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Chapter 5 – Surface Chemistry

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

The branch of studies that deals with the phenomena that occur on the surfaces of substances are called surface chemistry. Surface chemistry has a varied application not only in the industries but also in our day-to-day lives. The surface or the interface where this phenomenon occurs can be solid-gas, solid-liquid, solid-vacuum or liquid-gas. These notes provide you with an explanation on important surface chemistry topics that will help you in revision for exams.

At the interfaces, many phenomena occur. The most noticeable among them are:

  • Adsorption
  • Heterogeneous Catalysis
  • Corrosion
  • Crystallization

The accumulation of molecular species at the surface rather than the bulk of solid or liquid is termed as adsorption. The substance that accumulates on the surface is called an adsorbate, and the surface is called adsorbent. These molecular species accumulate on the surfaces because of intermolecular forces. 

A few examples of adsorption are

  1. Water Molecules are adsorbed by silica gel. That is why the air becomes dry in its presence.
  2. On being passed over animal charcoal, an aqueous solution of raw sugar loses its color. This happens because the coloring agent gets absorbed by the charcoal.
  3. Gases like Hydrogen, Oxygen and Nitrogen get adsorbed in the surface of animal charcoal.

 

Types of Adsorption

There are two types of adsorption

  • Physical adsorption or Physisorption: In physical adsorption, weak Van der Waals force holds gas on the surface of solids.

Characteristics:

  1. The nature of the force is a weak Van der Waals force.
  2. It is specifically not found in nature.
  3. The process can be reversed.
  4. The process is multi-layered.
  5. Desorption is very easy.

 

  • Chemical adsorption or Chemisorption: In chemical adsorption holds gas atoms or molecules to the solid surface.

Characteristics:

  1. Nature is a strong chemical force.
  2. It is very specifically found in nature.
  3. This process cannot be reversed.
  4. The process is single-layered.
  5. Desorption is very difficult.

 

Adsorption Isotherm

An adsorption isotherm is a curve that represents the ability of an adsorbent to adsorb gas at a pressure at a constant temperature.

Freundlich Adsorption Isotherm – This gives a relationship between the quantity of gas adsorbed by a unit mass of solid adsorbent and pressure at a particular temperature. The following equation represents the relation:

x/m = k.P1/n (n > 1)

x’ is mass of adsorbate on the mass of adsorbent

log K/m = 1/n (log P + log K)


It does not apply for the adsorption of gases on solids at a higher pressure

Application of adsorption:

  1. Production of a vacuum by absorption of gases by charcoals.
  2. Production of gas masks to be used in coal mines.
  3. Silica aluminum gel can control humidity.
  4. Animal charcoal can remove coloring impurities.
  5. Has medical application in adsorption of germs.

A catalyst is a substance that influences the rate of any chemical reaction without undergoing any change. The systematic study of such substances is called catalysis.

There are two types of catalysis:

  • Homogeneous Catalysis – When the reactants and the catalysts are in the same phase, it is called homogeneous catalysis.
  • Heterogeneous Catalysis – If the reactants and the catalysts are in different phases, then it is called heterogeneous catalysis.

Adsorption Theory of Heterogeneous Catalysis

The reactants in the gaseous state or a state of the solution are adsorbed on the surface of the solid catalysts. Adsorption is an exothermic process. As soon as the concentration of the reactants on the surface increases, the heat of adsorption increases the rate of reaction.

Catalysis in Industries

A few of the important catalysis processes from the industry are

  1. Haber’s process for the manufacturing of ammonia.
  2. Ostwald’s process for the manufacturing of nitric acid.
  3. Contact process for the manufacturing of sulphuric acid.

A colloid is a heterogeneous system where one substance is dispersed as very fine particles in another substance called dispersion medium.

Classification of Colloids

A colloid can be classified on the following basis

  1. The physical state of the dispersed phase and dispersed medium.
  2. Nature of interaction between them.
  3. Type of particle of the dispersed phase.

Based on the above criteria, there exist 8 types of colloids. Each of these types has certain examples of colloids around us. The following table captures the physical state of the dispersed state, dispersed medium and their examples

Dispersed PhaseDispersed MediumType of ColloidExamples
SolidSolidSolid SolGemstones and colored glasses
SolidLiquidSolPaints, cell fluids
SolidGasAerosolSmoke, dust
LiquidSolidGelCheese, butter, jellies
LiquidLiquidEmulsionMilk, hair cream
LiquidGasAerosolFog, mist, cloud, insecticide sprays
GasSolidSolid SolPumice stone, foam rubber
GasLiquidFoamFroth, whipped cream, soap lather

From the above table, we can see what emulsions are. Emulsions are colloidal solutions in which both the dispersed phase and the medium are liquids.

Classification of Emulsions

There are two types of emulsions:

NameDispersed phaseDispersed MediumExamples
Oil in waterOil/Fat dropletsWaterMilk
Water in oilWater dropletsOilVanishing cream

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