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Surface Chemistry

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Surface Chemistry PDF Notes, Important Questions and Formulas

Surface Chemistry

Surface chemistry deals with phenomena that occur at the surfaces or interfaces. Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallisation occur at interfaces. The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.

DISTINCTION BETWEEN TRUE SOLUTION, COLLODIAL AND SUSPENSION:

Name of property

True solution

Colloidal solution

Suspension

1. Particle size

<10-7cm

10-7 to 10-5cm

>10-5cm

2. Visibility

Not visible with any of the optical means

Not possible

Possible

3. Separation with filter paper

Not possible

Diffuse very slowly

Does not diffuse

4. Diffusion

Diffuses readily

Heterogeneous

Heterogeneous

5. Nature

Does not settle

Settle under centrifuge

Settle under influence of gravity

6. Setting

Does not show

Shows

May or may not show

7. Tyndall effect

Does not show

Shows

May show

8. Brownian movement

Does not show

Shows

May show

PARTICLE SIZE:

PHASE OF COLLOIDS:

A colloidal system is heterogeneous in character. It consists of two phases, namely a dispersed phase and a dispersion medium.

  1. Dispersed Phase (DP): It is the component present in small proportion and is just like a solute in a true solution. For example, in the colloidal state of sulphur in water, the former acts as a dispersed phase.
  2. Dispersion Medium (DM): It is normally the component present in excess and is just like a solvent in a solution. The particles of the dispersed phase are scattered in the dispersion medium in a colloidal system.

CLASSIFICATION OF COLLOIDS:

Colloids can be can be classified in a number of ways based upon some of their important characteristics. 

  1. Physical state of Dispersed Phase & Dispersion Medium:

Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gaseous, eight types of colloidal system are possible. A gas mixed with another gas forms a homogeneous mixture and not a colloidal system. Typical examples of various types along with their characteristic names are given in table.

Common Colloidal System

DP

DM

Colloidal system

Example

Gas

Liquids

Foam or froth

Soap sols,

Lemonade froth, whipped cream.

Gas

solid

Solid foam

Pumice stone, styrene, foam rubber.

Liquid

Gas

Aerosols of Liquids

Fog, clouds, fine insecticide spray, mist.

Liquid

Liquid

Emulsions

Milk, hair cream

 

Liquid

Solid

Gels

Cheese, butter, boot polish, table jellies, curd.

Solid

Gas

Aerosols of solid sols

Smoke, dust

Solid

Liquid

Sols

Most paint, starch dispersed in water, gold sol, muddy water, cell fluids, inks.

Solid

Solid

Solid sols

Ruby glass, some gem stones


2. Based on dispersion medium

1. Water      :    Hydrosols

2. Alcohol    :   Alcosols

3. Gases      :   Aerosols

4. Benzene   :   benzosol

5. Solid        :   gel

Some colloids, such as gelatin, can behave both as a sol and a gel. At high temperature and low concentration of gelatin, the colloid is a hydrosol. But at low temperature and high gelatin concentration, the hydrosol can change into a gel.

3. Based on interaction or affinity of phases: On the basis of the affinity or interaction between the dispersed phase and the dispersion medium, the colloids may be classified into two types:

  1. Lyophilic Colloids: The colloidal system in which the particle of dispersed phase have great affinity for the dispersion medium, are called lyophilic (solvent-loving) colloids. In such colloids, the dispersed phase does not get easily precipitated and the sols are more stable. Such colloidal systems, even if precipitated, may be reconverted to the colloidal state by simply agitating them with the dispersion medium. Hence lyophilic colloids are reversible. When the dispersion medium is water, these are called hydrophilic colloids. Some common examples of lyophilic colloids are gum, gelatin, starch, rubber, proteins, etc.
  2. Lyophobic colloids: The colloidal system in which the dispersed phase have no affinity for the dispersion medium are called lyophobic (solvent hating) colloids. They are easily precipitated (or coagulated) on the addition of small amounts of the electrolyte, by heating or by shaking. They are less stable and irreversible. When the dispersion medium is water, these are known as hydrophobic colloids. Examples of lyophobic colloids include sols of metals and their insoluble compounds like sulphides and oxides.

The essential differences between the lyophilic and lyophobic colloids are summarised in table.

Difference between Lyophilic and Lyophobic sols

Property

Lyophillic sols/hydrophilic

Lyophobic/hydrophobic sols

1. Nature

Reversible

Irreversible

2. Preparation

They are prepared very easily by shaking or warming the substance with dispersion medium. They do not required any electrolyte for stabilization.

They are difficult to prepare, special methods are used Addition of stabilizer is essential for their stability.

3. Stability

They are very stable and are not easily coagulated by electrolytes.

They are generally unstable and get easily coagulated on addition of electrolytes.

4. Charge

Particles carry no or very little charge depending upon the pH of the medium.

Colloidal particles have characteristics charge(positive or negative)

5. Viscosity

Viscosity is much higher than that of the medium.

Viscosity is nearly the same as that of the medium

6. Surface Tension

 

Surface tension is usually less than that of the medium.

Surface tension is nearly the same as that of the medium.

7. Migration in electric field

The particles may or may not migrate in an electric field.

The colloidal particles migrate either towards cathode or anode in an electric field.

8. Solvation

 

Particles are heavily solvated

Particles are not solvated

9. Visibility

The particles cannot be seen under ultra microscope.

The particles though invisible, can be seen under ultra microscope.

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