📚 AS Chemistry: Electrolysis Exam Essentials | AS化学：电解考点精讲
Electrolysis is a fundamental topic in AS Chemistry that involves using electricity to drive non-spontaneous chemical reactions. Exam questions on this topic test three things: the principles behind the process, the half-equations at each electrode, reactions, and the quantitative link between charge passed and amount of substance produced. This guide brings all of those essentials together in one place.
电解是AS化学中的基础课题，涉及利用电能驱动非自发化学反应。相关考题通常考查三件事：过程背后的原理、两极的半反应方程式，以及通过的电量与产物物质的量之间的定量关系。本文把这些考点系统地整理在一起。
Everything in this article follows the specification wording used by AQA, OCR, Edexcel and Cambridge International, so the definitions you see here can be quoted directly in an exam answer.
本文所有表述都遵循 AQA、OCR、Edexcel 与剑桥国际考纲的用语习惯，因此文中的定义可以几乎直接搬进答题卡。
1. What is Electrolysis? | 什么是电解？
Electrolysis is the process of decomposing an ionic compound by passing a direct electric current through it in its molten or dissolved state, causing chemical reactions at the electrodes. It converts electrical energy into chemical energy.
电解是通过将直流电通入熔融或溶解状态的离子化合物中，使其分解并在电极上发生化学变化的过程。它将电能转化为化学能。
It is the opposite of a galvanic cell, where a spontaneous redox reaction generates electricity. In electrolysis, an external power source forces the non-spontaneous reaction to occur.
它是原电池（自发的氧化还原反应产生电能）的反过程。在电解中，外部电源迫使非自发反应进行。
Three requirements must be satisfied before electrolysis can happen: the electrolyte must be molten or in aqueous solution so that its ions are free to move; there must be a direct current supply; and there must be two electrodes in contact with the electrolyte. A solid ionic compound such as sodium chloride crystals will not conduct, because its ions are locked in a lattice and cannot migrate.
电解发生前必须满足三个条件：电解质必须处于熔融或水溶液状态，使离子能够自由移动；必须有直流电源；必须有两个与电解质接触的电极。像氯化钠晶体这样的固态离子化合物不能导电，因为其中的离子被锁在晶格中，无法迁移。
2. Electrolytic Cell Setup | 电解池装置
An electrolytic cell consists of a direct current (DC) power supply, two electrodes (anode and cathode) dipped in an electrolyte, and connecting wires. The anode is connected to the positive terminal, and the cathode to the negative terminal.
电解池由直流电源、浸入电解质中的两个电极（阳极和阴极）以及连接导线组成。阳极与电源正极相连，阴极与电源负极相连。
Electrodes are often made of inert materials such as platinum or graphite to prevent them from taking part in the reaction and dissolving. Graphite is cheap and widely used in school practicals, while platinum is chosen when a chemically robust, non-reacting surface is needed.
电极通常由铂或石墨等惰性材料制成，以防它们参与反应而被消耗。石墨价格低廉，是学校实验中的常用选择；而需要化学性质非常稳定、不参与反应的表面时，则选用铂。
When a reactive electrode such as copper or silver is used instead, the electrode itself can be oxidised and enter the solution, which changes the products completely. In the external circuit the current is carried by electrons; inside the electrolyte it is carried by the movement of ions.
如果改用铜、银等活性电极，电极本身可能被氧化并进入溶液，产物会随之完全改变。在外电路中电流由电子传导；在电解质内部，电流则由离子的迁移来承担。
3. Electrode Reactions: Oxidation and Reduction | 电极反应：氧化与还原
At the cathode (the negative electrode) positively charged cations are attracted, gain electrons and are reduced. At the anode (the positive electrode) negatively charged anions are attracted, lose electrons and are oxidised.
在阴极（负极），带正电的阳离子被吸引过来，得到电子被还原。在阳极（正极），带负电的阴离子被吸引过来，失去电子被氧化。
Reduction always happens at the cathode and oxidation always happens at the anode, regardless of the type of cell. The memory aids "OIL RIG" (Oxidation Is Loss, Reduction Is Gain) and "An Ox, Red Cat" are worth writing in the margin of your notes.
无论在哪种电池中，还原总是发生在阴极，氧化总是发生在阳极。助记口诀 "OIL RIG"（氧化是失电子，还原是得电子）和 "An Ox, Red Cat"（阳极氧化、阴极还原）值得写在笔记页边上。
| Feature | Electrolytic cell 电解池 | Galvanic cell 原电池 |
| Anode polarity 阳极极性 | Positive 正极 | Negative 负极 |
| Cathode polarity 阴极极性 | Negative 负极 | Positive 正极 |
| Electron flow 电子流向 | Anode to cathode in external circuit 外电路由阳极到阴极 | Anode to cathode in external circuit 外电路由阳极到阴极 |
| Energy change 能量变化 | Electrical to chemical 电能→化学能 | Chemical to electrical 化学能→电能 |
The polarity of the electrodes is reversed between the two cell types, but the reaction happening at each named electrode is not. This is the single most common source of lost marks in this topic.
两类电池的电极极性正好相反，但同名电极上发生的反应类型并不改变。这是本考点最常见的失分点。
4. Electrolysis of Molten Electrolytes | 熔融电解质的电解
In a molten electrolyte only the ions of the compound itself are present, so there is no competition from water. The products are therefore predictable from the formula of the compound alone.
在熔融电解质中，只存在化合物自身的离子，没有水参与竞争，因此产物可以仅根据化合物的化学式加以预测。
a silvery bead of lead metal. At the anode, 2Br⁻ → Br₂ + 2e⁻, giving brown bromine vapour.
熔融溴化铅(Ⅱ) 是经典实验实例。在阴极：Pb²⁺ + 2e⁻ → Pb，析出银色的铅珠。在阳极：2Br⁻ → Br₂ + 2e⁻，产生棕色的溴蒸气。
olved in molten cryolite is electrolysed to extract aluminium, with Al³⁺ + 3e⁻ → Al at the cathode and oxide ions oxidised to oxygen at carbon anodes, which are gradually burnt away by the hot gas.
熔融氯化钠的表现完全一致：阴极 Na⁺ + e⁻ → Na，阳极 2Cl⁻ → Cl₂ + 2e⁻。工业上电解溶解在熔融冰晶石中的氧化铝以制取铝：阴极 Al³⁺ + 3e⁻ → Al，阳极的氧离子被氧化生成氧气，高温下会逐步消耗碳电极。
5. Aqueous Solutions: Competition at the Cathode | 水溶液：阴极放电竞争
In aqueous solution, water itself is a weak source of ions: H₂O ⇌ H⁺ + OH⁻. Both the cations of the dissolved compound and the H⁺ from water are attracted to the cathode, so a competition occurs.
在水溶液中，水本身是微弱的离子来源：H₂O ⇌ H⁺ + OH⁻。溶质的阳离子与水电离出的 H⁺ 都会被吸引到阴极，于是产生竞争。
ions with E° more positive than this, such as Cu²⁺ (+0.34 V) or Ag⁺ (+0.80 V), are discharged and the metal is deposited. Metal ions with a more negative E°, such as Na⁺ (−2.71 V), are not discharged and hydrogen is produced instead.
优先被还原的是标准电极电势更正的物种。由于 2H⁺ + 2e⁻ → H₂ 的 E° 为 0.00 V，凡是 E° 比它更正的金属离子（例如 Cu²⁺ 为 +0.34 V、Ag⁺ 为 +0.80 V）都会放电析出金属单质；而 E° 更负的金属离子（如 Na⁺ 为 −2.71 V）不会放电，取而代之的是氢气的生成。
drogen, the metal is deposited. Typical half-equations are Cu²⁺ + 2e⁻ → Cu and 2H⁺ + 2e⁻ → H₂, or 2H₂O + 2e⁻ → H₂ + 2OH⁻ in a neutral solution.
需要记住的实用规律：金属比氢活泼时，阴极放出氢气；金属不如氢活泼时，阴极析出金属。常见半反应为 Cu²⁺ + 2e⁻ → Cu 和 2H⁺ + 2e⁻ → H₂，中性溶液中也可写作 2H₂O + 2e⁻ → H₂ + 2OH⁻。
6. Aqueous Solutions: Competition at the Anode | 水溶液：阳极放电竞争
h concentration, it is oxidised and the halogen is released, for example 2Cl⁻ → Cl₂ + 2e⁻.
在阳极，竞争发生在溶质的阴离子与来自水的 OH⁻ 之间。若卤素离子浓度较高，它会被氧化并析出卤素单质，例如 2Cl⁻ → Cl₂ + 2e⁻。
n is released: 4OH⁻ → O₂ + 2H₂O + 4e⁻, which may also be written as 2H₂O → O₂ + 4H⁺ + 4e⁻.
若溶液中没有卤素离子，或卤素离子溶液非常稀，则由水中的氢氧根离子被氧化放出氧气：4OH⁻ → O₂ + 2H₂O + 4e⁻，也可写作 2H₂O → O₂ + 4H⁺ + 4e⁻。
Fluoride ions are never discharged in aqueous solution because the E° for F₂/F⁻ is far too positive; oxygen is produced instead. Sulfate and nitrate ions are also very difficult to oxidise and normally stay in solution, which is why dilute sulfuric acid electrolysis simply decomposes water.
在水溶液中氟离子永远不会放电，因为 F₂/F⁻ 的 E° 太正，实际析出的是氧气。硫酸根和硝酸根也很难被氧化，通常留在溶液中，这正是稀硫酸电解实质上只是分解水的原因。
