Chemistry Class 12 NCERT Solutions: Chapter 5 Surface-Chemistry Part 2

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Q: 4. What are the factors which influence the adsorption of a gas on a solid?


There are various factors that affect the rate of adsorption of a gas on a solid surface.

(1) Nature of the gas:

Easily liquefiable gases such as etc. are adsorbed to a great extent in comparison to gases such as etc. This is because Van der Waal’s forces are stronger in easily liquefiable gases.

(2) Surface area of the solid

The greater the surface area of the adsorbent, the greater is the adsorption of a gas on the solid surface.

(3) Effect of pressure

Adsorption is a reversible process and is accompanied by a decrease in pressure. Therefore, adsorption increases with an increase in pressure.

(4) Effect of temperature

Adsorption is an exothermic process. Thus, in accordance with Le-Chatelier’s principle, the magnitude of adsorption decreases with an increase in temperature.

Q: 5. What is an adsorption isotherm? Describe Freundlich adsorption isotherm.


Q 5 Describe Freundlich Adsorption Isotherm

Q 5 Describe Freundlich Adsorption Isotherm

The plot between the extent of adsorption against the pressure of gas (P) at constant temperature (T) is called the adsorption isotherm.

Freundlich adsorption isotherm:

Freundlich adsorption isotherm gives an empirical relationship between the quantity of gas adsorbed by the unit mass of solid adsorbent and pressure at a specific temperature.

From the given plot it is clear that at pressure reaches the maximum valve. is called the saturation pressure. Three cases arise from the graph now.

Case I- At low pressure:

The plot is straight and sloping, indicating that the pressure in directly proportional to i.e., .

(k is a constant)

Case II – At high pressure:

When pressure exceeds the saturated pressure, becomes independent of P values.

Case III – At intermediate pressure:

At intermediate pressure, depends on raised to the powers between and. This relationship is known as the Freundlich adsorption isotherm.

Now, taking log:

On plotting the graph between log and a straight line is obtained with the slope equal to and the intercept equal to.

Q 5 A Straight Line is Obtained with the Slope Equal to 1/n …

Q 5 a Straight Line with the Slope 1/N and the Intercept of …

Q: 6. What do you understand by activation of adsorbent? How is it achieved?


By activating an adsorbent, we tend to increase the adsorbing power of the adsorbent. Some ways to activate an adsorbent are:

(i) By increasing the surface area of the adsorbent. This can be done by breaking it into smaller pieces or powdering it.

(ii) Some specific treatments can also lead to the activation of the adsorbent. For example, wood charcoal is activated by heating it between 650 K and 1330 K in vacuum or air. It expels all the gases absorbed or adsorbed and thus, creates a space for adsorption of gases.

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