CHAPTER

SURFACE<br>CHEMISTRY

Syllabus

Adsorption-physisorption and chemisorption, factors affecting adsorption of gases on solids, catalysis, homogeneous and heterogeneous activity and selectivity, enzyme catalysis colloidal state, distinction between true solutions, colloids and suspension; lyophilic, lyophobic, multi-molecular and macromolecular colloids; properties of colloids; Tyndall effect, Brownian movement, electrophoresis, coagulation, emulsion-types of emulsions.

Chapter Analysis

2017 2018

List of Topics D OD OD D/OD

Adsorption, Absorption 1Q ▶ 1Q (3 marks)* (1 mark) Coagulation 1Q (3 marks)* Colloids 1Q 1Q 1Q

(3 marks)* (1 mark) (1 mark) 1Q (3 marks)#

Zeta Potential 1Q

(3 marks)#

Difference between type 1Q 1Q

(3 marks) <math>(3 \text{ marks})^{\wedge}</math>

Miscellaneous type 1Q

(3 marks) ^

Give reason type 1Q

(3 marks)

• One question of 3 marks on Adsorption, Absorption, Coagulation and Colloids was asked.

# One question of 3 marks on Colloids and Zeta Potential was asked.

^One question of 3 marks with two options i.e., one on Difference between various topics of Surface Chemistry and other miscellaneous type of question on topics of Surface Chemistry was asked.

On the basis of above analysis, it can be said that from exam point of view, Colloids, Adsorption and Absorption are the most important topics of the chapter. "Difference between type" question is also frequently asked.

TOPIC-1

Adsorption and its Types, Factors Affecting Adsorption

Revision Notes

➣ Surface chemistry: The branch of chemistry which deals with the phenomenon that occur at the surfaces is called surface chemistry. This phenomenon is studied with the help of adsorption and colloidal state.

76 Oswaal CBSE Chapterwise & Topicwise Question Bank, CHEMISTRY, Class – XII

• Adsorption: Adsorption is a surface phenomenon in which the substance gets accumulated on the surface of a solid rather than in the bulk of a solid or liquid. The surface that adsorbs is called adsorbent and the one that gets adsorbed is called an adsorbate. For example: Pt can adsorb large amount of hydrogen gas.

TOPIC - 2

> Types of adsorption :

1. Physisorption: Physisorption is also called physical adsorption. If the adsorbate is held on a surface of adsorbent by weak van der Waals forces, the adsorption is called physical adsorption or physisorption.
2. Chemisorption: Chemisorption is also called chemical adsorption. If the forces holding the adsorbate are as strong as in chemical bonds, the adsorption process is known as chemical adsorption or chemisorption.

• Desorption: The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.

TOPIC - 1

Adsorption and its Types, Factors

Affecting Adsorption ... P. 75

Catalysis and its Types, Enzyme

Catalysis .... P. 79

TOPIC - 3

Colloids, Types of Colloids,

Characteristics and Preparation

of Colloids .... P. 81

> In adsorption, molecules of a substance are uniformly distributed throughout the body of solid or liquid. For example: Ammonia absorbed by water, water absorbed by anhydrous CaCl₂.

Sorption : When adsorption and absorption take place simultaneously, it is called sorption, e.g., dying of cotton fabrics. The dye is adsorbed on the surface of cotton fibre but after it is dyed, the fibre has dye uniformly throughout.

➣ Enthalpy of adsorption : Adsorption generally occurs with release in energy, i.e., it is exothermic in nature. The enthalpy change for the adsorption of one mole of an adsorbate on the surface of adsorbent is called enthalpy or heat of adsorption.

➣ Differences between Adsorption and Absorption:

1. No. Adsorption Absorption
2. It is surface phenomenon. Adsorbate molecules are Absorption occurs in the bulk of absorbing held at the surface of adsorbent. substance.
3. The concentration of the adsorbate at the Absorbed material is uniformly distributed

adsorbent is much more than that in the bulk throughout the bulk. Thus, concentration is same

throughout.

1. Initially rate of adsorption is rapid. It decreases Absorption occurs with uniform rate. slowly till equilibrium is attained.
2. Example: Water vapours on silica gel. Example: Water vapours are absorbed by anhydrous CaCl₂.

➣ Factors affecting adsorption of gases on solids:

1. Nature of gas. (ii) Nature of adsorbent.
2. Specific area of the solid. (iv) Pressure of the gas.
3. Effect of temperature. (vi) Activation of adsorbent.

<math>\triangleright</math> Adsorption isobar A plot of extent of adsorption (x/m) vs. temperature at constant pressure is called adsorption isobar.

Adsorption isotherm : The plot of extent of adsorption (x/m) vs. pressure (p) at constant temperature is called adsorption isotherm, where, 'x' is the quantity of gas adsorbed by unit mass 'm' of the solid adsorbed. It is called adsorption isotherm.

Freundlich Adsorption Isotherm: It gives the relationship between magnitude of adsorption (x/m) and pressure at a constant temperature. It can be given by mathematical equation.

....(i)

<math>\frac{x}{m} = kp^{1/n}</math> b <math display="block">\log \frac{x}{m} = \log k + \frac{1}{n} \log p</math>

...(ii)

log x/m Slope = <math>\frac{1}{n}</math> log k (intercept)

A plot of <math>\log \frac{x}{m}</math> vs. <math>\log p</math> gives a straight line with slope <math>\frac{1}{n}</math> and y intercept = <math>\log k</math>.

In case of solution, the isotherm takes the form

0 <math>\log p</math> -

<math>\frac{x}{m} = k(C)^{1/n}</math>; <math>\log \frac{x}{m} = \log k + \frac{1}{n} \log C</math>

where x is the amount of adsorbate adsorbed on m gram of adsorbent at pressure p or concentration C of the adsorbate, k and n are constants, <math>n > 1</math>.