\[\Delta_\text{trs}S = \frac{\Delta_\text{trs}H}{T_\text{trs}}\]
含义Meaning
在相变温度（熔点、沸点）下可逆相变的熵变等于相变焓除以相变温度。熔化、汽化、升华熵均为正。
For a reversible transition at its transition temperature, ΔS = ΔH/T; fusion, vaporisation and sublimation entropies are positive.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(\Delta_\text{trs}H\) | 相变焓enthalpy of transition | J·mol⁻¹ |
| \(T_\text{trs}\) | 相变温度transition temperature | K |
推导Derivation
相变点两相平衡，过程可逆且恒温恒压，\(q_\text{rev} = \Delta H\)。
At the transition point the process is reversible with q_rev = ΔH.
\(\Delta S = q_\text{rev}/T\)。
ΔS = q_rev/T.
适用条件与局限Assumptions & limitations
只在平衡相变温度成立；过冷水结冰等非平衡相变需设计可逆途径计算。
Only at the equilibrium temperature; supercooled freezing needs a reversible path.
例题Worked example
水汽化：ΔS = 40.7×10³/373 = 109 J/(mol·K)；冰融化：6.01×10³/273 = 22.0 J/(mol·K)。
Water: ΔvapS = 109, ΔfusS = 22.0 J mol⁻¹ K⁻¹.