📚 Electrolysis Explained | IGCSE CIE 化学：电解 考点精讲
Electrolysis is one of the core topics in IGCSE Chemistry, bridging the gap between electricity and chemical change. Understanding how direct current can force non‑spontaneous redox reactions to occur is essential for explaining industrial processes like aluminium smelting, electroplating, and the manufacture of chlorine. This article breaks down every key idea CIE examiners expect you to know, from ion discharge series to half‑equations and the effect of concentration and electrode material.
电解是IGCSE化学的核心主题之一，它将电流与化学变化联系起来。理解直流电如何强制非自发的氧化还原反应发生，对于解释铝的冶炼、电镀以及氯的生产等工业过程至关重要。本文详细解析CIE考试重点涵盖的每一个关键概念，从离子放电顺序到半方程式，以及浓度和电极材料对产物的影响。
1. What Is Electrolysis? | 什么是电解？
Electrolysis is the process by which a compound is broken down into its elements using a direct electric current. The compound must be an ionic substance that is either molten or dissolved in water so that its ions are free to move.
电解是利用直流电将化合物分解为其组成元素的过程。该化合物必须是熔融状态或溶于水中的离子化合物，以便离子能够自由移动。
The substance undergoing electrolysis is called the electrolyte. Electrolysis is a key example of a redox reaction – reduction happens at the cathode and oxidation at the anode.
发生电解的物质叫做电解质。电解是氧化还原反应的一个典型例子——阴极发生还原，阳极发生氧化。
2. The Electrolytic Cell | 电解池的构成
An electrolytic cell consists of:
- a power supply (battery or DC source) that pushes electrons around the external circuit.
- two electrodes (usually inert materials such as graphite or platinum, or reactive if the electrode takes part in the reaction).
- an electrolyte (molten ionic compound or aqueous solution containing ions).
电解池由以下部分组成：
- 电源（电池或直流电源），驱动电子在外电路中流动。
- 两个电极（通常为惰性材料如石墨或铂；若电极参与反应则可为活性材料）。
- 电解质（熔融离子化合物或含离子的水溶液）。
The electrode connected to the negative terminal of the power supply is the cathode. The electrode connected to the positive terminal is the anode. Remember: Cations go to the Cathode, Anions go to the Anode.
连接电源负极的电极是阴极。连接正极的电极是阳极。记住：阳离子移向阴极，阴离子移向阳极。
3. Movement of Ions During Electrolysis | 电解过程中的离子移动
When the power is switched on, positively charged cations are attracted to the negative cathode, and negatively charged anions are attracted to the positive anode. This movement of ions conducts electricity through the electrolyte and allows chemical changes to take place at the electrodes.
接通电源后，带正电的阳离子被吸引到负极的阴极，带负电的阴离子被吸引到正极的阳极。离子的这种移动使电解质能够导电，并使电极上发生化学变化。
At the electrode surfaces, ions gain or lose electrons, forming neutral atoms or molecules. Without mobile ions, electrolysis cannot happen – that is why solid ionic compounds do not conduct and must be melted or dissolved.
在电极表面，离子得到或失去电子，形成中性原子或分子。如果没有自由移动的离子，电解就无法发生——这就是固态离子化合物不能导电而必须熔化或溶解的原因。
4. Reactions at the Electrodes – Cathode and Anode | 电极反应——阴极与阳极
At the cathode (negative electrode): reduction takes place. Positive ions gain electrons.
在阴极（负极）：发生还原反应，阳离子获得电子。
Example:
Cu²⁺ + 2e⁻ → Cu (copper metal deposits)
General pattern: the less reactive the metal, the more easily its ion is discharged at the cathode. Hydrogen ions from water compete with metal ions if the electrolyte is aqueous.
一般规律：金属越不活泼，其离子越容易在阴极放电。如果电解质是水溶液，水中的氢离子会与金属离子竞争放电。
At the anode (positive electrode): oxidation takes place. Negative ions lose electrons.
在阳极（正极）：发生氧化反应，阴离子失去电子。
Example:
2Cl⁻ → Cl₂ + 2e⁻
If the anode is made of an active metal (e.g. copper), the electrode itself can oxidise and enter the solution as ions, rather than the anion discharging.
如果阳极由活泼金属（如铜）制成，电极本身可能被氧化并进入溶液成为离子，而阴离子则不会放电。
5. The Discharge Series of Ions | 离子的放电顺序
When multiple cations or anions are present in an electrolyte, only one type is discharged at each electrode. The ion that is discharged is the one that most readily gains or loses electrons. The order of ease of discharge is given by the electrochemical series.
当电解质中存在多种阳离子或阴离子时，每种电极上只有一种离子放电。放电的离子是最容易获得或失去电子的离子。放电的难易顺序由电化学系列给出。
Cation discharge series (easiest to hardest to discharge):
阳离子放电顺序（从易到难）：
- Ca²⁺, Mg²⁺, Al³⁺ → never discharged from aqueous solution; hydrogen is discharged instead.
- Zn²⁺, Fe²⁺, Pb²⁺, H⁺
- Cu²⁺, Ag⁺
In a molten electrolyte of a single salt, the metal cation is always discharged because no competing ions exist.
在单一盐的熔融电解质中，由于没有竞争离子，金属阳离子总是会放电。
Anion discharge series (easiest to hardest to discharge):
阴离子放电顺序（从易到难）：
- m water or alkali) → discharged more readily than other common anions in aqueous solution.
- I⁻, Br⁻, Cl⁻
- O₃⁻ → very difficult to discharge; in aqueous solution, OH⁻ is oxidised instead giving O₂.
You must be able to apply these rules when predicting products of electrolysis of aqueous solutions.
在预测水溶液电解产物时，必须能够应用这些规律。
6. Effect of Concentration on Products | 浓度对产物的影响
When the concentration of an ion is very high, it can alter the usual discharge order. The classic example is concentrated sodium chloride solution (brine).
当某种离子浓度非常高时，它可能改变通常的放电顺序。典型例子是浓氯化钠溶液（盐水）。
In dilute NaCl(aq):
在稀氯化钠溶液中：
- Cathode: H⁺ is discharged → H₂ gas (because Na⁺ is too reactive).
- Anode: OH⁻ is preferentially discharged → O₂ gas.
In concentrated NaCl(aq):
在浓氯化钠溶液中：
- Cathode: still H⁺ → H₂ (Na⁺ remains in solution).
- Anode: Cl⁻ concentration is high enough to override OH⁻ → Cl₂ gas is produced.
This change is crucial in the chlor‑alkali industry, which yields hydrogen, chlorine, and sodium hydroxide.
这一变化在氯碱工业中至关重要，该工业可以生产氢气、氯气和氢氧化钠。
7. Molten Electrolytes vs. Aqueous Solutions | 熔融电解质与水溶液的对比
In a molten electrolyte, only the ions of the compound are present. Electrolysis always gives the pure elements. For example, molten lead(II) bromide forms lead at the cathode and bromine at the anode.
在熔融电解质中，只存在化合物自身的离子。电解总是得到纯净的单质。例如，熔融的溴化铅在阴极生成铅，在阳极生成溴。
In an aqueous solution, water also ionises slightly, providing H⁺ and OH⁻ ions. These ions compete with the solute’s ions for discharge at the electrodes. As a result, you must consider the discharge series and concentration effects to determine the products.
在水溶液中，水也会微弱电离，提供 H⁺ 和 OH⁻ 离子。这些离子会与溶质的离子竞争在电极上放电。因此，必须考虑放电顺序和浓度效应来确定产物。
Table comparing products:
产物比较表：
| Electrolyte | Cathode product | Anode product |
|---|---|---|
| Molten NaCl | Sodium (Na) | Chlorine (Cl₂) |
| Concentrated NaCl(aq) | Hydrogen (H₂) | Chlorine (Cl₂) |
| Dilute NaCl(aq) | Hydrogen (H₂) | Oxygen (O₂) |
| Molten Al₂O₃ dissolved in cryolite | Aluminium (Al) | Oxygen (O₂) (with carbon anodes forming CO₂) |
8. Key Example 1 – Electrolysis of Molten Lead(II) Bromide | 示例1——熔融溴化铅的电解
Lead(II) bromide is a classic demonstration of electrolysis using a molten salt. The electrolyte is heated until it melts, allowing Pb²⁺ and Br⁻ ions to move freely.
溴化铅是使用熔融盐演示电解的经典例子。将电解质加热至熔化，使 Pb²⁺ 和 Br⁻ 离子能够自由移动。
Cathode: Pb²⁺ + 2e⁻ → Pb (grey lead metal appears)
Anode: 2Br⁻ → Br₂ + 2e⁻ (red‑brown bromine gas bubbles off)
Observation: a silvery metal forms at the cathode; reddish‑brown vapour is seen at the anode. This experiment confirms that electrolysis decomposes the compound into its elements.
观察现象：阴极形成银白色金属；阳极出现红棕色蒸气。该实验证实电解能将化合物分解为组成它的元素。
9. Key Example 2 – Electrolysis of Concentrated Sodium Chloride (Brine) | 示例2——浓氯化钠溶液的电解（盐水电解）
This process is the foundation of the chlor‑alkali industry. The electrolyte is concentrated NaCl solution. The electrodes are inert (usually titanium anodes and steel cathodes in industry).
该过程是氯碱工业的基础。电解质为浓氯化钠溶液，电极采用惰性材料（工业中通常使用钛阳极和钢阴极）。
Ions present: Na⁺, Cl⁻, H⁺, OH⁻.
溶液中存在的离子：Na⁺, Cl⁻, H⁺, OH⁻。
Cathode: n gas). Na⁺ is too reactive to be discharged. The remaining solution becomes rich in NaOH.
Anode: as). OH⁻ ions are present but Cl⁻ concentration is high; Cl⁻ is preferentially discharged.
Overall products: hydrogen gas at the cathode, chlorine gas at the anode, and sodium hydroxide solution left behind. All three products are commercially very important.
总产物：阴极产生氢气，阳极产生氯气，溶液中留下氢氧化钠。这三种产物都具有重要的商业价值。
10. Key Example 3 – Electrolysis of Copper(II) Sulfate Solution | 示例3——硫酸铜溶液的电解
The products depend strongly on the anode material.
产物强烈依赖于阳极材料。
With inert electrodes (graphite or platinum):
使用惰性电极（石墨或铂）：
- Cathode: Cu²⁺ + 2e⁻ → Cu (pink‑brown copper metal plates onto the cathode).
- Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (oxygen gas is evolved). Sulfate ions are not discharged.
- Observation: cathode gains a reddish deposit; blue colour of solution fades as Cu²⁺ ions are removed.
观察：阴极上出现红色沉积物；随着 Cu²⁺ 离子被消耗，溶液蓝色逐渐褪去。
With copper anode (active electrode):
使用铜阳极（活性电极）：
- Cathode: Cu²⁺ + 2e⁻ → Cu (same as above).
- Anode: Cu → Cu²⁺ + 2e⁻ (the copper anode dissolves). So no gas is produced.
- Net effect: copper is transferred from the anode to the cathode. This is the basis of electrorefining of copper.
净效果：铜从阳极转移到阴极。这就是铜的电解精炼的基础。
11. Electroplating | 电镀
Electroplating is the process of coating a metal object with a thin layer of another metal using electrolysis. The object to be plated is made the cathode. The plating metal is made the anode. The electrolyte contains ions of the plating metal.
电镀是利用电解在金属物体表面镀上一层薄薄的其他金属的过程。待镀物件作为阴极，镀层金属作为阳极。电解液含有镀层金属的离子。
For example, to electroplate a steel spoon with silver:
- Cathode: steel spoon.
- Anode: pure silver bar.
- Electrolyte: silver nitrate solution (or a suitable silver complex).
- At the cathode: Ag⁺ + e⁻ → Ag (silver deposits on spoon).
- At the anode: Ag → Ag⁺ + e⁻ (silver dissolves to replenish the solution).
例如，在钢勺上镀银：
- 阴极：钢勺。
- 阳极：纯银棒。
- 电解液：硝酸银溶液（或合适的银配合物）。
- 阴极反应：Ag⁺ + e⁻ → Ag（银沉积在勺子上）。
- 阳极反应：Ag → Ag⁺ + e⁻（银溶解以补充溶液）。
Electroplating improves appearance and corrosion resistance.
电镀可以改善外观和耐腐蚀性。
12. Extraction of Aluminium – Electrolysis of Alumina | 铝的提取——氧化铝的电解
Aluminium is extracted from purified bauxite (Al₂O₃) by electrolysis because it is too reactive to be reduced by carbon. Alumina is dissolved in molten cryolite (Na₃AlF₆) to lower its melting point from over 2000 °C to about 950 °C, saving energy.
铝是通过电解从纯化的铝土矿（Al₂O₃）中提取的，因为铝过于活泼，不能用碳还原。将氧化铝溶解在熔融的冰晶石（Na₃AlF₆）中，可将其熔点从 2000 °C 以上降低到约 950 °C，从而节省能源。
The cell uses a carbon‑lined steel vessel as the cathode and large carbon blocks as the anodes.
电解槽使用碳衬里的钢容器作为阴极，大块碳块作为阳极。
Cathode: Al³⁺ + 3e⁻ → Al (molten aluminium collects at the bottom).
Anode: the carbon anodes at high temperature, producing CO₂, so anodes must be replaced regularly.
Overall equation for the decomposition:
2Al₂O₃ → 4Al + 3O₂
This is a major industrial application of electrolysis, consuming huge amounts of electricity.
这是电解的一个主要工业应用，需要消耗大量电能。
13. Quantitative Aspects – Faraday’s Laws (IGCSE Level) | 定量方面——法拉第定律（IGCSE 层次）
For IGCSE, you may be asked to relate the amount of product formed to the quantity of electric charge passed. The relationship is given by:
Q = I × t
where Q is charge in coulombs (C), I is current in amperes (A), and t is time in seconds (s).
其中 Q 是电量（库仑），I 是电流（安培），t 是时间（秒）。
One faraday ied by one mole of electrons. The mass of substance deposited or evolved can be calculated using the half‑equation and the number of electrons involved.
1 法拉第 = 96 500 C，相当于 1 摩尔电子的电量。利用半方程式和涉及的电子数，可以计算析出或生成物质的质量。
Example: Calculate the mass of copper deposited when a current of 2.0 A is passed through CuSO₄ solution for 30 minutes.
- t = 30 × 60 = 1800 s; Q = 2.0 × 1800 = 3600 C.
- Moles of electrons = 3600 / 96 500 ≈ 0.0373 mol.
- Half‑equation: Cu²⁺ + 2e⁻ → Cu ⇒ 2 mol e⁻ produce 1 mol Cu.
- Moles of Cu = 0.0373 / 2 ≈ 0.01865 mol; mass = 0.01865 × 63.5 ≈ 1.18 g.
This simple proportional reasoning is often tested in CIE structured papers.
这种简单的比例推理常见于 CIE 结构化试题中。
14. Summary of Key Points and Exam Tips | 要点总结与应试技巧
- Always identify the ions present in the electrolyte before predicting products.
- Use the discharge series to decide which cation and anion are discharged in aqueous solutions, but check concentration if chloride ions could be involved.
- Remember: cathode = reduction, anode = oxidation (both in terms of electron gain/loss).
- Practice writing half‑equations and combine them to show the overall redox change.
- Be familiar with the industrial applications: aluminium extraction, chlor‑alkali, electroplating, and copper refining.
- 在预测产物之前，一定要先识别电解质中存在哪些离子。
- 运用放电顺序确定水溶液中哪种阳离子和阴离子放电，但如果涉及氯离子，要注意浓度的影响。
- 牢记：阴极 = 还原，阳极 = 氧化（均以电子得失定义）。
- 练习书写半方程式，并组合它们表示总氧化还原变化。
- 熟悉工业应用：铝的提取、氯碱工业、电镀和铜的精炼。
Diagrams of electrolytic cells are frequently drawn in exams, so be able to label the direction of electron flow (from anode to cathode in the external circuit), ion movement, and the products at each electrode.
考试中经常要求绘制电解池示意图，要能够标出电子流动方向（外电路从阳极到阴极）、离子移动方向以及每个电极上的产物。
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