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Tancet notes: Chemistry : Adsorption

By : Engineer
On : 23:11

           The phenomenon of

           concentration of molecules 

          of a gas substances or liquid substances at a solid 

           is known as adsorption. 


Adsorption is a
1)    Surface phenomenon
2)    Rapid process
3)    Equilibrium is attained easily
4)    Concentration of molecules are more on the surface

 It is the substance which is held on the surface of the solid is called the adsorbate
The solid that takes up a gas or a solute from the solution is called adsorbent

Types of adsorption:
There are two types of adsorption:                                                                                                                                                                                                                                                                                                                                                                                                       
1)     Physical adsorption (or) physisorption (or) vanderwaal’s adsorption
2)     Chemical adsorption (or) chemisorptions (or) activated adsorption
Physical adsorption:

              Physical adsorption is the state 

                        in which the adsorbed molecules are seized on the surface of the adsorbent by vanderwaal’s force of attraction.

Chemical adsorption:

              Chemical adsorption is the state, 

                              in which the adsorbed molecules are seized on the surface of the adsorbent by pure chemical bonds.

properties of adsorption

Adsorption is specific, it depends on the nature of the adsorbent and Adsorbate
Adsorption is always accompanied by evolution of heat
Adsorption is selective process
Adsorption on surface of a solid is always spontaneous
Rate of adsorption depends on temperature
Adsorption is accompanied both by decrease in enthalpy and entropy of the system
Adsorption is a physical phenomenon, but accompanied by a chemical change

Adsorption of gases on solids:
   All solids adsorb gases to some measurable extent. The magnitude of adsorption of gases by solids depends on the following factors
  •    Thickness of the adsorbed layer of gas
  •   Heats( or) enthalpy of adsorption
  •   Activation of adsorbent
  •    Nature and surface area of adsorbents
  •    Reversible character of adsorbed gases
  • Temperature of gas
  •     Pressure of gas
Therefore, from above statements, some questions prediction    
1)    The amount of gas adsorbed increases with increase in pressure
2)    The amount of gas adsorbed decreases with increases in temperature(physical adsorption)
3)    The amount of gas adsorbed increases with increase in temperature(chemical adsorption)
4)    The amount of gas adsorbed increases with increase in the area of the adsorbent
5)    The adsorption is accompanied by a decrease in enthalpy (heat)of the system
6)    The adsorption is accompanied by decrease in entropy of the system

Adsorption of solutes from  different kind of solutions

An adsorbent adsorbs (solutes) from the solution in two behavior

     In addition Adsorbent adsorbs definite substances from the solution in preference to other substances.

1)             Solids adsorbs dissolved substances from solutions.
   Factors affecting adsorption of solutes from solutions

1)    Effect of temperature and concentration

                   Positive adsorption

                                     Adsorption of substance from solution increases with decrease of temperature and increase in concentration of solution. This type of adsorption is known as positive adsorption.

                  Negative adsorption
                                     Adsorption of substance from the solution decreases with rise of temperature and decrease in concentration of solution. This type of adsorption is known as negative adsorption.    


          Adsorption isotherms

                       The extent of adsorption (x/m) [where, x= mass of Adsorbate; m= mass of adsorbent] depends on pressure (P) and temperature (T). this can be understood by plotting  a graph between(x/m)against  (P)    

      Adsorption isotherm is a relationship (or a graph) between magnitude of adsorption with pressure at constant temperature

     Freundlich‘s adsorption isotherm:

      The relationship between the magnitude of adsorption (x/m) and pressure (P) can be expressed mathematically by empirical equation known as freundlich adsorption isotherm

                X/m = KP^1/n

      Disadvantages of freundlich’s adsorption isotherm

1)    Freundlich equation is purely empirical and has no theoretical basis

2)    The equation is valid only upto a certain pressure and invalid at higher pressure

3)    The constants K and  “n” are not temperature independents, that vary with temperature

4)    Freundlich’s adsorption isotherm fails , when the concentration of Adsorbate is very high   
       Langmuir’s adsorption isotherm

          The relation between the amounts of gas adsorbed to the pressure of the gas at constant temperature is known as Langmuir adsorption isotherm.
     Disadvantages of Langmuir’s adsorption:

      Langmuir’s adsorption isotherm is good at lower pressure but fails at high temperature
       Important application of adsorption

1)    It is used in gas masks, which adsorb all undesirable ( toxic) gases, ehile purified air passes its pores.

2)    It is used for removing coloring matter from the sugar solution and the decoloration in vinegar


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