Electrolysis | 电解 - aleveler.com

📚 Electrolysis | 电解

Electrolysis is a critical concept in IGCSE AQA Chemistry, linking electricity with chemical change. Understanding how and why ions discharge at electrodes unlocks many industrial processes, from metal extraction to electroplating. This guide breaks down every essential point you need for the exam, with paired English and Chinese explanations to reinforce learning and build confidence.

电解是 IGCSE AQA 化学的核心概念，它将电和化学变化联系在一起。理解离子如何在电极上放电，能帮助你掌握从金属提取到电镀的众多工业过程。本指南逐点梳理了考试所需的所有要点，采用中英双语对照讲解，帮助你加深理解、提升信心。

1. What is Electrolysis? | 什么是电解？

Electrolysis is the process of breaking down an ionic substance using direct current (DC) electricity. The ionic compound must be molten or dissolved in water so that its ions are free to move and carry charge. During electrolysis, electrical energy is converted into chemical energy.

电解是利用直流电（DC）将离子化合物分解的过程。离子化合物必须处于熔融状态或溶于水中，以便离子能自由移动并传导电荷。在电解过程中，电能转化为化学能。

2. Key Terminology | 关键术语

Electrolyte: the molten or aqueous ionic compound that conducts electricity and is decomposed. Electrodes: rods made of either graphite (inert) or metal that dip into the electrolyte. The anode is the positive electrode, attracting anions; the cathode is the negative electrode, attracting cations. Inert electrodes do not react, while active electrodes take part in the reaction.

电解质：能够导电并被分解的熔融态或水溶液中的离子化合物。电极：插入电解质中的棒状物，常用石墨（惰性）或金属制成。阳极是正极，吸引阴离子；阴极是负极，吸引阳离子。惰性电极不参与反应，而活性电极会参与反应。

Anode: positive, oxidation occurs, anions move here.
阳极：正极，发生氧化反应，阴离子移向此处。
Cathode: negative, reduction occurs, cations move here.
阴极：负极，发生还原反应，阳离子移向此处。

3. The Process of Electrolysis | 电解过程

When the DC power supply is switched on, cations migrate towards the cathode and anions migrate towards the anode. At the electrodes, ions gain or lose electrons to form neutral atoms or molecules. Reduction occurs at the cathode (gain of electrons), and oxidation occurs at the anode (loss of electrons). The circuit is completed by the flow of electrons in the external wires and the movement of ions in the electrolyte.

当直流电源接通后，阳离子向阴极迁移，阴离子向阳极迁移。在电极上，离子通过得到或失去电子变成中性原子或分子。阴极发生还原反应（得到电子），阳极发生氧化反应（失去电子）。外部导线中电子的流动和电解质中离子的移动共同构成完整的回路。

4. Electrolysis of Molten Compounds | 熔融化合物的电解

When a molten ionic compound such as lead(II) bromide (PbBr₂) is electrolysed, there are no water molecules to compete. The only ions present are Pb²⁺ and Br⁻. At the cathode, Pb²⁺ ions are reduced to lead metal. At the anode, Br⁻ ions are oxidised to bromine gas.

电解熔融离子化合物（例如溴化铅 PbBr₂）时，因为没有水分子参与竞争，体系中只有 Pb²⁺ 和 Br⁻ 离子。在阴极，Pb²⁺ 离子被还原为金属铅。在阳极，Br⁻ 离子被氧化为溴气。

Cathode: Pb²⁺ + 2e⁻ → Pb

阴极：Pb²⁺ + 2e⁻ → Pb

Anode: 2Br⁻ → Br₂ + 2e⁻

阳极：2Br⁻ → Br₂ + 2e⁻

This straightforward electrolysis is used to extract reactive metals such as aluminium from their molten compounds.

这种直接的电解方式常用于从熔融化合物中提取铝等活泼金属。

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

In aqueous solutions, water molecules dissociate slightly into H⁺ and OH⁻ ions, which also compete to discharge at the electrodes. Therefore, the products depend on the relative reactivity of the ions and the concentration of the solution. Graphite or platinum electrodes are typically used as inert electrodes.

在水溶液中，水分子会微弱电离出 H⁺ 和 OH⁻ 离子，这些离子也会在电极上竞争放电。因此，电解产物取决于离子的相对活泼性以及溶液的浓度。通常使用石墨或铂作为惰性电极。

For example, electrolysis of aqueous copper(II) sulfate using inert electrodes: Cu²⁺, H⁺, SO₄²⁻, OH⁻ are present. At the cathode, Cu²⁺ is discharged in preference to H⁺ because copper is less reactive than hydrogen. At the anode, OH⁻ is discharged to give O₂, because SO₄²⁻ is too stable to oxidise.

例如，用惰性电极电解硫酸铜水溶液：溶液中存在 Cu²⁺、H⁺、SO₄²⁻ 和 OH⁻。在阴极，Cu²⁺ 优先于 H⁺ 放电，因为铜的活泼性比氢弱。在阳极，OH⁻ 放电产生 O₂，因为 SO₄²⁻ 离子非常稳定，不容易被氧化。

6. Discharge Series and Predicting Products | 放电顺序与产物预测

The discharge series helps you predict which ion will be discharged. For cations at the cathode, the easier the ion is to reduce, the more likely it is to discharge. The general order (least easily discharged first) is: K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺, (H⁺ from water), Zn²⁺, Fe²⁺, Sn²⁺, Pb²⁺, Cu²⁺, Ag⁺. For anions at the anode, SO₄²⁻ and NO₃⁻ never discharge; OH⁻ discharges to give O₂ unless a halide ion (Cl⁻, Br⁻, I⁻) is present in reasonable concentration, in which case the halide discharges first.

放电顺序可以帮助你预测哪种离子会放电。对于阴极的阳离子，越容易被还原的离子越先放电。一般顺序（越难放电的排在越前面）：K⁺、Na⁺、Ca²⁺、Mg²⁺、Al³⁺、（来自水的 H⁺）、Zn²⁺、Fe²⁺、Sn²⁺、Pb²⁺、Cu²⁺、Ag⁺。对于阳极的阴离子，SO₄²⁻ 和 NO₃⁻ 永远不放电；OH⁻ 放电产生 O₂，但如果溶液中存在一定浓度的卤素离子（Cl⁻、Br⁻、I⁻），则卤素离子优先放电。

Cathode cation order (ease of discharge)	K⁺ < Na⁺ < Ca²⁺ < Mg²⁺ < Al³⁺ < (H⁺) < Zn²⁺ < Fe²⁺ < Sn²⁺ < Pb²⁺ < Cu²⁺ < Ag⁺
阳极阴离子顺序（放电难易）	SO₄²⁻, NO₃⁻ 不放电 → OH⁻ (生成 O₂) → Cl⁻, Br⁻, I⁻ (卤素优先)

7. Half Equations at Electrodes | 电极半方程式

Half equations show the gain or loss of electrons at each electrode. They must be balanced in terms of atoms and charge. At the cathode, reduction half equations show electrons on the left. At the anode, oxidation half equations show electrons on the right.

半方程式表示每个电极上电子的得失，必须满足原子和电荷守恒。在阴极，还原半方程式的电子写在左侧。在阳极，氧化半方程式的电子写在右侧。

Cathode (reduction): Cu²⁺ + 2e⁻ → Cu

阴极（还原）：Cu²⁺ + 2e⁻ → Cu

Anode (oxidation): 2Cl⁻ → Cl₂ + 2e⁻

阳极（氧化）：2Cl⁻ → Cl₂ + 2e⁻

When OH⁻ is discharged at the anode, the half equation is: 4OH⁻ → O₂ + 2H₂O + 4e⁻. For water oxidation in neutral or acidic conditions: 2H₂O → O₂ + 4H⁺ + 4e⁻.

当 OH⁻ 在阳极放电时，半方程式为：4OH⁻ → O₂ + 2H₂O + 4e⁻。在中性或酸性条件下，水的氧化：2H₂O → O₂ + 4H⁺ + 4e⁻。

8. Industrial Applications: Electrolysis of Brine | 工业应用：盐水的电解

Electrolysis of concentrated aqueous sodium chloride (brine) is a major industrial process. With inert electrodes, the products are hydrogen gas at the cathode, chlorine gas at the anode, and sodium hydroxide solution left in the electrolyte. This forms the basis of the chlor-alkali industry.

电解浓氯化钠水溶液（盐水）是一项重要的工业过程。使用惰性电极，阴极产生氢气，阳极产生氯气，溶液中留下氢氧化钠。这是氯碱工业的基础。

At the cathode, H⁺ ions are discharged rather than Na⁺ because hydrogen is less reactive: 2H⁺ + 2e⁻ → H₂. At the anode, Cl⁻ are discharged in preference to OH⁻ due to high chloride concentration: 2Cl⁻ → Cl₂ + 2e⁻.

在阴极，H⁺ 放电而不是 Na⁺，因为氢较不活泼：2H⁺ + 2e⁻ → H₂。在阳极，由于氯离子浓度高，Cl⁻ 优先于 OH⁻ 放电：2Cl⁻ → Cl₂ + 2e⁻。

9. Purification of Copper | 铜的精炼

Electrolysis is used to purify impure copper. The impure copper acts as the anode, a thin sheet of pure copper acts as the cathode, and copper(II) sulfate solution is the electrolyte. During electrolysis, the anode dissolves: Cu → Cu²⁺ + 2e⁻. Pure copper ions are reduced at the cathode: Cu²⁺ + 2e⁻ → Cu. Impurities such as silver and gold fall to the bottom as ‘anode sludge’.

电解可用于精炼粗铜。粗铜作为阳极，纯铜薄片作为阴极，硫酸铜溶液作为电解质。电解时，阳极溶解：Cu → Cu²⁺ + 2e⁻。纯铜离子在阴极被还原：Cu²⁺ + 2e⁻ → Cu。银、金等杂质会掉落到槽底，形成“阳极泥”。

The cathode gradually grows purer copper, while the anode shrinks. This process produces very high-purity copper suitable for electrical wiring.

阴极上逐渐沉积出更纯的铜，而阳极则逐渐消耗。这一方法可生产出适用于电线的高纯度铜。

10. Electroplating | 电镀

Electroplating uses electrolysis to coat a metal object with a thin layer of another metal for decoration or protection. The object to be plated is made the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal. For silver plating, the anode is silver, the electrolyte is silver nitrate solution, and the cathode object becomes coated with a thin layer of silver.

电镀是通过电解在金属物品表面覆盖一薄层其他金属的工艺，用于装饰或防腐蚀。待镀物件作为阴极，镀层金属作为阳极，电解质含有镀层金属的离子。例如镀银时，阳极为银，电解质为硝酸银溶液，阴极物件表面会沉积一层薄银。

Anode: Ag → Ag⁺ + e⁻

阳极：Ag → Ag⁺ + e⁻

Cathode: Ag⁺ + e⁻ → Ag

阴极：Ag⁺ + e⁻ → Ag

11. Extraction of Aluminium | 铝的提取

Aluminium is extracted from its ore, bauxite (purified to alumina, Al₂O₃), by electrolysis. Because Al₂O₃ has an extremely high melting point, it is dissolved in molten cryolite (Na₃AlF₆) to lower the temperature and save energy. The electrolysis takes place in a steel cell lined with graphite, which acts as the cathode. Graphite anodes are dipped into the molten mixture.

铝是通过电解从铝土矿（精炼为氧化铝 Al₂O₃）中提取的。由于 Al₂O₃ 的熔点极高，通常将其溶解在熔融的冰晶石（Na₃AlF₆）中，以降低操作温度并节约能源。电解在衬有石墨的钢槽中进行，石墨内衬作为阴极，石墨阳极插入熔融混合物中。

At the cathode: Al³⁺ + 3e⁻ → Al. Molten aluminium sinks to the bottom and is tapped off. At the anode: 2O²⁻ → O₂ + 4e⁻. The oxygen reacts with the carbon anodes, producing CO₂ and causing the anodes to burn away, so they need frequent replacement.

阴极反应：Al³⁺ + 3e⁻ → Al，生成的熔融铝液沉到底部被抽出。阳极反应：2O²⁻ → O₂ + 4e⁻。产生的氧气与碳阳极反应生成 CO₂，导致阳极消耗，因此需要定期更换阳极。

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

Always state the type of electrode (inert or active) when describing electrolysis. For aqueous solutions, check both the cation and anion discharge series carefully; many students forget that hydrogen may be produced at the cathode if the metal is more reactive than hydrogen. Practise writing half equations with electrons on the correct side and balanced charges. When drawing electrolytic cells, label the anode, cathode, electrolyte, and direction of electron flow correctly.

在描述电解时，务必说明电极类型（惰性或活性）。对于水溶液，要仔细对照阳离子和阴离子的放电顺序；很多学生常忘记，如果金属比氢活泼，阴极会析出氢气。要勤练写半方程式，确保电子在正确的一侧，并且电荷守恒。在绘制电解池示意图时，要正确标注阳极、阴极、电解质以及电子流动方向。

Common pitfalls: confusing anode and cathode (remember PANIC: Positive Anode, Negative Is Cathode), forgetting that water provides H⁺ and OH⁻ in aqueous electrolysis, and omitting state symbols when required. Always use the term ‘electrolysis’ precisely, and never say ‘ions move through the external circuit’ — ions move through the electrolyte and electrons flow in the wires.

常见误区：混淆阳极和阴极（记住 PANIC：Positive Anode, Negative Is Cathode），忘记水溶液电解时水能提供 H⁺ 和 OH⁻，以及在需要时遗漏状态符号。务必准确使用“电解”这一术语，且不要说“离子在外电路中移动”——离子在电解质中移动，而电子在导线中流动。

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Electrolysis | 电解