Engineering Chemistry S Datta

CATALYST 145
Q. 10. What are homogeneous and heterogeneous catalysts? Give examples.
Ans. If the catalyst is in the same phase as the reactants, it is a homogeneous catalyst.
If the catalyst is in a different phase, it is a heterogeneous catalyst.
Example: (i) Homogeneous catalyst = Hydrolysis of ester, both acid and base catalysed.
(ii) Heterogeneous catalyst (contact catalyst) = In Haber’s process for manufacture of
ammonia (Fe + Al 2
O 3
+ K 2
O).
Q. 11. What are enzymes?
Ans. Enzymes are proteins in nature and are the catalysts for biochemical reactions.
Q. 12. Do you know any use of enzymes in detergent?
Ans. Some washing powders contain proteases—enzymes that remove blood stains on
dirty clothes.
Q. 13. Mention the main characteristics of an enzyme.
Ans. Enzymes are highly specific. Each enzyme catalyses a particular reaction. Every
enzyme has an ‘active site’, that is, just right shape and size for the substrate molecules. Each
enzyme works best at a particular temperature and pH.
CATALYTIC APPLICATIONS OF ORGANOMETALLIC COMPLEXES
Effective Atomic Number (E.A.N.)
To explain the formation of a complex of a metal with a ligand, it was suggested by
Sidgwick that metal ions will tend to accept the electron pairs from donors, i.e., ligand until
they have obtained a sufficient number of electrons, so that a metal ion in the resulting complex
has an effective atomic number of the nearest inert gas. This rule will help to understand the
formation of a complex which in turn will lead to get a better explanation of complex compounds
acting as catalysts in various reactions.
So, the effective atomic number (E.A.N.) of a metal is obtained by deducting the number
of electrons lost in the metal ion formation, then adding the number of electrons gained by coordination
i.e., two electrons from each ligand having one co-ordination centre. The rule can be
exemplified by the following table:
E.A.N. Determination
Metal ion
Atomic no. Co-ordination Electrons Electrons
of metal number lost in ion added by
formation co-ordination
E.A.N.
Fe 2+ 26 6 2 12 36 (Kr)
Co 3+ 27 6 3 12 36 (Kr)
Cu + 29 4 1 8 36 (Kr)
Ni 2+ 28 4 2 8 34 (Se)
The 18-electron Rule
This is another way of expressing noble gas electron number rule i.e., effective atomic
number rule regarding complex formation. The statement of 18-electron rule is that the valence
shell of metal atom will attain 18 electrons by saturation of (n – 1) d, ns and np orbitals of
metals by the ligands. Thus, the saturation will occur as follows:
10 (for (n – 1) d subshell) + 2 (for ns subshell) + 6 (for np subshell) = 18 electrons.