Why is energy to break bonds and energy released on forming bonds described as they are?

Why is energy to break bonds and energy released on forming bonds described as they are?

• A. Breaking bonds releases energy and forming bonds absorbs energy.
• B. Breaking bonds absorbs energy; forming bonds releases energy; the net enthalpy depends on the balance of these energies.
• C. Bond energies are not related to enthalpy changes.
• D. Enthalpy changes are solely determined by temperature.

Answer: (B)

Energy to break bonds and energy released on forming bonds come from two opposite processes in a reaction. Breaking bonds takes in energy (it’s endothermic), while forming new bonds releases energy (it’s exothermic). The overall enthalpy change of a reaction, ΔH, is basically the balance between these two amounts: ΔH ≈ sum of energies needed to break bonds in the reactants minus sum of energies released by forming bonds in the products. So whether the reaction ends up releasing or absorbing heat depends on which side dominates. If breaking costs more energy than is released by forming new bonds, the reaction is endothermic; if forming bonds releases more energy than was needed to break them, it’s exothermic. Bond energies are averages used to estimate this, which is why we talk about the net enthalpy depending on the balance. For example, breaking the bonds in reactants costs energy, but forming the product bonds releases energy; in many reactions the energy released during bond formation outweighs the energy required to break bonds, giving an overall release of energy.

Energy to break bonds and energy released on forming bonds come from two opposite processes in a reaction. Breaking bonds takes in energy (it’s endothermic), while forming new bonds releases energy (it’s exothermic). The overall enthalpy change of a reaction, ΔH, is basically the balance between these two amounts: ΔH ≈ sum of energies needed to break bonds in the reactants minus sum of energies released by forming bonds in the products. So whether the reaction ends up releasing or absorbing heat depends on which side dominates. If breaking costs more energy than is released by forming new bonds, the reaction is endothermic; if forming bonds releases more energy than was needed to break them, it’s exothermic. Bond energies are averages used to estimate this, which is why we talk about the net enthalpy depending on the balance. For example, breaking the bonds in reactants costs energy, but forming the product bonds releases energy; in many reactions the energy released during bond formation outweighs the energy required to break bonds, giving an overall release of energy.