We know that, there are same properties of solids and liquids related to their surfaces. Shiele in the year 1776, observed the accumulation of gases on the surface of charcoal, this was then studied by H Kayser in 1881. This phenomenon was called as Adsorption. Adsorption is a surface phenomenon.
It was also observed that metals like platinum, nickel, and palladium have tendency to take up large quantities of hydrogen gas on their surfaces and also act as catalysts in hydrogenation of unsaturated organic compounds. All these phenomenon can be explained on the basis of surface property of solids or liquids, called adsorption.
What is Adsorption?
If we consider a liquid or soild substance then it is found that a molecule which is present in the bulk of the substance is attracted uniformaly from all sides by the other molecules and hence there is no unbalanced force on such molecules. On the other hand, if we consider a molecule present on the surface, it is noticed that such molecule is not surrounded forces.As a result of these unbalanced forces the molecules of other species on their surface. This results in increase in concentration of the substance at the surface than in bulk of soild or liquid. The phenomenon of accumulation of higher concentration of any molecular species on the surface than in bulk of a solid or liquid is called adsorption. It is a surface phenomenon.
What are Adsorption and Absorption:
Adsorption:
"The phenomenon of accumulation of one substance on the surface of other substance (solid or liquid) is defined as adsorption."The change in concentration at the interfacial layer between two phases of a system due to surface forces is known as adsorption.
Adsorption may be expected when two heterogeneous phases are brought in contact with each other. Hence following are the three cases of adsorption.
1) Gas- solid system e.g. adsorption of any gas on charcoal.
2) Liquid - solid system e.g. adsorption of acetic acid on charcoal.
3) Gas- liquid system e.g. adsorption of a gas on mercury.
Absorption:
The phenomenon in which a substance penetrates through the surface and gets uniformly distributed throughout the body of the other substance is called absorption.Thus absorption is a bulk phenomenon. e.g. absorption of water by a sponge or an ink by a chalk.
It may be noted that in certain absorption both take place simultaneously and are non-distinguishable. In such cases the word sorption is used to describe the phenomenon.
Example: Dyes get adsorbed as well as absorbed in cotton fibres i.e. sorption of dyes by cotton fibres.
Adsorbate:
The substance which itself gets adsorbed on the surface of another substance is called adsorbate.Examples: Gases like helium, neon, oxygen, nitrogen, sulphur dioxide, ammonia etc.
Adsorbent:
The substance which adsorbs the other substance on its surface is called as adsorbent. e.g. in the process of adsorption of acetic acid by charcoal powder, the charcoal is the adsorbent and acetic acid is adsorbate.Examples: Silica gel, colloidal substances, metals like nickel, copper, silver, platinum, palladium, tungsten etc.
Absorbate:
The substance which gets absorbed through the surface into the body of another substance and gets uniformaly distributed in it is called Absorbate.Absorbent:
The porus substance which absorbs the other substance in its body is called absorbent. e.g. absorption of water by sponge. In this case sponge is absorbent and water is absorbate. |
Concept of Adsorption and Absorption |
Forces between adsorbate and adsorbent:
The adsorption occurs because of attractive forces exerted by atoms or molecules present at the surface of the adsorbent. These attractive forces may be of two types:1) Physical forces or van der Waal's forces or cohesive foreces: These forces are weak and non-specific in nature.
2) Chemical forces: These forces are strong and specific in nature.
Difference between Adsorption and Absorption:
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| Difference between Adsorption and Absorbtion |
Physical and Chemical Adsorption:
Depending on nature of forces between the adsorbate and adsorbent which hold the particles of adsorbate on the surface of the adsorbent, the adsorption can be classified in the following two types :1) Physical adsorption:
When the molecules of adsorbate are held on the surface of adsorbent due to weak van der Waal's forces, the adsorption is called physical adsorption or physisorption or van der Waal's adsorption.e.g. Adsorption of H2 gas by charcoal.
2) Chemical adsorption:
When the molecules of an adsorbate are held on the surface of adsorbent by formation of chemical bonds the adsorption is termed as chemical adsorption. It is also called as chemisorption.e.g. Adsorption of oxygen on tungsten to form tungsten oxide on surface.
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| Difference between physical and chemical Adsorption |
Factors Affecting Adsorption:
1) Nature of adsorbate:
The gases get adsorbed more rapidly than liquids and solids. The easily liquefibale and highly water soluble gases like NH3, HCl, Cl2, SO2 etc. get adsorbed more rapidly than other gases.This is because these gases have greater van der Waal's forces of attraction as compared to those gases which are not easily liquefibale and less soluble in water like H2, O2, N2, He, Ne etc. The gases with high molecular weights also get adsorbed to greater extent.
Example: SO2 is adsorbed to a greater extent as compared to CO2.
2) Nature of adsorbent:
As adsorption is a surface phenomenon, the extent of adsorption depends upon surface area of adsorbent. Thus substances having porous nature, finely divided powders and rough surfaces provide larger surface area hence act as good adsorbents.Examples: Activated charcoal, silica gel, colloidal particles etc.
3) Temperature:
Adsorption is an exothermic process. According to LeChatelier's principle decrease in temperature favours adsorption. Thus at a constant pressure, adsorption varies inversely with temperature. |
| Effect of temperature on Adsorption |
The effect of temperature on extent of adsorption is studied by keeping pressure constant and curves obtained are called adsorption isobars.
4) pressure:
The increase in pressure helps the gaseous adsorbate molecules to get adsorbed at the faster rate. Hence at a constant temperature the rate of adsorption increases with the increase in pressure. But, at a certain high pressure the whole surface area of the adsorbent gets totally covered by adsorbate molecules.At this stage, even if the pressure is incresed further extent of adsorption remains constant. Thus initially the extent of adsorption remains constant. Thus initially the extent of adsorption is directly proportional to the pressure but at very high pressure it remains practically constant.
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| Effect of pressure on Adsorption |
Initially extent of adsorption increases with pressure due to availability of surface area. At high pressure due to unavailability of surface area of adsorbent extent of adsorption remains practically constant.
5) Concentration of adsorbate:
The extent of adsorption depends upon the concentration of the adsorbate. The rate of adsorption increases with the increase in concentration of solution of the adsorbate. The increase continues up to certain limit till equilibrium is achieved. Further increase in concentration has no effect on the extent of adsorption.6) Surface area of adsorbent:
As adsorption is a surface phenomenon, the extent of adsorption depends upon surface area of adsorbent. Thus substances having porous nature, finely divided powders and rough surfaces provide larger surface area hence act as good adsorbents.Applications of adsorption:
Some important applications of adsorption are as follows:
1) Purification of water:
The process of removal of insoluble and soluble impurities from water is purification of water.a) Water contains insoluble and turbid impurities like mud, rubbish etc. When impure water containing these impurities is passed through a bed of finely divided charcoal, the insoluble impurities are adsorbed on the surface of charcoal. Due to it's porous nature charcoal also acts as filter. After the adsorption of impurities on charcoal water is filtered through it.
b) Colloidal impurities from water are removed by adding alum (K2SO4.Al2(SO4)3.24H2O) to the impure water. A gelatinous precipitate of Al(OH)3 is formed by hydrolysis of alum, which acts as a good adsorbent. The insoluble, colloidal impurities from water are adsorbed on the surface of the precipitate. The precipitate becomes heavier and settles down at the bottom leaving pure water as supernatant liquid.
c) Removal of soluble impurities: Water contains soluble impurities like carbonates and bicarbonates of calcium and magnesium. This water is called as hard water. To remove these soluble impurities water is passed through a column of Zeolite or sodium permutit, Na2P(Na2Al2Si2O8). An ion exchange reaction (adsorption) takes place and Ca²+ and Mg²+ ions are removed from hard water and water is softened.
2) Adsorption indicator:
The indicator used to indicate the end point of a precipitation titration by the phenomenon of adsorption is called as adsorption indicator.The dyestuffs such as eosin, fluorescein, bromocresol blue etc are used as adsorption indicators.
3) Catalysis:
The process which increases the rate of a chemical reaction by adding a substance called catalyst is called catalysis. Many industrially important chemical substances are synthesised by the reactions that occur on the surface of an adsorbent. The concentration of the reactant increase on the surface of the catalyst and thus increase the rate of reaction.Many metals are used as catalysts. Metals like palladium, platinum, nickel, copper, have a tendency to absorb gases like H2, SO2, N2 and O2 on their surfaces. Hence these metals are used as catalysts. Adsorption is an exothermic process. The heat of adsorption is used to activate the reacting molecules. Thus process of adsorption also provides the energy of activation for reactants, hence rate of reaction increases.
Some examples are as follows:
1) Nickel is used for hydrogenation of oils.
2) Copper is used for oxidation of alcohols to aldehyde.
3) Platinized asbestos or vanadium pentoxide is used for oxidation of sulfur dioxide to sulfur trioxide in contact process for manufacture of sulfuric acid.
4) Finely divided iron is used as a catalyst in the manufacture of ammonia from nitrogen and hydrogen in Haber's process.
So, this is introduction about Adsorption which is surface phenomenon.