Gibbs Free Energy | 吉布斯自由能

📚 Gibbs Free Energy | 吉布斯自由能

Gibbs free energy is a thermodynamic potential that measures the maximum reversible work a system can perform at constant temperature and pressure. It is a central concept in chemical thermodynamics, used to predict the spontaneity of reactions and the position of equilibrium.

吉布斯自由能是一种热力学势，衡量系统在恒温恒压下所能做的最大可逆功。它是化学热力学的核心概念，用于预测反应的自发性和平衡位置。

1. What is Gibbs Free Energy? | 什么是吉布斯自由能？

Gibbs free energy, symbol G, combines enthalpy and entropy into a single value. It is defined by the equation G = H – TS, where H is enthalpy, T is absolute temperature in kelvin, and S is entropy. The change in Gibbs free energy for a process is given by ΔG = ΔH – TΔS at constant temperature.

吉布斯自由能，符号为 G，将焓和熵结合为一个值。其定义为 G = H – TS，其中 H 是焓，T 是开尔文绝对温度，S 是熵。对于恒温过程，吉布斯自由能的变化由 ΔG = ΔH – TΔS 给出。

The sign of ΔG determines whether a reaction is spontaneous: a negative ΔG indicates a spontaneous forward reaction, a positive ΔG means the reaction is non-spontaneous under those conditions, and if ΔG = 0, the system is at equilibrium.

ΔG 的符号决定反应是否自发：ΔG 为负表示正反应自发，ΔG 为正表示该条件下反应非自发，若 ΔG = 0，则系统处于平衡状态。

2. The Fundamental Equation: ΔG = ΔH – TΔS | 基本方程

The fundamental Gibbs equation is:

基本吉布斯方程为：

ΔG = ΔH – TΔS

ΔH is the enthalpy change (heat absorbed or released), ΔS is the entropy change (measure of disorder), and T is the kelvin temperature. This equation applies at constant temperature and pressure.

ΔH 是焓变（吸收或放出的热量），ΔS 是熵变（无序度的量度），T 是开尔文温度。该方程适用于恒温恒压条件。

Values must use consistent units: ΔH typically in kJ mol⁻¹, ΔS in J K⁻¹ mol⁻¹, so when calculating ΔG, convert ΔS to kJ K⁻¹ mol⁻¹ or ΔH to J mol⁻¹. In many exam problems, you calculate ΔG directly from given standard formation data or via this equation.

数值必须使用一致的单位：ΔH 通常为 kJ mol⁻¹，ΔS 为 J K⁻¹ mol⁻¹，因此在计算 ΔG 时，需将 ΔS 转换为 kJ K⁻¹ mol⁻¹ 或将 ΔH 转换为 J mol⁻¹。在许多考试题中，你需要根据给定的标准生成数据或通过该方程直接计算 ΔG。

3. Enthalpy and Entropy Contributions | 焓和熵的贡献

Enthalpy (ΔH) reflects the heat change: exothermic reactions (negative ΔH) tend to be favourable, while endothermic reactions (positive ΔH) are unfavourable. Entropy (ΔS) measures the dispersal of energy and matter: an increase in entropy (positive ΔS) favours spontaneity, while a decrease (negative ΔS) opposes it.

焓（ΔH）反映热量变化：放热反应（ΔH 为负）倾向有利，吸热反应（ΔH 为正）则不利。熵（ΔS）衡量能量和物质的分散程度：熵增加（ΔS 为正）有利于自发，熵减少（ΔS 为负）则不利于自发。

The TΔS term scales the entropy contribution by temperature: at high temperatures, the entropy term becomes more significant, potentially overriding an unfavourable enthalpy change.

TΔS 项通过温度来缩放熵的贡献：在高温下，熵项变得更加重要，可能压倒不利的焓变。

The following table summarises the combined effect on spontaneity:

下表总结了综合效应对自发性的影响：

4. Standard Gibbs Free Energy Change, ΔG° | 标准吉布斯自由能变

The standard Gibbs free energy change, ΔG°, refers to the ΔG when all reactants and products are in their standard states (1 bar pressure for gases, 1 mol dm⁻³ concentration for solutions, pure solids/liquids) at a specified temperature, usually 298 K.

标准吉布斯自由能变 ΔG° 是指所有反应物和产物均处于标准状态（气体压力为 1 bar，溶液浓度为 1 mol dm⁻³，纯固体/液体）时在指定温度（通常为 298 K）下的 ΔG。

ΔG° can be calculated from standard free energies of formation (ΔG°_f) using the formula:

ΔG° 可由标准生成自由能（

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