定义Definition
标准生成焓是在标准状态下（通常 298 K、100 kPa），由处于标准态的稳定单质生成 1 mol 某化合物时的焓变，符号 ΔfH°。稳定单质（如 O₂(g)、C(石墨)）的标准生成焓规定为零。
The standard enthalpy of formation, ΔfH°, is the enthalpy change when one mole of a compound forms from its elements in their standard states under standard conditions, usually 298 K and 100 kPa. Elements in their standard states, such as O₂(g) and graphite, have ΔfH° = 0 by definition.
数值与意义Values and meaning
常见数值（kJ mol⁻¹）：H₂O(l) −285.8，CO₂(g) −393.5，CH₄(g) −74.8，NH₃(g) −46.1，NaCl(s) −411.2。数值越负，化合物相对于组成元素越稳定。书写定义方程式时生成物必须为 1 mol，可出现分数系数，如 H₂(g) + ½O₂(g) → H₂O(l)。
Typical values (kJ mol⁻¹): H₂O(l) −285.8, CO₂(g) −393.5, CH₄(g) −74.8, NH₃(g) −46.1, NaCl(s) −411.2. The more negative, the more stable the compound relative to its elements. The defining equation must make one mole of product, so fractions can appear: H₂(g) + ½O₂(g) → H₂O(l).
用于计算反应焓Calculating reaction enthalpies
根据盖斯定律：ΔrH° = ΣΔfH°(生成物) − ΣΔfH°(反应物)。例：CH₄ + 2O₂ → CO₂ + 2H₂O(l)，ΔrH° = (−393.5 + 2 × (−285.8)) − (−74.8) = −890.3 kJ mol⁻¹。
By Hess's law, ΔrH° = ΣΔfH°(products) − ΣΔfH°(reactants). For CH₄ + 2O₂ → CO₂ + 2H₂O(l): ΔrH° = (−393.5 + 2 × (−285.8)) − (−74.8) = −890.3 kJ mol⁻¹.
年表Timeline
- 1840
盖斯定律发表，使生成焓的间接测定成为可能
Hess's law makes indirect determination of enthalpies of formation possible
- 1982
IUPAC 建议把标准压强由 101.325 kPa 改为 100 kPa
IUPAC recommends changing the standard pressure from 101.325 kPa to 100 kPa
相关公式Formulas
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.