Question

Which of the following pairs of molecules have the almost identical bond dissociation energy \[?\]
A.\[{F_2}\] and \[{H_2}\]
B.\[{N_2}\] and \[CO\]
C.\[{F_2}\] and \[{I_2}\]
D.\[HF\] and \[{O_2}\]

Answer 1

Hint: First we know bond dissociation energy is the amount of energy which is required to homolytically fracture a chemical bond. A homolytic fracture usually produces radical species. Shorthand notation for this energy is \[BDE\], \[{D_o}\], or \[D{H^o}\]. The units of bond dissociation energy are \[kJ/mol\] or \[kcal/mol\]. Bond dissociation energy may be measured experimentally using spectrometry, calorimetry, and electrochemical methods.

Complete answer: Bond dissociation energy is the energy required to break a chemical bond. It is one means of quantifying the strength of a chemical bond. The strongest bond dissociation energy is for the Si-F bond. The weakest energy is for a covalent bond and is comparable to the strength of intermolecular forces.
The bond enthalpies for the halogens are:

Halogen	Bond energy (\[kJ/mol\])
\[F - F\]	\[156\]
\[Cl - Cl\]	\[243\]
\[Br - Br\]	\[193\]
\[I - I\]	\[151\]

The pair of \[{F_2}\] and \[{I_2}\] have almost identical bond dissociation energy of \[156\]\[kJ/mol\] and \[151\]\[kJ/mol\], respectively. Since, bond dissociation energy equals bond energy only for diatomic molecules. As we go down a group in the periodic table, due to the larger radius of atoms the orbital overlap during bond formation is weak leading to low bond dissociation energy as in case of \[{I_2}\]. Although, \[F\] is on the top of the periodic table (period 2) and have good orbital overlap during bond formation, but due to its small size and large number of electrons in the valence sigh, the electron repulsion (the electrostatic repulsion between the nuclei) is high. This weakens the bond strength of \[{F_2}\]and the bond dissociation energy of \[{F_2}\] and \[{I_2}\]are identical.
Hence, the correct option is C.

Note:
Note that bond dissociation energy equals bond energy only for diatomic molecules. This is because the bond dissociation energy is the energy of a single chemical bond, while bond energy is the average value for all the bond dissociation energies of all bonds of a certain type within a molecule. Also note that the enthalpy change is temperature dependent.