Standard Enthalpy of Formation: Concepts and Calculations | 标准生成焓的概念与计算

📚 Standard Enthalpy of Formation: Concepts and Calculations | 标准生成焓的概念与计算

Standard enthalpy of formation (ΔHf°) is a cornerstone concept in thermochemistry and a key tool for calculating reaction enthalpies without direct measurement. In this article, we will explore its definition, conventions, and application to numerical problems.

标准生成焓(ΔHf°)是热化学中的基石概念，也是无需直接测量即可计算反应焓变的关键工具。本文将探讨其定义、约定及数值计算中的应用。

1. Standard Conditions and Standard States | 标准条件与标准状态

Standard enthalpy values are defined under standard conditions: a pressure of 100 kPa, a specified temperature (usually 298 K), and all substances in their standard states. The standard state of a pure solid or liquid is the pure substance under standard pressure; for a gas, it is the pure gas behaving ideally at standard pressure; for a solution, the standard state is a concentration of 1 mol dm⁻³.

标准焓值是在标准条件下定义的：压力为100 kPa，指定温度（通常为298 K），且所有物质均处于其标准状态。纯固体或液体的标准状态是标准压力下的纯物质；气体是标准压力下具有理想行为的纯气体；溶液的标准状态是浓度为1 mol dm⁻³的溶液。

These conditions are not to be confused with STP (273 K, 100 kPa). Standard enthalpy changes are usually reported at 298 K, but the pressure remains 100 kPa.

这些条件不应与STP（273 K，100 kPa）混淆。标准焓变通常在298 K下报告，但压力保持为100 kPa。

2. Definition of Standard Enthalpy of Formation | 标准生成焓的定义

The standard enthalpy of formation (ΔHf°) of a compound is the enthalpy change that occurs when one mole of the compound is formed from its constituent elements in their standard states under standard conditions.

化合物的标准生成焓（ΔHf°）是指在一定条件下，由标准状态下的组成元素生成一摩尔该化合物时的焓变。

For example, the formation of liquid water from hydrogen and oxygen gases:

例如，由氢气和氧气生成液态水的反应：

H₂(g) + ½O₂(g) → H₂O(l) ΔHf° = −285.8 kJ mol⁻¹

Note that the product must be exactly one mole, and the reactants are elements in their standard states (H₂ as gas, O₂ as gas).

注意产物必须恰好为一摩尔，反应物为处于标准状态的元素（氢为气体，氧为气体）。

3. Why Is ΔHf° of Elements Zero? | 为什么元素的标准生成焓为零？

By convention, the standard enthalpy of formation of an element in its most stable form is assigned a value of zero. For example, ΔHf° of graphite (C, s) is 0 kJ mol⁻¹, whereas diamond (C, s) has a non-zero

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