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Why are powdered substances more effective adsorbents than their crystalline forms?

Powderded substance are more effective adsorbent than their crystalline forms because when a substance is powdered its surface area increase and physisorption is directly proportional to the surface area of adsorbent,
1109 Views

What is the difference between physisorption and chemisorption?

321 Views

Adsorption isotherm: A graph between the extent of adsorption (x / m) and the pressure ‘p’ of the gas at constant temperature is called adsorption isotherm.

Freundlich Isotherm: The relationship between x/m and pressure of the gas at constant temperature is called adsorption isotherm and given as by  and n depend upon the nature of gas and the solid.

x/m first increase with the increase in pressure at low pressure but becomes independent of pressure at high pressure.

Thus three cases arise from the graph

At low pressure, the extent of adsorption is directly proportional to pressure (raised to power one).

At high pressure, the extent of adsorption is independent of pressure (raised to power zero).

Therefore at an intermediate value of pressure, adsorption is directly proportional to pressure raised to power 1/n.Here n is a variable whose value is greater than one.

Using constant of proportionality, k, also known as adsorption constant we get

The above equation is known as Freundlich adsorption equation.

Taking logarithm on both sides, we get

Here x is the weight of the gas absorbed by m mass of the adsorbent at a pressure p, k and n are constant (at a particular temperature) and for a particular adsorbate-adsorbent pair.

The equation above equation is comparable with comparable with the equation of a straight line,
y = m x + c where m represents the slope of the line and c represents intercept on y-axis.

Plotting a graph between log(x/m) and log p, we will get a straight line with the value of slope equal to 1/n and log k as y-axis intercept.

log(x/m) vs. log p graph.
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Discuss the effect of pressure and temperature on the adsorption of gases on solids.

Effect of pressure on adsorption. The extent of adsorption of a gas per unit mass of adsorbent (x / m) increases with increase of pressure at a constant temperature.
(i) For a lower range of pressure (x / m) is directly proportional to the applied pressure. Larger the pressure more is the amount of gas adsorbed, lower the pressure small is the amount of a gas adsorbed. That is
$\frac{\mathrm{x}}{\mathrm{m}}\propto \mathrm{p}$ (as a constant temperature)

(ii) For a high pressure range the extent of adsorption of a gas per unit mass of the adsorbent (x / m) is independent of the applied pressure. That is

(iii) For a moderate pressure range the value of x/m is proportional to a Fractional power of pressure. That is

$\frac{\mathrm{x}}{\mathrm{m}}\propto {\mathrm{p}}^{1/\mathrm{n}}$ (where T is constant)
where 1 / n is a fraction. Its value may be between 0 and 1. Fig. (a) and (b) show the variations of the extent of adsorption of a gas on an adsorbent as predicted by Frundlich equations (i) and (ii) respectively.
...(i)

and                 ...(ii)

Effect of temperature on adsorption: The amount of a gas adsorbed per unit mass of a solid surface (x / m) decreases with increase of temperature in case of physical adsorption. However, in case of chemical adsorption as the temperature increases x / m increases, attains a maximum value then decreases.

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How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?

A dispersed phase or dispersion medium in colloidal solution may be a solid, liquid or gas. Based on physical states, 8 types of colloidal systems are possible (gas in gas is not possible as gases are always miscible in all proportions forming homogeneous mixure). All other takes of combinations of gases, liquids and solids may exist as colloidal solutions. Examples are given in the following table:

 Internal phase of Dispersed phase External phase or Dispersion medium Colloidal name Example Solid Solid Solid sols Alloys, Ruby glass, Gems or precious stones, marbles, optical and vision glasses. Liquid sols Muddy water, gold sol, protein, starch, agar, gelatin in water, paints, pigments in water. Gas Aerosols (or solids Smoke, particulate clouds. Liquid Solids Gels Cheese, jems, jellies, plants, fruits, vegetables Liquid Emulsions Butter, milk, cosmetic products, e.g., shampoo, creams, emulsified oils, polish and medicines. Gas Aerosols (or liquids) Fog, clouds, mist. Gas Solid Solid foams Pumicestone, styrene foam, foamed rubber, porous pot. thermocole rubber pillows and mattresses. Liquid Foams and froths Lather, soap seeds, air bubble. Gas Homogeneous system Do not exist as colloids.

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Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.