IGCSE WJEC Chemistry: Electrochemistry | IGCSE WJEC 化学：电化学考点精讲

📚 IGCSE WJEC Chemistry: Electrochemistry | IGCSE WJEC 化学：电化学考点精讲

Electrochemistry is a core topic in the IGCSE WJEC Chemistry syllabus, covering the relationship between electricity and chemical reactions. This article provides a focused revision guide, breaking down key concepts such as electrolysis, electrochemical cells, and practical applications. Whether you are preparing for a unit test or the final examination, understanding the movement of ions, the discharge series, and electrode half-equations will be essential for success.

电化学是IGCSE WJEC化学考试中的核心主题，涵盖电与化学反应之间的关系。本文提供考点精讲，分解电解、电化学电池及实际应用等关键概念。无论你在准备单元测验还是大考，理解离子的移动、放电顺序以及电极半反应方程式都是取得高分的必要条件。

1. What is Electrochemistry? | 什么是电化学？

Electrochemistry is the study of chemical processes that involve the movement of electrons. It can be divided into two main branches: electrolysis, where electrical energy drives a non-spontaneous chemical change, and the generation of electricity from spontaneous chemical reactions in cells and batteries.

电化学是研究涉及电子转移的化学过程。它可以分为两大分支：电解，即电能驱动非自发的化学变化；以及通过自发的化学反应在电池中产生电能。

In electrolysis, an external power source forces electrons onto one electrode and removes them from another, causing ions to be discharged. In a simple cell, a spontaneous redox reaction produces a potential difference (voltage) between two different metals in an electrolyte.

在电解中，外部电源迫使电子进入一个电极并从另一个电极夺走电子，导致离子放电。在原电池中，自发的氧化还原反应使两种不同金属在电解质中产生电位差（电压）。

The WJEC syllabus expects you to be able to define these terms, draw and label electrolytic cells, and predict products at the electrodes.

WJEC考纲要求你能够定义这些术语，画出并标注电解池，以及预测电极产物。

2. Electrolytes and Non-electrolytes | 电解质与非电解质

An electrolyte is a substance that conducts electricity when molten or dissolved in water, due to the presence of mobile ions. Examples include ionic compounds such as sodium chloride, copper(II) sulfate, and acids like hydrochloric acid.

电解质是一种在熔融状态或溶于水时能够导电的物质，因为存在可自由移动的离子。例子包括离子化合物如氯化钠、硫酸铜，以及酸如盐酸。

A non-electrolyte does not conduct electricity in any state because it contains no ions; it consists of neutral molecules. Sugar (sucrose) and ethanol are typical non-electrolytes.

非电解质在任何状态下都不导电，因为它不含离子，由中性分子组成。糖（蔗糖）和乙醇是典型的非电解质。

For the exam, remember that solid ionic compounds do not conduct electricity because the ions are locked in a lattice and cannot move. Conductivity requires freely moving charged particles, which only exist in the liquid state or in solution.

考试时要记住，固态离子化合物不导电，因为离子被锁定在晶体点阵中无法移动。导电需要自由移动的带电粒子，这仅在液态或溶液中存在。

3. Key Terms in Electrolysis | 电解关键术语

Before diving deeper, you must master the following vocabulary: electrode – a rod or plate where electricity enters or leaves the electrolyte; anode – the positive electrode connected to the positive terminal of the power supply, where oxidation (loss of electrons) occurs; cathode – the negative electrode connected to the negative terminal, where reduction (gain of electrons) occurs.

在深入学习之前，你必须掌握以下词汇：电极 – 电流进入或离开电解质的棒或板；阳极 – 连接电源正极的正电极，发生氧化（失电子）；阴极 – 连接电源负极的负电极，发生还原（得电子）。

The mnemonic ‘OIL RIG’ helps: Oxidation Is Loss, Reduction Is Gain. At the anode, anions (negative ions) are attracted and lose electrons; at the cathode, cations (positive ions) are attracted and gain electrons.

助记口诀 ‘OIL RIG’ 很有用：氧化是失电子，还原是得电子。在阳极，阴离子（负离子）被吸引并失去电子；在阴极，阳离子（正离子）被吸引并得到电子。

WJEC questions often ask you to label a diagram of an electrolytic cell, showing the direction of electron flow in the external circuit (from anode to cathode) and the movement of ions in the electrolyte.

WJEC试题经常要求你在电解池示意图上标注电子在外电路中的流动方向（从阳极到阴极）以及离子在电解质中的移动方向。

4. Electrolysis of Molten Ionic Compounds | 熔融离子化合物的电解

When a molten ionic compound, such as lead(II) bromide, is electrolysed, it decomposes into its elements. The compound must be heated until it melts so that the ions are free to move.

当熔融的离子化合物，如溴化铅，被电解时，它会分解成其组成元素。该化合物必须加热至熔化，使离子能够自由移动。

In molten lead(II) bromide, Pb²⁺ ions migrate to the cathode and gain two electrons to form lead metal: Pb²⁺ + 2e⁻ → Pb. Bromide ions (Br⁻) migrate to the anode, lose one electron each, and form bromine gas: 2Br⁻ → Br₂ + 2e⁻. The overall reaction is PbBr₂(l) → Pb(l) + Br₂(g).

在熔融溴化铅中，Pb²⁺ 离子移向阴极，得到两个电子形成铅金属：Pb²⁺ + 2e⁻ → Pb。溴离子 (Br⁻) 移向阳极，各失去一个电子，形成溴气：2Br⁻ → Br₂ + 2e⁻。总反应为 PbBr₂(l) → Pb(l) + Br₂(g)。

You are expected to write similar half-equations for other molten salts, such as sodium chloride: cathode Na⁺ + e⁻ → Na; anode 2Cl⁻ → Cl₂ + 2e⁻. Always balance charge and atoms.

你应该能为其他熔融盐写出类似的半反应方程式，如氯化钠：阴极 Na⁺ + e⁻ → Na；阳极 2Cl⁻ → Cl₂ + 2e⁻。务必平衡电荷和原子。

This type of electrolysis is used industrially to extract reactive metals like sodium and aluminium from their ores.

这类电解在工业上用于从矿石中提取活泼金属，如钠和铝。

5. Electrolysis of Aqueous Solutions | 水溶液的电解

The electrolysis of aqueous solutions is more complex because water itself can be oxidised or reduced, competing with the dissolved ions. You must consider the reactivity of the ions and the role of water.

水溶液的电解更为复杂，因为水本身也能被氧化或还原，与溶解的离子竞争。你必须考虑离子的活泼性以及水的作用。

At the cathode, the cation that is less reactive than hydrogen will be discharged. For example, in copper(II) sulfate solution, Cu²⁺ ions are reduced to copper metal because copper is less reactive than hydrogen. But if the cation is more reactive than hydrogen (e.g. Na⁺, K⁺), hydrogen gas is produced from water instead: 2H₂O + 2e⁻ → H₂ + 2OH⁻.

在阴极，比氢不活泼的阳离子会放电。例如，在硫酸铜溶液中，Cu²⁺ 离子被还原为铜金属，因为铜没有氢活泼。但若阳离子比氢活泼（如 Na⁺、K⁺），则水释放出氢气：2H₂O + 2e⁻ → H₂ + 2OH⁻。

At the anode, the anion that is a halide (Cl⁻, Br⁻, I⁻) is generally discharged before hydroxide ions, giving the halogen. For example, in sodium chloride solution, Cl⁻ is discharged to form chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻. If no halide is present, or hydroxide ions are more easily discharged, oxygen gas is produced from water: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (or 2H₂O → O₂ + 4H⁺ + 4e⁻).

在阳极，卤素离子（Cl⁻、Br⁻、I⁻）通常优先于氢氧根离子放电，生成卤素单质。例如，在氯化钠溶液中，Cl⁻ 放电形成氯气：2Cl⁻ → Cl₂ + 2e⁻。若无卤素离子，或氢氧根离子更易放电，则水产生氧气：4OH⁻ → O₂ + 2H₂O + 4e⁻（或 2H₂O → O₂ + 4H⁺ + 4e⁻）。

The WJEC expects you to recall these general rules and apply them to solutions like dilute sulfuric acid, copper(II) sulfate, and sodium chloride.

WJEC要求你记住这些一般规则，并应用到如稀硫酸、硫酸铜和氯化钠等溶液中。

6. The Discharge Series and Selective Discharge | 放电顺序与选择性放电

When multiple ions are present in solution, only one type is discharged at each electrode. The ease of discharge depends on the ion’s position in the electrochemical series.

当溶液中存在多种离子时，每个电极上只有一种离子放电。放电的容易程度取决于离子在电化学序列中的位置。

For cations at the cathode, the order of ease of discharge (from easiest to hardest) is roughly: Ag⁺ > Cu²⁺ > H⁺ > Fe²⁺ > Zn²⁺ > Al³⁺ > Mg²⁺ > Na⁺ > K⁺. Hydrogen ions gain electrons more readily than metal ions that are more reactive than hydrogen.

对于阴极的阳离子，放电容易程度顺序（由易到难）大致为：Ag⁺ > Cu²⁺ > H⁺ > Fe²⁺ > Zn²⁺ > Al³⁺ > Mg²⁺ > Na⁺ > K⁺。氢离子比氢活泼的金属离子更易得电子。

For anions at the anode, the general order is: SO₄²⁻ < NO₃⁻ < OH⁻ < Cl⁻ < Br⁻ < I⁻. The halide ions discharge more easily than hydroxide, with iodide being the easiest. Sulfate and nitrate ions are never discharged in aqueous solution because water is oxidised instead.

对于阳极的阴离子，一般顺序为：SO₄²⁻ < NO₃⁻ < OH⁻ < Cl⁻ < Br⁻ < I⁻。卤素离子比氢氧根离子更易放电，碘离子最容易。硫酸根和硝酸根离子在水溶液中从不放电，因为水会优先被氧化。

Concentration also plays a role: in concentrated sodium chloride solution, chlorine is produced at the anode, but in very dilute solutions, oxygen may be produced instead. You should be able to explain product changes using these rules.

浓度也起一定作用：在浓氯化钠溶液中，阳极产生氯气；但在很稀的溶液中，可能产生氧气。你应能运用这些规则解释产物的变化。

7. Industrial Electrolysis: Brine and Aluminium | 工业电解：盐水与铝

The electrolysis of concentrated aqueous sodium chloride (brine) is a major industrial process, producing three useful products: chlorine gas, hydrogen gas, and sodium hydroxide solution.

电解浓氯化钠溶液（盐水）是一项重要的工业过程，产生三种有用产品：氯气、氢气和氢氧化钠溶液。

At the anode (positive electrode), chloride ions are discharged: 2Cl⁻ → Cl₂ + 2e⁻. At the cathode (negative electrode), hydrogen ions from water are discharged, leaving behind hydroxide ions: 2H₂O + 2e⁻ → H₂ + 2OH⁻. The sodium ions remain in solution, making sodium hydroxide. This is often called the chlor-alkali process.

在阳极（正极），氯离子放电：2Cl⁻ → Cl₂ + 2e⁻。在阴极（负极），水中的氢离子放电，留下氢氧根离子：2H₂O + 2e⁻ → H₂ + 2OH⁻。钠离子留在溶液中，形成氢氧化钠。这通常被称为氯碱工艺。

Aluminium is extracted by electrolysis of purified bauxite dissolved in molten cryolite. At the cathode, Al³⁺ + 3e⁻ → Al. At the anode, oxide ions are discharged: 2O²⁻ → O₂ + 4e⁻. The graphite anodes are consumed due to reaction with oxygen, forming CO₂, so they must be replaced regularly.

铝是通过电解溶解在熔融冰晶石中的纯化铝土矿提取的。在阴极：Al³⁺ + 3e⁻ → Al。在阳极，氧离子放电：2O²⁻ → O₂ + 4e⁻。石墨阳极因与氧气反应生成CO₂而被消耗，因此必须定期更换。

Questions on these processes often ask for electrode equations, reasons for using cryolite (to lower melting point, saving energy), and the need for periodic replacement of anodes.

关于这些工艺的试题经常要求写出电极方程式、解释使用冰晶石的原因（降低熔点，节约能源），以及阳极需要定期更换的原因。

8. Electroplating | 电镀

Electroplating is the process of depositing a thin layer of a metal onto the surface of another material using electrolysis. It is used to improve appearance, prevent corrosion, or reduce costs.

电镀是利用电解在另一种材料表面沉积一薄层金属的过程。它用于改善外观、防止腐蚀或降低成本。

In a typical setup for silver-plating a spoon: the spoon is the cathode, a pure silver bar is the anode, and the electrolyte contains silver ions (e.g. silver nitrate solution). At the anode: Ag → Ag⁺ + e⁻. At the cathode: Ag⁺ + e⁻ → Ag. The silver from the anode dissolves into solution and then plates onto the spoon. The concentration of silver ions in the electrolyte stays constant.

在给勺子镀银的典型装置中：勺子为阴极，纯银棒为阳极，电解质含有银离子（如硝酸银溶液）。阳极：Ag → Ag⁺ + e⁻；阴极：Ag⁺ + e⁻ → Ag。阳极的银溶解到溶液中，然后沉积在勺子上。电解质中银离子浓度保持恒定。

You must be able to identify the object to be plated as the cathode, describe the movement of ions, and write the half-equations. Common examples include chromium-plating of car parts and tin-plating of food cans.

你必须能够指出待镀物件是阴极，描述离子移动，并写出半反应方程式。常见的例子包括汽车零件的镀铬和食品罐头的镀锡。

9. Purifying Copper | 精炼铜

Copper extracted from its ore is about 98–99% pure and needs further purification by electrolysis for use in electrical wiring, where very high purity is required.

从矿石提取的铜纯度约为98–99%，需要用电解法进一步精炼，以用于对纯度要求极高的电线。

In the purification cell, the impure copper is made the anode, a thin sheet of pure copper is the cathode, and the electrolyte is copper(II) sulfate solution acidified with sulfuric acid. At the anode: Cu → Cu²⁺ + 2e⁻. Impure copper dissolves, and less reactive impurities (Ag, Au) fall to the bottom as ‘anode sludge’, while more reactive metals (Fe, Zn) remain in solution as ions.

在精炼池中，不纯的铜作阳极，纯铜薄片作阴极，电解质是酸化过的硫酸铜溶液。阳极：Cu → Cu²⁺ + 2e⁻。不纯的铜溶解，较不活泼的杂质（Ag、Au）落到底部形成“阳极泥”，而较活泼的金属（Fe、Zn）以离子形式留在溶液中。

At the cathode, only copper ions are discharged because they are reduced more easily than the reactive metal ions: Cu²⁺ + 2e⁻ → Cu. Pure copper is deposited, and the cathode gradually grows thicker. This produces copper of over 99.99% purity.

在阴极，只有铜离子放电，因为它们比活泼金属离子更容易被还原：Cu²⁺ + 2e⁻ → Cu。纯铜沉积，阴极逐渐变厚。这生产出纯度超过 99.99% 的铜。

10. Simple Cells and Batteries | 简单电池与原电池

A simple cell converts chemical energy into electrical energy through a spontaneous redox reaction. It consists of two different metals (electrodes) dipped in an electrolyte. The greater the difference in reactivity between the metals, the higher the voltage produced.

原电池通过自发的氧化还原反应将化学能转化为电能。它由两种不同金属（电极）浸入电解质中构成。金属间的活泼性差异越大，产生的电压越高。

For example, a zinc-copper cell with sulfuric acid electrolyte: zinc is more reactive and loses electrons (oxidation): Zn → Zn²⁺ + 2e⁻. These electrons flow through the external circuit to the copper electrode, where they reduce hydrogen ions from the acid: 2H⁺ + 2e⁻ → H₂. Copper acts as the positive electrode, and zinc as the negative electrode.

例如，以硫酸为电解质的锌-铜电池：锌更活泼，失去电子（氧化）：Zn → Zn²⁺ + 2e⁻。这些电子经外电路流到铜电极，在那里将酸中的氢离子还原：2H⁺ + 2e⁻ → H₂。铜作为正极，锌作为负极。

The WJEC syllabus includes the distinction between cells and batteries: a battery is two or more cells connected in series. You may be asked to interpret voltage measurements or predict the direction of electron flow.

WJEC考纲包括区分电池（cell）与电池组（battery）：电池组是串联的两个或多个电池。你可能会被问到分析电压测量值或预测电子流动方向。

11. Fuel Cells | 燃料电池

A fuel cell uses the reaction of a fuel (such as hydrogen) with oxygen to produce electricity directly, with water as the only chemical product. The hydrogen-oxygen fuel cell is the most common example and is considered a clean energy technology.

燃料电池利用燃料（如氢气）与氧气的反应直接发电，水是唯一的化学产物。氢氧燃料电池是最常见的例子，被认为是一种清洁能源技术。

In an alkaline hydrogen-oxygen fuel cell, hydrogen is fed to the anode, where it is oxidised: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻. Oxygen is fed to the cathode, where it is reduced: O₂ + 2H₂O + 4e⁻ → 4OH⁻. The overall reaction is 2H₂ + O₂ → 2H₂O. Electrons flow through an external circuit, doing useful work.

在碱性氢氧燃料电池中，氢气送入阳极被氧化：2H₂ + 4OH⁻ → 4H₂O + 4e⁻。氧气送入阴极被还原：O₂ + 2H₂O + 4e⁻ → 4OH⁻。总反应为 2H₂ + O₂ → 2H₂O。电子流经外电路做有用功。

Advantages of fuel cells include high efficiency, no harmful emissions (only water), and continuous operation as long as fuel is supplied. Challenges include hydrogen storage and the use of expensive catalysts. Compare this with rechargeable batteries, which store energy but eventually wear out.

燃料电池的优点包括高效率、无有害排放（仅水），以及只要供应燃料就能持续工作。挑战包括氢气储存和使用昂贵的催化剂。将它与可充电电池进行对比，后者储存能量但最终会耗尽。

12. Exam Tips and Common Misconceptions | 考试技巧与常见错误

Many students confuse the anode and cathode: remember that the anode is positive in electrolysis, but negative in a cell (discharge). In an electrolytic cell, the cathode is negative because it is connected to the negative terminal of the power supply; in a galvanic cell, the cathode is positive as it attracts cations.

许多学生混淆阳极和阴极：记住，电解中阳极为正，但在原电池中阳极为负。在电解池中，阴极是负极因为它连接电源负极；在自发电池中，阴极是正极因为它吸引阳离子。

Always write balanced half-equations showing electrons. State symbols are often required: (s), (l), (g), (aq). For molten electrolysis, no water is present, so products are straightforward elements. In aqueous electrolysis, water can be a reactant or product; use H₂O, H⁺ and OH⁻ correctly.

始终书写带电子数的平衡半反应方程式。通常需要标注状态符号：(s)、(l)、(g)、(aq)。熔融电解中没有水，产物是直接的元素。水溶液电解中，水可以是反应物或产物；正确使用 H₂O、H⁺ 和 OH⁻。

A common pitfall is forgetting that in aqueous sodium chloride electrolysis, the sodium ions are not discharged – hydrogen is produced instead. Explain this using the reactivity series of metals and the rule that metal ions more reactive than hydrogen stay in solution.

常见错误是忘记在电解氯化钠水溶液时，钠离子并不放电——而是析出氢气。用金属活泼性顺序以及比氢活泼的金属离子留在溶液中的规则来解释。

When describing electroplating, always mention that the anode is the plating metal and the cathode is the object to be plated. The electrolyte must contain ions of the plating metal. Marks are often lost for missing these key details.

描述电镀时，务必提及阳极是镀层金属，阴极是待镀物体。电解质必须含镀层金属离子。漏掉这些关键细节常会失分。

Practice drawing labelled diagrams of both electrolytic and galvanic cells, showing the direction of electron flow, ion migration, and identifying which electrode gains or loses mass. These visualisations are regularly tested in WJEC papers.

练习绘制电解池和自发电池的标注示意图，标出电子流动方向、离子迁移，并指明哪个电极质量增加或减少。这些图示在 WJEC 试卷中经常考查。

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IGCSE WJEC Chemistry: Electrochemistry | IGCSE WJEC 化学：电化学 考点精讲