--%>
Assignment Help - homework help

Describe physical adsorption and its characteristics.

When the forces of attraction existing between adsorbate and adsorbent are van der Waal's forces, the adsorption is called physical adsorption. This type of adsorption is also known as physisorption or van der Waal's adsorption. Since the forces existing between adsorbent and adsorbate are very weak, therefore, this type of adsorption can be easily reversed by heating or by decreasing the pressure.

Characteristics of Physisorption

Some of the important characteristics of physisorption are as follows:

(i) Deficient of specificity: since the van der Waal forces are universal, a given surface of adsorbent does not show any preference for any specific gas. It can adsorb all the gases but to a different extent.

(ii) Reversible nature: physical adsorption of a gas by the solid is reversible and thus equilibrium is reached rapidly

Solid + Gas  1387_Physical adsorption.png  gas/solid + Heat

Thus, according to Le-chatelier's principle,

(a) Increase of pressure pushes the equilibrium in forward direction leading to more adsorption of gas and decrease of pressure cause desorption to occur.

(b) Since process is exothermic, therefore, lowering of temperature favours more adsorption and increase of temperature leads to desorption.

(iii) Surface area of adsorbent: the extent of adsorption increase with the increase of surface area of adsorbent. Thus, finely divided metals and rough surfaces are good adsorbents.

(iv) Nature of adsorbate: the amount of gas adsorbed by solid depends on nature of gas. In general, easily liquefiable gases (i.e gases with higher critical temperature) are readily as van der Waal forces are stronger near the critical temperature.

(v) Enthalpy of adsorption: the enthalpy of adsorption is low (20-40 kJ mol-1). This is because of weak nature of van der Waal's forces.

(vi) State of adsorbate: molecular state of adsorbate remains unaltered.

(vii) Activation energy: physical adsorption does not involve any chemical reaction and therefore, it requires very low activation energy.

   Related Questions in Chemistry

  • Q : Relative lowering of vapour pressure

    Which of the following solutions will have a lower vapour pressure and why? a) A 5% aqueous solution of cane sugar. b) A 5% aqueous solution of urea.

    		•
  • Q : Describe Transformation Matrices. Each

    Each symmetry operation can be represented by a transformation matrix.You have seen what happens when a molecule is subjected to the symmetry operation that corresponds to any of the symmetry elements of the point group to which the molecule belongs. The m

    		•
  • Q : Pressure and power for adiabatic

    a) Air flowing at 1 m3/s enters an adiabatic compressor at 20°C and 1 bar. It exits at 200°C. The isentropic efficiency of the compressor is 80%. Calculate the exit pressure and the power required. b) Steam enter

    		•
  • Q : Explain various chemicals associated

    During processing of food, several chemicals are added to it to augment its shelf life and to make it more attractive as well. Main types of food addi

    		•
  • Q : Direction of dipole moment expected

    Illustrate the direction of the dipole moment expected for hydrogen bromide?

    		•
  • Q : Mole fraction and Molality Select the

    Select the right answer of the following question.What does not change on changing temperature : (a) Mole fraction (b) Normality (c) Molality (d) None of these

    		•
  • Q : How can enzymes act as catalyst?

    Enzymes are complex proteinous substances, produced by living bodies, such as act as catalysis in the physiological reactions. The enzymes are, also called biochemical catalysts and the phenomenon is known as bio-chemical catalysis because numerous reactions that occur the bodies of animals and p

    		•
  • Q : Simulate the column in HYSYS The

    The objective of this work is to separate a binary mixture and to cool down the bottom product for storage. (Check table below to see which mixture you are asked to study). 100 kmol of feed containing 10 mol percent of the lighter component enters a continuous distillation column at the m

    		•
  • Q : Describe the properties of the

    Briefly describe the properties of the carbohydrates?

    		•
  • Q : Strength of Nacl in solution To 5.85gm

    To 5.85gm of Nacl one kg of water is added to prepare of solution. What is the strength of Nacl in this solution (mol. wt. of nacl = 58.5)? (a) 0.1 Normal (b) 0.1 Molal (c) 0.1 Molar (d) 0.1 FormalAnswer:

    		•