April 10, 2012

Surface Chemistry Five Marks

1. Explain the intermediate compound formation theory of catalysis. Or Write briefly on intermediate compound formation theory of catalysis with an example.
Intermediate compound formation theory explains the mechanism of homogeneous catalysis. According to this theory, the catalyst first forms an intermediate compound with one of the reactants. The compound is formed with less energy consumption than needed for the actual reaction. The intermediate compound being unstable combines with other reactant to form the desired product and the catalyst is regenerated.
For example, a reaction of the type
           C
A + B AB
which occurs in presence of a catalyst C, may take place as
A + C   AC
       Catalyst  Intermediate compound
AC + B AB + C
                      Product Catalyst
The catalytic oxidation of SO2 to SO3 in the lead chamber process probably takes place as;
2NO + O2  2NO2
Catalyst                   Intermediate compound
NO2 + SO2  SO3 + NO
                          Product  Catalyst
2. Give any 5 main differences between physical adsorption and chemical adsorption.
S.No
Physical adsorption
Chemical adsorption
1
It is due to intermolecular Vander waal’s force.
It is due to chemical bond formation.
2
Depends on the nature of gas. Easily liquefiable gases are adsorbed readily.
More specific than the physical adsorption.
3
Heat of adsorption is small.
Heat of adsorption is large.
4
Reversible.
Irreversible.
5
If occurs rapidly at low temperature and decreases with increase of temperature.
Increases with increase of temperature.
6
Increase of pressure increases adsorption.
Change of pressure has no effect.
7
Forms multimolecular layers on adsorbent surface.
Forms unimolecular layer.
3. How are colloids prepared by chemical methods? Or Write any two chemical methods for the preparation of colloids.
The chemical methods involve chemical reaction in a medium in which the dispersed phase is sparingly soluble. Some of the methods are:
1. Double decomposition: An Arsenic sulphide sol is prepared by passing a slow stream of hydrogen sulphide gas through a cold solution of arsenious oxide.
As2O3 + 3H2S As2S3 + 3H2O
2.  Oxidation: A colloidal solution of sulphur is obtained by passing H2S into a solution of sulphur dioxide.
2H2S + SO2  2H2O + 3S
3. Reduction: Silver sols and gold sols can be obtained by treating dilute solution of silver nitrate or gold chloride with organic reducing agents like tannic acid or formaldehyde.
AgNO3 + tannic acid Ag Sol
AuCl3 + tannic acid Au Sol
4. Hydrolysis: A colloidal solution of ferric hydroxide is obtained by boiling a dilute solution of ferric chloride.
FeCl3 + 3H2O Fe(OH)3 + 3HCl
Colloidal solutions of the hydroxides of Cr, Al etc can be prepared by hydrolysis of their salts.
4. How can colloidal solutions be purified by dialysis?
Animal membranes (bladder) or those made of parchment paper and cellophane sheet, have very fine pores. These pores permit ions (or small molecules) to pass through but not the large colloidal particles.
When a sol containing dissolved ions (electrolyte) or molecules is placed in a bag of semi permeable membrane dipping in pure water, the ions diffuse through the membrane. By using a continuous flow of fresh water, the concentration of the electrolyte outside the membrane tends to be zero. Thus diffusion of the ions into pure water remains brisk all the time. In this way, practically all the electrolyte present in the sol can be removed easily.
The process of removing ions (or molecules) from a sol by diffusion through a permeable membrane is called Dialysis. The apparatus used for dialysis is called a Dialyser. 

5. What is electro-osmosis? Explain the experiment.
The movement of the dispersion medium under the influence of applied potential is known as electro-osmosis.
The phenomenon of electro-osmosis can be demonstrated by using a U-tube in which a plug of wet clay (a negative colloid) is fixed. The two limbs of the tube are filled with water to the same level. The platinum electrodes are immersed in water and potential applied across them. It will be observed that water level rises on the cathode side and falls on anode side. This movement of the medium towards the negative electrode shows that the charge on the medium is positive. Similarly, for a positively charged colloid electro-osmosis will take place in the reverse direction.
6. Write any three methods for the preparation of colloids by dispersion methods. Or How are colloids prepared by using i) mechanical dispersion method ii) electro dispersion method?
1. Mechanical dispersion: (Colloidal mill)
The solid along with the liquid is fed into a colloidal mill. The colloidal mill consists of two steel plates nearly touching each other and rotating in opposite directions with high speed. The solid particles are ground down to colloidal size and then dispersed in the liquid.
 

Use:
Colloidal graphite, Cement, Face powder and printing inks are made by this method.
2. Electro-dispersion method: (Bredig’s Arc Method)
An arc is struck between the metal electrodes under the surface of water containing some stabilising agent such as trace of alkali. The water is cooled by immersing the container in a cold bath. The intense heat of the arc vapourises some of the metal which condenses under cold water.
 

Use:
This method is suitable for the preparation of colloidal solution of metals like gold, silver, platinum etc.
3. Ultra-sonic dispersion:
The sound waves of high frequency are usually called ultra-sonic waves. Ultrasonic waves are passed through the solution containing larger particles. They break down to form colloidal solution.
4. Peptisation:
The dispersion of a precipitated material into colloidal solution by the action of an electrolyte in solution is termed as peptisation. The electrolyte used is called a peptizing agent.
Precipitate + Electrolyte (Peptizing agent) Colloidal solution (Sol)
Examples:
1. Silver chloride can be converted into a sol by adding hydrochloric acid
2. Ferric hydroxide yields a sol by adding ferric chloride
7. Write briefly about the adsorption theory of catalysis.
This theory explains the mechanism of heterogeneous catalysis. Here, the catalyst functions by adsorption of the reacting molecules on its surface.
 
Adsorption
In general, there are four steps involved in the heterogeneous catalysis.
                Catalyst
A(g) + B(g)  C(g) + D(g)
Step - 1 Adsorption of reactant molecules
The reactant molecules A and B strike the surface of the catalyst. They are held at the surface by weak vanderwaal’s forces or by partial chemical bonds.
Step - 2 Formation of Activated complex
The particles of the reactants adjacent to one another join to form an intermediate complex (A - B). The activated complex is unstable.
Step - 3 Decomposition of Activated complex
The activated complex breaks to form the products C and D. The separated particles of the products hold to the catalyst surface by partial chemical bonds.
Step - 4 Desorption of Products
The particles of the products are desorbed or released from the surface.
8. Write notes on i) Ultrafiltration and ii) Helmholtz double layer.
i) Ultrafiltration
Sols pass through an ordinary filter paper. Its pores are too large to retain the colloidal particles. However, if the filter paper is impregnated with collodion or a regenerated cellulose such as cellophane or visking, the pore size is much reduced. Such a modified filter paper is called an ultrafilter.
The separation of the sol particles from the liquid medium and electrolytes by filtration through an ultrafilter is called ultrafiltration.
Ultrafiltration is a slow process. Gas pressure (or suction) has to be applied to speed it up. The colloidal particles are left on the ultrafilter in the form of slime. The slime may be stirred into fresh medium to get back the pure sol. By using graded ultrafilters, the technique of ultrafiltration can be employed to separate sol particles of different sizes.
 

ii) Charge on Colloidal particles
The important property of colloidal dispersions is that all the suspended particles possess either a positive or negative charge. The mutual forces of repulsion between similarly charged particles prevent them from aggregating and settling under the action of gravity. This gives stability to the sol.
 

Adsorption of ions from dispersion medium gives charge to sol particles which do not settle on account of mutual repulsions
The surface of colloidal particle acquires a positive charge by selective adsorption of a layer of positive ions around it. This layer attracts counter ions from the medium which form a second layer of negative charges. The combination of the two layers of charges around the sol particle is called Helmholtz double layer.
9. Write notes on a i) Auto catalyst ii) Promoters.
i) Auto catalyst
When one of the products formed during the reaction acts as a catalyst for that reaction. Such type of catalyst is called auto catalyst and the phenomenon is known as auto catalysis.
Example:
In the oxidation of oxalic acid by potassium permanganate, one of the products MnSO4 acts as a auto-catalyst because it increases the speed of the reaction.
  COOH
5 |          + 2KMnO4 + 3H2SO4  2MnSO4 + K2SO4 + 10CO2 + 8H2O
  COOH
ii) Promoters
The activity of a catalyst can be increased by addition of a small quantity of a second material. A substance which, though itself not a catalyst, promotes the activity of a catalyst is called a promoter.
Example:
In the Haber’s process for the synthesis of ammonia, traces of molybdenum increase the activity of finely divided iron which acts as a catalyst.
                 Fe
N2 + 3H2 2NH3
                + Mo
10. Write the general characteristics of catalytic reactions.
1. The catalyst remains unchanged in mass and in chemical composition at the end of the reaction.
2. Only a small quantity of catalyst is generally needed.
3. A catalyst cannot initiate a reaction. The function of a catalyst is only to alter the speed of the reaction which is already occurring at a particular rate.
4. A catalyst does not alter the position of equilibrium in a reversible reaction.
5. The catalyst is generally specific in its action

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