📚 Oxford AQA International A-level Chemistry A2 Physical Unit 4: Thermodynamics, Kinetics & Equilibria | 牛津AQA国际A-Level化学A2物理单元4：热力学、动力学与平衡
This guide is written for the Oxford AQA International A-level Chemistry A2 Physical Unit 4 topic test. It consolidates definitions, equations, sign conventions and common calculation methods for thermodynamics, kinetics, equilibrium, acid-base chemistry and electrochemistry.
本指南为牛津AQA国际A-Level化学A2物理单元4主题测试而编写，系统梳理热力学、动力学、平衡、酸碱化学与电化学中的定义、公式、符号约定与常见计算方法。
1. Unit Overview | 单元概览
Unit 4 focuses on physical chemistry that builds on AS ideas. Expect questions on lattice enthalpy, entropy, Gibbs free energy, rate equations, Kp, buffers and electrode potentials, often combined with practical data.
单元4聚焦于在AS基础上拓展的物理化学内容。考试题目通常围绕晶格焓、熵、吉布斯自由能、速率方程、Kp、缓冲溶液和电极电势，并常结合实验数据。
Questions may require you to use Hess's law, analyse kinetic graphs, calculate pH of buffers, or decide whether a redox reaction is feasible from standard electrode potentials.
考题可能要求你运用赫斯定律、分析动力学图像、计算缓冲溶液的pH，或根据标准电极电势判断氧化还原反应是否可行。
2. Lattice Enthalpy and Born-Haber Cycles | 晶格焓与玻恩-哈伯循环
Lattice enthalpy is the enthalpy change when one mole of an ionic solid is formed from its gaseous ions. It is always exothermic, so values are negative.
晶格焓是指由气态离子生成一摩尔离子固体时的焓变。该过程总是放热，因此数值为负。
A Born-Haber cycle applies Hess's law to the formation of an ionic compound. For NaCl the cycle relates atomisation, ionisation, bond dissociation, electron affinity and lattice enthalpy.
玻恩-哈伯循环将赫斯定律应用于离子化合物的生成。对NaCl而言，该循环联系原子化、电离、键解离、电子亲和与晶格焓。
ΔH°f = ΔH°at(Na) + IE(Na) + ½ΔH°diss(Cl₂) + EA(Cl) + ΔH°latt
If the experimental lattice enthalpy is more negative than the theoretical value, the bonding has additional covalent character due to polarisation of the anion by a small, highly charged cation.
若实验晶格焓比理论值更负，说明由于小半径、高电荷阳离子对阴离子的极化作用，键中带有额外共价成分。
3. Enthalpy of Solution and Hydration | 溶解焓与水合焓
Enthalpy of solution is the enthalpy change when one mole of an ionic solute dissolves in a large excess of water to give an infinitely dilute solution.
溶解焓是一摩尔离子溶质在大量水中溶解形成无限稀释溶液时的焓变。
Hydration enthalpy is the enthalpy change when one mole of gaseous ions is surrounded by water molecules. It is exothermic for both cations and anions.
水合焓是一摩尔气态离子被水分子包围时的焓变。对阳离子和阴离子而言该过程均为放热。
ΔH°soln = −ΔH°latt + ΔH°hyd(cation) + ΔH°hyd(anion)
The cycle is useful because you can calculate one unknown enthalpy if the others are known. Smaller, more highly charged ions have more exothermic hydration enthalpies because they attract water molecules more strongly.
该循环可用来在已知其他焓变时计算某一未知焓变。离子半径越小、电荷越高，水合焓越负，因为离子吸引水分子的能力更强。
4. Entropy and Gibbs Free Energy | 熵与吉布斯自由能
Entropy measures disorder or the dispersal of energy. Gases have higher molar entropy than liquids, and liquids have higher molar entropy than solids.
熵用于衡量系统混乱度或能量分散程度。气体的摩尔熵高于液体，液体高于固体。
For a reaction to be feasible, the total entropy change of the system and surroundings must be positive or zero.
反应可以自发进行时，体系与环境的熵变总和必须大于等于零。
ΔS°total = ΔS°system − ΔH/T
Gibbs free energy change links enthalpy, entropy and temperature. A reaction is feasible when ΔG is negative or zero.
吉布斯自由能变将焓变、熵变与温度联系。当ΔG小于等于零时反应可行。
ΔG = ΔH − TΔS
When ΔH and ΔS have the same sign, temperature becomes the deciding factor. For example, an endothermic reaction with a positive ΔS can become feasible at high temperature.
当ΔH和ΔS同号时，温度成为决定因素。例如，吸热且ΔS为正的反应在高温下可能变得可行。
5. Rate Equations and Reaction Order | 速率方程与反应级数
The rate equation links reaction rate to reactant concentrations. The orders m and n are determined by experiment and are not generally equal to the stoichiometric coefficients.
速率方程将反应速率与反应物浓度联系起来。反应级数m和n由实验测定，一般不等于化学计量系数。
rate = k[A]ᵐ[B]ⁿ
Use initial rates or concentration-time graphs to determine order. Zero order gives a linear decrease in [A]; first order gives a constant half-life; second order gives a steeper curve at high concentration.
可使用初始速率法或浓度-时间图判断反应级数。零级反应[A]随时间线性下降；一级反应半衰期恒定；二级反应曲线在高浓度时下降更快。
| Overall order | Units of k |
| 0 | mol dm⁻³ s⁻¹ |
| 1 | s⁻¹ |
| 2 | mol⁻¹ dm³ s⁻¹ |
| 3 | mol⁻² dm⁶ s⁻¹ |
For first order reactions, half-life is independent of concentration. This gives a useful diagnostic test: constant half-life means first order.
对于一级反应，半衰期与浓度无关。这提供了一个有用的判断方法：半衰期恒定意味着反应为一级。
6. Arrhenius Equation and Rate-Determining Step | 阿伦尼乌斯方程与速率控制步骤
The rate constant increases with temperature because a greater fraction of molecules have energy equal to or greater than the activation energy.
速率常数随温度升高而增大，因为更多分子具有大于或等于活化能的能量。
k = Ae−Eₐ/RT
The logarithmic form gives a straight line when ln k is plotted against 1/T. The slope is −Eₐ/R.
使用对数形式，将ln k对1/T作图可得直线，斜率为−Eₐ/R。
ln k = ln A − Eₐ/(RT)
The rate-determining step must include species that appear in the rate equation. If a reactant does not appear in the rate equation,
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