Surface Chemistry Notes

Surface Chemistry Notes

Intorduction :

→ The upper or top layer of any substance is known as its surface. Various properties of the substances mainly of solids and liquids are related to their surface. For example, the behaviour of the molecules of liquid present on the surface is quite different to those molecules of liquid which are present in the interior. We can understand it as the molecule present within the liquid is attracted equally in all directions by the surrounding molecules.

Surface Chemistry Notes

→ As a result, the resultant attraction on this molecule is nil. But the molecule present on the surface of the liquid has some inward attraction force which is created by the molecules lying just below and besides it. As a result, these molecules of the liquid have an urge to go inside the liquid hence such molecules move inside the liquid. From the above example, it is clear that the chemical phenomenon occurring at surface is quite different to those occurring inside.

→ Hence, the branch of chemistry which deals with the study of the phenomenon occurring at the surface or interface i.e., at the boundary separating two bulk phases is called surface chemistry. For example: Adsorption of hydrogen gas on the surface of nickel metal has studied under surface chemistry.

In this unit, we will study some important surface phenomena like adsorption, catalysis, colloids, emulsion and micelles etc.

Chapter in Brief and Glossary :

Adsorption : It is a phenomenon of attracting and retaining the molecules of a substance at the surface or interface of a solid or a liquid resulting into higher concentration of the molecules on the surface than in the bulk.

Adsorbate : The substance which gets adsorbed at any surface is called adsorbate.

  • Adsorbent: The solid or liquid at the surface of which adsorption takes place is termed as adsorbent.
  • Desorption: The removal of adsorbate from the surface of adsorbent is termed as desorption.
  • Sorption : When adaorption and absorption occur simultaneously then the process is called sorption.
  • Interface : The surface of adsorbent, at which adsorbate is adsorbed or concentrated is known as interface.
  • Active centres of Adsorption : The surface of adsorbent has some free valencies, these free valencies are known as active centers.
  • Positive Adsorption : When the concentration of adsorbate is higher at surface relative to the concentration in the bulk then this type of adsorption is called positive adsorption.
  • Negative Adsorption : When the concentration of adsorbate is lower at the surface relative to the concentration in the bulk then this type of adsorption is called negative adsorption.
  • Heat of Adsorption : It is the amount of heat released during the adsorption of one mole of adsorbate at the surface of adsorberrt.
  • Physical Adsorption : When the particles or molecules of the adsorbate are held to the surface of adsorbent by some physical forces like van der Waal’s forces then such type of adsorption is termed as physical adsorption or physiosorption.

Chemical Adsorption: When the forces of attraction existing between adsorbate and adsorbent are strong chemical bonds, then the adsorption is called chemical adsorption.

Critical Temperature : The minimum temperature, above which a gas cannot be liquified even at a very high pressure, is called critical temperature.

Adsorption Isotherm : The relationship between amount of adsorption and pressure of gas at constant temperature is called adsorption isotherm.

Freundlich Adsorption isotherm curve equation

log \(\frac{x}{m}\) = log k \(\frac{1}{n}\) log p

Homogeneous Catalysis: If the reactants, products and catalyst are in same phase then this type of catalysis is known as homogeneous catalysis.

SO2(g)(g) + O2(g) + [NO](g) → 2SO3(g) + [NO](g)

Heterogeneous Catalysis: If the catalyst present is not in same phase as reactants and products in a chemical reaction then this type of catalysis is called heterogeneous catalysis.

Surface Chemistry Notes 1

Positive Catalyst : The catalyst which can increase the rate of a chemical reaction is called positive catalyst.

Surface Chemistry Notes 2

Here, MnO2 is positive catalyst.

Negative Catalyst : The catalyst which can decrease the rate of a chemical reaction is known as negative catalyst.

Surface Chemistry Notes 3

Here, glycerol is negative catalyst.

Auto Catalysis : During a chemical reaction, when one of the products formed behaves as a catalyst and increases the rate of reaction then such catalyst is known as auto catalyst.

Surface Chemistry Notes 4

Catalytic Poison or Inhibitor : The substance which destroys the activity of a catalyst is called catalytic poison.

Surface Chemistry Notes 5

Catalytic Promotor : The substances which can increase the efficiency of catalyst but themselves are not catalysts are known as catalytic promoters.

Surface Chemistry Notes 6

→ Colloidal Solution : It is an intermediate state between their solution and suspension. The size of their particles is in between 1 nm to 1000 nm.

It has two phases :

  • Dispersed phase
  • Dispersion medium.

→ Positive Colloids : The colloidal solutions in which the particles of dispersed phase have positive charge then they are called positive colloids. Examples:
Metal hydroxides Al(OH)3 Fe(OH)3 etc

Surface Chemistry Notes

→ Negative Colloids : The colloidal solutions in which the particles of dispersed phase have negative charge then they are called negative colloids. Examples:
Metals like Au, Ag. Cu, Pt, Fe, Cd, Soil etc.

→ Lyophilic Colloids : The colloidal solutions which are directly formed by mixing substances like gum, gelatin, starch, rubber etc., with a suitable dispersion medium are called lyophilic colloids. Here the dispersed phase and dispersion medium have high affinity for each other.

  • Lyophobic Colloids : The colloidal solutions which are not easily formed, known as lyophobic calloids, here the dispersed phase and dispersion medium have very little affinity for each other.
  • Critical Micelle Concentration : The concentration below which substance behaves as electrolyte and above which behaves as colloid is known as critical micelle concentration.
  • Kraft Temperature : The formation of micelles takes place only above a particular temperature which is called kraft temperature.
  • Micelle : On increasing the concentration of solution of soaps and detergents from CMC then the molecules of soaps and detergents associated and form aggregate of colloidal size. The aggregates are called micelles.
  • Peptization: The process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte is called peptization.
  • Dialysis. The process of removing excess dissolved impurities of electrolytes from a colloidal solution by means of diffusion through a parchment paper or animal membrane is called dialysis.
  • Electrodialysis : Dialysis is a slow process. It can be made faster by carried out this process under the influence of an electric field. Then this process is called electrodialysis.
  • Tyndall Effect: The scattering of light by the colloidal particles is known as Tyndall effect.
  • Brownian Movement : The zig-zag motion of colloidal particle is known as Brownian movement.
  • Electrophoresis When electric field is applied on colloidal solution then colloidal particles show movement either towards cathode or anode. The movement of particle towards oppositely charged electrode is known as electrophoresis.
  • Gold Number : The number of milligrams or a hydrophilic colloid which just prevent the precipitation of 10 mL of a gold sol on the addition of 1 mL of ten percent sodium chloride solution is known as gold number.
  • Emulsifying Agent : Emulsion of oil and water is unstable and sometimes they are distributed into two layers. So to make the system stable, a third component is added to it. It is known as emulsifying agent or emulsifier.

Surface Chemistry Notes

→ Other Important Points:

  • Substance which alters the rate of a chemical reaction is known as catalyst.
  • In homogeneous catalysis, the physical state of reactants, products and catalysts are similar.
  • In heterogeneous catalysis, the physical state of reactants, products and catalysts are not similar.
  • Positive catalysts increase the rate of reaction.
  • Negative catalysts decrease the rate of reaction.
  • Enzymes are complex nitrogeneous compounds having high molecular weights.
  • The binding site of the enzyme is such that it can bind only one substrate.
  • The particular shape and location of the atoms at the binding site account for the extra ordinary specificity of the enzyme.
  • Activator increases the activity of enzyme.
  • Inhibitor or poison destroys the enzyme.
  • Optimum temperature and optimum pH is favourable for the good activity of enzyme.
  • The term colloid does not apply to a particular class of substances but is a state of aggregation of matter like solid, liquid or gas.
  • A suspension is a heterogeneous mixture containing small insoluble particles.
  • In lyophilic colloids, the particles of the dispersed phase have great affinity for the dispersion medium.
  • In the lyophobic colloids, the particles of the dispersed phase have no affinity for dispersion medium.
  • The rapid zig-zag movement of colloidal particle is known as Brownian movment.
  • Tyndall effect is the scattering of light by sol particles.
  • Blue colour of sky, visibility of tails of comets is due to scattering of light.
  • An emulsion is a colloidal system having one liquid dispersed in another liquid.
  • A gel is a colloidal system having a liquid dispersed in a solid phase.
  • The purification of blood by artificial kidney is based on the process of dialysis.
  • Colloidal solution of gold is called “purple of cassius.”
  • Colloidal solution of graphite is called aqua dag.
  • Colloidal solution of antimony is used for curing Kalzar.
  • Argyrol is a silver sol used as an eye lotion.
  • Lyophilic sols are known as emulsoid while lyophobic sols are known as suspensoid.

Chemistry Notes