Final answer:
To estimate ΔH°rxn for the given reaction , calculate the bonds broken and formed in the reaction and subtract their bond dissociation energies. With the given bond dissociation energies, the estimated value of ΔH°rxn is 116 kcal/mol.
Step-by-step explanation:
To estimate the value of ΔH°rxn for the equation C2H4(g) + HBr(g) ⟶ C2H5Br(g), you need to consider the bond dissociation energies of the bonds being broken and formed. In this case, the reactant C2H4(g) has one C-C bond and four C-H bonds, and the product C2H5Br(g) has one C-Br bond and three C-H bonds. Refer to Table 7.2.1 for the bond dissociation energies:
C-C bond: 83 kcal/mol
C-H bond: 100 kcal/mol
C-Br bond: 67 kcal/mol
Using the formula for enthalpy change, ΔH°rxn = (bonds broken) - (bonds formed), the calculation would be:
ΔH°rxn = [(1 C-C bond x 83 kcal/mol) + (4 C-H bonds x 100 kcal/mol)] - [(1 C-Br bond x 67 kcal/mol) + (3 C-H bonds x 100 kcal/mol)]
ΔH°rxn = (83 + 400) - (67 + 300) = 116 kcal/mol