📚 Gibbs Free Energy | GCSE WJEC 化学：吉布斯自由能 考点精讲
In GCSE Chemistry, you learn that exothermic reactions release heat and endothermic reactions absorb heat. But have you ever wondered why some exothermic reactions do not happen at room temperature? The answer lies in a powerful concept called Gibbs free energy, which combines enthalpy, entropy and temperature to predict whether a reaction is feasible. This article will guide you through the key ideas, equations and calculations you need for your WJEC GCSE exams.
在 GCSE 化学中，你学到了放热反应释放热量，吸热反应吸收热量。但你是否想过，为什么有些放热反应在室温下不会发生？答案在于一个强大的概念——吉布斯自由能，它结合了焓、熵和温度来预测反应是否可行。本文将带你掌握 WJEC GCSE 考试所需的核心概念、方程和计算。
1. From Energy Changes to Feasibility | 从能量变化到可行性
At GCSE level, you already know that chemical reactions involve energy changes. However, energy change alone does not determine whether a reaction will proceed. For a reaction to be feasible, the overall change in a quantity called Gibbs free energy (ΔG) must be negative. This idea builds on your existing knowledge of exothermic and endothermic processes.
在 GCSE 阶段，你已经知道化学反应涉及能量变化。然而，仅凭能量变化并不能决定反应是否会发生。要使反应可行，一个称为吉布斯自由能（ΔG）的量的总变化必须为负。这一概念建立在你对放热和吸热过程已有知识的基础上。
2. Revisiting Enthalpy Change (ΔH) | 回顾焓变 (ΔH)
Enthalpy change (ΔH) is the heat energy transferred during a reaction at constant pressure. In WJEC GCSE, you calculate ΔH using bond energies: ΔH = total energy absorbed to break bonds – total energy released when making bonds. A negative ΔH indicates an exothermic reaction; a positive ΔH indicates an endothermic reaction.
焓变（ΔH）是在恒压条件下反应过程中传递的热量。在 WJEC GCSE 中，你使用键能计算 ΔH：ΔH = 断裂键吸收的总能量 – 形成键释放的总能量。负的 ΔH 表示放热反应；正的 ΔH 表示吸热反应。
3. Introducing Entropy (ΔS) | 引入熵 (ΔS)
Entropy (S) is a measure of disorder or randomness of a system. A gas has higher entropy than a liquid, which has higher entropy than a solid. When a reaction produces more gas molecules or more disordered products, the entropy change (ΔS) is positive. GCSE students often encounter entropy as the ‘messiness’ of particles.
熵（S）是系统无序度或随机性的量度。气体的熵高于液体，液体的熵高于固体。当反应产生更多气体分子或更无序的产物时，熵变（ΔS）为正值。GCSE 学生通常将熵理解为粒子的“混乱度”。
4. The Gibbs Free Energy Equation | 吉布斯自由能方程
Gibbs free energy change (ΔG) is given by the equation:
吉布斯自由能变化（ΔG）由以下方程给出：
ΔG = ΔH – TΔS
where ΔH is the enthalpy change, T is the absolute temperature in kelvin (K), and ΔS is the entropy change. This equation links energy, disorder and temperature to determine spontaneity.
其中 ΔH 是焓变，T 是开尔文绝对温度 (K)，ΔS 是熵变。该方程将能量、无序度和温度联系起来，以确定反应的自发性。
5. Units and Calculation of ΔG | ΔG 的单位与计算
In typical GCSE problems, ΔH is given in kJ mol⁻¹ and ΔS in J K⁻¹ mol⁻¹. You must convert ΔS to kJ K⁻¹ mol⁻¹ by dividing by 1000 before calculation. The resulting ΔG will have units of kJ mol⁻¹. Always check and convert units carefully.
在典型的 GCSE 题目中，ΔH 的单位为 kJ mol⁻¹，ΔS 为 J K⁻¹ mol⁻¹。在计算前必须将 ΔS 除以 1000 转换为 kJ K⁻¹ mol⁻¹。计算得到的 ΔG 单位为 kJ mol⁻¹。务必仔细检查并转换单位。
6. Sign of ΔG and Feasibility | ΔG 的符号与可行性
A reaction is feasible (can occur) if ΔG < 0. If ΔG > 0, the reaction is not feasible under those conditions. When ΔG = 0, the system is at equilibrium. Remember: negative Gibbs free energy change is the thermodynamic condition for a spontaneous reaction at constant temperature and pressure.
如果 ΔG < 0，反应可行（可以发生）。如果 ΔG > 0，在该条件下反应不可行。当 ΔG = 0 时，系统处于平衡状态。记住：在恒温恒压下，吉布斯自由能变化为负是反应自发热力学条件。
7. Effect of Temperature: Sign Combinations | 温度的影响：ΔH 和 ΔS 符号组合
The feasibility of a reaction depends on the signs of ΔH and ΔS, and the temperature. There are four possibilities:
反应的可行性取决于 ΔH 和 ΔS 的符号以及温度。有四种可能性：
| ΔH | ΔS | ΔG | Feasibility |
|---|---|---|---|
| – | + | Always – | Feasible at all T |
| + | – | Always + | Never feasible |
| – | – | – at low T, + at high T | Feasible only at low T |
| + | + | + at low T, – at high T | Feasible only at high T |
When ΔH is negative and ΔS is positive, the reaction is feasible at all temperatures. When both are positive, feasibility requires high temperature to make the TΔS term dominate and drive ΔG negative.
当 ΔH 为负且 ΔS 为正时，反应在所有温度下均可行。当两者均为正时，需要高温使 TΔS 项占主导，从而使 ΔG 为负，反应才可行。
8. Worked Example: Ice Melting | 实例计算：冰融化
Consider the melting of ice: H₂O(s) → H₂O(l). ΔH = +6.01 kJ mol⁻¹, ΔS = +22.0 J K⁻¹ mol⁻¹. Convert ΔS to 0.0220 kJ K⁻¹ mol⁻¹. At 298 K, ΔG = 6.01 – 298(0.0220) = 6.01 – 6.556 = –0.546 kJ mol⁻¹ (approx –0.5 kJ mol⁻¹). Since ΔG < 0, melting is feasible at 298 K (25 °C). This explains why ice melts at room temperature.
考虑冰的熔化：H₂O(s) → H₂O(l)。ΔH = +6.01 kJ mol⁻¹，ΔS = +22.0 J K⁻¹ mol⁻¹。将 ΔS 转换为 0.0220 kJ K⁻¹ mol⁻¹。在 298 K 时，ΔG = 6.01 – 298(0.0220) = 6.01 – 6.556 = –0.546 kJ mol⁻¹（约 –0.5 kJ mol⁻¹）。由于 ΔG < 0，熔化在 298 K（25 °C）可行。这解释了为什么冰在室温下会融化。
9. Worked Example: Decomposition of Calcium Carbonate | 实例计算：碳酸钙分解
The decomposition of CaCO₃(s) → CaO(s) + CO₂(g) has ΔH = +178 kJ mol⁻¹ and ΔS = +161 J K⁻¹ mol⁻¹. Convert ΔS to 0.161 kJ K⁻¹ mol⁻¹. We find the temperature at which ΔG = 0: 178 – T(0.161) = 0 → T = 178 / 0.161 ≈ 1106 K (833 °C). Above this temperature, ΔG < 0 and decomposition becomes feasible. This matches industrial lime production conditions.
碳酸钙分解：CaCO₃(s) → CaO(s) + CO₂(g)，ΔH = +178 kJ mol⁻¹，ΔS = +161 J K⁻¹ mol⁻¹。将 ΔS 转换为 0.161 kJ K⁻¹ mol⁻¹。求 ΔG = 0 时的温度：178 – T(0.161) = 0 → T = 178 / 0.161 ≈ 1106 K（833 °C）。高于此温度，ΔG < 0，分解变得可行。这与工业石灰生产条件一致。
10. Graphical Interpretation: ΔG vs Temperature | 图解：ΔG 与温度的关系
For a given reaction, ΔH and ΔS are approximately constant with temperature. The equation ΔG = ΔH – TΔS is a straight line with gradient –ΔS and y-intercept ΔH. Plotting ΔG against T helps visualise the temperature at which ΔG changes sign. When ΔS is positive, the line slopes downward; when ΔS is negative, the line slopes upward.
对于给定反应，ΔH 和 ΔS 随温度变化不大。方程 ΔG = ΔH – TΔS 是一条直线，斜率为 –ΔS，y 截距为 ΔH。绘制 ΔG 对 T 的图有助于直观了解 ΔG 改变符号的温度。当 ΔS 为正时，直线向下倾斜；ΔS 为负时，直线向上倾斜。
11. Gibbs Free Energy and Activation Energy | 吉布斯自由能与活化能
A negative ΔG tells us a reaction is thermodynamically feasible, but it does not guarantee that the reaction will happen at an observable rate. Kinetics matters too. Many reactions with ΔG < 0 still require a high activation energy or a catalyst. For example, combustion of diamond has ΔG < 0 but is extremely slow at room temperature.
负的 ΔG 告诉我们反应在热力学上可行，但并不保证反应会以可观察的速率发生。动力学也同样重要。许多 ΔG < 0 的反应仍需要较高的活化能或催化剂。例如，金刚石的燃烧 ΔG < 0，但在室温下极其缓慢。
12. Summary and Exam Tips | 总结与应试技巧
- Memorise the equation: ΔG = ΔH – TΔS and know how to use it.
- 牢记方程：ΔG = ΔH – TΔS 并知道如何使用。
- Watch the units: Convert ΔS from J K⁻¹ mol⁻¹ to kJ K⁻¹ mol⁻¹.
- 注意单位：将 ΔS 从 J K⁻¹ mol⁻¹ 换算为 kJ K⁻¹ mol⁻¹。
- Temperature in kelvin: Add 273 to Celsius temperatures.
- 温度用开尔文：摄氏温度加 273。
- Interpret the result: ΔG < 0 means feasible; ΔG > 0 means not feasible.
- 解读结果：ΔG < 0 表示可行；ΔG > 0 表示不可行。
- Link to entropy: More gas molecules usually mean positive ΔS.
- 联系熵：气体分子越多，通常 ΔS 为正。
- Practice the temperature calculation: Be able to find the temperature at which feasibility changes (ΔG = 0).
- 练习温度计算：能够求出可行性转变的温度（ΔG = 0）。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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