Surface Chemistry
Surface Chemistry is the branch of chemistry which deals with the phenomenon that occurs on the surfaces or interfaces, such phenomenon includes corrosion. catalysis, crystallization, etc.
Adsorption
Due to unbalanced attraction forces, accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption. The molecular species accumulates at the surface is termed as adsorbate and the material on the surface of which the adsorption takes place is called adsorbent, e.g..
(i) O2, H2, C12, NB3 gases are adsorbed on the surface of charcoal.
(ii) Silica gels adsorb water molecules from air.
Charcoal, silica gel, metals such as Ni, Cu, Ag, Pt and colloids are some adsorbents.
Important Characteristics of Adsorption
- It is specific and selective in nature.
- Adsorption is spontaneous process, therefore change in free energy (ΔG)is negative.
- ΔG= ΔH – TΔS,
- For the negative value of ΔG,in a system, in which randomness decreases, ΔH must be negative.
- Hence, adsorption is always exothermic.
- Adsorption of hydrogen over Pt is called occlusion.
Desorption
It is a process of removing an adsorbed substance from a surface on which it is adsorbed, is known as desorption.
Distinction between Adsorption and Absorption
Sorption
It is a process in which both adsorption and absorption take place simultaneously, the term sorption is simply used.
Positive and Negative Adsorption
When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk, it is called positive adsorption. On the other hand, if the concentration of the adsorbate is less relative to its concentration in the bulk, it is called negative adsorption, e.g., when a dilute solution of KCl is shaken with blood charcoal, it shows negative adsorption.
Distinction between Physio sorption and Chemisorption
Adsorption Isobars
These are plots of x / m us temperature t at constant pressure.
For physical and chemical adsorption, they are shown below.
Adsorption Isostere
These are the plot of temperature versus pressure for a given amount of adsorption
Applications of Adsorption
1. For production of high vacuum.
2. Gas masks containing activated charcoal is used for breathing in coalmines. They adsorb poisonous gases.
3. Silica and aluminium gels are used as adsorbents for controlling humidity.
4. Removal of colouring matter from solutions.
5. It is used in heterogeneous catalysis.
6. In separation of inert gas.
7. As adsorption indicators.
8. In chromatographic analysis.
9. Qualitative analysis, e.g., lake test for Al3 +. Catalysis Catalyst is a chemical substance which can change the rate of reaction without being used up in that reaction and this process is known as catalysis
A catalyst may be positive (i.e., increases rate of reaction) or negative (i.e., decreases rate of reaction).
Types of Catalysis
(a) Homogeneous catalysis In this catalysis, and the catalyst reactants are in the same physical state [phase], e.g.,
(b) Heterogeneous catalysis In heterogeneous catalysis, catalyst is present in a different phase than that of reactants, e.g.,
(c) Autocatalysis
When one of the product of a reaction acts as catalyst, the process is called autocatalysis.
Characteristics of Catalysts
1 The catalyst remains unchanged in mass and chemical composition.
2. In case of reversible reactions, the catalyst does not influence the composition of reaction mixture at equilibrium.
It only helps to attain the equilibrium quickly. Promoters and Poisons
Promoters are chemical substances that enhance the activity of a catalyst while poisons decreases the activity of a catalyst
Adsorption Theory of Heterogeneous Catalysis
The mechanism involves five steps:
(i) Diffusion of reactants to the surface of the catalyst
(ii) Adsorption of reactant molecules on the surface of the catalyst.
(ill) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate.
(iv) Desorption of reaction products from t he catalyst surface.
(v) Diffusion of reaction products away from the catalyst’s surface
Important Features of Solid Catalysts
(i) Activity
The activity of a catalyst depends upon the strength of chemisorption to a large extent. The adsorption should be reasonably strong but not so strong that they become immobile and no space is available for other reactants to get adsorbed
. (ii) Selectivity
The selectivity of a catalyst is its ability to direct a reaction to yield a particular product, e.g., starting with Hz and CO using different catalysts, we get different products. Shape–selective catalysis 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.
Cracking Isomerization of hydrocarbons in the presence of zeolites is an example of shape selective catalysis.
An important zeolite catalyst used in the petroleum industry is ZSM-S.lt converts alcohols directly into gasoline.
Enzyme Catalysis
Enzymes are complex nitrogenous organic compounds which are Produced by living plants and animals. They are actually protein molecules of high molecular mass and form colloidal solutions in water. They are also known as biochemical catalysis.
Mechanism of Enzyme Catalysis
Some examples of enzyme catalysed reactions are:
(Source of invertase, zymase and maltose is yeast and that of diastase is malt. Soybean is the source of urease.)
(v) In stomach, the pepsin enzyme converts proteins into peptides while in intestine, the pancreatic trypsin converts proteins into amino acids by hydrolysis.
(vi) Lactobacilli is used to convert milk into curd.
Characteristics of Enzyme Catalysis
High efficiency
One molecule of an enzyme may transform one million molecule of reactant per minute.
Highly specific nature Each enzyme catalyst cannot catalyse more than one reaction.
Optimum temperature
Enzyme catalyst gives higher yield at optimum temperature i.e., at 298-310 K. Human body temperature, i.e., at being 310 K is suited for enzyme catalysed reactions.
Optimum pH The rate of an enzyme catalysed reaction is maximum at optimum pH range 5 to 7.
Activators Activators like ions such as Na+ ,Ca 2+, Mn2+ help in the activation of enzymes which cannot act on their own strength.
Co-enzyme Co-enzymes are the substance having nature similar to the enzyme and their presence increases the enzyme activity. Mostly vitamins act as co-enzymes.
Effect of Inhibitors Inhibitors slow down the rate of enzymatic reaction. The use of many drugs is based on enzyme inhibition action of those drugs in the body.
1. This phenomenon of attracting and retaining the molecules of a substance by a solid (or a liquid) on its surface resulting into a higher concentration of the molecules on the surface is known as adsorption.
2. The substance that is adsorbed is called adsorbate and the substance which adsorbs is called adsorbent.
3. Desorption is a process of removing an adsorbed substance from a surface on which it is adsorbed.
4. Absorption is different from adsorption. In absorption, the substance is uniformly distributed throughout the body of a solid or a liquid.
5. When the adsorbate is held on the surface by weak van der Waals forces, the process is called physical adsorption or physical adsorption. This type of adsorption can be reversed by heating or decreasing the pressure.
6. When the forces holding the adsorbate on the surface are of the magnitude of chemical bond forces, the process is called chemical adsorption or chemisorption. This type of adsorption is irreversible.
7. Adsorption is generally accompanied by evolution of heat, i.e., it is an exothermic process.
8. The extent of adsorption of a gas on a solid depends upon the following factors:
(a) Nature of the adsorbate,
(b) Nature of the adsorbent,
(c) Temperature, and
(d) Pressure.
9. A relation or a graph between the magnitude of adsorption x/m and the pressure P of the gas at a constant temperature is called adsorption isotherm.
10. Freundlich adsorption isotherm:
Plot of log x/m Vs log P will be a straight line with a slope of 1 In. It holds good at moderate temperature. At low pressure, n = 1.
11. Langmuir adsorption isotherm is based on following assumptions:
(i) Every adsorption site is equivalent in all respect.
(ii) The ability of a particle to bind at a particular site is independent of whether the nearby sites are occupied or not.
12. Langmuir derived the following relation.
where a and b are Langmuir parameters.
13. A substance that can influence the rate of a chemical reaction but itself remains unchanged chemically at the end is called a catalyst.
14. In a homogeneous catalysis, the catalyst is present in the same phase as the reactants.
15. In heterogeneous catalysis, the catalyst is present in a different phase than that of the reactants.
16. Enzymes also called biological catalysts are proteins which catalyse the reactions in living systems.
17. The colloidal solutions are intermediate between true solutions and suspensions. The diameter of colloidal particles varies from 1 to 1000 nm.
18. A colloidal system is a heterogeneous system which consists of disperse phase and dispersion medium.
19. The disperse phase constitutes the colloidal particles whereas the dispersion medium constitutes the medium in which the colloidal particles are dispersed.
20. There are eight types of colloidal systems based on the disperse phase and the dispersion medium.
21. Sols are the colloidal system in which the solid is disperse phase and the liquid is dispersion medium.
22. Hydrosols-Colloids in water.
Alcosols – Colloids in alcohol.
23. Lyophillic colloids (solvent loving) are those substances that directly pass into the colloidal state when brought in contact with the solvent, e.g., proteins, starch, rubber, etc.
These sols are quite stable because of the strong attractive forces between the particles and the, dispersion medium.
24. Lyophobic colloids (solvent hating) are those substances that do not form the colloidal sol readily when mixed with the dispersion medium. These sols are less stable than the lyophilic sols.
25. The colloids are also classified as multimolecular, macro-molecular and associated colloids.
26. Lyophobic sols can be prepared by the following methods:
(a) Chemical methods:
(i) Oxidation,
(ii) Reduction,
(iii) Hydrolysis ,
(iv) Double decomposition,
(b) Physical methods:
(i) Exchange of solvent:
(ii) Excessive cooling: A colloidal sol of ice in an organic solvent (CHCl3 or ether) can be obtained by freezing a solution of water in the solvent.
(c) Dispersion methods:
(i) Mechanical dispersion:
(ii) Bredig’s arc method:
(iii) Peptization method:
27. Lyophilic sols are readily prepared by warming the substance with a dispersion medium, e.g., starch, gelatin, gumarabic, etc., are easily brought into the colloidal state by warming with water.
28. The process of separating a soluble crystalloid from a colloid is called dialysis.
29. Characteristics of colloidal solution:
(a) The zig-zag and random motion of the colloidal particles is called Brownian movement.
(b) When a beam of light is passed through a colloidal solution, its path becomes visible.
This phenomenon is known as Tyndall effect.
It is due to the scattering of light by colloidal particles.
(c) This movement of colloidal particles under applied electric field is known as
electrophoresis.
(d) Diffusion of colloidal particles takes place from a region of higher concentration to lower concentration.
30. Emulsions: It is a colloidal system in which both the dispersed phase and the dispersion medium are liquids, e.g., milk consists of small drops of liquid fat dispersed in water.
31. Emulsification is the process of making an emulsion.
32. Types of Emulsions
(a) Oil-in-water type in which small droplets of an oil are dispersed in water, e.g., milk, cod- liver oil, etc.
(b) Water-in-oil type in which water droplets are
dispersed in an oil medium, e.g., butter.
