\[\Delta_rH = \sum_i \Delta H_i\ (\text{各步之和，与路径无关})\]\[\Delta_rH = \sum_i \Delta H_i\ (\text{sum of steps , path-independent})\]
含义Meaning
化学反应的焓变只取决于始态和终态，与反应经过的途径无关（盖斯，1840）。因此可由已知反应的焓变加减求出难以直接测定的反应焓。
A reaction’s enthalpy change depends only on initial and final states, not on the route (Hess, 1840), so unmeasurable reaction enthalpies can be found by combining known ones.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(\Delta H_i\) | 各步反应焓变enthalpy of each step | kJ·mol⁻¹ |
推导Derivation
焓是状态函数，其变化只由始末态决定。
Enthalpy is a state function.
热化学方程式可像代数式一样相加减（ΔH 同步加减），乘以系数时 ΔH 同倍乘，反写时 ΔH 变号。
Thermochemical equations combine algebraically, with ΔH scaled and signed accordingly.
适用条件与局限Assumptions & limitations
各步须在相同条件（温度、压强、物态）下比较。
Steps must refer to the same conditions and phases.
例题Worked example
C(石墨) + ½O₂ → CO 无法直接测：由 C + O₂ → CO₂（−393.5）减去 CO + ½O₂ → CO₂（−283.0）得 −110.5 kJ/mol。
C + ½O₂ → CO: −393.5 − (−283.0) = −110.5 kJ/mol.