Chemistry · CED Unit 6 Thermodynamics · 14 min read · Updated 2026-05-11
Bond Enthalpy — AP Chemistry
AP Chemistry · CED Unit 6 Thermodynamics · 14 min read
1. What Is Bond Enthalpy?★★☆☆☆⏱ 3 min
Bond enthalpy (also called bond energy or average bond dissociation enthalpy) is defined as the enthalpy change required to homolytically break one mole of a specific covalent bond in the gaseous state, averaged across many different compounds containing that bond. Common notation is $E( ext{X-Y})$ or $ ext{bond enthalpy}$, with standard units of kilojoules per mole ($ ext{kJ mol}^{-1}$).
Because bond enthalpies are averaged across different chemical environments, they are approximate values, not exact for any single molecule. This topic is a core skill in AP Chemistry Unit 6, appearing regularly in both multiple-choice and free-response sections.
2. Calculating Reaction Enthalpy from Bond Enthalpies★★★☆☆⏱ 5 min
To calculate the net enthalpy change of a reaction, we first account for all energy required to break all reactant bonds into gaseous atoms, then subtract the energy released when those atoms form all product bonds. Breaking bonds contributes positive energy, while forming bonds contributes negative energy, leading to the core formula:
This method only works for reactions where all reactants and products are in the gaseous state. Bond enthalpies do not account for extra enthalpy changes from phase changes of solids or liquids, so those reactions require Hess’s law or enthalpy of formation instead. Accurate counting of bonds is the most critical step for getting the correct result; drawing full Lewis structures for all species avoids miscounting.
3. Bond Enthalpy, Strength, and Bond Length Trends★★☆☆☆⏱ 3 min
Bond enthalpy directly measures bond strength: the higher the bond enthalpy, the more energy required to break the bond, so the stronger the bond. For bonds between the same pair of atoms, bond enthalpy is inversely related to bond length: shorter bonds have higher bond enthalpy and are stronger. This trend follows bond order: as bond order increases (single → double → triple), bond length decreases and bond enthalpy increases. AP questions regularly ask to rank bonds by strength, or justify a given trend using bond enthalpy arguments, often in FRQ sections.
4. Predicting Reaction Thermicity from Bond Enthalpy★★★☆☆⏱ 3 min
AP FRQ frequently asks to justify whether a reaction is endothermic or exothermic using bond enthalpy arguments, without requiring a full numerical calculation. If the total energy required to break reactant bonds is greater than the total energy released when forming product bonds, ΔH is positive (endothermic). If the total energy released from forming product bonds exceeds the energy required to break reactant bonds, ΔH is negative (exothermic). This skill is often tested in contextual questions about polymerization, fuel combustion, or bond cleavage reactions.
5. AP-Style Practice Problems★★★★☆⏱ 6 min
Common Pitfalls
Why: Students miscount multiple bonds within a single molecule
Why: Confusion over the sign convention for bond formation
Why: Forgetting bond enthalpies only apply to gas-phase bonds and ignore phase change enthalpy
Why: Overgeneralizing the inverse bond length-bond enthalpy trend
Why: Counting bonds per molecule but not scaling to the reaction as written
Why: Forgetting bond enthalpies are averaged across many different compounds