Mistake Master
Home› Unit 6 · Thermodynamics› 6.1· 6.2· 6.3· 6.4· 6.5· 6.6· 6.7· 6.8· 6.9

Bond Enthalpies

▶︎  Watch it animatedinteractive step-through · ~3 min · optional

Bond enthalpies estimate a reaction's ΔH by accounting for the bonds broken and formed. Breaking bonds absorbs energy (endothermic); forming bonds releases energy (exothermic). So ΔH ≈ (energy to break reactant bonds) − (energy released forming product bonds). Because tabulated bond enthalpies are averages, the result is an estimate.

UNIT 6 TOPIC 6.7 • BOND ENTHALPIES BOND LEDGER ESTIMATE ΔH°rxn FROM AVERAGE BOND ENTHALPIES ΔH°rxn ≈ Σ (bond enthalpies BROKEN) − Σ (bond enthalpies FORMED) energy absorbed to break reactant bonds − energy released when product bonds form BREAKING BONDS costs energy · ENDOTHERMIC · sign + FORMING BONDS releases energy · EXOTHERMIC · sign − WORKED EXAMPLE H₂(g) + Cl₂(g) → 2 HCl(g) BONDS BROKEN (reactants · absorbed, +) H–H 1 × 436 = +436 kJ/mol Cl–Cl 1 × 243 = +243 kJ/mol Σ broken = +679 kJ/mol absorbed BONDS FORMED (products · released, −) H–Cl 2 × 431 = −862 kJ/mol Σ formed = −862 kJ/mol released ENERGY LEDGER IN — break bonds +679 OUT — form bonds −862 183 extra out OUT bar is longer than IN bar: more energy released than absorbed → EXOTHERMIC. ΔH ≈ 679 − 862 = −183 kJ/mol exothermic (releases heat) CED ANCHOR ΔH is estimated as the energy to break reactant bonds minus the energy released when product bonds form (average bond enthalpies). REMEMBER AVERAGE enthalpies → an ESTIMATE only Works best for GAS-PHASE reactions Stronger / shorter bond = higher enthalpy AP Chemistry · Unit 6 · Thermodynamics
Bond enthalpies estimate ΔH by accounting for bonds broken and formed. Breaking bonds absorbs energy (endothermic); forming bonds releases it (exothermic). So ΔH ≈ (energy to break reactant bonds) − (energy released forming product bonds).
Bond Accountant · Open the sandbox →

The central error is thinking breaking bonds releases energy — it costs energy; forming bonds releases it. Other traps swap the bond and formation sums, or treat average bond enthalpies as exact. Break costs, form releases; the difference is ΔH.

The work

3 ways in · any order
Lesson
Bond Enthalpies ›

ΔH from bond enthalpies is bonds broken (cost) minus bonds formed (release), using average values. The lesson works the ledger and the sign logic, then closes with a ten-scenario check.

Skill check · 10 scenarios
Open lesson →
Diagnostic
10-item topic check ›

Ten items spanning the Topic 6.7 misconceptions: breaking bonds thought to release energy, bond and formation sums swapped, average bond enthalpies treated as exact, and signs mishandled.

Not started · 10 items · ~15 min
Start diagnostic →
Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first