\[\Delta_rH^\circ = \sum\nu\,\Delta_fH^\circ(\text{产物}) - \sum\nu\,\Delta_fH^\circ(\text{反应物})\]\[\Delta_rH^\circ = \sum\nu\,\Delta_fH^\circ(\text{products}) - \sum\nu\,\Delta_fH^\circ(\text{reactants})\]
由标准燃烧焓（有机物常用）from standard combustion enthalpies
\[\Delta_rH^\circ = \sum\nu\,\Delta_cH^\circ(\text{反应物}) - \sum\nu\,\Delta_cH^\circ(\text{产物})\]\[\Delta_rH^\circ = \sum\nu\,\Delta_cH^\circ(\text{reactants}) - \sum\nu\,\Delta_cH^\circ(\text{products})\]含义Meaning
标准生成焓是由稳定单质生成 1 mol 物质的标准焓变。用生成焓数据表可计算任意反应的标准焓变。
The standard enthalpy of formation is for making 1 mol of a substance from its elements in their standard states; tables of ΔfH° give any reaction’s ΔrH°.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(\Delta_fH^\circ\) | 标准摩尔生成焓（稳定单质为 0）standard molar enthalpy of formation (0 for elements) | kJ·mol⁻¹ |
| \(\Delta_cH^\circ\) | 标准摩尔燃烧焓standard enthalpy of combustion | kJ·mol⁻¹ |
推导Derivation
设想把反应物先分解为单质（−ΣνΔfH°反应物），再由单质生成产物（+ΣνΔfH°产物），由盖斯定律相加。
Decompose reactants into elements, then form products; add by Hess’s law.
燃烧焓：把反应物和产物都燃烧到 CO₂、H₂O，方向相反。
With combustion enthalpies the signs reverse.
适用条件与局限Assumptions & limitations
注意物态（H₂O(l) 与 H₂O(g) 的 ΔfH° 差 44 kJ/mol）；数据对应 298.15 K。
Watch phases; data are for 298.15 K.
例题Worked example
CH₄ + 2O₂ → CO₂ + 2H₂O(l)：ΔrH° = (−393.5) + 2(−285.8) − (−74.8) = −890.3 kJ/mol。
Methane combustion: −890.3 kJ/mol.