acts as dispersed phase and oil acts as dispersion medium.

E.g. butter, cold cream, etc.

TEST FOR THE IDENTIFICATION OF EMULSION

 

DILUTION TEST: - Add water to the emulsion if it gets diluted then it is oil in water type emulsion. On the other hand if not then it is water in oil emulsion.

 



Q: - What is sol?

Ans :- A colloidal solution in which dispersed phase consists of a solid and dispersion medium consists of liquid is called sol. E.g. colloidal solution of Fe(OH)₃ in water.

Q: - Define gel and gelation?

Ans: - A colloidal solution in which dispersed phase consists of liquid and dispersion medium consists of a solid is called gel. E.g. gum, cheese etc

GELATION: - The process of formation of gel is known as gelation.

 

Q: - What is thixotropy?

Ans: - The conversion of gel into sol by simply shaking, which again changes into gel on standing is called thixotropy. E.g. gelatin and silica gel

Q: - What is colloidion?

Ans: - It is a colloidal solution of cellulose nitrate in ethyl alcohol.

Q: - What is occulusion?

Ans: - The adsorption of gases on the surface of metals.

Q: - Give an example of shape selective catalysis.

Ans: - Conversion of alcohol into gasoline by ZSM-5

Q: - Why does beam of light has visible path in colloidal solutions?

Ans: - Due to Tyndall effect.

Q: -What happens when NaCl is added to ferric hydroxide sol?

Ans: - The Cl^- ions of sodium chloride neutralize the positive charge of ferric hydroxide sol particles and coagulation takes place.

Q: - What is a micelle?

Ans: - A particle of colloidal size formed by the association of several ions or molecules is called micelle. E. g. molecules of soap in water get associated to form micelles. A micelle may contain 100 or more molecules. The micelle forming molecules consist of two parts polar water soluble part and a non polar water insoluble part.







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