📚 5.10 Electrolysis | 电解
Electrolysis is the process of using direct current electricity to drive a non-spontaneous chemical reaction, typically splitting an ionic compound into its elements. It is a crucial topic in IGCSE Science, linking electricity to chemical change and underpinning many industrial processes.
电解是利用直流电驱动非自发化学反应的过程，通常将离子化合物分解为组成它的元素。这是IGCSE科学中的一个重要主题，它将电与化学变化联系起来，并支撑着许多工业流程。
1. What is Electrolysis? | 什么是电解？
Electrolysis takes place in an electrolytic cell, which contains an electrolyte – an ionic compound either molten or dissolved in water – through which an electric current is passed. The cell has two electrodes connected to a direct current (DC) supply: the positive electrode (anode) and the negative electrode (cathode). When the current flows, positive ions (cations) move to the cathode and negative ions (anions) move to the anode, where they gain or lose electrons to become neutral atoms or molecules.
电解发生在电解池中，池内装有电解质——一种熔融态或溶于水中的离子化合物——电流通过电解质。电解池有两个与直流电源相连的电极：正极（阳极）和负极（阴极）。当电流通过时，阳离子移向阴极，阴离子移向阳极，在电极上得失电子后变成中性原子或分子。
The essential components for electrolysis are: an electrolyte containing free-moving ions, two electrodes that make contact with the electrolyte, and a DC power source that provides the energy needed to force the ions to discharge at the electrodes.
电解的必要组成包括：含有自由移动离子的电解质，与电解质接触的两根电极，以及提供能量以迫使离子在电极上放电的直流电源。
2. Key Terminology | 关键术语
It is essential to use precise terminology when describing electrolysis. The cathode is the negative electrode, where reduction takes place because cations gain electrons. The anode is the positive electrode, where oxidation takes place because anions lose electrons. Remember the mnemonic: “Cation to Cathode, Anion to Anode” and “OIL RIG” (Oxidation Is Loss, Reduction Is Gain of electrons).
描述电解时必须使用准确的术语。阴极是负极，阳离子在此获得电子，发生还原反应。阳极是正极，阴离子在此失去电子，发生氧化反应。记住口诀：”阳离子移向阴极，阴离子移向阳极”以及”氧化失电子，还原得电子”。
3. 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 (PbBr₂) is electrolysed, the only ions present are those from the compound itself. At the cathode, lead(II) ions (Pb²⁺) are reduced to lead atoms. At the anode, bromide ions (Br⁻) are oxidised to bromine molecules. The half-equations are:
当对熔融的离子化合物（如溴化铅 PbBr₂）进行电解时，存在的离子仅来自该化合物本身。在阴极，铅离子（Pb²⁺）被还原成铅原子；在阳极，溴离子（Br⁻）被氧化成溴分子。半方程式如下：
Cathode: Pb²⁺(l) + 2e⁻ → Pb(l)
Anode: 2Br⁻(l) → Br₂(g) + 2e⁻
Overall, solid lead is deposited at the cathode and brown bromine gas is released at the anode. This method is used to extract reactive metals like sodium and aluminium from their molten compounds.
总的来说，固态铅沉积在阴极，棕色的溴气在阳极释放。这种方法用于从熔融化合物中提取钠、铝等活泼金属。
4. Electrolysis of Aqueous Solutions: General Rules | 水溶液电解的一般规则
When an ionic compound is dissolved in water, the solution contains not only the compound’s ions but also hydrogen ions (H⁺) and hydroxide ions (OH⁻) from the slight ionisation of water. This introduces competition at the electrodes. The ion that is discharged depends on its position in the reactivity series (for cations) and on the type of anion present.
当离子化合物溶于水时，溶液中不仅含有化合物自身的离子，也含有水微弱电离产生的氢离子（H⁺）和氢氧根离子（OH⁻）。这就引发了电极上的竞争放电。哪种离子先放电取决于其在金属活动性顺序中的位置（对于阳离子）以及阴离子的种类。
The discharge preferences at the cathode: if the metal is more reactive than hydrogen (e.g. K, Na, Ca, Mg, Al), hydrogen gas (H₂) is produced from the reduction of H⁺ ions. If the metal is less reactive than hydrogen (e.g. Cu, Ag), the metal ion itself is reduced and a solid metal is deposited.
阴极的放电顺序：如果该金属比氢活泼（如钾、钠、钙、镁、铝），则会从氢离子还原生成氢气（H₂）。如果金属不如氢活泼（如铜、银），则金属离子自身被还原，析出固态金属。
The discharge preferences at the anode: for concentrated solutions of halides (Cl⁻, Br⁻, I⁻), the halide ion is discharged to give the halogen. However, in dilute solutions, or when a sulfate (SO₄²⁻) or nitrate (NO₃⁻) is present, hydroxide ions (OH⁻) are oxidised to produce oxygen gas and water.
阳极的放电顺序：如果溶液是浓卤化物（Cl⁻, Br⁻, I⁻），卤离子放电生成卤素单质。但在稀溶液中，或者当存在硫酸根（SO₄²⁻）或硝酸根（NO₃⁻）时，氢氧根离子（OH⁻）被氧化生成氧气和水。
5. Electrolysis of Concentrated Sodium Chloride Solution | 浓缩氯化钠溶液的电解
Concentrated brine (NaCl(aq)) contains Na⁺, Cl⁻, H⁺ and OH⁻. At the cathode, hydrogen ions are discharged in preference to sodium ions because sodium is more reactive than hydrogen, so hydrogen gas is produced. At the anode, chloride ions are discharged to give chlorine gas. The half-equations are:
浓盐水（NaCl(aq)）中含有 Na⁺, Cl⁻, H⁺ 和 OH⁻。在阴极，氢离子优先于钠离子放电，因为钠比氢活泼，因此生成氢气。在阳极，氯离子放电生成氯气。半方程式如下：
Cathode: 2H⁺(aq) + 2e⁻ → H₂(g)
Anode: 2Cl⁻(aq) → Cl₂(g) + 2e⁻
The remaining Na⁺ and OH⁻ form sodium hydroxide (NaOH) in the solution. This is the basis of the chlor-alkali industry, which produces hydrogen, chlorine and sodium hydroxide, all of which are immensely useful chemicals.
溶液中剩余的 Na⁺ 和 OH⁻ 形成氢氧化钠（NaOH）。这就是氯碱工业的基础，该工业生产氢气、氯气和氢氧化钠，三者都是用途极为广泛的化学品。
6. Electrolysis of Dilute Sodium Chloride Solution | 稀氯化钠溶液的电解
If the brine is very dilute, the concentration of chloride ions is low, and the competition at the anode shifts. Now, hydroxide ions are discharged in preference to chloride ions, producing oxygen gas instead of chlorine. The cathode reaction still produces hydrogen. Hence the overall electrolysis of a very dilute NaCl solution effectively splits water into hydrogen and oxygen, with sodium chloride remaining unchanged.
如果盐水非常稀，氯离子浓度低，阳极上的竞争放电就会改变。此时，氢氧根离子优先于氯离子放电，产生氧气而非氯气。阴极仍然产生氢气。因此，对极稀的氯化钠溶液进行电解，本质上是将水分解为氢气和氧气，氯化钠保持不变。
7. Electrolysis of Copper(II) Sulfate Solution with Inert Electrodes | 用惰性电极电解硫酸铜(II)溶液
Using graphite or platinum electrodes (which do not react) in CuSO₄(aq), Cu²⁺, SO₄²⁻, H⁺ and OH⁻ are present. At the cathode, copper ions are discharged because copper is less reactive than hydrogen, forming a pink-brown coating of copper metal. At the anode, hydroxide ions are discharged (since sulfate ions are not easily oxidised), releasing oxygen gas and water. The blue colour of the solution fades as Cu²⁺ ions are removed, and the solution becomes acidic due to the formation of sulfuric acid (H⁺ and SO₄²⁻ remain).
在硫酸铜溶液中使用石墨或铂电极（不参与反应），存在 Cu²⁺, SO₄²⁻, H⁺ 和 OH⁻。在阴极，铜离子放电，因为铜不如氢活泼，形成红棕色的金属铜镀层。在阳极，氢氧根离子放电（因为硫酸根离子不易被氧化），释放出氧气和水。随着 Cu²⁺ 离子的消耗，溶液蓝色逐渐褪去，同时由于生成了硫酸（H⁺ 和 SO₄²⁻ 残留），溶液变得酸性。
Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)
Anode: 4OH⁻(aq) → 2H₂O(l) + O₂(g) + 4e⁻
8. Electrolysis Using Copper Electrodes (Purification of Copper) | 使用铜电极的电解（铜的精炼）
In copper purification, both electrodes are copper: the anode is impure copper, and the cathode is a thin sheet of pure copper, with CuSO₄(aq) as the electrolyte. At the anode, copper atoms lose electrons and enter the solution as Cu²⁺ ions. Insoluble impurities fall to the bottom as ‘anode sludge’. At the cathode, Cu²⁺ ions gain electrons and deposit as pure copper. The cathode gradually grows thicker, while the anode dissolves.
在铜的精炼中，两个电极都是铜：阳极为不纯铜，阴极为纯铜薄片，电解质为硫酸铜溶液。在阳极，铜原子失去电子，以 Cu²⁺ 离子形式进入溶液。不溶性杂质沉入池底成为”阳极泥”。在阴极，Cu²⁺ 离子得到电子，析出纯铜。阴极逐渐变厚，而阳极不断溶解。
Anode: Cu(s) → Cu²⁺(aq) + 2e⁻
Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)
The concentration of Cu²⁺ in the solution remains constant throughout, making this a highly efficient process for obtaining very pure copper for electrical wiring.
整个过程中溶液中 Cu²⁺ 的浓度保持恒定，这使它成为一种高效获取极纯铜的方法，满足电线制造的要求。
9. Electroplating | 电镀
Electroplating uses electrolysis to coat a metal object with a thin layer of another metal, often for corrosion resistance or decoration. The object to be plated is made the cathode, the plating metal is made the anode, and the electrolyte contains ions of the plating metal. For example, to electroplate a steel spoon with silver, the spoon is the cathode, a silver bar is the anode, and the electrolyte is a silver nitrate or silver cyanide solution. Silver ions are reduced onto the spoon’s surface, giving a shiny, protective layer.
电镀利用电解在金属物体表面镀上一层其他金属的薄层，通常是为了防腐蚀或装饰。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。例如，给钢勺镀银时，钢勺为阴极，银棒为阳极，电解液为硝酸银或氰化银溶液。银离子在钢勺表面被还原，形成光亮的保护层。
10. Writing Half-Equations | 书写半方程
Half-equations are essential for showing the electron transfer at each electrode. Always identify the species being oxidised or reduced, and balance atoms and charge using electrons (e⁻). For example, the reduction of aluminium ions in molten aluminium oxide is written as: Al³⁺ + 3e⁻ → Al. The oxidation of oxide ions is: 2O²⁻ → O₂ + 4e⁻. In aqueous solutions, water molecules or H⁺/OH⁻ may appear, and it is vital to balance hydrogen and oxygen atoms with H⁺ and H₂O as needed.
半方程对于表示每个电极上的电子转移至关重要。始终先确定被氧化或还原的物种，然后用电子（e⁻）平衡原子和电荷。例如，熔融氧化铝中铝离子的还原写成：Al³⁺ + 3e⁻ → Al。氧离子的氧化写成：2O²⁻ → O₂ + 4e⁻。在水溶液中，可能出现水分子或 H⁺/OH⁻，需要适当地用 H⁺ 和 H₂O 来平衡氢原子和氧原子。
11. Applications of Electrolysis | 电解的应用
Electrolysis has numerous industrial and everyday applications. It is used to extract reactive metals like aluminium from their ores (the Hall-Héroult process). It produces chlorine, hydrogen and sodium hydroxide via the chlor-alkali process. Electrolysis refines copper to high purity for electrical applications. Electroplating protects steel from rusting (e.g. galvanising with zinc, chromium plating) and enhances appearance. It is also used to manufacture clean fuels such as hydrogen gas from water electrolysis, an increasingly important technology for sustainable energy.
电解有许多工业和日常应用。它用于从矿石中提取铝等活泼金属（霍尔-埃鲁法）。通过氯碱工艺生产氯气、氢气和氢氧化钠。电解将铜精炼至高纯度用于电气领域。电镀可防止钢铁生锈（如镀锌、镀铬）并改善外观。它还用于通过水电解制造清洁燃料氢气，这是一项日益重要的可持续能源技术。
12. Summary Table and Key Points | 总结表与要点
The following table summarises the products at inert electrodes for common electrolytes, a critical revision tool for IGCSE Science examinations.
下表总结了常见电解质在惰性电极上的产物，这是IGCSE科学考试的重要复习工具。
| Electrolyte | Cathode Product | Anode Product |
|---|---|---|
| Molten PbBr₂ | Lead | Bromine |
| Concentrated NaCl(aq) | Hydrogen | Chlorine |
| Dilute NaCl(aq) | Hydrogen | Oxygen |
| CuSO₄(aq) (inert electrodes) | Copper | Oxygen |
| CuSO₄(aq) (copper electrodes) | Copper deposition | Copper dissolution |
Mastering electrolysis requires a solid understanding of ion migration, the discharge series, and the impact of concentration and electrode material. With this knowledge, you can predict products in any electrolytic cell and connect theory to real-world chemical industries.
掌握电解内容需要清晰理解离子迁移、放电顺序以及浓度和电极材料的影响。有了这些知识，你就能预测任何电解池中的产物，并将理论与现实的化学工业联系起来。
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