The Time Evolution of the Electronegativity - Modern Trends in Chemistry and Chemical Engineering - page 36

k = (c/R) 1/2 + d, (56)

where c and d are the constants.
Remick [46] pointed out that the bond stretching force constant (k) is a function of
the product of the electronegativities of the atoms constituting the molecule. Thus for
a diatomic molecule, the bond stretching force constant (k AB ) was given by:

k AB = ƒ(χ A χ B ) (57)

The stretching force constant is not only functions of product of the electronega-
tivity of the atoms present on the molecule but it is also dependent on several other
factors such as (i) bond order (N), and (ii) bond distance (R).
Gordy [47] considered all the factors and modifi ed the Reimick's equation as:

k AB = ƒ(N,χ A χ B ,R) (58)

Thus, he proposed the general equation for the bond stretching force constant as
follows:

k AB = a N(χ A χ B /R 2 ) 3/4 + b, (59)

where a and b are constants for certain broad classes of molecules. To determine
the constants, a and b, Gordy compared calculated N(χ A χ B /R 2 ) 3/4 with the experimental
bond stretching values of some molecules and evaluated the constants as a = 1.67 and
b = 0.30.
It should be notated here that k AB is measured in dynes × cm 10 -5 , R is in Å. The
value of the constants “a” and “b” varies from a class of molecule to another class. A
detail study was made by Gordy and proposed different constant values for different
classes and bond order in his paper [47].
On the basis of Pasternak's [21] electronegativity defi nition and SBC model [18-
21], Ray et al. [17] proposed a direct method for the evaluation of the force constant
of A-B bond from the equalized electronegativity as:

k AB = (7/4) χ 2 AB /R AB C 2 (60)

where, C is a constant which depends on bond type.

STANDARD ENTHALPIES OF FORMATION AND BOND DISSOCIATION
ENERGY
Bratsch [48] pointed out that the Pauling Scale of electronegativity can be used to
predict standard enthalpies of formation of binary compounds:

ΔH 0 f (kilo joule/mole) = - 96.5n [χ A - χ B ] 2 (61)

where n is the number of equivalents in the compound formula, χ A , χ B are the Paul-
ing electronegativity in (eV) 1/2 unit.
Knowledge of bond dissociation energies of chemical bonds in molecules is essen-
tial for understanding chemical processes [49]. At the outset, it should be emphasized
that there is a distinction [50] between the bond dissociation energy and the average