Chemistry Project Report on “Adsorption”
I, ____________ student of ___________________ is doing project report entitled “Adsorption” being submitted to ___________________is an original piece of work done by me.
The situation existing at the surface of Liquid or a solid is different from them in interior. For example, a molecule in the interior of a liquid is completely surrounded by other molecules on all sides and hence the intermolecular forces of attraction are exerted equally in all directions. however, a molecule at the surface of a liquid is surrounded by large number of particles in the liquid phase and fewer number of particles in vapour phase i.e. in the space above the liquid surface. As a result these molecules lying at the surface, experiences some net inward forces of attraction which cause surface tension. Similar inward forces of attraction exist at the surface of a solid.
Because of the unbalanced inward forces of attraction of free valances at the surface, liquid and solids have the property to attract and retain the molecules of a gas or dissolved substance on the their surfaces with which they come in contact.
Definition of Adsorption : The phenomenon of attracting and retaining the molecules of a substance on the surface of a Liquid or a solid resulting in the higher concentration of the molecules on the surface is called Adsorption.
The substance thus adsorbed on surface is called Adsorbate and the substance on which it is adsorbed is called Adsorbent.
Adsorption arises at the surface of solids as a result of presence of unbalanced forces at the surface. These forces develop either during the crystallization of solids or by virtue of the presence of unpaired e- in d-orbital.
1. It is specific and selective in nature.
2. It is accompanied by decrease in the free energy of the system. When G becomes zero, Adsorption equilibrium is established.
3. Adsorption is spontaneous process therefore change in free energy (G) for the process is negative.
According to Gibb's Helmhotz equation :
G = H - TS
G = -Ve ; H = -Ve
( it is exothermic process)
And S is -Ve because adhering of gas molecules to the surface lowers the randomness.
1. Surface area of Adsorbent :
Greater the surface Area of Adsorbent, greater is the volume of gas adsorb
2. Nature of gas being Adsorb :
Higher the critical temperature of gas, greater is the among of that gas adsorbed.
3. Temperature :
Adsorption decreases with increase in temperature and vice - versa.
4. Pressure :
Adsorption of a gas increase with increase of pressure because on applying pressure gas molecules comes close to each other.
1. Physical Adsorption :
When a gas is bled on the surface of a solid by Vander wall's forces without resulting in to formation of any chemical bond between the Adsorbate and Adsorbent. It is called physical adsorption or vender wall's adsorption or also known as physicosorption.
2. Chemical Adsorption :
When a gas is held on the surface of solid by forces similar to those of a chemical bond, the type of adsorption is called chemical adsorption or chemisorptions. It is also known as Longmuir adsorption.
Freundlich's adsorption isotherm
A graph between the amount adsorbed by an adsorbent and the equilibrium pressure of the adsorbate at a constant temperature is called the Adsorption isotherm.
At low value of p, the graph is nearly straight and sloping this is represented by follow equation :
or = constant x p1 ------------------ (a)
At higher pressure becomes independent of the values of p. in this range of pressure :
p0 or = constant x p0 ------------------ (b)
in the intermediate range of pressure, will depend on p raised to the powers between 1 and 0. i.e. fractions. For a small range of pressure values, we can write :
Adsorption Isobars :
A graph drawn between the amount adsorbed () and temperature 't' at a constant equilibrium pressure of adsorbate gas is known as Adsorption isobars.
(i) Physical Adsorption isobar (ii) Chemical Adsorption isobar.
1. In preserving vacuum : In Dewar flasks activated charcoal is placed between the walls of the flask so that any gas which enter in to the annular space either due to glass imperfection or diffusion through glass is adsorbed.
2. In gas masks : All gas masks are devices containing suitable adsorbent so that the poiseness gases present in the atmosphere are preferentially absorbed and the air for breathing is purified.
3. In clarification of sugar : Sugar is decolorized by treating sugar solution with charcoal powder. The later adsorbs the undesirable colours present.
4. In softening of hard water : The use of ion exchangers for softening of hard water is based upon the principle of competing adsorption just as in chromatography.
5. In removing moisture from air in the storage of delicate instruments : Such instruments which may be harmed by contact with the moist air, are kept out of contact with moisture using silica gel.
6. In Adsorption indicator : Varous dyes, which owe their use to adsorption, have been introduced as indicator particularly in precipitation titration. For example, KBr is easily titrated with AgNo3 using eosin as indicator.