Engineering Chemistry S Datta

18 ENGINEERING CHEMISTRY
Highlights:
• Born-Haber cycle is a thermochemical cycle that can be used to calculate the lattice
energy for a compound of a metal with a nonmetal.
• The cycle is an application of Hess’s Law (see Chapter 4).
• A Born-Haber cycle can also help to determine whether the bonding in a compound
is truly ionic.
• The lattice energy for NaCl is –780 kJ mol –1 , that for KCl is –711 kJ mol –1 and that
for AgCl is –905 kJ mol –1 calculated as per Born-Haber cycle. The lattice energy for
AgCl is greater than that of ionic bondings in NaCl and KCl indicating that there is
a contribution of covalent bonding in AgCl.
• We have utilised Born-Haber cycle in determining the electron affinity of ‘Cl’ atom.
Chemical Bonding (Wave-Mechanical Concept)
The classical concept of covalency doesn’t give any idea of the directional characteristic
of a bond and also of the bond energy. The application of wave mechanics gives a satisfactory
explanation of some of these problems. In this connection, two main approaches are —
1. Valence-bond theory
2. Molecular orbital theory.
Molecular orbital theory is gaining much importance. Both the theories admit that —
(i) A covalent bond is formed as a result of overlapping of the atomic orbitals,
(ii) When the overlapping takes place along the axes of the atomic orbitals maximum
overlapping of atomic orbitals can occur,
(iii) Maximum overlapping gives a strong bond known as sigma (σ) bond.
H H
H : H
Fig. 2.3 Formation of H 2
molecule by the overlapping of
two 1s atomic orbitals of two H atoms.
(iv) Each covalent bond requires overlapping of a pair of orbitals.
(v) The overlapping orbitals must be valence orbitals and must contain an odd electron.
Schematic representation of the formation of some simple molecules on the basis of
valence bond theory:
(i) s-s overlapping ⇒ H 2
molecule
(ii) s-p overlapping ⇒ the formation of HF, NH 3
, H 2
O molecules
(i) HF molecule
H + H
F
F
Fig. 2.4 (a) Formation of HFmolecule.