Surface chemistry MSc previous
Introduction
- Residual unbalanced forces exist on the surface of solid.
- As a result of these residual forces, the surface of solid has a tendency to attract and retain molecules of other species with which it is brought into contact.
- As these molecules remain only at the surface, their concentration is more at the surface than in the bulk of the solid is known as adsorption.
- Solids when finely divided have a large surface area shows adsorption to a large extent.
- Solid that takes up a gas or vapour or a solute from a solution is called the adsorbent while the gas or vapour or the solute, which is held to the surface of the solid is called adsorbate.
- Collides such as charcoal, silica gel, alumina gel, clay, etc. Are good adsorbents.
- Reason:- this is because they have extremely small dimensions, possess enormous surface area per unit mass .
Differentiate between adsorption and absorption :-
- 1. Adsorption is a surface phenomenon ,i.e. , it occurs only at the surface of the adsorbent.
- Where as absorption is a bulk phenomenon, i.e. , occurs throughout the body of the material.
- 2. In adsorption, the concentration on the surface of adsorbent is different from that in the bulk.
- Where as in absorption, the concentration is same throughout the material.
- 3.Rate of adsorption is high in the beginning and then decreases till equilibrium is attained.
- Whereas rate of absorption remain throughout the process.
Absorption implies that a substance is uniformly distributed throughout the body of a solid or a liquid.
Example :- water vapours are absorbed by anhydrous calcium chloride.
Water vapour are adsorbed by charcoal.
When a hot crucible is cooled in atmosphere, a film of moisture collects at its surface. (Adsorption of water vapour on the material of the crucible)
Charcoal when mixed with a coloured solution of sugar, adsorbs the colouring matter and is used as a decoloriser.
Sorption :- a process in which both adsorption and absorption take place simultaneously.
Type of adsorption :- Two types
1. Physical adsorption (physisorption)
2. Chemical adsorption (chemisorption)
Physisorption :- When a gas is adsorbed on the surface of a solid by van Der Waals forces without resulting into the formation of any chemical bond between the adsorbate and the adsorbent, it is called physical adsorption or van Der waals adsorption or physisorption.
Chemisorption:- Chemical process involving chemical interaction between the surface atoms of the adsorbent and the atoms of the adsorbate.
Example :- Oxygen is chemisorbed by carbon ;
Hydrogen is chemisorbed by nickel under suitable conditions.
In each case, a stable surface compound is formed called surface complex.
Differentiate between chemisorption and physisorption :-
- Physisorption occurs only at very low temperature falling below the boiling point of the adsorbate. Chemisorption can occur at all temperature.
- Magnitude of chemisorption increase with rise in temperature.
- Magnitude of physisorption decrease with rise in temperature.
- Heat evolved in chemisorption is very high varying generally between 40-400 KJ /mol.
- Heat evolved in physisorption is quite low, varying generally between 4 - 40 KJ / mol.
- Chemisorption is irreversible as the gas adsorbed cannot be recovered from the adsorbent as such on lowering the pressure of the system at the same temperature.
- Physisorption is reversible as the gas adsorbed can be recovered from the adsorbent easily on lowering the pressure of the system at the same temperature.
- Chemisorption operate within short distances only and does not extend beyond a monolayer of gas molecules or atoms on the surface of the solid.
- Physisorption may extend beyond a monolayer also.
- In chemisorption, the adsorbate molecules are held by comparatively strong Valence forces. Hence the activation energy of desorption is very high.
- In physisorption, the adsorbate molecules are held by comparatively weak van Der Waals forces. Hence , the activation energy of desorption, in physisorption is very low.
Applications of adsorption:-
1. A very good method of creating a high vacuum is to connect a bulb of charcoal cooled in liquid air to a vessel which has already been exhausted as far as possible by a vacuum pump.
Since the magnitude of adsorption at such a low temperature is quite high, the remaining trace of air , in spite of the low pressure, are adsorbed by the charcoal almost completely.
2. Activated charcoal is used in gas masks in which all toxic gases and vapours are adsorbed by the charcoal while pure air passes through its pores practically unchanged .
3. Silica and alumina gels are used as adsorbent for removing moisture and for controlling humidity of rooms.
4. Animal charcoal is used as a decoloriser in the manufacture of cane sugar.
5. Soil contains small amounts of colloidal fractions in the form of very fine particles of clay. It can always adsorb and retain certain amount of moisture in which nutrients, such as compounds of nitrogen, phosphorus and potassium can dissolve and pass up to the plant through the roots.
6. Adsorption also plays an important role in heterogeneous catalysis .
Example:- role of finely divided iron in the manufacture of ammonia and that of finely divided nickel in the hydrogenation of oil.
Adsorption of gases by solids:- Several methods for determining adsorption of gases on solid adsorbents under the given conditions have been devised .
In one such method , the gas is contained in a vessel of known volume at a given temperature.
The pressure of the gas is measured on a manometer attached to the vessel.
The adsorbent is then introduced into the vessel by a suitable device.
Adsorption takes fairly quickly and the pressure of the gas falls .
This is noted on the manometer.
Knowing the fall of pressure, the quantity of the gas adsorbed by the solid can be calculated, assuming Boyle's law to hold good.
It is necessary to apply correction due to the volume of the adsorbent added.
