Monday, September 22, 2014

Free Energy Change of Adsorption

Since adsorption is Exothermic Reaction. Therefore for adsorption ΔH is always negative and favours the process. Also adsorption leads to decrease in entropy i.e. ΔS is negative because the adsorbate molecules gets lesser opportunity to move about on the surface of adsorbent. According to Gibbs - Helmholtz Equation.
Since adsorption is Spontaneous Process, hence ΔG is negative or ΔH > TΔS. As adsorption continues, the difference between two opposing factors becomes lesser and lesser till they becomes equal i.e ΔH = TΔS or ΔG = 0. At this stage, Equilibrium called Adsorption Equilibrium is established and hence no adsorption takes place at this stage.

Applications of Adsorption

The phenomenon of adsorption finds extensive application in field of Industry, laboratory and various other technical processes. A few important applications of adsorption are as under:-
1.     Manufacture of Gas Masks
               Activated charcoal used in gas masks removes all undesirable toxic and poisonous gases while purified air passes through its pores.
2.      Removing Of Colouring Matter From Sugar Juice & Vegetable Oils
                 Activated animal charcoal is used as decolourizer in removing colouring matter of sugar solution and other vegetable materials.
3.      Dehydration & Purification Of Gases
                  Silica gel and Alumina are used as good adsorbents for removing moisture and for controlling humidities in rooms which store delicate articles.
4.      Dyeing of Clothes
                  Modrants used in dyeing adsorb the colouring matter which does not attach to fabric otherwise.
5.      Creation Of High Vacuum In Laboratory
                  Finely divided coconut charcoal adsorbs all gases creating a vacuum as low as 10–4 mm.
6.      Heterogenous Catalysis
                 The action of heterogeneous catalysts like finely divided nickel, finely divided platinum, finely divided iron in hydrogenation of oils, conversion of SO2 to SO3 and manufacture of ammonia respectively is based on the phenomenon of adsorption.
7.      Ion-Exchange Resins
                   The organic high polymers containing groups such as –COOH, –SO3H or –NH2 possess the property of selective adsorption of ions from solution. These resins are largely used in Industrial softening of water and separation of rare earths from their mixture.
8.      Chromotography
                   There are a number of chromotographic techniques like adsorption chromotography, paper chromotography and vapour phase chromotography which are based on selective adsorption of different substances of the adsorbent.
9.      Qualitative Inorganic Analysis
                  The confirmatory lake test for Al3+ ions is based on the adsorption of colour of litmus by Al(OH)3.
10.    Drugs
         Various drugs get adsorbed on the tissues which then are heated. The germicidal action of medicines is also based on the property of adsorption.

Difference Between Physical Adsorption and Chemical Adsorption

1. Molecules are held due to vanderwaal’s forces. Molecules are held due to chemical bond forces.
2. Heat of adsorption are in the range of 20 – 40KJ/Mol. Heat of adsorption are in the range of 40 – 400 KJ/Mol.
3. Activation energy are small. Activation energy are appreciable.
4. Usually occurs rapidly at low temperatures and decreases with increase in temperature. Can occur at high temperature.
5. Usually completely reversible. Often irreversible.
6. The extent of adsorption is approximately related to the ease of liquification of gas. No such correlation is there.
7. Not very specific. Often highly specific.
8. Form multilayer on the surface of adsorbent. Forms unimolecular layer.
9. No surface compound is formed Surface compounds are formed.