--%>
Assignment Help - homework help

What is chemisorption or chemical adsorption?

When the forces of attraction existing between adsorbate particles and adsorbent almost of the same strength as chemical bonds, the adsorption is called chemical adsorption. This type of adsorption is also known as chemisorptions. Since forces of attraction existing between adsorbent and adsorbate are relatively strong, therefore, this type of adsorption cannot be easily reversed. 

Characteristics of chemisorptions

Some important characteristics of chemisorptions are as follows:

(i) High specificity: chemisorptions is highly specific in nature. It occurs only if there is a possibility of bond formation between adsorbent and adsorbate molecules. For example O2 is adsorbed on metals by virtue of oxide formation and H2 is absorbed by transition metals due to hydride formation.

(ii) Irreversibility: as chemisorptions involve the compound formation between adsorbent and adsorbate, it is generally irreversible.

(iii) Enthalpy of adsorption: attractive forces between adsorbent and adsorbate molecules are strong chemical bonds and therefore, molar heat of adsorption is high and is of the order of 200-400 kJ mol-1.

(iv) High activation energy: although chemisorptions is exothermic, yet the process is slow at low temperature. It is because of high activation energy required for chemical process to occur. Like most of the chemical changes the extent of chemisorption increases initially with rise in temperature. High pressure is also supporting for chemisorption.

(v) Surface area: like physisorption, chemisorption also increases with increase in surface area of adsorbent.

(vi) State of adsorbate: since chemical reaction takes place in this type of adsorption, therefore, the molecular state of adsorbate molecules may be altered. For example, oxygen exists as O2, but on the surface where it is chemisorbed, it may exist as O2-, O22-, O-, O, O3-, etc.

(vii) Activation energy: chemical adsorption involves a chemical reaction between adsorbent and adsorbate; therefore, it requires high activation energy.

The adsorption of N2 on iron under two different conditions provides distinction between physisorption and chemisorption. At 83 K nitrogen gas undergoes physical adsorption on iron surface. N2 molecules are amount of N2 adsorbed decreases with further rise in temperature. At room temperature there is almost again shows adsorption as N atoms on the iron surface. This mode of adsorption is chemical adsorption as atoms form chemical bonds with iron atoms.

   Related Questions in Chemistry

  • Q : Problem on making solutions The weight

    The weight of pure NaOH needed to made 250cm3 of 0.1 N solution is: (a) 4g  (b) 1g  (c) 2g  (d) 10g Choose the right answer from above.

    		•
  • Q : Determining mole fraction of water in

    A mixture has 18 g water and 414 g ethanol. What is the mole fraction of water in mixture (suppose ideal behaviour of mixture): (i) 0.1  (ii) 0.4  (iii) 0.7  (iv) 0.9 Choose the right answer from abo

    		•
  • Q : Dipole moment of chloro-octane Describe

    Describe the dipole moment of chloro-octane in brief?

    		•
  • Q : Molarity of Barium hydroxide 25 ml of a

    25 ml of a solution of barium hydroxide on titration with 0.1 molar solution of the hydrochloric acid provide a litre value of 35 ml. The molarity of barium hydroxide solution will be: (i) 0.07 (ii) 0.14 (iii) 0.28 (iv) 0.35

    		•
  • Q : What is depression in freezing point?

    Freezing point of a substance is the temperature at which solid and liquid phases of the substance coexist. It is defined as the temperature at which its solid and liquid phases have the same vapour pressure. The freezing point o

    		•
  • Q : Describe various systems for

    Common system According to this system, the individual members are named according to alkyl groups att

    		•
  • Q : What is covalent radii? Explain its

    Average covalent radii can be assigned on the basis of molecular structures. The accumulation of structural data by spectroscopic studies and both electron and x-ray diffraction studies allows one to investigate the possibili

    		•
  • Q : Question on Raoults law Give me answer

    Give me answer of this question. For a dilute solution, Raoult's law states that: (a) The lowering of vapour pressure is equal to mole fraction of solute (b) The relative lowering of vapour pressure is equal to mole fraction of solute (c) The relative lowering of v

    		•
  • Q : Molarity of HCl solution 20 ml of HCL

    20 ml of HCL solution needs 19.85 ml of 0.01M NaOH solution for complete neutralization. Morality of the HCL solution is:  (i) 0.0099 (ii) 0.099 (iii) 0.99 (iv) 9.9 Choose the right answer from above.

    		•
  • Q : Molar mass of solute The boiling point

    The boiling point of benzene is 353.23 K. If 1.80 gm of a non-volatile solute was dissolved in 90 gm of benzene, the boiling point is increased to 354.11 K. Then the molar mass of the solute is: (a) 5.8g mol-1  (b)

    		•