You are provided with the following average bond dissociation energies: , , and . Compute the absolu — Thermodynamics and Thermochemistry Chemistry Question
Question
You are provided with the following average bond dissociation energies: $E_{H-H} = 433 \text{ kJ/mol}$, $E_{Br-Br} = 192 \text{ kJ/mol}$, and $E_{H-Br} = 364 \text{ kJ/mol}$. Compute the absolute magnitude of the standard enthalpy of reaction ($\Delta H^\circ$) in kJ for the synthesis equation: $H_2(g) + Br_2(g) \rightarrow 2HBr(g)$.
💡 Solution & Explanation
The overall enthalpy change of a gas-phase reaction can be estimated from bond energies via $\Delta H_{rxn} = \sum BE(reactants) - \sum BE(products)$. Applying this to the reaction: $\Delta H = [BE(H-H) + BE(Br-Br)] - [2 \times BE(H-Br)]$. Inserting numerical data yields: $\Delta H = [433 + 192] - [2 \times 364] = 625 - 728 = -103 \text{ kJ}$. Since the question asks for the absolute magnitude of the energy change, the answer is $103 \text{ kJ}$.