平均键焓Mean bond enthalpies
键焓是在气态下断裂 1 mol 某种化学键所需的能量。除双原子分子外，表中数值是该键在多种化合物中的平均值，因此用来估算反应焓变时只能得到近似结果。估算式为 ΔH ≈ Σ（断裂的键焓）− Σ（生成的键焓）。
Bond enthalpy is the energy needed to break one mole of a given bond in the gas phase. Except for diatomic molecules, the values are averages over many compounds, so reaction enthalpies estimated from them are approximate: ΔH ≈ Σ(bonds broken) − Σ(bonds formed).
键焓 / kJ mol⁻¹Bond enthalpy / kJ mol⁻¹
含氢的键 Bonds to hydrogen
碳的单键 Carbon single bonds
多重键 Multiple bonds
其他单键 Other single bonds
数值取自通用化学教材中的常见数据，不同资料可能相差几 kJ mol⁻¹。Values follow a common general-chemistry table; other sources may differ by a few kJ mol⁻¹.
说明Notes
C=O 键焓随化合物不同而变化：表中 799 kJ mol⁻¹ 为 CO₂ 中的数值，醛、酮中通常取 745 kJ mol⁻¹ 左右。
The C=O value varies between compounds: 799 kJ mol⁻¹ is the value in CO₂; about 745 kJ mol⁻¹ is typical for aldehydes and ketones.
键越短，键焓通常越大：卤化氢的键长从 H–F 到 H–I 依次增大，键焓依次减小。
Shorter bonds are usually stronger: bond length increases from H–F to H–I and the bond enthalpy falls in the same order.
双键并不等于两个单键之和：C=C（614）小于 2 × C–C（694），因为 π 键比 σ 键弱。
A double bond is not twice a single bond: C=C (614) is less than 2 × C–C (694), because the π bond is weaker than the σ bond.