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NCERT Class 12 Chemistry Textbook Chapter 5 With Answer PDF Free Download
Chapter 5: Surface Chemistry
Surface chemistry deals with phenomena that occur at the surfaces or interfaces. The interface or surface is represented by separating the bulk phases by a hyphen or a slash.
For example, the interface between a solid and gas may be represented by solid gas or solid/gas.
Due to complete miscibility, there is no interface between the gases. The bulk phases that we come across in surface chemistry may be pure compounds or solutions.
The interface is normally a few molecules thick but its area depends on the size of the particles of bulk phases.
Many important phenomena, noticeable among these being corrosion, electrode processes, heterogeneous catalysis, dissolution, and crystallization occur at interfaces.
The subject of surface chemistry finds many applications in industry, analytical work, and daily life situations.
To accomplish surface studies meticulously, it becomes imperative to have a really clean surface.
Under a very high vacuum of the order of 10–8 to 10–9 pascal, it is now possible to obtain an ultra-clean surface of the metals.
Solid materials with such clean surfaces need to be stored in a vacuum otherwise these will be covered by molecules of the major components of air namely dioxygen and dinitrogen.
In this Unit, you will be studying some important features of surface chemistry such as adsorption, catalysis, and colloids including emulsions and gels.
There are several examples, which reveal that the surface of a solid has the tendency to attract and retain the molecules of the phase with which it comes into contact.
These molecules remain only at the surface and do not go deeper into the bulk. The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.
The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate and the material on the surface of which the adsorption takes place is called adsorbent.
Adsorption is essentially a surface phenomenon. Solids, particularly in a finely divided state, have a large surface area, and therefore, charcoal, silica gel, alumina gel, clay, colloids, metals in a finely divided state, etc. act as good adsorbents.
It is clear from the above examples that solid surfaces can hold the gas or liquid molecules by virtue of adsorption.
The process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.
In adsorption, the substance is concentrated only at the surface and does not penetrate through the surface to the bulk of the adsorbent, while in absorption, the substance is uniformly distributed throughout the bulk of the solid.
For example, when a chalk stick is dipped in ink, the surface retains the color of the ink due to the adsorption of colored molecules while the solvent of the ink goes deeper into the stick due to absorption.
On breaking the chalk stick, it is found to be white from the inside. A distinction can be made between absorption and adsorption by taking the example of water vapor.
Water vapors are absorbed by anhydrous calcium chloride but adsorbed by silica gel.
In other words, in adsorption, the concentration of the adsorbate increases only at the surface of the adsorbent, while in absorption the concentration is uniform throughout the bulk of the solid.
| Author | NCERT |
| Language | English |
| No. of Pages | 26 |
| PDF Size | 1209 KB |
| Category | Chemistry |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 12 Chemistry Chapter 5 Surface Chemistry
Question 5.1: Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.
Soln: The surface phenomenon of crowding up molecules of a substance at the base surface rather than in the mass (bulk) of a solid or liquid is called Adsorption. Adsorbate is the substance that gets adsorbed and the adsorbent is the substance on whose surface the adsorption takes place.
Here, the concentration of the adsorbate on the surface of the adsorbent increases. The concentration of the substance remains at the surface only in the case of adsorption.
It does not penetrate through the surface to the bulk of the solid or liquid. For example, on dipping a chalk stick into an ink solution, there is only a color change on its surface and it will be found to be white from the inside when the chalk stick is broken.
On the other hand, the process of absorption is a bulk phenomenon. In absorption, the substance gets uniformly distributed throughout the bulk of the solid or liquid.
Question 5.3: Give a reason why a finely divided substance is more effective as an adsorbent.
Soln: Adsorption is directly proportional to the surface area as adsorption is a surface phenomenon. A finely divided substance behaves like a good adsorbent because it has a large surface area. Both physisorption and chemisorption increase with an increase in the surface area.
Question 5.4: What are the factors that influence the adsorption of a gas on a solid?
Soln: The rate of adsorption of a gas on a solid surface depends on various factors:
(1) Nature of the gas:
There is a strong Van der Waal’s forces in easily liquefiable gases hence, easily liquefiable gases such as NH3, HCl, etc. are adsorbed to a great extent in comparison to gases such as H2, O2 etc.
(2) Surface area of the solid
The adsorption rate of gas on the solid surface increases with the increase in the surface area of the adsorbent.
(3) Effect of pressure
Adsorption increases with an increase in pressure and, as a result, adsorption is a reversible process and is accompanied by a decrease 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.
Question 5.6: What do you understand by activation of adsorbent? How is it achieved?
Soln: Increasing the adsorbing power of the adsorbent is known as activating an adsorbent.
Adsorbents are activated in the following ways:
(i) The surface area of the adsorbent can be increased. Breaking it into smaller pieces or powdering it, is one way of doing it.
(ii) Activation of the adsorbent can be done by some specific treatments. For example, wood charcoal’s activation is done 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.
Question 5.7: What role does adsorption play in heterogeneous catalysis?
Soln: Heterogeneous catalysis: It is a catalytic process that involves the presence of catalysts and reactants in different phases. The adsorption theory explains this type of heterogeneous catalytic action. The mechanism of catalysis involves the following steps:
(i) The reactant molecules are adsorbed on the catalyst surface.
(ii) The formation of an intermediate is caused by the occurrence of a chemical reaction.
(iii) The products are desorbed from the catalyst surface
(iv) The products are diffused away from the catalyst surface.
In this process, the catalyst is present in the solid state and the reactants are usually present in the gaseous state. Gaseous molecules are then adsorbed on the surface of the catalyst.
The rate of reaction increases when the concentration of reactants on the surface of the catalyst increases.
In such reactions, the products have very less affinity for the catalyst and are quickly desorbed, thereby making the surface free for other reactants.
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