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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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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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Why is it essential to wash the precipitate with water before estimating it quantitatively?

Answer:

When a colloidal solution is prepared, quite often it contains certain impurities of electrolytes which are crystalloidal in nature and tend to destabilize the solution. Hence it is essential to wash the precipitate with water before estimating it quantitatively to remove impurities of electrolytes.
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Answer;
The greater the valence of the flocculating ion added, the greater is its power to cause precipitation.
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Give reason why a finely divided substance is much more effective as an adsorbent. 

Answer:
Adsorption is surface phenomenon therefore adsorption is directly proportional to the surface area.
Finely divided substance has greater surface area. Greater is the surface area of the adsorbent, greater would be its absorbent power. That is why a finely divided surface is more effective as an absorbent.
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