📚 Gibbs Free Energy Exam Essentials | 吉布斯自由能考点精讲
Understanding Gibbs free energy is crucial for predicting the spontaneity of chemical reactions and their equilibrium positions. This revision guide covers key concepts required for IB and AQA Chemistry, including the Gibbs equation, temperature dependence, relationship with equilibrium constants, and electrochemical applications. You'll learn how to calculate ΔG, interpret its sign, and apply the principles to exam-style questions.
理解吉布斯自由能对于预测化学反应的自发性及其平衡位置至关重要。本复习指南涵盖IB和AQA化学所需的核心概念，包括吉布斯方程、温度依赖性、与平衡常数的关系以及电化学应用。你将学习如何计算ΔG、解释其符号，并将这些原理应用于考试题型。
1. Introduction to Gibbs Free Energy | 吉布斯自由能简介
s thermodynamically favorable at constant temperature and pressure. It is defined as G = H – TS, where T is the absolute temperature in kelvin.
吉布斯自由能（G）结合了焓（H）和熵（S），用以确定在恒温恒压下反应是否在热力学上有利。其定义为 G = H – TS，其中 T 是以开尔文为单位的绝对温度。
G = H – TS
s is spontaneous, while a positive ΔG means it is non-spontaneous. At equilibrium, ΔG = 0.
过程的自由能变化 ΔG 是关键指标：ΔG 为负（ΔG < 0）意味着过程是自发的，ΔG 为正意味着非自发。在平衡时，ΔG = 0。
2. The Gibbs Equation: ΔG = ΔH – TΔS | 吉布斯方程：ΔG = ΔH – TΔS
The change in Gibbs free energy for a reaction is given by ΔG = ΔH – and ΔG in kJ mol⁻¹, and ΔS in J K⁻¹ mol⁻¹, so remember to convert ΔS to kJ K⁻¹ mol⁻¹ by dividing by 1000.
反应的吉布斯自由能变化由 ΔG = ΔH – TΔS 给出，其中 ΔH 是焓变，ΔS 是熵变，T 是以开尔文为单位的温度。所有项的单位必须一致：通常 ΔH 和 ΔG 使用 kJ mol⁻¹，而 ΔS 使用 J K⁻¹ mol⁻¹，因此务必记得将 ΔS 除以 1000 转换为 kJ K⁻¹ mol⁻¹。
ΔG = ΔH – TΔS
Standard conditions (298 K, 100 kPa, 1 mol dm⁻³ solutions) yield the standard free‑ hange, denoted ΔG°. For example, for the Haber process: N₂(g) + 3H₂(g) → 2NH₃(g), if ΔH° = –92.4 kJ mol⁻¹ and ΔS° = –198.3 J K⁻¹ mol⁻¹, at 298 K: ΔG° = –92.4 – [298 × (–198.3/1000)] = –92.4 – (–59.1) = – at the reaction is spontaneous under standard conditions, despite the decrease in entropy.
标准条件（298 K，100 kPa，1 mol dm⁻³ 溶液）下得到标准自由能变，记作 ΔG°。例如哈伯法：N₂(g) + 3H₂(g) → 2NH₃(g)，若 ΔH° = –92.4 kJ mol⁻¹，ΔS° = –198.3 J K⁻¹ mol⁻¹，则在 298 K 时：ΔG° = –92.4 – [298 × (–198.3/1000)] = –92.4 – (–59.1) = –33.3 kJ mol⁻¹。负的 ΔG° 表明即使熵减少，该反应在标准条件下仍是自发的。
3. Spontaneity and Sign of ΔG | 自发性和 ΔG 的符号
is at equilibrium; no net reaction occurs. A positive ΔG means the forward reaction is non‑spontaneous – however, the reverse reaction would be spontaneous.
要使反应在给定条件下自发（热力学可行），ΔG 必须为负。若 ΔG = 0，系统处于平衡状态，没有净反应发生。正的 ΔG 表示正向反应非自发——然而，逆反应将是自发的。
Importantly, spontaneity does not say anything about the rate of reaction: a thermodynamically favorable process can be extremely slow due to high activation energy. For instance, the combustion of diamond is spontaneous at room temperature but does not occur noticeably.
重要的是，自发性与反应速率无关：热力学上有利的过程可能由于高活化能而极其缓慢。例如，金刚石的燃烧在室温下是自发的，但并不会明显发生。
Always distinguish between 'spontaneous' and 'fast'. Exam commentary questions often ask you to explain why a reaction with ΔG < 0 still needs heating or a catalyst.
务必区分“自发”与“快速”。考试中的评论题经常要求解释为什么 ΔG < 0 的反应仍然需要加热或催化剂。
4. Effect of Temperature on Spontaneity | 温度对自发性的影响
on the balance between ΔH and ΔS, heavily influenced by temperature. By analyzing ΔG = ΔH – TΔS, you can predict how changing temperature switches spontaneity.
ΔG 的符号取决于 ΔH 与 ΔS 的平衡，并受温度的强烈影响。通过分析 ΔG = ΔH – TΔS，你可以预测温度变化如何改变自发性。
| ΔH sign | ΔS sign | Sign of ΔG = ΔH – TΔS | Spontaneity |
|---|---|---|---|
| – (exothermic) | + (more disorder) | Always – | Always spontaneous |
| – | – (less disorder) | – at low T; + at high T | Spontaneous only at low temperatures |
| + (endothermic) | + | + at low T; – at high T | Spontaneous only at high temperatures |
| + | – | Always + | Never spontaneous |
ulate the crossover temperature where ΔG = 0, i.e., T = ΔH / ΔS (with ΔS in kJ K⁻¹ mol⁻¹).
此表总结了 ΔH 和 ΔS 的组合如何决定自发性的温度窗口。你还可以计算出 ΔG = 0 的转折温度，即 T = ΔH / ΔS（ΔS 以 kJ K⁻¹ mol⁻¹ 计）。
Published by TutorHao | IB Chemistry Revision Series | aleveler.com
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