SURFACE CHEMISTRY
Adsorption
Adsorbate & Adsorbent.
Desorption.
Adsorption and Absorption
Sorption
Enthalpy of adsorption
Surface area & Adsorption
Physisorption
Chemisorption
Adsorption Isotherm
Freundlich isotherm
Applications of Adsorption
Promoters and Poisons
Homogeneous catalysis
Heterogeneous catalysis
Activity of a solid catalyst
Selectivity of a catalyst
Shape- Selective Catalysis
by Zeolites
Characteristics of enzyme
catalysis
Catalysts used in industrial
process.
Colloids
Lyophilic colloids
Lyophobic colloids
Multimolecular colloids
Macromolecular colloids
Associated
colloids
(Micelles)
Kraft Temperature (Tk)
CMC
Cleansing action of soaps
Electrical disintegration or
Bredig’s Arc method
Peptization
Dialysis:
Tyndall effect
Conditions
effect
for
Tyndall
Accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.
The substance adsorbed is called adsorbate. And the surface provider is called adsorbent.
The process of removing an adsorbed substance from a surface.
In adsorption, the substance is concentrated only at the surface while in absorption, the
substance is uniformly distributed throughout the bulk of the solid.
Both adsorption and absorption can take place simultaneously.
Adsorption is exothermic ΔH is -ve. There is decrease in entropy, ΔS is -ve. For a process
to be spontaneous, ΔG must be negative, so ΔH should have sufficiently high-ve value.
Adsorption increases with the increase of surface area of the adsorbent. Thus, finely
divided metals and porous substances having large surface areas are good adsorbents.
van der Waals’ forces / not specific /reversible /easily liquefiable gases adsorbed readily./
Enthalpy of adsorption is low/ Low temperature is favourable / Multimolecular layers.
chemical bond /highly specific / irreversible/Enthalpy of adsorption high / High
temperature is favourable / / unimolecular layer.
Variation of amount of gas adsorbed by the adsorbent with pressure at constant temperature
can be expressed by means of a curve termed as adsorption isotherm.
X / m = k.P1/n (n > 1)
log x/m = log k + 1/n log P
High vacuum/masks/Control of humidity/ Removal of colour/ catalysis/ curing diseases/
N2(g) + 3H2(g) ………… 2NH3(g)
Reactants and the catalyst are in the same phase.
Reactants and the catalyst are in different phases.
Depends upon strength of chemisorption .Reactants must get adsorbed reasonably strongly.
The selectivity of a catalyst is its ability to direct a reaction to yield a particular product.
Depends upon the pore structure of the catalyst and the size of the reactant and product
molecules. Example:- ZSM-5 (a zeolite) converts alcohols directly into gasoline (petrol).
Highly efficient/ Highly specific / active under optimum temperature& pH/ Increasing
activity in presence of activators and co-enzymes/ Influence of inhibitors and poisons
Haber’s process (NH3) Finely divided Fe, Mo - promoter; Ostwald’s process(HNO3) Pt
Contact process(H2SO4) Pt or vanadium pentoxide (V2O5);
Diameters of the particles is between 1 and 1000 nm.
Liquid-loving/ can be reconstituted/ reversible/easily prepared/stable/ eg- gum, starch.
Liquid-hating/ not be reconstituted/ irreversible/ prepared by special methods/ precipitated
on addition of electrolytes/heating/shaking/ not stable./ eg –metal sols , sulphide sols.
Large number of atoms or molecules of a substance aggregate eg- gold sol , sulphur sol.
Macromolecules in suitable solvents. Eg-starch, cellulose, proteins and enzymes; polythene.
Substances at low conc behave as strong electrolytes, but at high conc exhibit colloidal
behaviour due to the formation of aggregates(micelles.) eg- soaps and detergents.
Formation of micelles takes place only above a particular temperature.
Formation of micelles takes place only above a particular conc.
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 . 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.
This process involves dispersion as well as condensation. Colloidal sols of metals such as
gold, silver, platinum, etc., can be prepared.
Process of converting a ppt into colloid by shaking it with dispersion medium in the
presence of a small amount of electrolyte. The electrolyte used for this purpose is called
peptizing agent.
Removal of dissolved substance from colloidal by diffusion through a suitable membrane.
Colloidal particles scatter light in all directions. Scattering of light illuminates the path of
beam of the light.
(i)
Diameter of particles is not much smaller than wavelength of the light used;
(ii)
Refractive indices of the phase and medium differ greatly in magnitude
+ve charged sols  Hydrated metallic oxides,/methylene blue sol. /Haemoglobin (blood)
-ve charged sols  Metals,/ sulphides/ eosin, congo red / starch, gum, gelatin, clay,charcoal.
Preferential adsorption of ions . The sol particles acquire positive or negative charge by
preferential adsorption of +ve or –ve ions. When two or more ions are present in the
dispersion medium, preferential adsorption of the ion common to the colloidal particle
usually takes place.
AgI/I–
Zeta potential
Potential difference between the fixed layer and the diffused layer.
Electrophoresis
The movement of colloidal particles under an electric potential is called electrophoresis.
Electroosmosis
If electrophoresis is prevented , it is observed that the dispersion medium begins to move in
an electric field.
Coagulation(ppt)
The process of settling of colloidal particles is called coagulation or precipitation of the sol.
Reason of Coagulation
Electrophoresis/mix oppositely charged sols/boiling/persistent dialysis/addition of
electrolytes.
Hardy-Schulze rule
The greater the valence of the flocculating ion added, the greater is its power to cause
precipitation.
For coagulation of a negative sol, : Al3+>Ba2+>Na+
For coagulation of a negative sol :[Fe(CN)6]4– > PO4 3– > SO4 2– > Cl–
Coagulating value
Min. conc of an electrolyte in millimoles/L required to cause precipitation of a sol in 2 hours
Emulsions
Liquid-liquid colloida (i) Oil dispersed in water (O/W type) - milk and vanishing cream .
(ii) Water dispersed in oil (W/O type).- butter and cream.
Demulsification
Emulsions can be broken into constituent liquids by heating, freezing, centrifuging, etc.
Artificial Rain
By throwing electrified sand , spraying a sol of opposite charge to the clouds from aeroplane
Formation of Delta
River water is a colloidal solution of clay. Sea water contains a number of electrolytes.
When river water meets the sea water, the electrolytes present in sea water coagulate the
colloidal solution of clay resulting in its deposition.
Electrical precipitation of The smoke, before it comes out from the chimney, is led through a chamber containing
smoke(Cottrellppter)
plates having a charge opposite to that carried by smoke particles. The particles on coming
in contact with these plates lose their charge and get precipitated.
Charge
on
colloidal
particles
Origin of charge on
colloidal particles