A-Level WJEC Chemistry: Electrochemistry Key Points | A-Level WJEC 化学：电化学 考点精讲

📚 A-Level WJEC Chemistry: Electrochemistry Key Points | A-Level WJEC 化学：电化学 考点精讲

Electrochemistry explores the interplay between chemical change and electrical work. In WJEC A-Level Chemistry, it bridges oxidation–reduction theory, cell potentials, electrolysis, and quantitative relationships. Mastery of these topics is essential for solving structured questions on cell EMF, predicting reaction feasibility, and applying Faraday’s laws to mass–charge calculations. This article distils the core concepts, standard definitions, and problem-solving strategies that appear consistently in exam papers.

电化学研究化学变化与电能之间的相互转换。在 WJEC 化学 A-Level 中，它连接了氧化还原理论、电池电动势、电解以及定量关系。掌握这些知识点是解答电池电动势计算、判断反应可行性以及运用法拉第定律进行质量–电量计算的基础。本文提炼了考试中反复出现的核心概念、标准定义和解题方法。

1. Redox Foundations and Oxidation Numbers | 氧化还原基础与氧化数

you to assign oxidation numbers using rules (e.g., uncombined element = 0, oxygen usually – n). Use these to identify the oxidising agent (reduced) and the reducing agent (oxidised).

氧化还原反应涉及电子的转移。氧化是失电子（氧化数升高），还原是得电子（氧化数降低）。WJEC 要求使用规则分配氧化数（如单质为 0、氧通常为 –2、氢为 +1、总和等于离子电荷），并据此判断氧化剂（被还原）和还原剂（被氧化）。

In a disproportionation reaction, the same species is simultaneously oxidised and reduced, showing two different oxidation numbers in the products. For example, Cl₂ in water gives Cl⁻ (–1) and ClO⁻ (+1). Always check changes in oxidation state to balance half-equations.

在歧化反应中，同一物质同时被氧化和还原，产物中表现两种不同氧化数。例如 Cl₂ 在水中生成 Cl⁻（–1）和 ClO⁻（+1）。检查氧化数变化是配平半反应的基础。

2. Half-Equations and Ion–Electron Method | 半反应与离子–电子法

Any redox process can be split into two half-equations: one for oxidation and one for reduction. Balance atoms (except H and O) first, then add H₂O to balance O, H⁺ to balance H, and finally electrons to balance charge. In alkaline solutions, add OH⁻ after H⁺ to convert H⁺ to water. The WJEC mark schemes emphasise the inclusion of state symbols and correct charges.

任何氧化还原反应都可拆分为两个半反应：氧化半反应和还原半反应。先配平除氢氧外的原子，再用水配平氧、用 H⁺ 配平氢，最后用电子配平电荷。在碱性条件下，用 OH⁻ 将 H⁺ 转化为水。WJEC 评分标准强调标注状态符号和正确电荷。

For example, the manganate(VII)– 5Fe³⁺ + Mn²⁺ + 4H₂O. These ionic equations are fundamental to constructing cell diagrams.

例如高锰酸根与亚铁离子滴定：MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O；Fe²⁺ → Fe³⁺ + e⁻。合并时使电子数相等，整体反应为 5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O。这类离子方程式是构建电池符号的基础。

3. Electrochemical Cells: Construction and Principles | 电化学电池：构造与原理

An electrochemical cell consists of two half-cells connected externally by a wire and internally by a salt bridge (or porous barrier). Each half-cell contains an electrode in contact with a solution of its ions. The salt bridge (e.g., filter paper soaked in KNO₃) completes the circuit by allowing ion migration without mixing the solutions. WJEC expects you to draw and label a standard cell, showing the direction of electron flow from the more negative electrode to the more positive electrode.

电化学电池由两个半电池组成，外部用导线连接，内部用盐桥（或多孔隔膜）连接。每个半电池含有一个电极及相应的离子溶液。盐桥（例如浸有 KNO₃ 的滤纸）通过允许离子迁移来闭合回路，而不使溶液混合。WJEC 要求能够绘制并标注标准电池，标明电子从电势更负的电极流向更正的电极。

Cell notation follows the convention: anode (oxidation) on the left, cathode (reduction) on the right, with phases separated by vertical lines and phase boundaries by single lines. For example: Zn(s) | Zn²⁺(aq) ‖ Cu²⁺(aq) | Cu(s). The double line represents the salt bridge. The cell EMF is positive when the right-hand electrode has a more positive reduction potential.

电池符号惯例：阳极（氧化）在左侧，阴极（还原）在右侧，不同相之间用竖线分隔，盐桥用双竖线表示。例如 Zn(s) | Zn²⁺(aq) ‖ Cu²⁺(aq) | Cu(s)。双竖线代表盐桥。当右侧电极的还原电势更正时，电池电动势为正。

4. Standard Electrode Potential and the Standard Hydrogen Electrode | 标准电极电势与标准氢电极

The standard electrode potential (E°) of a half-cell is the EMF measured under standard conditions (298 K, 1 atm pressure, 1 mol dm⁻³ ion concentration) when connected to the standard hydrogen electrode (SHE). By definition, the SHE has a potential of 0.00 V. It consists of platinum black in H⁺ (1 mol dm⁻³) with H₂ gas at 1 atm passing over it.

半电池的标准电极电势（E°）是在标准条件（298 K、1 atm 压强、1 mol dm⁻³ 离子浓度）下，与标准氢电极（SHE）连接时测得的电动势。根据定义，标准氢电极的电势为 0.00 V。它由铂黑置于 1 mol dm⁻³ H⁺ 溶液中，并通入 1 atm H₂ 气体构成。

WJEC questions frequently ask for the standard conditions and why they are necessary: ion concentration affects the position of the redox equilibrium; temperature and pressure alter the kinetics and equilibrium position. Inability to name these conditions often loses marks. Remember that E° values refer to reduction potentials (gain of electrons).

WJEC 题目常要求写出标准条件及其必要性：离子浓度影响氧化还原平衡的位置；温度和压力改变动力学与平衡位置。若不能准确写出这些条件，容易失分。记住 E° 数值指的是还原电势（得电子方向）。

5. The Electrochemical Series and Predicting Feasibility | 电化学系列与反应方向预测

Arranging half-cells in order of their standard electrode potentials gives the electrochemical series. The more positive (or less negative) the E° value, the stronger the oxidising agent on the left of the equilibrium. Conversely, the more negative the E°, the stronger the reducing agent on the right. WJEC candidates must be able to use E° to predict whether a reaction is feasible under standard conditions.

将半电池按其标准电极电势排序即得电化学系列。E° 值越正（或负得越少），平衡左侧的氧化型氧化能力越强；反之，E° 值越负，右侧的还原型还原能力越强。WJEC 考生需要能够使用 E° 预测标准条件下反应的可行性。

A reaction is thermodynamically feasible if the cell EMF calculated from E°(right) – E°(left) is positive. However, feasibility does not guarantee a reaction will occur at a measurable rate; activation energy may prevent observation. A classic exam trap is to state that a reaction with a small positive EMF may not happen due to slow kinetics, as is the case with the Cu/Fe³⁺ redox system.

如果由 E°(右) – E°(左) 计算出的电池电动势为正，则反应在热力学上可行。但可行性并不保证反应能以可观测的速率进行；活化能可能导致观察不到现象。一个经典考试陷阱是：电动势略正的反应可能因动力学缓慢而实际上不发生，如 Cu/Fe³⁺ 氧化还原体系。

6. Cell EMF and Spontaneous Reactions | 电池电动势与自发反应

tween the two electrodes when no current flows. It is calculated as E°(cell) = E°(cathode) – ction in the direction that gives a positive E°, corresponding to a spontaneous reaction (ΔG negative, using ΔG° = –nFE°).

电池的电动势（EMF）是在无电流通过时两电极间的最大电势差。计算公式为 E°(电池) = E°(阴极) – E°(阳极)，其中阴极发生还原，阳极发生氧化。始终选择使 E° 为正的方向书写反应，对应自发过程（ΔG° 为负，依据 ΔG° = –nFE°）。

When using standard potentials from a data table, do not multiply E° values by stoichiometric coefficients. The potential is an intensive property. Common exam errors include adding potentials for two half-cells and ignoring the need to reverse the sign of the oxidation potential. WJEC mark schemes reward clear identification of the stronger oxidising agent and the correct subtraction.

使用数据表中的标准电势时，不要因为化学计量数而对 E° 值乘倍。电势是强度性质。常见考试错误包括将两个半电池的电势简单相加，以及忘记氧化半反应需要变号。WJEC 评分标准对正确识别强氧化剂和正确减法步骤给予分数。

7. The Nernst Equation and Effect of Concentration on EMF | 能斯特方程与浓度对电动势的影响

Under non-standard conditions, electrode potentials shift according to the Nernst equation. For a general half-equation a Ox + n e⁻ ⇌ b Red, at 298 K the potential is given by:

E = E° − (0.0592/n) log₁₀([Red]ᵇ/[Ox]ᵃ)

在非标准条件下，电极电势根据能斯特方程发生偏移。对于一般的半反应 a Ox + n e⁻ ⇌ b Red，在 298 K 时电势由下式给出：

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E = E° − (0.0592/n) log₁₀([Red]ᵇ/[Ox]ᵃ)

ol⁻¹. At 298 K, converting to log base 10 gives the factor 0.0592 V. As a reactant concentration increases or product concentration decreases, the potential becomes more positive, favouring reduction. WJEC questions may ask you to calculate the EMF of a concentration cell, where both half-cells use the same chemistry but different concentrations.

完整形式为 E = E° − (RT/nF) ln Q，其中 R = 8.31 J K⁻¹ mol⁻¹，F = 96,500 C mol⁻¹。在 298 K 下，转换为以 10 为底的对数得到系数 0.0592 V。当反应物浓度增加或产物浓度降低时，电势变得更正，有利于还原。WJEC 可能要求计算浓差电池的电动势，即两个半电池化学相同但浓度不同。

In a concentration cell, the electrode in contact with the more dilute solution is the anode (oxidation) and the more concentrated solution acts as the cathode. The EMF is small, given by (0.0592/n) log₁₀(c₂/c₁) when using the same electrode. Understanding this explains why a voltmeter reading can change when a cell is left operating.

在浓差电池中，与较稀溶液接触的电极作为阳极（氧化），较浓溶液一侧为阴极。电动势较小，由 (0.0592/n) log₁₀(c₂/c₁) 计算（相同电极）。理解这一点可以解释为什么电池工作时电压表读数会变化。

8. Practical Cells: Fuel Cells and Common Batteries | 实用电池：燃料电池与常见电池

WJEC expects knowledge of the hydrogen– s include high efficiency, no polluting emissions, and continuous operation with a fuel supply. Limitations relate to hydrogen storage and cost of noble-metal catalysts.

WJEC 要求掌握氢氧燃料电池，它直接将化学能转化为电能，碱性条件下唯一的产物是水。反应：H₂ + 2OH⁻ → 2H₂O + 2e⁻（阳极）；O₂ + 2H₂O + 4e⁻ → 4OH⁻（阴极）。优点包括效率高、无污染排放、可连续运行；局限性在于氢气储存和贵金属催化剂的成本。

ity, operating voltage, and environmental impact. Non-rechargeable alkaline cells and lead–acid accumulators also appear in past papers. Know the half-equations for the lead– tery: Pb + SO₄²⁻ → PbSO₄ + 2e⁻ (anode); PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O (cathode).

要能比较氢燃料电池与可充电锂离子电池，注意能量密度、工作电压和环境影响。不可充电的碱性电池和铅酸蓄电池也出现在历年考题中。掌握铅酸电池的半反应：Pb + SO₄²⁻ → PbSO₄ + 2e⁻（阳极）；PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O（阴极）。

9. Electrolysis and Electrolytic Cells | 电解与电解池

Electrolysis is the decomposition of a compound by an electric current. It occurs in an electrolytic cell where the anode (positive) attracts anions and the cathode (negative) attracts cations. Unlike a galvanic cell, the non-spontaneous process requires an external power source. WJEC questions often involve predicting products at inert electrodes (e.g., graphite or platinum) from aqueous solutions.

电解是利用电流使化合物分解的过程。它发生在电解池中，阳极（正极）吸引阴离子，阴极（负极）吸引阳离子。与自发电池不同，电解是非自发过程，需要外部电源驱动。WJEC 常考惰性电极（如石墨或铂）下从水溶液中预测电解产物。

The priority of discharge at the cathode depends on the oxidation potential of the cation: the easiest to reduce (most positive E°) is discharged first. At the anode, the easiest to oxidise (most negative E°) is discharged, but for aqueous solutions, concentration and overpotential complicate the sequence, as OH⁻ may be oxidised in preference to SO₄²⁻ or NO₃⁻.

阴极的放电顺序取决于阳离子的还原电势：最容易还原的（E° 最正）先放电。阳极上，最容易氧化的（E° 最负）先放电；但在水溶液中，浓度和过电位使顺序复杂化，例如 OH⁻ 可能比 SO₄²⁻ 或 NO₃⁻ 先被氧化。

Overpotential effects mean that even if thermodynamics predicts oxygen, the higher activation energy for O₂ formation may lead to chlorine evolution at the anode from concentrated solutions. Exam answers must acknowledge this where appropriate.

过电位效应意味着即使热力学上预测生成氧气，但由于析氧反应的活化能更高，在浓溶液中阳极可能析出氯气。答题时需要视情况说明这一点。

10. Faraday’s Laws and Quantitative Electrolysis | 法拉第定律与定量电解

, for a half-reaction, mass of substance m = (Q × M) / (nF), where n is the number of electrons transferred per ion, M is molar mass.

法拉第定律将通过的电量与生成或消耗的物质的量联系起来。第一定律：Q = It（电量，库仑 = 电流，安培 × 时间，秒）。第二定律：1 法拉第（96,500 C）沉积 1 摩尔一价离子（或 1 摩尔电子）。因此，对于半反应，物质质量 m = (Q × M) / (nF)，其中 n 为每离子转移电子数，M 为摩尔质量。

WJEC problems may involve electrolysis of molten salts (e.g., Al₂O₃ in cryolite) or aqueous solutions, using a series circuit where the same current passes through multiple cells. In such cases, equating the number of moles of electrons passed allows calculation of masses or gas volumes deposited at different electrodes. Always convert volumes to moles (24 dm³ mol⁻¹ at room temperature and pressure unless otherwise stated).

WJEC 题目可能涉及熔融盐电解（如冰晶石中的 Al₂O₃）或水溶液电解，并使用串联电路使相同电流通过多个电解池。此时，通过电子摩尔数相等的关系，可计算不同电极上产物的质量或气体体积。始终将体积转换为摩尔（通常室温常压下 24 dm³ mol⁻¹，除非特殊说明）。

11. Corrosion and its Prevention | 电化学腐蚀与防护

²⁺ is further oxidised to Fe³⁺, forming hydrated iron(III) oxide (rust). Impurities or stress promote local cells, accelerating corrosion in the presence of a conducting electrolyte (e.g., salt water).

铁的生锈是一个电化学过程，需要水和氧。阳极区：Fe → Fe²⁺ + 2e⁻；阴极区：O₂ + 2H₂O + 4e⁻ → 4OH⁻。Fe²⁺ 进一步被氧化为 Fe³⁺，生成水合氧化铁（铁锈）。杂质或应力促进局部电池形成，在导电电解质（如盐水）存在时加速腐蚀。

Prevention strategies exploit electrochemical principles: coating (paint, grease) excludes O₂ and H₂O; sacrificial protection uses a more reactive metal (e.g., zinc or magnesium) that corrodes preferentially, supplying electrons to the iron and keeping it cathodic. Galvanising coats iron with zinc, combining barrier and sacrificial protection. Tin plating, however, only provides barrier protection and accelerates corrosion once scratched, because iron is more reactive than tin. WJEC frequently asks students to explain these differences.

防护策略利用电化学原理：涂层（油漆、油脂）隔绝 O₂ 和 H₂O；牺牲保护使用更活泼的金属（如锌或镁），它们优先腐蚀，为铁提供电子，使铁保持为阴极。镀锌（热浸锌）结合了隔离与牺牲保护。而镀锡仅提供隔离保护，一旦划伤会加速腐蚀，因为铁比锡更活泼。WJEC 经常要求解释这些差异。

12. Common Calculation and Problem-Solving Techniques | 常见计算与解题技巧

f-cell with the more positive E° as the cathode and then write the cell EMF as E°(cathode) – It, then use the balanced half-equation to find moles of product, and finally convert to mass or volume.

考试成功取决于系统的解题方法。对于电池符号，先确定 E° 更正的半电池为阴极，然后计算电池电动势 E°(阴极) – E°(阳极)。如果电动势为负，则反应非自发。合并半反应时确保电子数相等。在电解中，首先由 Q = It 计算电子摩尔数，再利用配平的半反应求产物物质的量，最后换算为质量或体积。

WJEC often features tables of E° values; always keep in mind the direction of the half-reaction you are using. Do not alter the sign of the tabulated reduction potential when writing the cell EMF expression, because the subtraction automatically accounts for the required oxidation. Practicing with past questions that integrate standard potentials, Faraday’s laws, and the Nernst equation will build fluency in multi-step reasoning.

WJEC 常提供 E° 数据表；始终注意所使用的半反应方向。列写电池电动势表达式时不要手动改变表中还原电势的符号，因为减法已自动考虑了氧化过程。通过练习整合标准电势、法拉第定律和能斯特方程的历年真题，将有助于提高多步推理的熟练度。

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