The bond enthalpies for six selected chemical bonds are shown below. For a reaction in which 2 moles of H₂(g) reacts with 1 mole of O₂(g) to form 2 moles of H₂O(g), what is the heat of the reaction?

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11ecba2d_1241_5a17_a3bf_6592b813ef96_MD0008_00 According to Hess' law, if a reaction occurs in several steps, the heat of the reaction (overall enthalpy) is equal to the sum of the enthalpy changes for each step, because enthalpy is a state function. Accordingly, a chemical reaction can be viewed as a combined series of steps in which bonds within the reactants are broken as new bonds are formed to make the products. Forming a bond is an exothermic process that releases energy (negative enthalpy), whereas breaking a bond is an endothermic process that requires energy (positive enthalpy). As a result, the enthalpies associated with breaking and forming a given type of bond are equal in magnitude but opposite in sign. When a bond between two atoms is broken in a homolytic cleavage, the electrons within the bond are equally divided between the atoms. 11ecba2d_1241_5a18_a3bf_bdec763aaad6_MD0008_00 The bond enthalpy ΔH°₂₉₈ (bond dissociation energy) is the energy needed to homolytically break 1 mole of a given type of bond between two atoms in the gas phase at a temperature of 298 K. Applying Hess' law using measured bond enthalpies, the heat of the reaction ΔH°_rxn can be determined as: 11ecba2d_1241_5a19_a3bf_bf3ccb7463e3_MD0008_00 In the reaction given in this question, 2 moles of (H-H) bonds are broken, 1 mole of O=O bonds is broken, and 4 moles of O-H bonds are formed. 11ecba2d_1241_5a1a_a3bf_31601f6abc00_MD0008_00 11ecba2d_1241_812b_a3bf_f19cf6311c0b_MD0008_00 Substituting the relevant bond enthalpies from the table into the summation from Hess' law, the overall heat of the reaction is evaluated as: 11ecba2d_1241_812c_a3bf_f78bc8fdd85d_MD0008_00 11ecba2d_1241_812d_a3bf_1fac989506ed_MD0008_00 11ecba2d_1241_812e_a3bf_1d483b3b05f1_MD0008_00 (Choice A) 1367 kJ is only the energy required to break the bonds in the reactants. (Choice B) 369 kJ is obtained if the number of moles of water formed is used in the calculation rather than the number of moles of O-H bonds formed. (Choice D) −1996 kJ is only the energy released due to forming the O-H bonds in the products. Educational objective: Bond enthalpy is the energy needed to homolytically break 1 mole of a given type of bond between two atoms in the gas phase at 298 K. According to Hess' law, a reaction can be assessed as a series of endothermic bond-breaking and exothermic bond-forming steps, and the heat of the reaction is equal to the sum of the enthalpy changes for each step.

The Bond Enthalpies for Six Selected Chemical Bonds Are Shown