How is the energy change of a reaction calculated using bond energies?

• A. Energy change = energy of bonds formed minus energy of bonds broken.
• B. Energy change = energy of bonds broken plus energy of bonds formed.
• C. Energy change = mass times specific heat.
• D. Energy change = energy of bonds broken minus energy of bonds formed.

Answer: (D)

When you predict how much energy a reaction changes the system by, focus on two things: the energy needed to break the bonds in the reactants and the energy released when new bonds form in the products. Breaking bonds is an energy input (absorbed), while forming bonds releases energy. The net energy change is the energy required to break bonds minus the energy released in forming new bonds: ΔH ≈ ΣE(bonds broken) − ΣE(bonds formed). This way, a negative result means the reaction releases more energy than it absorbs (exothermic), and a positive result means it absorbs more than it releases (endothermic).

For example, breaking H–H and Cl–Cl costs 436 kJ and 242 kJ, totaling 678 kJ, while forming two H–Cl bonds releases 2×431 = 862 kJ. The net ΔH is 678 − 862 = −184 kJ, indicating an exothermic process.

So the expression that matches energy broken minus energy formed is the correct one. The other forms would misrepresent the energy flow: adding energies mixes inputs and outputs, and reversing the subtraction would flip the sign; mass times specific heat doesn’t apply to bond energies.