Page 65 - 11-chem-6

6 CHEMICAL BONDING eLearn.Punjab

Table (6.6) Comparison of experimental

and theoretical bond energies

Bond Bond energies (kJmol-1)

X=F X=Cl X=Br X=I

X-X 155 242 193 151

H-X 293 336 311 291

(calculated)

H-X 567 431 366 299

(observed)

difference 274 95 55 8

Let us, now consider, the bond in HCl. This bond is polar, but we consider the electron pair to be
equally shared. On adding up the bond energy contributions of H-atom and Cl-atom, we expect the
bond energy of H-Cl to be 56.14 x 10-23kJ per molecule which is the sum of 36.21xl0-23kJ and 19.93x10-
23kJ. For Avogadro’s number of HCl molecules, the calculated bond energy is 337.96 kJmol-1 which
is obtained by multiplying 56.14 x 10 -23 with 6.02 x 10-23 The experimentally found bond energy
for HCl is 431 kJmol-1.The observed bond energy is significantly greater than the calculated value
and that means a more stable H-Cl bond. This stability is due to the ionic character present in the
molecule.The decreasing polarity from HF to HI shows a trend toward equal sharing of electrons
which is consistent with decreasing electronegativity from F to I.
The bonds with higher bond energy values have shorter bond lengths. The bond energies of
C to C bonds being in the order C ≡ C >C=C>C-C. Their bond lengths are in the reverse order i.e. C
-C > C=C > C ≡ C.

6.5.3 Bond Length

The distance between the nuclei of two atoms forming a covalent bond is called the bond
length. The bond lengths are experimentally determined by physical techniques. The techniques
may be electron diffraction, X-ray diffraction or spectral studies.
The covalent bond length between two atoms is often but not always independent of the nature of
the molecules. For instance, in most of the aliphatic hydrocarbons, the C-C bond length is very close
to 154 pm. The C-C bond length is also found to be the same in diamond.
The covalent radii for different elements are almost additive in nature. The single bond
covalent radius of carbon is 77 pm which is half of the C-C bond length (154 pm). Similarly, the
covalent radius of Cl is 99 pm i.e. one half of the Cl-Cl bond length (198 pm). So the bond length of
C-Cl bond will be 77 + 99= 176 pm. Some selected bond lengths are given in Table (6.7).

65