Surface Chemistry Class 12th Notes

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Surface Chemistry Class 12th Notes | Class 12th chemistry easy notes Surface Chemistry Class 12th Notes

Adsorption– The phenomenon of attraction of particles and there penetration on the surface of solid and liquid material is known as adsorption.

Adsorbent– Adsorbent is a substance on the surface of which adsorption takes place.

Adsorbate– Adsorbate is the species which is adsorbed.

Desorption– Desorption is the process of removing an absorbed substance from the surface on which it is adsorbed.

Mechanism of Adsorption– Adsorption arises due to forces of attraction between the adsorbent and adsorbate. In such reactions, entropy is negative. Thus, for \Delta G = \Delta H - T \Delta S should be negative and the \Delta H must be sufficient negative.

There are two types of adsorption

Physical adsorption – In physical adsorption, forces of attraction between adsorbent and adsorbate is weak van der Waal’s type with low heat of adsorption. Easily liquefiable gases are readily absorbed.                                                                                    Characteristics of physisorption are

  • It lacks specificity.
  • At a given temperature and pressure with the increases in the surface area of the adsorbent, the extent if adsorption increases too.
  • The amount gas adsorbed by solid depends on the nature of gas.
  • It is an exothermic process and has low enthalpy of of adsorption ( 20-40          kJ{mol}^{-1} )

Chemical adsorption – In chemical adsorption, the force of attraction between adsorbent and adsorbate is strong chemical bond.

Characteristics of chemisorption are

  • It is highly specific in nature in nature and forms unimolecular layer.
  • It has high heat of adsorption ( 80-240  kJ  {mol}_{-1} ).
  • With the increase in the surface area, rate of chemisorption rises.
  • It first increases and then decreases at high temperatures.

Factor Affecting Adsorption of Gases on Solids

  • Presence of active centers in the surface of solid which helps to provide the necessary force of attraction.
  • Value of \Delta H for adsorption of a gas on a solid should be sufficiently negative.
  • The system (solid+gas) are at suitable temperature and pressure.

Adsorption Isotherm – A mathematical equation that represent the relationship between the amount of gas adsorbed at the equilibrium pressure and at a constant temperature is known adsorption isotherm.

\therefore    Value  of  adsorption  isotherm = \frac {x}{m}

Freundlich adsorption isotherm– This is applicable on those adsorptions where adsorbate form a monoatomic layer on the surface of adsorbent.

According to it \frac {x}{m} = {kp}^{ \frac{1}{n}}

This is not applicable at high pressure.

Langmuir adsorption isotherm– Erving Langmuir in 1916 explained adsorption isotherm on the basis of kinetic theory of gases, which is known as Langmuir adsorption isotherm.

According to it   \frac {x}{m} = \frac {ap}{1+ap}

where, a and b are constant whose value depend on the nature of gas , solid adsorbent nature and temperature .

Application of Adsorption-

  • Production of high vacuum
  • Control of humidity
  • Heterogeneous
  • Gas mask
  • Separation of inert gases
  • In curing diseases
  • Froth floatation process
  • Removal of colouring matter from solutions

Catalyst– When a substance changes the rate of a reaction but is recovered chemically unchanged at the end of the reaction then the phenomenon is called catalysis and substance is known as catalyst.

e.g. Haber’s process, N+ 3H2 \xrightarrow [ Fe ]{ Mo } 2NH3

Those catalyst which increases the rate of a reaction are called positive catalyst and those catalyst which decreases the rate of reaction are called negative catalyst.

Homogeneous Catalysis – In homogeneous catalysis, the reactants and the catalysts are in the same phase ( that is liquid or solid ).

e.g. \frac{3}{2}{O}_{2}{(g)} \xrightarrow{NO(g)}  {O}_{3}{(g)}               (Homogeneous Catalysis)

Heterogeneous Catalysis– In heterogeneous catalysis, the reactants and the catalysts are in different phases.

e.g. {4NH}_{3}{(g)} {+} {5O}_{2}{(g)} \xrightarrow {Pt(s)} {4NO(g)} {+} {{6H}_{2} O(g)}   (Heterogeneous Catalysis)

Auto Catalysis– In certain reactions, one if the product acts as catalyst. This type of phenomenon in which one if the products itself acts as a catalyst is known as auto catalysis.

Induced Catalysis– When one reaction foes not occur under ordinary conditions, but influenced by other reaction, the  phenomenon is termed as induced catalysis.

Promoters and Poisons (Inhibitors) – Promoters are the substances which increase the activity of a catalyst while poisons are substances which decrease the activity of a catalyst.

Activity of catalyst– The activity of a catalyst depends on the strength if chemisorption to a larger extent. It is the ability of a catalysts to accelerate chemical reaction. e.g. H2 and O2 presence of platinum react to form water explosively.

{2H}_(2) {+} {O}_{2} \xrightarrow {Pt} {{2H}_{2}O}

Selectivity of catalyst– The selectivity of a catalyst implies that the catalyst has the ability to convert the reactant into a specific product. e.g.

{CO(g)} {+} {H}_{2}{(g)} \xrightarrow{Cu} {HCHO(g)}

Shape Selective Catalysis by Zeolites– When catalytic reaction depends on the pore structure and size of the catalyst, they are called shape selective catalysis. Zeolites are one of the best examples of shape-selective catalysts.

Enzyme Catalyst– Enzymes are nitrogen containing organic compounds which are found in living tissues of organisms. They act as catalysts in the chemical reactions taking taking place in organisms.

Enzyme Reaction

                      {Sucrose} \xrightarrow{Invertase} {Glucose + Fructose }
                          {Starch} \xrightarrow{Amylase} {Glucose }
                        {Glucose} \xrightarrow{Zymase} {Ethyl Alcohol + {CO}_{2}}
                          {Lactose} \xrightarrow{Lactose} { Glucose + Galactose }

 

Colloidal State– The state of substance in which dispersed phase [solute] is distributed in dispersion medium [solvent].

Properties of Colloids

  • Colloidal particles form heterogeneous mixture consisting of particles of dispersed phase and dispersion medium.
  • The colour of a colloidal solution depends on the wavelength being absorbed and reflected by dispersed phase particles.
  • Size of colloidal particles is between 10Å- 100Å.

On the basis of  nature colloidal system are of two types-

  1. Lyophilic Colloids– Lyophilic colloids have strong interaction between dispersed phase and dispersion medium. These are reversible and more stable.
  2. Lyophobic Colloids– Lyophobic colloids have very less interaction between dispersed phase and dispersion medium. These are irreversible and less stable. They need stabilising agents for their preservation.

Multimolecular Colloidal Solution– Formed by combination of infinite number of minute particles to a colloidal size.

Macromolecular Colloidal Solution– A single particle of colloidal size. e.g. Polymer.

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