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1.2.1—Bond energies

Syllabus
First assessment 2025
Objective
1.2.1
Level
HL

Average Bond Energies

ΔH≈Σ(bondsbroken)−Σ(bondsformed)ΔH ≈ Σ(bonds broken) − Σ(bonds formed)

Breaking bonds absorbs energy; forming bonds releases energy. Count every bond with its stoichiometric multiplicity before applying the signed sum.

For H₂ + Cl₂ → 2HCl, break one H–H and one Cl–Cl bond, then form two H–Cl bonds. Average bond enthalpies give an estimate because the tabulated value averages that bond across different gaseous molecules.

Worked example — bond enthalpies: for CX2HX4(g)+HBr(g)→CX2HX5Br(g)\ce{C2H4(g) + HBr(g) -> C2H5Br(g)}, the local course book gives C−H=414\ce{C-H}=414, C=C=614\ce{C=C}=614, H−Br=366\ce{H-Br}=366, C−C=346\ce{C-C}=346 and C−Br=285 kJ mol−1\ce{C-Br}=285\,\mathrm{kJ\,mol^{-1}}. ΔH=[4(414)+614+366]−[5(414)+346+285]=2636−2701=−65 kJ mol−1\Delta H=[4(414)+614+366]-[5(414)+346+285]=2636-2701=-65\,\mathrm{kJ\,mol^{-1}}. The negative estimate means the bonds formed release more energy than the bonds broken absorb; it remains approximate because the values are gaseous averages.

Calculating from Bond Energies

Assessment in practice

Representative question

Question 1

[Maximum number: 3]

Calculate the enthalpy change for the reaction, ΔH\Delta H. Use section 12 of the data booklet.

Energy Cycles Summary

Retrieve the route: count bond breaking/forming, manipulate Hess equations, define standard formation/combustion values, apply product–reactant sums, and track every Born–Haber energy term.

Check equation direction, coefficients, signs, standard states, lattice enthalpy convention, and one-versus-two-electron steps.