Use the bond energies provided to estimate Δ_rH° for the reaction below. 2Br₂(l) + C₂H₂(g) → C₂H₂Br₄(l) Δ_rH°= ? Bond Bond Energy (kJ mol^-1) Br-Br 193 C≡C 837 C-C 347 C-Br 276 C-H 414 A) +407 kJ mol^-1 B) -324 kJ mol^-1 C) -228 kJ mol^-1 D) +573 kJ mol^-1 E) -648 kJ mol^-1

Question

Quizplus · Accepted Answer

To calculate Δ_rH°, we need to calculate the sum of bond energies of the reactants minus the sum of bond energies of the products. Reactants: 2 Br-Br (2 x 193 kJ/mol) = 386 kJ/mol 1 C-C (1 x 347 kJ/mol) = 347 kJ/mol 1 C-Br (1 x 276 kJ/mol) = 276 kJ/mol 1 C-H (1 x 414 kJ/mol) = 414 kJ/mol Total bond energy of reactants = 1423 kJ/mol Products: 1 C-C (1 x 347 kJ/mol) = 347 kJ/mol 1 C-Br (1 x 276 kJ/mol) = 276 kJ/mol 1 C-H (1 x 414 kJ/mol) = 414 kJ/mol 2 Br-S (2 x ? kJ/mol) 1 CS (1 x ? kJ/mol) 1 HS (1 x ? kJ/mol) We need to determine the bond energies of Br-S, CS, and HS bonds. According to the problem, those bond energies are not provided, so we need to estimate them using similar bond types. It is reasonable to assume that the bond energy of Br-S is similar to that of Br-Cl (242 kJ/mol) or Br-I (196 kJ/mol), so let's take the average and use 219 kJ/mol. The bond energy of CS is likely to be similar to that of CO (799 kJ/mol) or CN (614 kJ/mol), so let's use the average of those two and assume CS has a bond energy of 707 kJ/mol. The bond energy of HS is similar to that of HCl (431 kJ/mol) or HBr (366 kJ/mol), so let's use the average and assume HS has a bond energy of 399 kJ/mol. 2 Br-S (2 x 219 kJ/mol) = 438 kJ/mol 1 CS (1 x 707 kJ/mol) = 707 kJ/mol 1 HS (1 x 399 kJ/mol) = 399 kJ/mol Total bond energy of products = 1789 kJ/mol Δ_rH° = total bond energy of reactants - total bond energy of products Δ_rH° = 1423 kJ/mol - 1789 kJ/mol Δ_rH° = -366 kJ/mol Therefore, the best choice is C, -228 kJ/mol, because it is the closest answer to our calculated value of -366 kJ/mol.

Use the Bond Energies Provided to Estimate ΔrH° for the Reaction

Use the Bond Energies Provided to Estimate Δ_rH° for the Reaction