Breaking bonds requires energy. Forming bonds releases energy. The overall change depends on the difference between these two totals, not on either process alone.
Use the balanced equation to count molecules, then count bonds within each molecule. A coefficient multiplies the number of molecules, so it also multiplies the total number of bonds.
Calculate energy required for breaking minus energy released on formation. Negative results are exothermic; positive results are endothermic. State which amounts in the balanced equation your energy change refers to.
The supplied values are mean bond energies for a gas-phase estimate. Water vapour and liquid water are different product states; condensation energy is not included unless stated.
Worked example
H₂ + Cl₂ → 2HCl breaks one H–H and one Cl–Cl bond, but forms two H–Cl bonds.
Using 436, 243 and 432 kJ mol⁻¹ of bonds gives 679 − 864 = −185 kJ mol⁻¹ for the reaction as written.
Specification references
Higher only: AQA 4.5.1.3; Edexcel 7.14. Supplied mean bond values and named gas-phase products define the calculation.
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Explain the two counting errors in “2H₂ + O₂ forms two O–H bonds, and breaking the reactant bonds releases energy.”
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Count four O–H bonds in two water molecules.
Identify bond breaking as energy input.
Use the two totals with their correct roles.
Example answer
Two water molecules contain four O–H bonds. Breaking the two H–H bonds and one O=O bond requires energy; forming the four O–H bonds releases energy. Subtract the energy released when bonds form from the energy needed to break bonds.
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