Question

Quizplus · Accepted Answer

The only difference between the two molecules is the substitution of a fluorine atom for a hydrogen atom. This indicates a difference in electronegativity between the two atoms and suggests that the bond dissociation energy of the C-H bond is greater than that of the C-F bond. Therefore, the molecule with the C-H bond (11eb6c5c_3ab9_d3aa_8d9d_bbd2722f69ec_TB8482_00) will have a higher bond dissociation energy and a larger standard enthalpy of formation. The difference in standard enthalpy of formation between the two molecules can be calculated using Hess's law as follows: 
ΔHf(11eb6c5c_3ab9_fabb_8d9d_4b966be53bb3_TB8482_00) = ΔHf(11eb6c5c_3ab9_d3aa_8d9d_bbd2722f69ec_TB8482_00) + energy required to break C-F bond - energy released by forming C-H bond 
The energy required to break a C-F bond is approximately 484 kJ/mol, while the energy released by forming a C-H bond is approximately 413 kJ/mol. Therefore, the difference in standard enthalpy of formation is approximately (484 - 413) kJ/mol = 71 kJ/mol ≈ 21.9 kJ/mol.

A) 34.7 KJ/mol B) 21.9 KJ/mol