Q9.80CP - Chemistry: Molecular Nature Of Matter And Change

Question

In the developing concept of electronegativity, Pauling used the term excess bond energy for the difference between the actual bond energy X-Y and the average bond energies of X-X and Y-Y (see text discussion for the case of HF). Based on the values in figure 9.20, p. 364, which of the following substances contains bonds with no excess bond energy?

(a) ${\mathbf{PH}}_3$        (b) ${\mathbf{CS}}_2$         (c) $\mathbf{BrCl}$          (d)  ${\mathbf{BH}}_3$        (e)  ${\mathbf{Se}}_8$

Step-by-Step Solution

Answer

a) excess bond energy of ${\mathbf{PH}}_3$  is zero.

b) excess bond energy of ${\mathbf{CS}}_2$ is zero. 

c) excess bond energy of $\mathbf{BrCl}$  is not zero.

d) excess bond energy of ${\mathbf{BH}}_3$  is not zero.

(e) the excess bond energy of ${\mathbf{Se}}_8$ is zero.

the concept of electronegativity

"The ability of an atom in a molecule to attract electrons to itself," was how Linus Pauling defined electronegativity. Basically, the electronegativity of an atom is a relative value of that atom’s ability to attract electron density toward itself when it bonds to another atom.   

Electronegativity values of given atoms 

B = 2.0

H = 2.1

P = 2.1

Se = 2.4

C = 2.5

S = 2.5

Br = 2.8

Cl = 3.0

excess bond energy

excess bond energy is the difference between the actual bond energy X-Y and the 

average bond energies of X-X and Y-Y. It can be formulated as:

excess bond energy = ${\mathbf{BE}}_{X-Y}\mathbf{-}\frac{1}{2}\mathbf{(}\mathbf{B}{\mathbf{E}}_{X-X}\mathbf{+}{\mathbf{BE}}_{Y-Y}\mathbf{)}$  

when X and Y atoms are similar or have the same electronegativity, then excess bond energy will be zero.

excess bond energy of PH 3

Electronegativity of P is 2.1 and electronegativity of H is 2.1. Since, both atoms have same electronegativities then the excess bond energy will be zero.

excess bond energy of CS 2

Electronegativity of C atom is 2.5 and electronegativity of S atom is 2.5. Since, both the atom has the same electronegativities then the excess bond energy will be zero.

excess bond energy of BrCl

Electronegativity of Bromine atom is 2.8 and electronegativity of Cl atom is 3.0. Since, both the atom has different electronegativities then the excess bond energy will not be zero.

excess bond energy of BH 3

Electronegativity of B atom is 2.0 and electronegativity of H atom is 2.1. Since there is a small difference between electronegativities of atoms, the excess bond energy will not be zero.

excess bond energy of Se 8

 ${\mathbf{Se}}_8$ contains 8 Se atoms. The electronegativity of all Se atoms is 2.4, hence the excess bond energy will be zero.