Surface Chemistry – Adsorption

Adsorption:

  • Adsorption is defined as the phenomenon in which there is the accumulation of one substance on the surface of the other substance. It can also be defined as the change in concentration at the interfacial layer between two phases of the system due to surface forces. OR It can also be defined as the change in concentration at the interfa­cial layer between two phases of the system due to surface forces.
  • Examples: 
    • If animal charcoal is shaken with a diluted solution of acetic then it is observed that acetic acid is concentrated on the surface of charcoal. If the ink is shaken with charcoal then the intensity of colour decreases.  This is due to the adsorption of ink molecules on the charcoal.

Adsorption 01

  • Explanation:
    • The molecules below the surface of the substance i.e. in the bulk (Molecule A) are equally attracted by other molecules from all sides but molecules on the surface (Molecule B)  are subjected to an unbalanced attraction from molecules in the bulk. These unbalanced forces try to drag the molecules inside. So as to satisfy these unbalanced residual forces surface molecules tend to attract and retain particles or molecules of other substances on the surface with which they come in contact.
    • When a solid surface is exposed to a gas or liquid, the molecules from the gas or liquid accumulate or concentrate at the surface.  This process is called as adsorption.
    • Since the surface molecules are responsible for adsorption, adsorption is a surface phenomenon and its extent depends on the surface area of the absorbent. Adsorption may occur when two heterogeneous phases are in contact with each other.

Terms Used in Surface Chemistry:

Adsorbent:

  • The substance whose surface adsorbs the gas or solute molecules from solution is called as an adsorbent.
  • e.g.  If animal charcoal is shaken with a dilute solution of acetic then it is observed that acetic acid is concentrated on the surface of charcoal. Thus charcoal is adsorbent.

Adsorbate:

  • The substance which gets adsorbed on the surface of solid or a liquid is called as adsorbate or adsorbed phase.
  • e.g. If animal charcoal is shaken with a dilute solution of acetic then it is observed that acetic acid is concentrated on the surface of charcoal. Thus acetic acid is adsorbate.

Heat of Adsorption:

  • Adsorption is an exothermic process. The heat evolved per mole of adsorbate is called heat of adsorption.

Desorption:

  • The removal of an adsorbed substance from the surface is known as desorption. It is an endothermic process. i.e. Increase in temperature increases the rate of desorption.

Absorption:

  • Absorption is a phenomenon in which a substance penetrates through the surface and gets distributed uniformly throughout the body or bulk of another substance.  e.g. 1. Water (absorbate)  is absorbed by the sponge (absorbent).2.  Ammonia gas is absorbed in water.

Characteristics of Adsorption:

  • It is a surface phenomenon.
  • It takes place due to the presence of residual surface forces.
  • It is dependent upon temperature and  pressure.
  • It is affected by the surface area of adsorbent.
  • It is in an exothermic process.
  • It is a reversible process and a state of dynamic equilibrium is attained.
  • Example: Accumulation of acetic acid molecules on the surface by charcoal.


Characteristics of Absorption:

  • It is a bulk phenomenon.
  • It takes place due to porous nature of the substance.
  • It is independent of temperature and pressure.
  • It is not affected by the surface area of the absorbent.
  • It is neither exothermic nor endothermic process.
  • It is not reversible process and a state of static equilibrium may be reached.
  • Example: Absorption of water by a sponge.

Experiment to Demonstrate Adsorption:

  • Take about 100 ml of 0.1 N  acetic acid solution in a beaker. Add about 5 gram of activated and powdered charcoal to the solution. Stir the solution containing charcoal and keep it standing for 30 minutes. Filter the solution and find the strength of this solution (filtrate) by titration against 0.1 N NaOH.
  • It is observed that The concentration of an acetic acid solution is decreased as acetic acid is adsorbed by charcoal. This experi­ment proves that the molecules of acetic acid must be adsorbed by activated charcoal. It is a liquid-solid system in which acetic acid is adsorbate and charcoal is adsorbent.
  • This is reversible physical adsorption as if the charcoal in above experiment is boiled with water, the adsorbed acetic acid molecules will be desorbed.

Types of Adsorption:

  • It is found that the forces operative in adsorption are not the same in all cases.  Depending upon the forces which hold the particles or molecules of adsorbate on the surface of adsorbent there are two types of adsorption namely a) Physical adsorption and b) Chemical adsorption or chemisorption.

Physical Adsorption:

  • The adsorption in which molecules of adsorbate are held on the surface of adsorbent by Van der Waals forces or weak physical forces is called as physical adsorption or Van der Waals adsorption or physisorption.
  • Examples:
    • Accumulation of ammonia gas on the surface of activated charcoal.
    • Accumulation of hydrogen gas on the surface by platinised platinum.
    • Accumulation of acetic acid on surface by charcoal.

Characteristics of Physical Adsorption:

  • In physical adsorption, the adsorbed molecules are held on the surface of adsorbent by weak Van der Waals forces.
  • As the Van der Waals forces or physical forces are not specific in character, it is said to be general in character.
  • Van der Waals forces or physical forces are weak therefore it is reversible and it has a low heat of adsorption.
  • Van der Waals forces or physical forces may operate between adsorbed molecules and the other non adsorbed molecules this makes it multilayer in character.
  • The energy of activation is low.
  • It takes place at low temperature.
  • It adsorption is a fast process.

Chemical Adsorption:

  • The adsorption in which molecules of adsorbate are held on the surface of adsorbent by chemical forces (chemical bonds) is called as chemical adsorption or chemisorption.
  • Examples:
    • Accumulation of hydrogen gas on nickel.
    • Accumulation of oxygen or carbon monoxide on the surface of tungsten.

Characteristics of Chemical Adsorption:

  • In chemical adsorption, the adsorbed molecules are held on the surface of adsorbent by strong chemical bonding forces.
  • As the chemical bonds are highly specific in character, it is said to be specific in character.
  • Chemical bonds are strong therefore it is irreversible and it has a high  heat of adsorption.
  • For it to take place, there must be a direct contact between adsorbate and adsorbent molecules, therefore, it results in monolayer formation of adsorbate on the surface of the adsorbent.
  • The energy of activation is high.
  • It takes place at high temperature.
  • It is a slow process.

Extent of Adsorption:

  • The quantity of an adsorbate that is adsorbed per unit mass of adsorbent is called extent of adsorption.

Thus,   a = x/m

Where a = Extent of adsorption

x = The mass of gas adsorbed



m = Mass of adsorbent

Factors Affecting Adsorption:

Temperature:

  • Adsorption is an exothermic process, Hence according to Le Chatelier’s principle at given pressure low temperature favours adsorption.
  • If the temperature is increased, adsorbate molecules get removed from the adsorbent and this process is called as desorption.
  • Thus, adsorption is inversely proportional to the temperature. This is true for physical adsorption. In chemical adsorption due to the high energy of activation, the extent of adsorption increases initially and decreases as the temperature increases further.

Pressure:

  • When a gas is adsorbed on the surface of adsorbent there is a decrease in the volume of adsorbent. Thus, by Le Chatelier’s principle If the temperature is kept constant, the quantity of gas adsorbed by metal adsorbent increases with the increase in pressure.  Adsorption is directly proportional to the pressure of a gas over a limited range of pressure.
  • Graphical representation:

Adsorption 03

  •  At low pressure:

at low pressure   1/ n = 1

∴  x/m ∝  P



 

  • Thus at low pressure, the extent of adsorption is directly proportional to the pressure and hence for low pressure, the graph is a straight line.
  • At medium pressure:

At medium pressure  0 < 1/ n <  1

∴  x/m ∝  P1/n

  • Thus at medium pressure, the extent of adsorption increases less rapidly with increase in the pressure hence for medium pressure the graph is a curve.
  • At high pressure :

At high pressure  1/ n = 0

∴  x/m = constant



  • Thus at high pressure, the extent of adsorption is independent of pressure hence for high pressure, the graph is parallel to pressure axis.

Nature of Adsorbent:

  • Since adsorption is a surface phenomenon, adsorption increases with the increase in the surface area of the adsorbent.
  • More finely divided or rougher the surface of adsorbent, the greater will be the surface area and hence the greater will be the adsorption.
  • Metal catalysts in the finely divided form, colloidal form, rough surfaces, activated adsorbent provides more surface area.
  • Chemisorption is preferential and more specific. A gas will be chemisorbed on a given solid only if it can provide large surface area. For e.g. Hydrogen gas is adsorbed by nickel but not by iron.
  • The chemical nature of the adsorbent should be such so that it can cause chemisorption of adsorbate on the adsorbent.

Nature of Adsorbate:

  • In case of adsorption of gases by solids, it has been found that more easily liquefiable and highly water-soluble gases are adsorbed more readily due to greater van der Walls forces.
  • Hence ammonia, hydrogen chloride, chlorine and sulphur dioxide are more adsorbed than hydrogen, nitrogen, oxygen.
  • In physical adsorption, the extent of adsorption depends on the boiling point of the gases.
  • Gases are adsorbed on solids more readily than liquids.

Concentration of Adsorbate:

  • When liquid is adsorbed on solid, at higher concentration of adsorbate, the extent of adsorption is greater, provides that the temperature is kept constant.
  • The concentration of adsorbate has a similar type of effect as that of pressure.

Adsorption Isotherm:

  • An adsorption isotherm is a relation between the extent of adsorption (amount of a substance adsorbed per unit mass of an adsorbent) and the equilibrium pressure or concentration at a constant temperature.
  • Actually, it is a curve obtained by plotting extent of adsorption (amount of a substance adsorbed per unit mass of an adsorbent) against the equilibrium pressure or concentration at a constant temperature.


Freundlich Adsorption Isotherm:

  • Adsorption isotherm is a curve obtained by plotting extent of adsorption (amount of a substance adsorbed per unit mass of an adsorbent) against the equilibrium pressure or concentration at a constant temperature.

Adsorption 03

  • An empirical equation for the variation of gas adsorption with pressure at constant temperature over a limited range of pressure which was put forwarded by Freundlich and is known as “Freundlich adsorption isotherm.”
  • Mathematically the equation is,

Adsorption 04

Where x  = mass of adsorbate (gas) adsorbed.

m = mass of adsorbent (solid).

P = equilibrium pressure of adsorbate.



C = equilibrium concentration of adsorbate in the solution.

In this equation ‘K’ and ‘n’ are constant and ‘n’ is less than one.

  • At low pressure, the extent of adsorption is directly proportional to the pressure and hence for low pressure, the graph is a straight line. At medium pressure, the extent of adsorption increases less rapidly with increase in the pressure hence for medium pressure the graph is a curve. At high pressure, the extent of adsorption is independent of pressure hence for high pressure the graph is parallel to pressure axis. i.e.   x/m = constant. Thus Freundlich adsorption isotherm holds good only at moderate pressure.

By taking logs of both the side of isotherm,

Adsorption 05

  • The form of the equation is y = mx + c. On plotting (log x/m) against (log P), a straight line graph is ob­tained which indicates the validity of the equation.  Thus the slope of the graph = 1/n and its y-intercept is log k. From this graph values of k and n can be found out.

Adsorption 06



 

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