Edexcel A-Level Chemistry Topic 13.9: Entropy, Free Energy and Reaction Feasibility | Edexcel A-Level 化学 13.9：熵、自由能与反应可行性

📚 Edexcel A-Level Chemistry Topic 13.9: Entropy, Free Energy and Reaction Feasibility | Edexcel A-Level 化学 13.9：熵、自由能与反应可行性

Entropy and Gibbs free energy are central to Edexcel A-Level Chemistry Topic 13.9. This topic explains why some exothermic reactions do not happen, why some endothermic reactions can go at high temperature, and what ‘feasible’ really means in a thermodynamic sense. It links enthalpy, entropy, temperature and free energy into one quantitative tool for predicting whether a reaction is possible under standard conditions.

熵和吉布斯自由能是 Edexcel A-Level 化学 13.9 的核心内容。本主题解释了为什么有些放热反应不会发生，为什么有些吸热反应在高温下可以进行，以及从热力学角度看“可行”究竟意味着什么。它将焓、熵、温度和自由能整合为一个定量工具，用来预测反应在标准条件下是否可能发生。

1. Why Topic 13.9 Matters | 为什么 13.9 很重要

Topic 13.9 is the final part of the Energetics II topic in the Edexcel A-Level Chemistry specification. It pulls together enthalpy changes, entropy changes and temperature to introduce the Gibbs free energy equation. Exam questions regularly ask you to calculate ΔG⦵, explain the temperature at which a reaction becomes feasible, or discuss limitations of thermodynamic predictions.

13.9 是 Edexcel A-Level 化学课程“能量学 II”中的最后一部分。它把焓变、熵变和温度联系起来，引入吉布斯自由能方程。考试题经常要求你计算 ΔG⦵、解释反应变得可行的温度，或讨论热力学预测的局限性。

You should be able to use the equation ΔG⦵ = ΔH⦵ – TΔS⦵ in both directions: calculate ΔG⦵ from data, and rearrange to find the temperature when ΔG⦵ = 0. You also need to explain why a thermodynamically feasible reaction might not actually happen, which is a classic high-mark exam point.

你应能双向使用方程 ΔG⦵ = ΔH⦵ – TΔS⦵：既要从数据计算 ΔG⦵，也要会变形求出 ΔG⦵ = 0 时的温度。你还需要解释为什么热力学上可行的反应可能实际上不会发生，这是典型的高分考点。

2. Defining Entropy (S) | 定义熵 S

Entropy, given the symbol S, is a measure of the disorder of a system. More precisely, it is related to the number of possible ways that energy and particles can be arranged. A system with greater entropy has more available microstates and is more disordered.

熵用符号 S 表示，是系统混乱程度的量度。更准确地说，它与能量和粒子可能排列方式的数量有关。系统的熵越大，可用的微观状态越多，系统越混乱。

At A-Level, use simple comparisons: gases have higher entropy than liquids, and liquids have higher entropy than solids. A reaction that produces more gas molecules usually has a positive entropy change. For example, CaCO₃(s) → CaO(s) + CO₂(g) has a positive ΔS because one solid is converted into a solid and a gas, increasing disorder.

在 A-Level 考试中，可以使用简单比较：气体的熵高于液体，液体的熵高于固体。产生更多气体分子的反应通常熵变为正。例如，CaCO₃(s) → CaO(s) + CO₂(g) 的 ΔS 为正值，因为一种固体转化为一种固体和一种气体，混乱程度增大。

3. Standard Entropy and Units | 标准熵与单位

Standard entropy, S⦵, is the entropy of one mole of a substance under standard conditions: 100 kPa pressure, 298 K temperature, and 1 mol dm⁻³ for solutions. Entropy is usually quoted in J K⁻¹ mol⁻¹, so be careful to convert when combining with enthalpy values in kJ mol⁻¹.

标准熵 S⦵ 是一摩尔物质在标准条件下的熵：压力 100 kPa，温度 298 K，溶液浓度为 1 mol dm⁻³。熵通常以 J K⁻¹ mol⁻¹ 表示，因此与 kJ mol⁻¹ 的焓值结合时要注意单位换算。

Unlike enthalpy, entropy has an absolute scale. The third law of thermodynamics states that a perfect crystal at 0 K has zero entropy. This means standard entropies are absolute values, not changes relative to an arbitrary zero.

与焓不同，熵有绝对标度。热力学第三定律指出，0 K 时完美晶体的熵为零。这意味着标准熵是绝对值，不是相对于任意零点的变化值。

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