📚 Electrochemistry Key Points for IB & AQA Chemistry | IB与AQA化学电化学考点精讲
Electrochemistry bridges the study of redox reactions and electricity, forming a cornerstone of both IB and AQA Chemistry curricula. From predicting spontaneity using electrode potentials to calculating yields in electrolysis, these concepts are essential for mastering chemical energetics and practical applications like batteries and corrosion control.
电化学架起了氧化还原反应与电能之间的桥梁，是IB和AQA化学课程的核心基石。从利用电极电势预测反应自发性到计算电解产量，这些概念对于掌握化学能量学以及电池、腐蚀控制等实际应用至关重要。
1. Redox Reactions and Oxidation Numbers | 氧化还原反应与氧化数
A redox reaction involves simultaneous reduction (gain of electrons) and oxidation (loss of electrons). The oxidation number (or state) is a bookkeeping tool that tracks electron transfer: it increases upon oxidation and decreases upon reduction.
氧化还原反应同时包含还原（得电子）和氧化（失电子）。氧化数（或氧化态）是一种记录电子转移的工具：氧化时氧化数升高，还原时氧化数降低。
The oxidation number of an atom in a free element is zero; for a monatomic ion it equals the charge; in compounds, hydrogen is usually +1, oxygen –2, and the sum of oxidation numbers equals the overall charge.
游离态单质原子的氧化数为零；单原子离子的氧化数等于其所带电荷；在化合物中，氢通常为 +1，氧为 –2，所有原子氧化数的代数和等于总电荷。
Identifying changes in oxidation numbers allows you to determine which species is the oxidising agent (electron acceptor) and which is the reducing agent (electron donor).
通过识别氧化数的变化，可以判断哪个物质是氧化剂（电子接受者），哪个是还原剂（电子给予者）。
2. Balancing Redox Half-Equations | 氧化还原半反应配平
Redox equations are split into oxidation and reduction half-equations. In acidic solution, balance O atoms with H₂O and H atoms with H⁺; in basic solution, add OH⁻ to neutralise H⁺ after balancing as in acid.
氧化还原方程式可拆分为氧化半反应和还原半反应。在酸性溶液中，用 H₂O 平衡 O 原子，用 H⁺ 平衡 H 原子；在碱性溶液中，先按酸性条件配平，再加 OH⁻ 中和 H⁺。
For example, the reduction of MnO₄⁻ to Mn²⁺ in acid: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. Charge and mass must be conserved in each half-equation.
例如，酸性条件下 MnO₄⁻ 还原为 Mn²⁺：MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O。每个半反应必须同时满足电荷守恒与质量守恒。
Combine half-equations so that electrons cancel. The balanced overall equation for the reaction of Fe²⁺ with MnO₄⁻ in acid is: 5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O.
将半反应合并使电子抵消。酸性条件下 Fe²⁺ 与 MnO₄⁻ 反应的总配平方程式为：5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O。
3. Components of Electrochemical Cells | 电化学电池的构成
A simple electrochemical cell (galvanic or voltaic cell) consists of two half-cells connected by a wire and a salt bridge. Each half-cell contains an electrode immersed in an electrolyte solution containing the relevant ion.
简单的电化学电池（原电池或伏打电池）由两个通过导线和盐桥连接的半电池组成。每个半电池包含一个浸在含有关离子的电解质溶液中的电极。
The electrode where oxidation occurs is the anode (negative in a galvanic cell), and the electrode where reduction occurs is the cathode (positive). Electrons flow externally from anode to cathode.
发生氧化的电极是阳极（原电池中为负极），发生还原的电极是阴极（正极）。电子通过外电路从阳极流向阴极。
The salt bridge (e.g., a strip of filter paper soaked in KNO₃) allows ion migration to maintain electrical neutrality, preventing charge build-up that would stop the reaction.
盐桥（如浸有 KNO₃ 的滤纸条）允许离子迁移以维持电中性，防止电荷积累导致反应停止。
4. Standard Hydrogen Electrode and Standard Electrode Potentials | 标准氢电极与标准电极电势
The standard hydrogen electrode (SHE) is the primary reference, assigned a potential of 0.00 V under standard conditions: 1 mol dm⁻³ H⁺, 298 K, 100 kPa H₂ gas, with a platinum electrode.
标准氢电极（SHE）是基准参比电极，在标准条件下（1 mol dm⁻³ H⁺，298 K，100 kPa H₂ 气体，铂电极）其电势定义为 0.00 V。
A standard electrode potential (E°) is measured by connecting the half-cell of interest to the SHE and reading the cell EMF. By convention, the half-cell potential is for the reduction process.
标准电极电势（E°）通过将待测半电池与标准氢电极相连并读取电池电动势来测定。按惯例，半电池电势针对的是还原过程。
Standard conditions ensure comparability: all solutions at 1 mol dm⁻³, gases at 100 kPa, temperature 298 K. A more positive E° implies a greater tendency to be reduced.
标准条件保证可比性：所有溶液浓度为 1 mol dm⁻³，气体压强为 100 kPa，温度为 298 K。E° 值越正，表示该物质越容易被还原。
5. Electrochemical Series and Spontaneity of Redox Reactions | 电化学系列与反应自发性
The electrochemical series arranges half-cells in order of decreasing (or increasing) standard reduction potentials. Species with more positive E° values are stronger oxidising agents.
电化学系列按标准还原电势递减（或递增）的顺序排列半电池。E° 值越正的物质，其氧化性越强。
For a spontaneous redox reaction under standard conditions, the overall cell EMF must be positive. This is calculated as: E°cell = E°cathode – E°anode, where both values are reduction potentials.
在标准条件下，若氧化还原反应能自发进行，则总电池电动势必须为正。计算公式为：E°cell = E°cathode – E°anode，其中两个数值均为还原电势。
For example, Zn²⁺/Zn has E° = –0.76 V and Cu²⁺/Cu has E° = +0.34 V. Connecting them gives E°cell = +0.34 – (–0.76) = +1.10 V, so the reaction Zn + Cu²⁺ → Zn²⁺ + Cu is spontaneous.
例如，Zn²⁺/Zn 的 E° = –0.76 V，Cu²⁺/Cu 的 E° = +0.34 V。将它们连接，E°cell = +0.34 – (–0.76) = +1.10 V，因此反应 Zn + Cu²⁺ → Zn²⁺ + Cu 是自发的。
6. Cell EMF and Gibbs Free Energy | 电池电动势与吉布斯自由能
The thermodynamic feasibility of a redox reaction is linked to the cell EMF by the equation: ΔG = –nFE_cell, where n is the number of moles of electrons transferred and F is the Faraday constant (96 485 C mol⁻¹).
氧化还原反应的热力学可行性通过方程 ΔG = –nFE_cell 与电池电动势关联，其中 n 是转移电子的物质的量，F 是法拉第常数（96 485 C mol⁻¹）。
ΔG° = –nFE°cell
A negative ΔG indicates a spontaneous reaction, corresponding to a positive E°cell. This relationship also allows calculation of equilibrium constants using ΔG° = –RT ln K.
ΔG 为负表示反应自发，对应于 E°cell 为正。此关系还可通过 ΔG° = –RT ln K 计算平衡常数。
For IB HL and AQA students, you may be asked to determine E°cell from ΔG° or vice versa, or to predict the effect of concentration on cell voltage using the Nernst equation.
对于 IB HL 和 AQA 学生，可能需要根据 ΔG° 求算 E°cell 或反之，或者利用能斯特方程预测浓度对电池电压的影响。
7. The Nernst Equation | 能斯特方程
The Nernst equation adjusts the electrode potential for non‑standard conditions. For a reduction half‑reaction: aOx + ne⁻ ⇌ bRed, at 298 K it simplifies to:
能斯特方程用于修正非标准条件下的电极电势。对于还原半反应 aOx + ne⁻ ⇌ bRed，在 298 K 时可简化为：
E = E° – (0.0592 / n) log Q
where Q = [Red]ᵇ / [Ox]ᵃ. If Q = 1, E = E°. The equation can also be written as E = E° – (RT/nF) ln Q.
其中 Q = [Red]ᵇ / [Ox]ᵃ。若 Q = 1，则 E = E°。此方程也可写作 E = E° – (RT/nF) ln Q。
Using the Nernst equation, you can explain why a cell voltage drops during discharge as reactant concentrations change, and how concentration cells produce a voltage from the same half‑cells at different concentrations.
利用能斯特方程，可以解释放电过程中反应物浓度变化导致电池电压下降的原因，以及为何浓度差电池能通过相同半电池的不同浓度产生电压。
8. Electrolysis and Faraday’s Laws | 电解与法拉第定律
Electrolysis uses an external power source to drive a non‑spontaneous redox reaction. It takes place in an electrolytic cell where the anode is positive and the cathode is negative.
电解利用外部电源驱动非自发的氧化还原反应。它在电解池中进行，其中阳极接正极，阴极接负极。
Faraday’s first law: the mass of substance liberated at an electrode is directly proportional to the quantity of electricity passed. Q = It, where I is current in amperes and t is time in seconds.
法拉第第一定律：电极上析出物质的质量与通过的电量成正比。Q = It，其中 I 为电流（安培），t 为时间（秒）。
Faraday’s second law: the number of moles of electrons transferred, n(e⁻) = Q / F. The mass deposited is then m = (n(e⁻) × M) / (number of electrons per ion).
法拉第第二定律：转移电子的物质的量 n(e⁻) = Q / F。然后沉积质量 m = (n(e⁻) × 摩尔质量) / (每个离子的电子数)。
For example, to deposit 1 mole of copper from Cu²⁺, 2F of charge is required. If a current of 2.0 A is passed for 30 minutes, Q = 2.0 × 1800 = 3600 C, giving 0.0187 mol Cu, or 1.19 g.
例如，从 Cu²⁺ 中沉积 1 摩尔铜需要 2F 电荷。若通以 2.0 A 电流 30 分钟，Q = 2.0 × 1800 = 3600 C，可得 0.0187 mol Cu，即 1.19 g。
9. Measurement of EMF and the Role of Salt Bridge | 电动势测量与盐桥作用
Cell EMF is measured with a high‑resistance voltmeter to prevent current flow, ensuring the reading corresponds to the equilibrium potential. Under standard conditions, the reading equals the standard cell potential.
电池电动势用高阻伏特计测量，以防止电流通过，确保读数对应平衡电势。在标准条件下，读数等于标准电池电动势。
The salt bridge completes the circuit by allowing ions to flow without mixing the two half‑cell solutions. It maintains charge balance; without it, electrode surfaces would build up charge and halt the reaction.
盐桥通过允许离子流动而不混合两个半电池溶液来构成回路。它维持电荷平衡；没有盐桥，电极表面积累电荷会使反应停止。
A common exam question asks how the EMF would be affected if the salt bridge were removed or if a different salt were used. The correct answer: the EMF would drop to zero or the cell would stop working.
常见考题：移除盐桥或使用其他种类的盐桥会对电动势产生什么影响？正确答案：电动势会降为零，或电池停止工作。
10. Corrosion and Electrochemical Protection | 腐蚀与电化学防护
Corrosion, especially rusting of iron, is an electrochemical process requiring oxygen and water. Iron acts as the anode (oxidation to Fe²⁺), while a cathode region reduces O₂ to OH⁻. Fe²⁺ further oxidises to form rust (Fe₂O₃·xH₂O).
腐蚀，特别是铁锈生成，是一个需要氧气和水的电化学过程。铁作为阳极（氧化为 Fe²⁺），阴极区域则将 O₂ 还原为 OH⁻。Fe²⁺ 进一步氧化形成铁锈（Fe₂O₃·xH₂O）。
Protective methods include barrier layers (paint, grease), sacrificial protection (attaching a more reactive metal like zinc, which corrodes preferentially), and impressed current cathodic protection.
防护方法包括隔离层（油漆、油脂）、牺牲保护（连接更活泼的金属如锌，使其优先腐蚀）以及外加电流阴极保护。
Galvanising (coating iron with zinc) protects even when scratched because zinc has a more negative E° and acts as a sacrificial anode. This links directly to the electrochemical series.
镀锌（铁表面覆盖锌）即使在涂层破损时也能提供保护，因为锌具有更负的 E° 值并充当牺牲阳极。此原理直接与电化学系列相关。
11. Common Electrochemical Cells | 常见电化学电池
Understanding cell diagrams and notations is essential. For a Daniell cell, the cell diagram is: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s). The double vertical line represents the salt bridge.
理解电池图示和符号至关重要。丹尼尔电池的电池图示为：Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)。双竖线代表盐桥。
A lead–acid battery (rechargeable) uses Pb and PbO₂ electrodes in H₂SO₄. Discharge: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O. Recharging reverses the reaction; E° of each cell is about 2 V.
铅酸蓄电池（可充电）使用 Pb 和 PbO₂ 电极，电解液为 H₂SO₄。放电反应：Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O。充电时反应逆向进行；每个电池的 E° 约为 2 V。
Lithium‑ion cells have high energy density and are widely used in portable electronics. The key half‑reactions involve Li⁺ intercalation into graphite and lithium metal oxides.
锂离子电池能量密度高，广泛用于便携电子设备。其关键半反应涉及 Li⁺ 嵌入石墨及锂金属氧化物。
12. Factors Affecting Electrode Potentials | 影响电极电势的因素
Concentration, temperature, and pressure can shift electrode potentials as described by the Nernst equation. Increasing concentration of the oxidised form makes E more positive; increasing reduced form makes E more negative.
浓度、温度和压力均可通过能斯特方程影响电极电势。增加氧化型物质浓度使 E 更正；增加还原型物质浓度使 E 更负。
Complex formation or precipitation can drastically alter potentials. For instance, adding NH₃ to Ag⁺ lowers [Ag⁺] via [Ag(NH₃)₂]⁺ formation, making E much less positive, which explains why Ag can reduce H⁺ in the presence of NH₃.
配位或沉淀可显著改变电势。例如，向 Ag⁺ 中加入 NH₃ 会通过形成 [Ag(NH₃)₂]⁺ 降低 [Ag⁺]，使 E 值变得远小于标准值，这解释了为何在 NH₃ 存在下 Ag 可还原 H⁺。
The choice of electrode material influences kinetics but not thermodynamics. A platinum electrode is often used for gas electrodes because it is inert and provides a surface for electron transfer.
电极材料的选择影响动力学而非热力学。铂电极常用于气体电极，因为它惰性且为电子转移提供表面。
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