SURFACE CHEMISTRY

Contents

• Adsorption• Absorption• Differentiation!?#$• Factors influencing adsorption of gases• Types of adsorption• Characteristics of physical adsorption• Characteristics of chemical adsorption

Adsorption

• The process of accumulation of a liquid or a gas molecule on a solid surface is called adsorption.

• adsorption is a process in which a substance takes another substance on its surface , i.e. they remain on the surface.

adsorption

Solid adsorbent

Absorbed gas molecules

Examples

• When methylene blue is treated with activated charcoal, then the charcoal adsorbs the methylene blue molecules, as a result the blue colour fades.

• When a gas like NH3 is treated with charcoal powder in a closed container, the charcoal adsorbs the gas molecules. Thus the pressure of the gas decreases.

Adsorbent and adsorbate

• The solid on the surface of which gas or a liquid molecules accumulate is called ‘adsorbent’.

• The substance (gas or liquid) whose molecules accumulate on the solid surface (adsorbent) is called ‘adsorbate’.

Example description

• When the molecules of NH3 accumulate on the surface of charcoal, _________ is the adsorbate and ___________is the adsorbent.

Example description

• When the molecules of NH3 accumulate on the surface of charcoal, ammonia is the adsorbate and ___________is the adsorbent.

Example description

• When the molecules of NH3 accumulate on the surface of charcoal, ammonia is the adsorbate and charcoalis the adsorbent.

NOTE :

1. Adsorbent is solid whereas adsorbate is gas or liquid.

2. The release of the adsorbed molecules from the surface of the adsorbent is called desorption.

Absorption

• Absorption is a process in which one substance penetrates into another substance, i.e. the molecules penetrate into the body of the solid.

• A piece of cotton dipped in water absorbs the water molecules.

Differentiation!?#$ adsorption Absorption

1. It involves the unequal distribution of molecules on the surface and in the bulk.

2. The process is fast in the beginning and gradually becomes slower.

3. At the end, molecules of the adsorbate are found only on the surface.

1. It involves the uniform distribution of molecules on the surface and in the bulk.

2. It happens at an uniform rate.

3. At the end, absorbed molecules penetrate into the body of the solid.

Factors which influence adsorption of gases by solids

1) Nature of the gas (adsorbate)2) Nature of the solid (adsorbent)3) Surface area of the solid 4) Pressure of the gas5) Temperature6) Nature of the surface of the adsorbent

Types of adsorption

1> Physical or Van der Waal’s adsorption

2> Chemical adsorption or chemisorption.

Physical or Van der Waal’s adsorption

• The process by which the adsorbed molecules are held on the solid by weak Van der Waal’s forces is called Physical or Van der Waal’s adsorption.

• Example : Adsorption of Cl on the surface of charcoal.

Chemical adsorption or chemisorption

• The process by which the adsorbed molecules are held on the solid by strong chemical bonds is called Chemical adsorption or chemisorption.

• Example : Adsorption of H2 on the surface of Pt or Pd.

Characteristics of physical adsorption

• Surface area of the adsorbent• Nature of the gas• Heat of Adsorption• Effect of Pressure• Reversible Nature• Effect of temperature• Thickness of the adsorbed layer

Surface area of the adsorbent

• The extent of the adsorption of gases on the surface is directly proportional to the surface area of the solid.

Nature of the gas

• Since the forces that bind the molecule are Van der Waal’s forces, is easily liquefiable gases like SO2 and NH3 are readily adsorbed .

• For example, 1 g of activated charcoal can adsorb 380 cm3 of SO2 gas(easily liquefiable ) whereas it can adsord only 4.5 cm3 of H2 gas (difficult to liquefy)

Heat of Adsorption

• The adsorption process is always exothermic. Since the Van der Waal’s forces is weak, the heat of adsorption is only 20 - 40 kJ / mol.

Effect of Pressure

• Pressure is directly proportional to extent of adsorption.

Reversible Nature

• At high pressure adsorption takes place whereas decrease in pressure causes desorption .

Effect of temperature

• Van der Waal’s force of attraction is weak at high temperature. Hence physical adsorption is favored at low temperatures.

Thickness of the adsorbed layer

• At low pressure unimolecular thick layer is formed on the surface. However at high pressure the multi-molecular thick layer is formed.

Characteristics of chemical adsorption

• Surface area of the adsorbent• Nature of the gas• Heat of Adsorption• Effect of Pressure• Reversible Nature• Effect of temperature• Thickness of the adsorbed layer

Surface area of the adsorbent

• The extent of the adsorption of gases on the surface is directly proportional to the surface area of the solid.

Nature of the gas

• Chemical adsorption is highly specific and is formed if only the adsorbed molecules are capable of forming a chemical bond with the surface.

• H2 can be adsorbed on Pt but not CO2.

Heat of Adsorption

• Since the chemical bond is much stronger, the heat of adsorption is much higher.

Effect of Pressure

• Pressure is directly proportional to extent of adsorption only until the surface gets saturated.

• After the unimolecular thick layer is formed pressure has no effect.

Reversible Nature

• Decrease in pressure does not cause desorption as molecules are strongly bound to the surface.

Effect of temperature

• As any chemical reaction is fast at high temperature, chemisorption is favored at high temperature.

Thickness of the adsorbed layer

• In chemisorption , there is only a unimolecular thick layer on the surface.

Comparison Physical adsorption Chemisorption

1. Caused by intermolecular van der Waal’s forces.

1. Caused by chemical bonds

2. Depends on the nature of the gas

2. Highly specific

3. Heat of adsorption is small (20 - 40 kJ / mol.)

3. Heat of adsorption is small (80 - 240 kJ / mol.)

4. Reversible 4. Irreversible

5. Occurs readily at low temperature

5. Occurs readily at high temperature

6. Increase in pressure increases adsorption

6. Pressure has no affect after a unimolecular thick layer is formed

7. Multilayered 7. Unilayered

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Contd....... By ameya