定义Definition
盖斯定律指出：化学反应的焓变只取决于反应的始态和终态，与反应经过的途径无关。因此可以把难以直接测定的反应焓变，由一组已知反应的焓变相加减得到。
Hess's law states that the enthalpy change of a reaction depends only on the initial and final states, not on the route taken. Enthalpy changes that are hard to measure can therefore be found by adding and subtracting those of known reactions.
常用计算方法Common methods
由生成焓：ΔH = ΣΔfH(生成物) − ΣΔfH(反应物)，单质的标准生成焓为零。由燃烧焓：ΔH = ΣΔcH(反应物) − ΣΔcH(生成物)。例如无法直接测定 C(s) + 2H₂(g) → CH₄(g) 的焓变，可由 C、H₂、CH₄ 的燃烧焓求得 ΔfH(CH₄) = −393.5 + 2 × (−285.8) − (−890.3) = −74.8 kJ mol⁻¹。
From enthalpies of formation: ΔH = ΣΔfH(products) − ΣΔfH(reactants), with ΔfH of elements zero. From enthalpies of combustion: ΔH = ΣΔcH(reactants) − ΣΔcH(products). The enthalpy of C(s) + 2H₂(g) → CH₄(g) cannot be measured directly, but from combustion data ΔfH(CH₄) = −393.5 + 2 × (−285.8) − (−890.3) = −74.8 kJ mol⁻¹.
原理与应用Basis and uses
盖斯定律是能量守恒（热力学第一定律）的直接结果，因为焓是状态函数。它是玻恩–哈伯循环求晶格能、用键能估算反应热以及各种热化学循环计算的基础。
Hess's law follows from conservation of energy, the first law of thermodynamics, because enthalpy is a state function. It underlies the Born–Haber cycle for lattice energies, estimating reaction enthalpies from bond energies, and all thermochemical cycle calculations.
年表Timeline
- 1840
盖斯在圣彼得堡发表热量总和不变定律
Hess publishes the law of constant heat summation in St Petersburg
- 1847
亥姆霍兹系统阐述能量守恒定律
Helmholtz sets out the conservation of energy
相关公式Formulas
例题Worked example
NaCl：−411 = 107 + 496 + 122 − 349 + U_L → U_L = −787 kJ/mol。
NaCl: −411 = 107 + 496 + 122 − 349 + U_L → U_L = −787 kJ/mol.
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.