What Are Enthalpy Changes? | 什么是焓变？

📚 What Are Enthalpy Changes? | 什么是焓变？

Enthalpy changes are central to A-Level thermochemistry. They describe the heat energy absorbed or released by a chemical system at constant pressure. Understanding enthalpy changes allows chemists to compare fuel energy, explain reaction feasibility, and design industrial processes.

焓变是 A-Level 热化学的核心内容。它描述化学体系在恒压条件下吸收或释放的热能。理解焓变可以帮助化学家比较燃料能量、解释反应可行性并设计工业流程。

In this article, we define enthalpy, distinguish exothermic from endothermic changes, explore standard conditions, examine calorimetry and Hess’s law, and review the main enthalpy changes tested by Cambridge International A-Level Chemistry.

本文将定义焓，区分放热与吸热变化，介绍标准条件、量热法和盖斯定律，并复习剑桥国际 A-Level 化学考查的主要焓变类型。

1. Defining Enthalpy and Enthalpy Change | 焓与焓变的定义

Enthalpy, symbol H, is the total heat content of a system at constant pressure. It cannot be measured directly, but changes in enthalpy can be determined experimentally.

焓（符号 H）是体系在恒压下的总热含量。虽然焓本身无法直接测量，但焓的变化可以通过实验测定。

The enthalpy change, symbol ΔH, is the heat transferred when a reaction takes place at constant pressure. It is calculated as the enthalpy of the products minus the enthalpy of the reactants:

焓变（符号 ΔH）是反应在恒压条件下转移的热量，等于生成物的焓减去反应物的焓：

ΔH = H(products) − H(reactants)

The unit of enthalpy change is usually kilojoules per mole, kJ mol⁻¹. A negative ΔH means heat is released to the surroundings; a positive ΔH means heat is absorbed from the surroundings.

焓变的单位通常为千焦每摩尔（kJ mol⁻¹）。ΔH 为负表示向环境释放热量；ΔH 为正表示从环境吸收热量。

2. System, Surroundings and Energy Transfer | 体系、环境与能量转移

In thermochemistry, the system is the chemical reaction being studied. The surroundings are everything else, including the solvent, container and laboratory.

在热化学中，体系指被研究的化学反应；环境指其他一切，包括溶剂、容器和实验室。

Energy transfer is observed as a temperature change in the surroundings. At constant pressure, the heat gained or lost by the system is equal to the enthalpy change, ΔH.

能量转移表现为环境的温度变化。在恒压条件下，体系获得或失去的热量等于焓变 ΔH。

Because a calorimeter measures the heat change in the surroundings, the sign of ΔH for the system is reversed when calculating from measured temperature change.

由于量热计测量的是环境的热量变化，在通过温度变化计算时，体系的 ΔH 符号需要反过来。

3. Exothermic and Endothermic Reactions | 放热反应与吸热反应

An exothermic reaction releases energy to the surroundings, so ΔH is negative. Common examples are combustion of fuels, neutralisation and respiration.

放热反应向环境释放能量，因此 ΔH 为负。常见的例子包括燃料燃烧、中和反应和呼吸作用。

An endothermic reaction absorbs energy from the surroundings, so ΔH is positive. Common examples are thermal decomposition of carbonates and photosynthesis.

吸热反应从环境吸收能量，因此 ΔH 为正。常见例子包括碳酸盐的热分解和光合作用。

The sign of ΔH refers to the system, not the surroundings. An exothermic reaction makes the surroundings warmer; an endothermic reaction makes the surroundings cooler.

ΔH 的符号是针对体系而不是环境。放热反应使环境变热；吸热反应使环境变冷。

4. Standard Enthalpy Changes and Standard States | 标准焓变与标准状态

To compare enthalpy changes fairly, chemists use standard conditions. A standard enthalpy change is shown by the symbol ΔH°.

为了公平比较焓变，化学家使用标准条件。标准焓变用符号 ΔH° 表示。

Standard conditions are: a pressure of 100 kPa, a temperature of 298 K, solutions at 1.00 mol dm⁻³, and substances in their standard states.

标准条件为：压强 100 kPa，温度 298 K，溶液浓度为 1.00 mol dm⁻³，且物质处于标准状态。

The standard state is the most stable physical form of a substance under standard conditions. For example, carbon is graphite, oxygen is O₂(g), and water is H₂O(l).

标准状态是物质在标准条件下最稳定的

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