Surface Chemistry

Dr. SURENDRAN PARAMBADATH (M.Sc, M.Phil, M.Tech)

Formerly: Post Doctoral Research Associate,Nano-Information Materials Research Laboratory,

Pusan National University, Busan-South Korea

Currently: Assistant ProfessorGovt. Polytechnic College, Perinthalmanna

Bulk

Surface

Surface of solids and liquids has the tendency to attract and retain other molecules with which it is brought in to contact.

This is due to unbalanced residual inward forces of attraction or free valencies at the surface of solids and liquids.

Due to these residual forces, surface of solid or liquid has a higher concentration of other molecular species than the bulk.

Adsorption

Free materials

Adsorbed material

Adsorption: The accumulation of molecular species at the surface of a solid or liquid rather than in the bulk is called adsorption.

Adsorbate: The molecular species that accumulate at the surface is termed adsorbate.

Adsorbate

Adsorbent: The material on the surface of which adsorption takes place is called adsorbent.

Adsorbent

Desorption: The removal of adsorbate from the surface is called desorption.

Occlusion: The adsorption of gases on the surface of metals is called occlusion.

Examples for adsorption…………

1. If a gas like H2, O2, Cl2 etc is taken in a vessel containing powdered charcoal, pressure of the gas slowly decreases as the gas is adsorbed on the surface of charcoal.

2. Air becomes dry in presence of silica gel because adsorption of water takes place on the surface of the gel.

3. Aqueous solution of raw sugar becomes colourless when passed over a bed of animal charcoal. The coloring matter is adsorbed by the charcoal.

4. Litmus solution or a solution of a dye like methylene blue when shaken with animal charcoal turns colourless due to adsorption of coloring material.

Absorption

Absorption: The distribution of substance uniformly throughout the bulk of the adsorbent.

Sorption: If both adsorption and absorption takes place simultaneously is called sorption.

Adsorption Absorption

It is a surface phenomenon

It is a bulk phenomenon

Adsorbed species is accumulated in the surface

It is uniformly distributed throughout the bulk

It is a fast process It is a slow processes

Rate of adsorption decreases gradually

Absorption takes place at steady rate

Distinction between adsorption & Absorption

Types of Adsorption

Physisorption Chemisorption

Adsorption

Physical Adsorption

or Physisorption

If accumulation of gas molecules on the surface of solids occurs due to weak van der Waals’ forces of attraction, the adsorption is called physisorption.

Chemical Adsorption

or Chemisorption

When atoms or molecules of gases are held by solids on its surface by chemical bonds, the adsorption is called chemisorption.

Characteristics of Physisorption

1. Non-specific nature: An adsorbent does not show any preference for a gas as the van der Waals’ forces are universal.

2. Easily liquefiable gases like CO2, SO2, NH3 etc, are readily adsorbed.

3. Reversible nature: Physisortion of a gas by a solid is reversible.

4. Increases by increase of pressure. 5. Surface area of adsorbant: When the surface area of the adsorbent

increases, more gas is adsorbed, ie extent of adsorption increases. 6. Enthalpy of adsorption: Enthalpy of adsorption of physisorption

is very low (20-40 KJ mol-1)

Since adsorption is exothermic, physisorption takes place readily at low temperature and desorption takes place at higher temperature.

Characteristics of Chemisorption

1. High specificity: It is highly specific and will occur only if chemical bond formation takes between adsorbate and adsorbent.

2. Irreversibility: Chemisorption is irreversible because the chemical bond formed is difficult to break.

3. Chemisorption increases with temperature. 4. Increases by increase of pressure. 5. Surface area of adsorbent: When the surface area of the

adsorbent increases, more gas is adsorbed, ie extent of adsorption increases.

6. Enthalpy of adsorption: Enthalpy of adsorption of chemisorption is high (80-240 KJ mol-1)

Physisorption Chemisorption

1 Occurs due to van der Waals’ force Chemical Bond

2 Reversible Irreversible

3 Not specific Specific

4 Enthalpy of adsorption is low Enthalpy of adsorption is high

5 More liquefiable gases are adsorbed readily

Gases which form compounds with adsorbent alone undergo chemisorption

6 Decreases with increase of temperature Increases with increase of temperature

7 Low temperature is favorable. High temperature is favorable

8 High pressure favors physisorption and decrease of pressure causes desorption

High pressure is favorable but decreases of pressure does not cause desorption

9 Results in multimolecular layers Only unimolecular layer are formed

10 No activation energy is needed High activation energy is needed

11 It is instantaneous It is a slow process

Factors Influencing Adsorption

2. Surface Area of the Adsorbent

Same gas is adsorbed to different extent by different solids at identical conditions. Greater the surface area of the adsorbent greater the volume of gas adsorbed.

3. Temperature

Adsorption of a gas generally decreases with rise in temperature. This is because adsorption is exothermic and increases of temperature favors the backward process which is desorption.

Heat

4. Pressure

Adsorption of a gas by an adsorbent at constant temperature increases with increase of pressure.

Applications Adsorption

1. In Gas Masks

2. Production of High Vacuum

3. Softening of Hard water

Ca2+ Ca2+Ca2+Ca2+

Exchange resin

4. Heterogeneous Catalysis

5. Refining of Petroleum

6. Chromatographic Seperation

Thank You