\[H = U + pV,\qquad \Delta H = q_p\ (\text{恒压、只做体积功})\]\[H = U + pV,\qquad \Delta H = q_p\ (\text{const. p , pV work only})\]
含义Meaning
焓是为方便恒压过程而定义的状态函数：恒压下吸收的热等于焓变。化学反应多在敞口（恒压）条件下进行，所以反应热通常用 ΔH 表示。
Enthalpy is a state function defined so that heat at constant pressure equals ΔH. Most reactions run at constant pressure, so reaction heats are given as ΔH.
符号说明Symbols
| 符号Symbol | 含义 Meaning | 单位 Unit |
|---|---|---|
| \(H\) | 焓enthalpy | J |
| \(q_p\) | 恒压热heat at constant pressure | J |
推导Derivation
恒压只做体积功：\(\Delta U = q_p - p\Delta V\)。
At constant pressure with only expansion work, ΔU = q_p − pΔV.
移项 \(q_p = \Delta U + p\Delta V = \Delta(U + pV) = \Delta H\)。
Rearranging gives q_p = Δ(U + pV) = ΔH.
同理恒容 \(q_V = \Delta U\)，弹式量热计测的是 ΔU。
Similarly q_V = ΔU (bomb calorimetry).
适用条件与局限Assumptions & limitations
焓本身没有绝对值，只有差值有意义（规定稳定单质标准生成焓为零）。
Only differences matter; elements in standard states have ΔfH° = 0.