Average Bond Enthalpy
Average bond enthalpy = the energy required to break one mole of a specific covalent bond in the gaseous state, averaged over a range of similar compounds. Always positive (endothermic process).
The Calculation
Enthalpy from Bond Enthalpies
\( \Delta H = \sum(\text{bonds broken}) - \sum(\text{bonds formed}) \)
Breaking = positive (endothermic) | Forming = negative (exothermic)
Key Bond Enthalpies (from Data Booklet)
| Bond | kJ mol⁻¹ | Bond | kJ mol⁻¹ |
|---|---|---|---|
| H-H | 436 | C-C | 346 |
| C-H | 414 | C=C | 614 |
| O-H | 463 | O=O | 498 |
| C=O | 804 | N≡N | 945 |
Why Bond Enthalpy Calculations Are Approximate
- Bond enthalpies are averages across many molecules. The actual C–H bond strength varies slightly between CH₄, C₂H₆, etc.
- Data applies to the gaseous state only: liquid or solid reactants/products require additional energy for changes of state (e.g. enthalpy of vaporisation).
- Resonance structures (e.g. benzene, carbonate) make average single/double bond values significantly inaccurate.
The Exceptional Strength of the N≡N Triple Bond
The triple bond in N₂ (945 kJ mol⁻¹) is one of the strongest chemical bonds known. This explains why nitrogen gas is so inert under ambient conditions and why industrial nitrogen fixation (the Haber process) requires high temperatures and a catalyst.
Calculating ΔH for Methane Combustion
Calculate ΔH for: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
Bonds broken (endothermic, positive):
4 × C–H = 4 × 414 = 1656 kJ
2 × O=O = 2 × 498 = 996 kJ
Total broken = 2652 kJ
Bonds formed (exothermic, negative):
2 × C=O = 2 × 804 = 1608 kJ
4 × O–H = 4 × 463 = 1852 kJ
Total formed = 3460 kJ
ΔH = bonds broken − bonds formed = 2652 − 3460 = −808 kJ mol-1
Tip: A negative result confirms the reaction is exothermic, as expected for combustion.
Average vs Exact Bond Enthalpy Values
Bond enthalpy values in the data booklet are averages compiled across hundreds of diverse molecules. The actual bond strength in any given molecule depends on surrounding electron density and steric environment. Calculations using bond enthalpies give approximate answers, not exact thermochemical values.
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