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Surface Chemistry

Long Answer Type

101. 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

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.

As per 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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Short Answer Type

102. What do you understand by activation of adsorbent? How is it achieved?

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103. What role does adsorption play in heterogeneous catalysis?

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104. Why is adsorption always exothermic?

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Long Answer Type

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

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

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107. What are lyophilic and lyophobic sols? Give one example of each type. Why are hydrophobic sols easily coagulated?

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108. What is the difference between multimolecular and macromolecular colloids? Give one example of each. How are these two types of colloids different from these two types of colloids?

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109. What are enzymes? Write in brief the mechanism of enzyme catalysis.

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110. How are colloids classified on the basis of
(i) physical states of components
(ii) nature of dispersed phase and
(iii) interaction between dispersed phase and dispersion medium?

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