Given the bond dissociation energies below (in kcal/mol) ,estimate the ΔH° for the propagation step (CH₃) ₂CH∙ + Cl₂ → (CH₃) ₂CHCl + Cl∙.
CH₃CH₂CH₂-H 98
(CH₃) ₂CH-H 95
Cl-Cl 58
H-Cl 103
CH₃CH₂CH₂-Cl 81
(CH₃) ₂CH-Cl 80

Question

Quizplus · Accepted Answer

To estimate the ΔH° for the reaction, we need to calculate the bond dissociation energies broken and formed in the reaction. Breaking the bond between Cl and Cl requires 58 kcal/mol, breaking the bond between H and Cl requires 103 kcal/mol, and breaking the bond between the carbon and hydrogen in the reactant requires 95 kcal/mol. Forming the bond between carbon and chlorine requires 80 kcal/mol, and forming the chlorine radical requires 58 kcal/mol.

Therefore, the total energy change in the reaction can be estimated as follows: 
ΔH° = (energy required to break bonds in reactants) - (energy released in forming bonds in products) 
ΔH° = (95 kcal/mol + 103 kcal/mol + 80 kcal/mol) - (80 kcal/mol + 58 kcal/mol) 
ΔH° = 22 kcal/mol

Thus, the answer is A.

Given the Bond Dissociation Energies Below (In Kcal/mol),estimate the ΔH°