Adsorption

What is Adsorption?

Adsporption is the phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or solid resulting into a higher concentration of molecules on the surface. The substance adsorbed on the surface is called adsorbate and the substance on which it is adsorbed is called adsorbent. The surface where the process takes place is called interface.

When there is an increase of concentration or a condensation of matter on a surface, the phenomenon is called positive adsoprtion, while if there is a decrease of concentration, it is called negative adsorption.

The examples cited above indicated two main types of adsorption:

  • Adsorption of gases on solids, and

  • Adsorption of solutes on solids.

What is the difference between adsorption and absorption?Adsorption and Absorption

The distinction between absorption and adsorption should be clearly understood. Some of the difference are:

  • Adsorption is surface phenomenon, while absorption concerns with the whole mass of the absorbent.

  • In adsorption, the substance is only retained on the surface and does not go into the bulk or interior of the solid or liquid. Absorption implies that substance is uniformly distributed throughout the body of the solid or liquid.

  • In adsorption, the concentration of the adsorbed molecules is always greater in the immediate vicinity of the surface than in the free phase. Absorption involves bulk penetration of the molecules into the structure of the solid or liquid by some process of diffusion.

  • Adsorption is a rapid process and equilibrium is attained in a short time. In absorption the equilibrium takes place slowly.

Such substance is said to be sorbed and the phenomenon is known as sorption. 

Refer to the folloiwing video for adsorption and absorption

Examples of Adsorption and Absorption

  • Water vapours is absorbed by anhydrous calcium chloride, while it is adsorbed by silica gel.

  • Ammonia is absorbed in water forming NH4OH,  while absorbed by charcoal.

  • Acetic acid and iodine are adsorbed.

  • Nitrogen gas is adsorbed on mica.

  • Oxygen is adsorbed by tungsten surface.

  • Decolourisation of sugar solution by activated charcoal is adsorption of colouring matter on charcoal.

  • If a blotting paper is kept in contact with ink, the latter is absorbed, because it penetrates uniformly into the absorbent, the blotting paper.

Examples of Adsorbent and Adsorbate

Activated CharcoalThe common adsorbents are charcoals (both vegetable and animal charcoal), silica gel (it can be prepared by heating a mixture of sodium silicate with 10% HCl at 50°C). Metals, such as Ni, Cu, At, Pt and Pd (these  are prepared by reducing their oxide or their salts under suitable experimental conditions), and colloids, as they posses high surface area per unit mass.

Various gases, such He, Ne, H2, N2, O2,SO2, NH3 etc. and substances, such as NaCl, KCl etc.  in solution can be adsorbed by suitable adsorbents.

Reversible and irreversible adsorption

The adsorption is reversible, if the adsorbent can be easily removed from the surface of the adsorbent by physical methods. It is called irreversible adsorption, if the adsorbate can not be removed from the surface of the adsorbent. A gas adsorbed on a solid surface can be completely removed in vacuum. It is, therefore, reversible adsorption. Examples of irreversible adsorption are adsorption of oxygen on tungsten adsorbate and adsorption of CO on tungsten surface.

Characteristics of Adsorption

  • The phenomenon of adsorption strictly refers to the existence of a higher concentration of any particular component at the surface of a liquid or a solid phase than is present in the bulk or interior. For example, five types of interfaces can exist (i) gas solid (ii) liquid solid (iii) liquid – liquid, (iv) solid – solid  and gas solid.

  • Adsorption is a specific and selective phenomenon. In other words, to what extent a certain substance will be adsorbed by an adsorbent depends on the physical and chemical nature of  both adsorbent as well as the adsorbate. It is not possible to predict  the extent of adsorption in any combination. However, it has actually been found that easily liquefiable gases are more easily adsorbed.

  • Adsorption is accompanied by a decrease in the free energy of the system (dG is negtive). Adsorption will continue to such an extent that dG continues to be negative. When dG becomes zero, i.e., dG = 0, adsorption equilibrium is said to be established. Since decrease in dH (Heat content) appears as heat, adsorption process is always exothermic.

Enthalpy of adsorption: 
The adsorption of one substance on the surface of another leads to the existence of new types of forces between them. Therefore, it is an exothermic process and is accompanied by the release of energy. The enthalpy or heat of adsorption is defined as the heat energy evolved when one mole of adsorbate is adsorbed on the surface of adsorbent. Since physical adsorption involves weak forces of attraction between the molecules of the adsorbent and the adsorbate, the heat of physisorption is generally low, of the order of 20-40 kJ mol–1. Chemical adsorption, on the other hand involves strong chemical bond formation and the heat of chemisorptions is quite high, of the order of 80-400 kJ mol–1

Types of Adsorption

 Physisorption (Physical adsorption) 

   Chemical adsorption

Weak  Vander Waal’s forces are present between adsorbent and adsrbate.

Chemical bonds are formed between adsorbent and adsrbate molecules.

Depends on nature of gas. Easily liquefiable gases are adsorbed readily.

Much more specific and depends upon the nature of the both the adsorbate and adsorbent. 

Heat of adsorption is small (about 5 kcal per mol) 

Very large eat of adsorption (80-400 kcal per mol). 

Reversible in nature

Irreversible in nature

Forms multimolecular layers on adsorbent surface.

Forms unimolecular layer. 

Occurs at low temperature; decreases with increase in temperature.

Takes place at high temperature. first increases and then starts decreasing with rise in temperature

Increase of pressure increases adsorption 

High pressure is favourable. Decrease of pressure does not cause desorption. 

Equilibrium is attained readily and it is reversible. 

Equilibrium is attained slowly and mostly not reversible. 

Mechanism of adsorption

Factors Affecting Adsorption:

The magnitude of gaseous adsorption depends upon the following factors:

  • Temperature: An increase of temperature leads to a decrease in amount adsorbed and vice – versa.

  • Pressure or concentration: It has been found that in most cases,  the adsorption is reversible and an increased pressure of a gases vapour or an increase in concentration of a solute causes increased adsorption.
    Effect of pressure on adsorption

  • Nature of Adsorbate and Adsorbent: The amount of the gas adsorbed depends upon the nature of adsorbent and the gas (adsorbate), which is to be adsorbed. It has been found that easily liquifiable gases  such as NH3, HCl, Cl, SO2 CO2  etc. are more readily adsorbed than so the called permanent gases such as O2,N2, Hetc. This is because that molecules of the former type of gases have greater Vander waal’s or molecular force of attraction.

 Applications of Adsorption

  • Activates charcoal is used in gas masks to remove poisonous gases such as carbon monoxide, methane etc. Animal charcoal is used to remove colouring matter from sugarcane juice in the manufacture of sugar. 

  • Ion exchange resin is used to remove hardness of water. 

  • Several organic compounds are purified by chromatographic adsorption.

  • Silica gel is used for removing and controlling humidity.

  • The catalytic effect of a number of catalysts like spongy iron (in the manufacture of ammonia) and nickel, platinum or palladium (used in the reduction of unsaturated hydrocarbons is based on the principle of adsorption. 

  • Production of high vacuum. 

  • Gas masks-It is a device which consists of activated charcoal. This is used to adsorb poisonous gases. 

  • Humidity control-Silica gel and aluminium gel are used for this purpose. 

  • Removal of colouring matter from solutions-Animal charcoal is used for decolorising cane sugar. 

  • Heterogeneous catalysis 

  • Separation of inert gases by coconut charcoal 

  • Softening of hard water 

  • De-jionising of water 

  • Cleaning agents 

  • Froth floatation process 

  • Adsorption indicators 

  • Chromatographic analysis 

  • In curing diseases. 

Question 1: Which of the following statements regarding adsorption is incorrect?

a. Adsorption is surface phenomenon.

b. In adsorption, the substance is only retained on the surface and does not go into the bulk or interior of the solid or liquid.

c. An increase of temperature leads to a decrease in amount adsorbed and vice – versa.

d. Adsorption is always reversible in nature.

Question 2: Which of the following is not a character of chemisorption ?

a. Irreversible  nature 

b. Formation of  unimolecular layer.

c. Small heat of adsorption

d. Chemical bonds are present between adsorbent and adsrbate.

Question 3: Which of the following substances is not used as an adsorbent?

a. Charcoal

b. Silica gel

c. Calcium carbonate

d. Platinum metal

Question 4: Those substances on which both adsorption and absorption takes place are called

a. sorbed 

b. adabsorbet

c. adsorbent

d. activated adsorbate

Q.1

Q.2

Q.3

Q.4

d

c

c

a

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