📚 Gibbs Free Energy – AS Chemistry Revision | 吉布斯自由能 – AS化学考点精讲
Gibbs free energy, named after J. Willard Gibbs, is a central concept in AS Chemistry that helps us predict whether a chemical reaction is feasible under a given set of conditions. By combining enthalpy and entropy changes, it provides a single thermodynamic criterion: a reaction is feasible if the change in Gibbs free energy (ΔG) is negative. This does not tell us about the rate of reaction, only its thermodynamic possibility.
吉布斯自由能以 J. Willard Gibbs 命名，是 AS 化学的核心概念，用于判断化学反应在特定条件下是否可行。它将焓变与熵变结合起来，给出了一个统一的热力学判据：当吉布斯自由能变 ΔG 为负时，反应是可行的。这仅给出热力学上的可能性，不涉及反应速率。
1. What is Gibbs Free Energy? | 什么是吉布斯自由能？
Gibbs free energy (G) is a thermodynamic state function defined as G = H − TS, where H is enthalpy, T is the absolute temperature in kelvin, and S is entropy. For a chemical reaction at constant temperature, the change in Gibbs free energy is given by ΔG = ΔH − TΔS. ΔG has units of kJ mol⁻¹.
吉布斯自由能 (G) 是一个热力学状态函数，定义为 G = H − TS，其中 H 是焓，T 是热力学温度（开尔文），S 是熵。对于恒温下的化学反应，吉布斯自由能变由 ΔG = ΔH − TΔS 给出。ΔG 的单位是 kJ mol⁻¹。
It is essential to understand that ΔG only tells us about thermodynamic feasibility, not about kinetics. A reaction with a negative ΔG may still be extremely slow, and a positive ΔG only means the forward reaction is not feasible under the specified conditions.
必须明白 ΔG 只说明热力学可行性，不涉及动力学。ΔG 为负的反应也有可能极其缓慢；ΔG 为正仅意味着在指定条件下正向反应不可行。
2. The Gibbs Free Energy Equation | 吉布斯自由能方程式
The key equation you must memorise for AS Chemistry is:
ΔG = ΔH − TΔS
where ΔH is the enthalpy change (kJ mol⁻¹), T is the temperature in kelvin (K), and ΔS is the entropy change (J K⁻¹ mol⁻¹). Because the units of ΔH and ΔS differ, you must be careful to convert ΔS into kJ K⁻¹ mol⁻¹ (÷1000) before substituting into the equation.
AS 化学必须牢记的关键方程式是：ΔG = ΔH − TΔS，其中 ΔH 是焓变 (kJ mol⁻¹)，T 是温度 (K)，ΔS 是熵变 (J K⁻¹ mol⁻¹)。由于 ΔH 与 ΔS 的单位不同，代入前务必先将 ΔS 转换为 kJ K⁻¹ mol⁻¹（除以 1000）。
Under standard conditions (298 K, 100 kPa), equation becomes ΔG° = ΔH° − TΔS°. When calculating, use the data given and always write the equation first to remind yourself of the relationship.
在标准条件下 (298 K, 100 kPa)，方程写作 ΔG° = ΔH° − TΔS°。计算时请先写出该方程，以提醒自己各项之间的关系。
3. Relating ΔG, ΔH and ΔS | ΔG、ΔH 与 ΔS 的关系
ΔG represents the balance between the enthalpy change and the temperature‑multiplied entropy change. Favorable factors for a negative ΔG are: an exothermic reaction (ΔH negative) and an increase in entropy (ΔS positive). However, these factors compete; a large exothermic ΔH can overcome an unfavorable entropy decrease, especially at low temperatures, while a large positive ΔS can make a reaction feasible at high temperatures even if it is endothermic.
ΔG 体现了焓变与温度乘以熵变之间的平衡。有利于 ΔG 为负的因素是：放热反应 (ΔH 为负) 和熵增加 (ΔS 为正)。然而这两个因素会相互竞争；较大的放热 ΔH 能克服不利的熵减小，尤其在低温下；而较大的正 ΔS 即使在吸热反应中，也能在高温下使反应变得可行。
In short, the term TΔS becomes more influential as temperature rises, which is the basis for predicting temperature‑dependent feasibility.
简而言之，随着温度升高，TΔS 项的影响增大，这是预测温度依赖可行性的基础。
4. Feasibility Criteria | 反应可行性判据
For a reaction at constant temperature and pressure:
- ΔG < 0: The reaction is thermodynamically feasible (spontaneous) in the forward direction.
- ΔG < 0：正向反应在热力学上可行（自发）。
- ΔG > 0: The forward reaction is not feasible; the reverse reaction would be feasible.
- ΔG > 0：正向反应不可行；逆向反应则可行。
- ΔG = 0: The system is at equilibrium; there is no net tendency to change.
- ΔG = 0：体系处于平衡状态，没有净变化趋势。
Note that feasibility does not guarantee a reaction will happen at an observable rate. A catalyst cannot change ΔG; it only lowers activation energy to make a feasible reaction faster.
注意，可行并不保证反应会以可观察的速率进行。催化剂不能改变 ΔG，它只能降低活化能，使可行的反应变快。
5. Temperature Dependence | 温度对反应自发性的影响
The term TΔS can completely alter the sign of ΔG, especially when ΔH and ΔS carry the same sign. At low temperatures, the TΔS term is small, so ΔG ≈ ΔH. At high temperatures, TΔS becomes large and can dominate over ΔH, potentially reversing the sign of ΔG.
温度项 TΔS 能彻底改变 ΔG 的符号，尤其是在 ΔH 和 ΔS 符号相同时。低温下，TΔS 项很小，因而 ΔG ≈ ΔH。高温下，TΔS 变大且可能超过 ΔH，从而逆转 ΔG 的符号。
Therefore, a reaction that is exothermic but has a negative ΔS may be feasible only at low temperatures, while an endothermic reaction with a positive ΔS may become feasible only at high temperatures.
因此，放热但 ΔS 为负的反应可能仅在低温下可行，而吸热且 ΔS 为正的反应则可能仅在高温下可行。
6. Combining Signs of ΔH and ΔS | ΔH 和 ΔS 的符号组合
The following table summarises how the signs of ΔH and ΔS determine feasibility at different temperatures. This is a frequent AS exam topic.
下表总结了 ΔH 与 ΔS 的符号如何决定反应在不同温度下的可行性，这是 AS 常见考点。
| ΔH | ΔS | Feasibility (Spontaneity) | 可行性（自发性） |
| Negative (–) | Positive (+) | Always feasible at any temperature | 任何温度下均可行 |
| Negative (–) | Negative (–) | Feasible only at low temperatures (T < ΔH/ΔS) | 仅在低温下可行 (T < ΔH/ΔS) |
| Positive (+) | Positive (+) | Feasible only at high temperatures (T > ΔH/ΔS) | 仅在高温下可行 (T > ΔH/ΔS) |
| Positive (+) | Negative (–) | Never feasible at any temperature | 任何温度下均不可行 |
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