The standard enthalpy of reaction ($\Delta_r H^\circ$) can be calculated from the bond energies of the reactants and products using the formula: $\Delta_r H^\circ = \sum \text{Bond energies of reactants} - \sum \text{Bond energies of products}$ For the reaction: $\text{H}_2(g) + \text{Br}_2(g) \rightarrow 2\text{HBr}(g)$ $\Delta_r H^\circ = [\text{BE}(\text{H}-\text{H}) + \text{BE}(\text{Br}-\text{Br})] - [2 \times \text{BE}(\text{H}-\text{Br})]$ Given values: $\text{BE}(\text{H}-\text{H}) = 433\text{ kJ mol}^{-1}$ $\text{BE}(\text{Br}-\text{Br}) = 192\text{ kJ mol}^{-1}$ $\text{BE}(\text{H}-\text{Br}) = 364\text{ kJ mol}^{-1}$ Substituting the values: $\Delta_r H^\circ = [433 + 192] - [2 \times 364]$ $\Delta_r H^\circ = 625 - 728$ $\Delta_r H^\circ = -103\text{ kJ}$