--%>
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 reversible and irreversible

    The second law states that  dS ≥ (dQ/T), where dS = dQ/T for a reversible process and dS > dQ/T for an irreversible process.   a. Show that since dW12 = -dW21 (dWreverse = -dWforward) for a r

    		•
  • Q : Molarity of solution Help me to go

    Help me to go through this problem. When 7.1gm Na2SO4 (molecular mass 142) dissolves in 100ml H2O , the molarity of the solution is: (a) 2.0 M (b) 1.0 M (c) 0.5 M (d) 0.05 M

    		•
  • Q : Vapour pressure of benzene Give me

    Give me answer of this question. The vapour pressure of benzene at a certain temperature is 640mm of Hg. A non-volatile and non-electrolyte solid weighing 2.175g is added to 39.08g of benzene. The vapour pressure of the solution is 600,mm of Hg . What is the mo

    		•
  • Q : Pressure Phase Diagrams The occurrence

    The occurrence of different phases of a one component system can be shown on a pressure temperature. The phases present in a one line system at various temperatures can be conveniently presented on a P- versus-T diagram. An example is pro

    		•
  • Q : How to establish nomenclature for

    In the common chemistry terminologies, aliphatic halogen derivatives are named as alkyl halides. The words, n-, sec-, tert-, iso-, neo-, and amyl are

    		•
  • Q : Calculate PH value for a acetic acid 1.

    1. A solution of 0.100 M acetic acid is prepared. a) What is its pH value? b) If 20% of the initial acetic acid is converted to the acetate form by titration with NaOH, what is the resultant pH?

    		•
  • Q : Relative reactivity Which is more

    Which is more reactive towards nucleophilic substitution aryl halide or vinyl halides

    		•
  • Q : Problem associated to vapour pressure

    Provide solution of this question. 60 gm of Urea (Mol. wt 60) was dissolved in 9.9 moles, of water. If the vapour pressure of pure water is P0 , the vapour pressure of solution is:(a) 0.10P0 (b) 1.10P0 (c) 0.90P0 (d) 0.99P0

    		•
  • Q : Problem based on molarity Choose the

    Choose the right answer from following. The molarity of a solution of Na2CO3 having 10.6g/500ml of solution is : (a) 0.2M (b)2M (c)20M (d) 0.02M

    		•
  • Q : Infrared Adsorption The adsorption of

    The adsorption of infrared radiation by diatomic molecules increases the vibrational energy fo molecules and gives information about the force constant for the "spring" of the molecule.;The molecular motion that has the next larger energy level spacing aft

    		•