CLASS 11–12 CHEMISTRY · THERMODYNAMICS

Reaction enthalpy from bond enthalpies

ΔH = Σ(bond enthalpies of bonds broken in reactants) − Σ(bond enthalpies of bonds formed in products). Breaking bonds needs energy; forming bonds releases it.

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Worked examples

Example 1 · level 1

Using the average bond enthalpies (in kJ/mol) H–H 436, F–F 158, H–F 568, estimate ΔH in kJ/mol for the gas-phase reaction H₂ + F₂ → 2HF. Include the sign.

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Bonds broken: 436 (H–H) + 158 (F–F) = 594 kJ.
Bonds formed: 2 × 568 (H–F) = 1136 kJ.
ΔH = 594 − 1136 = −542 kJ/mol (exothermic).

Answer: -542

Example 2 · level 2

Using the average bond enthalpies (in kJ/mol) H–H 437, O=O 501, O–H 460, estimate ΔH in kJ/mol for the gas-phase reaction 2H₂ + O₂ → 2H₂O. Include the sign.

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Bonds broken: 2 × 437 (H–H) + 501 (O=O) = 1375 kJ.
Bonds formed: 4 × 460 (O–H) = 1840 kJ.
ΔH = 1375 − 1840 = −465 kJ/mol (exothermic).

Answer: -465

Example 3 · level 3

Using the average bond enthalpies (in kJ/mol) C–H 417, O=O 495, C=O 801, O–H 461, estimate ΔH in kJ/mol for the gas-phase reaction CH₄ + 2O₂ → CO₂ + 2H₂O. Include the sign.

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Bonds broken: 4 × 417 (C–H) + 2 × 495 (O=O) = 2658 kJ.
Bonds formed: 2 × 801 (C=O) + 4 × 461 (O–H) = 3446 kJ.
ΔH = 2658 − 3446 = −788 kJ/mol (exothermic).

Answer: -788