焓Enthalpy

由热力学第一定律 ΔU = q + w，恒压下 w = −pΔV，所以 q_p = ΔU + pΔV = ΔH。ΔH < 0 为放热反应，ΔH > 0 为吸热反应。焓的绝对值无法测定，只能测量变化值；规定稳定单质在标准状态下的标准生成焓为零，由此建立各物质的 ΔfH° 数据表。

From the first law, ΔU = q + w, and at constant pressure w = −pΔV, so q_p = ΔU + pΔV = ΔH. ΔH < 0 means an exothermic reaction and ΔH > 0 an endothermic one. Absolute enthalpies cannot be measured, only changes; by setting the standard enthalpy of formation of stable elements to zero, tables of ΔfH° for every substance are built.

吉布斯在 1875 年前后的论著中使用了“恒压热函数”。荷兰物理学家卡末林·昂内斯在 1909 年前后提出 enthalpy 一词，源自希腊语 enthalpein（加热其中）。1922 年波特把符号 H 引入，此后通行。中文“焓”是根据“含热”之意新造的字。

Gibbs used a 'heat function for constant pressure' in his work around 1875. The Dutch physicist Kamerlingh Onnes coined the word enthalpy around 1909, from Greek enthalpein, to warm in. Porter introduced the symbol H in 1922, and it became standard. The Chinese character 焓 was newly created from the idea of 'containing heat'.

盖斯定律指出反应焓变只与始末状态有关，可由已知反应组合计算未知反应焓变：ΔrH° = ΣνΔfH°(产物) − ΣνΔfH°(反应物)。基尔霍夫定律给出焓变随温度的变化：d(ΔH)/dT = ΔCp。焓与熵共同决定吉布斯自由能 G = H − TS，从而判断反应方向。

Hess's law says the enthalpy change depends only on initial and final states, so unknown values follow from known reactions: ΔrH° = ΣνΔfH°(products) − ΣνΔfH°(reactants). Kirchhoff's law gives the temperature dependence, d(ΔH)/dT = ΔCp. Together with entropy, enthalpy fixes the Gibbs energy G = H − TS, which decides the direction of a reaction.

反应焓变用量热计测定：弹式量热计测恒容热 ΔU，再换算为 ΔH = ΔU + ΔnRT；溶液反应用恒压量热计（咖啡杯量热计）直接得到 ΔH。焓数据用于燃料热值、化工过程能量衡算和材料稳定性评估。

Reaction enthalpies are measured by calorimetry: a bomb calorimeter gives the constant-volume heat ΔU, converted by ΔH = ΔU + ΔnRT; a constant-pressure (coffee-cup) calorimeter gives ΔH directly for solution reactions. Enthalpy data are used for fuel heating values, energy balances in chemical plants and assessments of material stability.