📚 AQA International A-level Chemistry Unit 3 (CH03) Core Revision: Thermodynamics, Equilibria, Redox and Transition Metals | AQA 国际 A-level 化学第三单元 CH03 核心考点精讲：热力学、平衡、氧化还原与过渡金属
This revision guide covers the essential physical and inorganic chemistry topics examined in AQA International A-level Chemistry Unit 3 (CH03). It brings together energetics, entropy, equilibrium constants, acid-base equilibria, electrode potentials, transition metal chemistry, kinetics and periodicity. Use the worked equations and paired explanations to consolidate your understanding and prepare for data-analysis, calculation and extended-response questions.
本复习指南涵盖 AQA 国际 A-level 化学第三单元（CH03）的核心物理化学与无机化学考点。内容包括能量学、熵、平衡常数、酸碱平衡、电极电势、过渡金属化学、动力学与周期性。利用给出的方程式与中英对照讲解，巩固理解，并为数据分析、计算与论述题做好准备。
1. Energetics and Hess's Law | 能量学与盖斯定律
Enthalpy change (ΔH) is the heat change at constant pressure. Standard conditions are 100 kPa, 298 K and 1 mol dm⁻³ for solutions. Standard enthalpy changes include ΔHf°, ΔHc°, ΔHat°, ΔHion°, ΔHea° and lattice enthalpy ΔHLE°.
焓变（ΔH）是恒压下的热量变化。标准条件为 100 kPa、298 K 和 1 mol dm⁻³（溶液）。标准焓变包括标准生成焓 ΔHf°、标准燃烧焓 ΔHc°、标准原子化焓 ΔHat°、标准电离焓 ΔHion°、标准电子亲和焓 ΔHea° 以及晶格焓 ΔHLE°。
Hess's Law states that the total enthalpy change for a reaction is independent of the route taken. This allows you to construct Born-Haber cycles for ionic compounds and calculate lattice enthalpy from experimental data.
盖斯定律指出，反应的总焓变与反应路径无关。因此可以构建离子化合物的 Born-Haber 循环，并利用实验数据计算晶格焓。
ΔHf° = ΔHat° + ΔHion° + ΔHea° + ΔHLE°
For NaCl, the cycle includes atomisation of Na(s) and Cl₂(g), ionisation of Na(g), electron affinity of Cl(g) and lattice formation. Always check whether the question uses electron affinity as a positive or negative value, because sign conventions differ between exam boards and textbooks.
对于 NaCl，循环包括 Na(s) 和 Cl₂(g) 的原子化、Na(g) 的电离、Cl(g) 的电子亲和以及晶格形成。务必确认题目中电子亲和的符号约定，因为不同考试局和教材的正负号可能不同。
2. Entropy and Gibbs Free Energy | 熵与吉布斯自由能
Entropy (S) is a measure of disorder or the number of ways energy can be distributed. Gases have higher entropy than liquids and solids, and more particles generally mean higher entropy.
熵（S）表示系统的混乱度或能量分布方式的数目。气体的熵高于液体和固体；通常粒子数越多，熵越高。
The total entropy change for a process is the sum of the system entropy change and the surroundings entropy change. A reaction is feasible when total entropy change is positive.
过程的总熵变等于系统熵变与周围环境熵变之和。当总熵变为正值时，反应可以自发进行。
ΔStotal = ΔSsystem − ΔH/T
erature: ΔG = ΔH − TΔS. A reaction is feasible when ΔG is negative; at equilibrium ΔG = 0.
吉布斯自由能将在恒温下的焓变与熵变结合：ΔG = ΔH − TΔS。当 ΔG 为负值时反应可行；平衡时 ΔG = 0。
ΔG = ΔH − TΔS
Temperature can change feasibility when ΔH and ΔS have the same sign. If ΔH is positive and ΔS is positive, the reaction becomes feasible at high temperature where TΔS outweighs ΔH.
当 ΔH 与 ΔS 同号时，温度可以改变反应的自发性。若 ΔH 为正且 ΔS 为正，则高温下 TΔS 超过 ΔH，反应变得可行。
3. Chemical Equilibria and Equilibrium Constants | 化学平衡与平衡常数
Dynamic equilibrium occurs when forward and reverse rates are equal and concentrations remain constant. Homogeneous equilibria involve a single phase; heterogeneous equilibria involve different phases but pure solids and liquids are omitted from K expressions.
动态平衡在正逆反应速率相等、各物质浓度保持不变时建立。均相平衡只涉及一个相；非均相平衡涉及多个相，但纯固体和纯液体不写入平衡常数表达式。
For the general reaction aA + bB ⇌ cC + dD, the equilibrium constant Kc uses concentrations and Kp uses partial pressures. Partial pressure of a gas is its mole fraction multiplied by total pressure.
对于一般反应 aA + bB ⇌ cC + dD，平衡常数 Kc 用浓度表示，Kp 用分压表示。气体的分压等于其摩尔分数乘以总压。
pA = xA × Ptotal
Kp = (pCc pDd) / (pAa pBb)
Only temperature changes the value of K. Increasing temperature shifts the equilibrium in the endothermic direction, increasing K for endothermic forward reactions and decreasing K for exothermic forward reactions. Catalysts do not affect K because they speed up both forward and reverse reactions equally.
只有温度能改变 K 的数值。升高温度使平衡向吸热方向移动：正向吸热时 K 增大，正向放热时 K 减小。催化剂不影响 K，因为它同等程度地加快正逆反应速率。
4. Acid-Base Equilibria, pH and Buffers | 酸碱平衡、pH 与缓冲溶液
Brønsted-Lowry acids are proton donors and bases are proton acceptors. Strong acids dissociate fully in water, while weak acids dissociate partially and are described by the acid dissociation constant Ka.
Brønsted-Lowry 酸是质子给予体，碱是质子接受体。强酸在水中完全解离，弱酸部分解离，其解离程度用酸解离常数 Ka 描述。
Ka = [H⁺][A⁻] / [HA]
t 298 K. pH is defined as −log₁₀[H⁺], so a tenfold increase in [H⁺] lowers pH by one unit.
水发生微弱的自偶电离：Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ mol² dm⁻⁶（298 K）。pH 定义为 −log₁₀[H⁺]，因此 [H⁺] 增大十倍，pH 降低一个单位。
pH = −log₁₀[H⁺]
Acidic buffer solutions contain a weak acid and its conjugate base, for example ethanoic acid and sodium ethanoate. They resist pH changes when small amounts of acid or base are added. The pH of an acidic buffer can be calculated from Ka and the ratio of conjugate base to weak acid.
酸性缓冲溶液由弱酸及其共轭碱组成，例如乙酸和乙酸钠。加入少量酸或碱时，缓冲溶液能抵抗 pH 变化。酸性缓冲溶液的 pH 可由 Ka 以及共轭碱与弱酸的浓度比计算。
pH = pKa + log₁₀([A⁻] / [HA])
5. Redox Equilibria and Electrode Potentials | 氧化还原平衡与电极电势
Oxidation is loss of electrons and reduction is gain of electrons. Oxidation states are assigned using electronegativity: elements in their standard state have oxidation state 0, fluorine is always −1, oxygen is usually −2 except in peroxides, and hydrogen is usually +1 except in metal hydrides.
氧化是失去电子，还原是得到电子。氧化态根据电负性确定：单质氧化态为 0，氟总是 −1，氧通常为 −2（过氧化物除外），氢通常为 +1（金属氢化物除外）。
A half-cell is made by placing a metal in a solution of its own ions or by using a platinum electrode with a gas or an ion mixture. The standard hydrogen electrode (SHE) has a potential of 0.00 V under standard conditions.
半电池由金属与其离子溶液构成，或用铂电极与气体或离子混合物构成。标准氢电极（SHE）在标准条件下的电极电势为 0.00 V。
E°cell = E°(right-hand electrode) − E°(left-hand electrode)
When both half-equations are written as reductions, the cell potential is the difference between the more positive reduction potential and the less positive reduction potential. A positive E°cell means the reaction is thermodynamically feasible under standard conditions.
当两个半反应都写成还原形式时，电池电动势等于更正极还原电势减去较负极还原电势。E°cell 为正值表示该反应在标准条件下在热力学上可行。
Electrode potentials are intensive properties and are not multiplied by stoichiometric coefficients. Kinetic barriers may prevent a reaction with positive E°cell from occurring at a noticeable rate, so feasibility does not guarantee the reaction proceeds quickly.
电极电势是强度性质，不随化学计量系数翻倍。动力学障碍可能使 E°cell 为正的反应速率极慢，因此热力学可行并不保证反应快速进行。
6. Transition Metals and Complex Ions | 过渡金属与配位离子
Transition metals are d-block elements that form at least one stable ion with a partially filled d subshell. Their electron configurations show characteristic exceptions at chromium and copper because a half-filled or fully filled d subshell is more stable.
过渡金属是 d 区元素，它们至少能形成一种具有部分填充 d 亚层的稳定离子。铬和铜的电子构型出现特征性例外，因为半充满或全充满的 d 亚层更稳定。
Cr: [Ar] 3d⁵ 4s¹ Cu: [Ar] 3d¹⁰ 4s¹
A complex ion consists of a central metal ion surrounded by ligands. Ligands are electron-pair donors: water, ammonia and chloride ions are common monodentate ligands. Coordination number is the number of coordinate bonds; common shapes are octahedral (coordination number 6), tetrahedral (4) and square planar (4).
配位离子由中心金属离子和围绕其周围的配体组成。配体是电子对给予体：水、氨和氯离子是常见的单齿配体。配位数是配位键的数目；常见形状为八面体（配位数 6）、四面体（4）和平面正方形（4）。
| Complex ion | Colour | Shape |
| [Cu(H₂O)₆]²⁺ | Blue | Octahedral |
| [Fe(H₂O)₆]²⁺ | Pale green | Octahedral |
| [Fe(H₂O)₆]³⁺ | Yellow-brown | Octahedral |
| [Co(H₂O)₆]²⁺ | Pink | Octahedral |
Colour in transition metal complexes arises from d-d electron transitions. When visible light is absorbed, an electron is promoted from a lower energy d orbital to a higher energy d orbital; the transmitted light shows the complementary colour.
过渡金属配离子的颜色来自 d-d 电子跃迁。吸收可见光后，电子从较低能量的 d 轨道跃迁到较高能量的 d 轨道；透射光呈互补色。
7. Ligand Substitution and Isomerism in Complexes | 配体取代与配位化合物中的同分异构现象
Ligand substitution occurs when one ligand is replaced by another. For example, adding excess ammonia to [Cu(H₂O)₆]²⁺ gives [Cu(NH₃)₄(H₂O)₂]²⁺, with a distinctive deep blue colour.
配体取代发生在一种配体被另一种配体替换时。例如，向 [Cu(H₂O)₆]²⁺ 中加入过量氨水，得到 [Cu(NH₃)₄(H₂O)₂]²⁺，呈特征的深蓝色。
The chelate effect explains why multidentate ligands such as ethane-1,2-diamine or EDTA form more stable complexes than similar monodentate ligands. Substitution reactions that increase the number of particles are entropically favourable.
螯合效应解释了为什么乙二胺、EDTA 等多齿配体形成的配合物比类似的单齿配体更稳定。使粒子数增加的取代反应在熵上有优势。
Octahedral complexes can show cis-trans isomerism when two different ligands each occupy two positions. Octahedral complexes with three bidentate ligands, such as [Co(en)₃]³⁺, can show optical isomerism because the mirror images are non-superimposable.
八面体配合物若两种不同配体各占据两个位置，则可能出现顺反异构。具有三个双齿配体的八面体配合物，如 [Co(en)₃]³⁺，能表现出旋光异构，因为其镜像无法重叠。
8. Kinetics and Rate Equations | 动力学与速率方程
m and n are the orders with respect to each reactant and are determined experimentally, not from the stoichiometric coefficients.
速率方程表示初始速率与反应物浓度的关系：rate = k[A]ᵐ[B]ⁿ。指数 m 和 n 分别是各反应物的反应级数，必须由实验确定，不能从化学计量系数直接得出。
The rate constant k is only constant at a fixed temperature. Arrhenius equation links k to temperature and activation energy.
速率常数 k 仅在温度一定时为常数。阿伦尼乌斯方程将 k 与温度和活化能联系起来。
k = A e−Eₐ/RT
ln k = ln A − Eₐ/RT
A plot of ln k against 1/T gives a straight line with gradient −Eₐ/R. Increasing temperature increases k because a larger proportion of molecules have energy equal to or greater than the activation energy.
以 ln k 对 1/T 作图得到直线，斜率为 −Eₐ/R。升高温度使 k 增大，因为更多分子具有大于或等于活化能的能量。
9. Periodicity: Period 3 Oxides and Chlorides | 周期性：第三周期氧化物与氯化物
Period 3 oxides show a transition from basic to acidic character across the period. Na₂O and MgO are basic; Al₂O₃ is amphoteric; SiO₂ is weakly acidic; P₄O₁₀, SO₂ and SO₃ are acidic.
第三周期氧化物从左到右表现出从碱性向酸性过渡的趋势。Na₂O 和 MgO 为碱性；Al₂O₃ 为两性；SiO₂ 为弱酸性；P₄O₁₀、SO₂ 和 SO₃ 为酸性。
Na₂O + H₂O → 2NaOH
MgO + H₂O → Mg(OH)₂
P₄O₁₀ + 6H₂O → 4H₃PO₄
SO₃ + H₂O → H₂SO₄
Aluminium oxide reacts with both acids and alkalis, showing amphoteric behaviour. With hot aqueous sodium
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