Surface ChemistryModule-1/10

IntroductionBranch of chemistry which deals with the nature of surfaces and physical and chemical changes occurring on the surface /interface (i.e) at the boundary separating two bulk phases which can be pure compounds or solutions.

The interface can exist between solid-Liquid, solid-gas, liquid-gas and in some cases between liquid-liquid.

No gas-gas interface is possible as gases intermix with each other due to diffusion.

Study of interface plays a vital role in our daily life and is of great interest to chemists.

The common phenomena observed are Dissolution, crystallisation, electrode processes, dyeing, corrosion, emulsification etc.

In this unit we will be discussing 3 important features of surface chemistry. They are:

Adsorption, Catalysis and

Colloids including Emulsions and Gels.

AdsorptionThe phenomenon in which the surface of liquid or solid attract, retain, or accumulate molecular, atomic, or ionic species of the substance they come in contact.

It is a surface phenomenon.

There is uniform distribution of a substance at surface of another. Ex- H2 on Pd.

The concentration of a substance at the interface ofor boundary between heterogeneous phases

Ex- solid/gas, two immiscible liquids

Adsorbate-The substance which gets adsorbed on s any surface. Ex-Gas gets adsorbed on the surface of a solid like metal, then gas is the adsorbate.

Adsorbent- The substance on the surface of which adsorption occurs. These may be solids or liquids. Ex- Metal powders, Powdered charcoal be it animal or activated, Clay, Silica gel, Alumina etc.

Desorption- The removal of adsorbed molecules from the surface of adsorbent either by heating or by lowering pressure.

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Absorption- The phenomenon in which molecules of a substance penetrate uniformly into another substance. It is a bulk phenomenon. Ex- Sponge absorbing water, Roots of a plant absorb water.

Sorption- The phenomenon in which both adsorption and absorption occur simultaneously.

Occlusion-Adsorption of gases on surface of metals is called Occlusion. Ex- H2 on platinum or palladium metal surface.

Competing Adsorption- Different adsorbates are adsorbed to different extent on same adsorbent at same temperature and pressure. Ex- Adsorption of poisonous gases like Cl2, SO2, CH4 etc on charcoal which already has O2, N2 etc adsorbed on it. These gases get displaced by poisonous gases. (mask used by miners)

OCCULSION

Cause of adsorption

The forces of interaction between molecules/atoms/ions present in substances lead to two types of phenomena - Bulk and Surface. The forces may be balanced as in case of bulk of matter or may be unbalanced as observed at the surface of solid or liquid. It is these unbalanced forces which is the basis for the study of surface chemistry and its wide application

S.no. Adsorption Absorption

1. Surface phenomenon Bulk phenomenon2. Substance gets adsorbed on the

surface of the adsorbent. The accumulation of molecules is more on surface rather than in bulk

Substance gets absorbed inside and is distributed uniformly throughout the bulk

3. Is rapid in the beginning and then slows down gradually until equilibrium is reached

Proceeds at a steady speed.

4. Exothermic process Endothermic process5. Favoured at low temperature Is unaffected by temperature

Distinguish between Adsorption and Absorption

Types of AdsorptionBased on concentration of adsorbate:

Positive adsorption- Concentration of adsorbate is more on surface than in the bulk of adsorbent. Ex- H2 gas on metal surface.

Negative adsorption- Concentration of adsorbate is less on surface relative to its concentration in the bulk. Ex- When dilute KCl solution is shaken with blood charcoal.

Based on nature of forces which hold adsorbate on surface of adsorbent:

Physical adsorption / Physisorption/ Vander Waal’s adsorption- When the adsorbate is held to the surface of adsorbent by weak Vander Waal’s forces of attraction.

Ex- Adsorption of gases on animal charcoal, Adsorption of water vapour on silica gel

Chemical adsorption/ Chemisorption/ Activated adsorption/Langmuir adsorption- When the adsorbate is held to the surface of adsorbent by chemical bonds which may either be ionic or covalent.

Ex- Hydrogenation of alkenes, Corrosion etc.

Types of Adsorption

Positive AdsorptionMore adsorbate on surface than in bulk

Negative AdsorptionLess Adsorbate on surface than in bulk

Physisorption/ Vander Waal’s AdsorptionAdsorbate held by Vander Waal’s forces to Adsorbent

Chemisorption/Activated AdsorptionAdsorbate held to adsorbent by Chemical bonds-Ionic or covalent

S.no. Physical adsorption Chemical adsorption1. Involves Vander Waal’s forces Involves chemical forces like ionic or covalent bond

2. No compound is formed Surface compound is formed

3. Nonspecific in nature, surface has no preference for particular gas

Highly specific in nature

4. ReversibleAdsorption Desorption

Irreversible New substance is formed with bonds

5. Enthalpy of adsorption is low (20Kj-40Kj)Kinetic energy increases so molecules will leave surface

Enthalpy of adsorption is high (200Kj-400Kj)Strong bonds so more energy is required to leave surface

6. Favoured at low temperature Favoured at high temperature

7. No appreciable activation energy is required

High activation energy is required

8. Forms multi layers on surface of adsorbent

Forms mono layer on surface of adsorbent

Distinguish between Physical and Chemical adsorption:

Acknowledgement

•NCERT XIIth Chemistry Textbook- 1•New Course Chemistry XIIth - Volume 1 Publishers- Pradeep • ABC of Chemistry XIIth - Volume 1 Publishers- Modern • Conceptual Chemistry XIIth - Volume 1 Publishers- S. Chand• ISC chemistry XIth -Volume 1 Publishers- S. Chand

Slide 1IntroductionSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Cause of adsorptionSlide 13Slide 14Slide 15Slide 16Slide 17Slide 18Types of AdsorptionSlide 20Slide 21Slide 22Slide 23