Bond Enthalpies
Count broken and formed bonds and state the gas-phase average-bond-enthalpy approximation.
AP Chemistry Topic 6.7 · CED printed p. 119 / PDF p. 126. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- System and surroundings as a chosen boundary
- Moles, balanced equations and limiting reactants
- Intermolecular versus intramolecular interactions
- Activation energy versus overall energy change
Bond enthalpy is a positive energy cost for breaking a specified bond in gas-phase species. Forming the same type of bond releases an approximately equal magnitude under matching conditions. A reaction changes potential energy because the pattern of bonds differs. Count bonds from molecular structure, not from atom formulas alone: H2 has one H–H bond, while NH3 has three N–H bonds.
Estimate reaction enthalpy with . Multiply bond counts by equation coefficients. Unchanged bonds can cancel if treated consistently. If formation release outweighs breaking cost, ΔH is negative. Tabulated average bond enthalpies average over molecular environments, usually for gas-phase bonds; they estimate rather than exactly reproduce a reaction involving a particular molecule or liquid/solid product. Include phase-change information separately if supplied. Stronger reactant bonds can raise breaking cost, but the full sum decides the sign.
Let's work through it
Estimate H2(g) + Cl2(g) → 2HCl(g) with D(H–H) =436, D(Cl–Cl) =243, D(H–Cl) =431 kJ mol−1 of bonds.
- Break one H–H and one Cl–Cl: input =436+243 =679 kJ per reaction as written.
- Form two H–Cl: release magnitude =2×431 =862 kJ.
- ΔH ≈679−862 =−183 kJ. The greater formation release makes it exothermic; breaking itself does not release heat.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
What sign does the bond-breaking sum have in an energy ledger?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
What sign does the bond-breaking sum have in an energy ledger?
Worked answer and reason
Positive: separating bonded atoms requires energy input.
Bond attractions stabilize the bonded configuration.
Retrieval 2
Why is a bond-enthalpy result usually an estimate?
Worked answer and reason
Average gas-phase values do not exactly capture each molecular environment or other physical phases.
The table’s reference conditions and averaging limit precision.
Catch the tempting mistake
- Use formed minus broken.
- Formation contributes release, so subtract its positive tabulated magnitude from the breaking cost.
- One molecule always has one bond.
- Count its structure; coefficients then count how many such molecules react.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 6 problem 11 · ap application · symbolic
- Unit 6 problem 12 · ap application · data
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For the teacher
- Require a bond-count table before the arithmetic.
- State the gas-phase average-value limitation in comparisons with formation-data results.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 6.7.A
- Calculate the enthalpy change of a reaction based on the average bond energies of bonds broken and formed in the reaction.
- 6.7.A.1
- During a chemical reaction, bonds are broken and/or formed, and these events change the potential energy of the system.
- 6.7.A.2
- The average energy required to break all of the bonds in the reactant molecules can be estimated by adding up the average bond energies of all the bonds in the reactant molecules. Likewise, the average energy released in forming the bonds in the product molecules can be estimated. If the energy released is greater than the energy required, the reaction is exothermic. If the energy required is greater than the energy released, the reaction is endothermic.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Enthalpy — Scientific check: stoichiometric enthalpy, formation values, sign reversal/scaling and Hess cycles.
Glossary and deeper reading · 5 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Limiting reactant
The limiting reactant is consumed first under the specified complete-reaction model and limits the product amount. Identifying it requires comparing available amounts with the balanced equation's ratios. For example, abundant hydrogen cannot produce more water after the available oxygen has been consumed.
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Activation energy
Activation energy is the energy barrier associated with a reaction pathway in an introductory energy-profile model. It concerns the pathway and is distinct from the reactant-to-product energy change. For example, a thermodynamically favored reaction can remain slow when its activation barrier is large.
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Bond enthalpy
A bond enthalpy is the enthalpy required to break a specified bond in the gas phase. Tables often give average values, so estimates for complex reactions are approximate. For example, a reaction estimate adds the costs of bonds broken and subtracts the energy released by bonds formed.
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Enthalpy
Enthalpy is a state function defined from internal energy, pressure, and volume. At constant pressure with only pressure–volume work, an enthalpy change equals the heat transferred to the system. For example, a negative reaction enthalpy indicates heat release for the written reaction under those conditions.