\[\Delta_\text{vap}S = \frac{\Delta_\text{vap}H}{T_b} \approx 85\text{–}88\ \mathrm{J\,mol^{-1}\,K^{-1}}\]
含义Meaning
许多非极性液体在正常沸点的汽化熵近似相同（特鲁顿，1884），可由沸点估算汽化焓。形成氢键的液体（水 109、乙醇 110）偏大，因为液体更有序。
Many non-polar liquids have about the same vaporisation entropy at their normal boiling point (Trouton, 1884), so boiling points estimate ΔvapH. Hydrogen-bonded liquids (water 109, ethanol 110) are higher because the liquid is more ordered.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(T_b\) | 正常沸点normal boiling point | K |
推导Derivation
汽化时体积从液体变为同温同压气体，增加约 10³ 倍，对应的位置熵增大致相同。
Vaporisation expands volume ~10³-fold, giving similar positional entropy gains.
所以 \(\Delta H_\text{vap} \approx 88T_b\)。
So ΔvapH ≈ 88T_b.
适用条件与局限Assumptions & limitations
氢键液体、极低沸点（H₂、He）和缔合气体（乙酸蒸气二聚）偏离明显。
Fails for H-bonded liquids, quantum liquids and associating vapours.
例题Worked example
苯 T_b = 353 K：ΔvapH ≈ 88×353 = 31 kJ/mol（实测 30.7）。
Benzene: ΔvapH ≈ 31 kJ/mol (observed 30.7).