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Class 10 Class 12
Why are powdered substances more effective adsorbents than their crystalline forms?

Answer:
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?

Answer:

Physisorption

Chemisorption

1. The forces of attraction between adsorbent and adsorbate are of Van der Waals type (weak forces).

2. This predominates at low temperatures.

3. Almost all gases show this type of adsorption at low temperatures.

4. The heat of adsorption is low and has a value of about 40 kJ per mole or less.

5. This type of adsorption attains equilibrium very rapidly on changing the temperature and pressure of the system.

6. This is reversible in nature.

7. The activation energy involved in this adsorption is small and is often less than 5 kJ. It is for this reason, this is even attained at low temperatures.

8. Adsorption in this case is often multilayer.

1. The forces of attraction between adsorbent and absorbate are of a chemical nature (strong forces).

2. This usually occurs at high temperatures.

3. It is highly specific in nature.

4. The heat of adsorption is high and has a value of the order of 80 to 420 kJ per mole.

5. This type of adsorption is relatively slower.

6. This is usually irreversible in nature. For example, O2 adsorbed on charcoal, when adsorbed also releases CO and CO2.

7. The activation energy involved in this adsorption is high. It is for this reason, it is attained only at high temperatures.

8. Adsorption in this case in monolayer.

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What is an adsorption isotherm ? Describe Freundlich adsorption isotherm. 

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 straight x over straight m space equals space kp to the power of 1 divided by straight n end exponent space left parenthesis straight n greater than 1 right parenthesis 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).

straight x over straight m proportional to straight P to the power of 1

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

straight x over straight m proportional to straight P to the power of 0

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.

therefore space straight x over straight m space proportional to space straight P to the power of 1 divided by straight n end exponent

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

straight x over straight m equals space kP to the power of 1 divided by straight n end exponent

The above equation is known as Freundlich adsorption equation.

As per Freundlich adsorption equation

straight x over straight m equals space kP to the power of 1 divided by straight n end exponent


Taking logarithm on both sides, we get

log space straight x over straight m space equals space log space straight k space plus space 1 over straight n space log space straight p

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.
765 Views

Discuss the effect of pressure and temperature on the adsorption of gases on solids. 

Answer:

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
xmp (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

xmp1/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.
                xm= k p1/n                                 ...(i)

and logxm = log k + 1nlog p                ...(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? 

Answer:
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.

 

218 Views

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

Answer:

Absorption

Adsorption

1. It is the phenomenon in which the particles of gas or liquid get uniformly distributed throughout the body of the solid.

2. The concentration is the same through the material. Therefore, it is a bulk phenomenon.

3. Absorption occurs at uniform rate.

1. It is the phenomenon of higher concentra-tion of particles of gas or liquid on the surface than in the bulk of the solid.

2. The concentration on the surface of the adsorbent is different from that in the bulk. Therefore, it is a surface phenomenon.

3. Adsorption is rapidly in the beginning and its rate slowly increases.

Example of adsorption: NH3, adsorbed by charcoal, H2O adsorbed by silica, ink adsorbed by chalk.
Example of absorption: NH3 absorbed by water, water absorbed by anhydrous CaCl2, water absorbed by a sponge.

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