Engineering Chemistry S Datta

VALENCY AND CHEMICAL BONDING 17
Highlights:
• The dipole moment of H 2
O molecule is 1.8 D. i.e., its dipole moment is 1.8 × 10 –18
esu-cm.
• HCl, H 2
O, NH 3
, CH 3
OH are polar molecules i.e., their centre of action of positive
charges and centre of action of negative charges are not same and the molecule is not
symmetrical.
• Cl 2
, H 2
, CH 4
, C 6
H 6
, CO 2
, etc. are non-polar molecules. The molecules are symmetrical,
and in those molecules centre of positive charge action and centre of negative charge
action are at the same place.
i.e., d ⇒ 0; ∴ µ ⇒ 0.
• The dipole moment of a molecule can be calculated from the formula:
2
2
R = µ 1 + µ 2 + 2µ 1µ 2 cos α
where, µ 1
and µ 2
= dipole moments of the different bonds of the molecule.
R = Resultant moment of the molecule.
If R is zero, the molecule becomes non-polar and if R has got a positive value the
molecule becomes polar.
α = Angle between the covalent bonds
Born-Haber Cycle (Determination of Electron Affinity):
The electron affinity of an element is the quantity of energy released when an electron is
gained or lost by an atom in the gaseous state to form an anion.
−
A( g) + e ( g) ⎯→ A ( g)
+ Energy released
Neutral gaseous atom Anion (Electron affinity of atom A, E ) A
−
Electron affinity is represented by the ‘–ve’ sign as energy is liberated in this case.
When energy is supplied it is represented by ‘+ve’ sign and called second electron affinity (E 2
).
Born-Haber cycle is represented diagrammatically with the formation of NaCl (s) from
Na (s) and 1 2 Cl 2
(g) by direct combination to determine electron affinity of ‘Cl’ atom:
Na(s) +
1
Cl (g)
2 2
Q = –98.2 kcal
NaCl(s)
Crystal
Na(g)
+
Na (a)
S = +26.0 kcal
I = +118.0 kcal
(– e)
1
2
Cl(g)
–
Cl (g)
D = +28.9 kcal
E = ? (+e)
U = –180.4 kcal
0
Determination of electron
affinity of Na atom.
U0 Lattice energy of
NaCl crystal.
Q = S + 1 2 D + I + E + U 0
– 98.2 = + 26 + 28.9 + 118 + E – 180.4
∴
E = – 90.7 kcal/g atom of Cl
Negative sign indicates that energy is released in the formation,
Cl(g) ⎯⎯→ Cl – (g)