Electrolysis for CCEA A-Level Chemistry | CCEA A-Level 化学：电解考点精讲

📚 Electrolysis for CCEA A-Level Chemistry | CCEA A-Level 化学：电解考点精讲

Electrolysis is a core topic in CCEA A-Level Chemistry, bridging the gap between redox theory and real-world industrial processes. Understanding how electrical energy drives non-spontaneous chemical reactions is essential for answering both qualitative and quantitative questions on the exam. This article provides a concise yet thorough revision guide covering all key concepts from molten salt electrolysis to Faraday's laws, tailored to the CCEA specification.

电解是 CCEA A-Level 化学的核心主题，连接了氧化还原理论与工业实践。理解电能如何驱动非自发的化学反应，对于应对考试中的定性和定量问题至关重要。本文提供简明而全面的复习指南，涵盖从熔盐电解到法拉第定律的所有关键概念，并针对 CCEA 考试大纲进行讲解。

1. What is Electrolysis? | 什么是电解？

Electrolysis is the process of using direct current (DC) electricity to drive a non-spontaneous chemical reaction. It involves the passage of electric current through an electrolyte, which is a substance that contains mobile ions and can be either molten or in aqueous solution. During electrolysis, chemical decomposition occurs at the electrodes, where reduction takes place at the cathode and oxidation at the anode.

电解是利用直流电驱动非自发化学反应的过程。它涉及电流通过电解质，电解质是含有可移动离子的物质，可以是熔融态或水溶液。在电解过程中，化学分解发生在电极上：阴极发生还原反应，阳极发生氧化反应。

2. Electrolytic Cell Components | 电解池组成

An electrolytic cell consists of a power source (DC), two electrodes (cathode and anode), an electrolyte, and often a container. The cathode is the negative electrode connected to the negative terminal of the battery; it attracts cations and gives electrons to them (reduction). The anode is the positive electrode connected to the positive terminal; it attracts anions and takes electrons from them (oxidation). The electrodes must be made of an inert material (such as platinum or graphite) unless they are designed to participate in the reaction.

电解池包括电源（直流）、两个电极（阴极和阳极）、电解质以及通常一个容器。阴极是连接电池负极的负电极；它吸引阳离子并向它们提供电子（还原）。阳极是连接正极的正电极；它吸引阴离子并从它们夺取电子（氧化）。除非电极本身需要参与反应，否则电极必须由惰性材料制成（如铂或石墨）。

In an electrolytic cell, unlike a galvanic cell, the cathode is negative and the anode is positive, because the external power source forces current to flow in the opposite direction to the spontaneous cell potential.

在电解池中，与原电池不同，阴极是负极、阳极是正极，因为外接电源迫使电流以与自发电位相反的方向流动。

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

When an ionic compound is melted, its ions become mobile and can conduct electricity. Electrolysis of a molten salt decomposes it into its elements. For example, molten lead(II) bromide (PbBr₂) yields lead metal at the cathode and bromine gas at the anode:

当离子化合物熔化时，其离子变得可移动并能够导电。电解熔融盐会将其分解为单质。例如，熔融溴化铅 (PbBr₂) 在阴极产生金属铅，在阳极产生溴气：

Cathode: Pb²⁺ + 2e⁻ → Pb (liquid)
Anode: 2Br⁻ → Br₂(g) + 2e⁻

阴极：Pb²⁺ + 2e⁻ → Pb（液态）
阳极：2Br⁻ → Br₂(g) + 2e⁻

Similarly, molten sodium chloride (NaCl) produces sodium metal at the cathode and chlorine gas at the anode. Aluminium extraction from bauxite (Al₂O₃) dissolved in molten cryolite is another crucial example where the oxide ions discharge at the anode to give oxygen.

类似地，熔融氯化钠 (NaCl) 在阴极产生金属钠，在阳极产生氯气。从铝土矿 (Al₂O₃) 溶于熔融冰晶石中提取铝是另一个重要例子，其中氧离子在阳极放电产生氧气。

4. Electrolysis of Aqueous Solutions: General Rules | 水溶液电解：一般规则

In aqueous solutions, water itself can be oxidised or reduced, competing with the ions of the dissolved solute. The products depend on the relative electrode potentials of the possible half-reactions. At the cathode, the cation with the most positive E° value (easiest to reduce) is discharged first. At the anode, the anion with the most negative E° value (easiest to oxidise) is discharged first, or water is oxidised if no simple anion is present.

在水溶液中，水本身可以被氧化或还原，与溶质离子竞争。产物取决于可能发生的半反应的相对电极电势。在阴极，具有最正 E° 值的阳离子（最容易还原）首先放电。在阳极，具有最负 E° 值的阴离子（最容易氧化）首先放电，或者如果没有简单阴离子，水则被氧化。

The general rule: for cations of very reactive metals (K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺), water is reduced in preference, producing hydrogen gas and hydroxide ions. For cations of less reactive metals (Zn²⁺, Fe²⁺, Pb²⁺, Cu²⁺, Ag⁺), the metal ion is reduced to the metal. For anions, if the solution contains a halide ion (Cl⁻, Br⁻, I⁻), the halogen is usually produced at the anode; otherwise, oxygen from water is produced.

一般规则：对于极活泼金属的阳离子（K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺），水优先还原，产生氢气和氢氧根离子。对于较不活泼金属的阳离子（Zn²⁺, Fe²⁺, Pb²⁺, Cu²⁺, Ag⁺），金属离子被还原为金属。对于阴离子，如果溶液中含有卤离子（Cl⁻、Br⁻、I⁻），通常在阳极产生卤素；否则，水被氧化产生氧气。

5. The Electrochemical Series and Discharge Order | 电化学系列与放电顺序

The electrochemical series (reactivity series) helps predict which species will be discharged at each electrode. Cations lower in the series (less reactive metals) are easier to reduce and thus discharge in preference to those higher up. The order of cation discharge (ease of reduction) is:

电化学系列（活泼性顺序）有助于预测哪种物质会在每个电极放电。系列中位置越低的阳离子（越不活泼的金属）越容易还原，因而优先于位置更高的阳离子放电。阳离子放电顺序（易还原性）如下：

• Ag⁺ > Cu²⁺ > H⁺ > Pb²⁺ > Sn²⁺ > Ni²⁺ > Fe²⁺ > Zn²⁺ > Al³⁺ > Mg²⁺ > Na⁺ > Ca²⁺ > K⁺

• Ag⁺ > Cu²⁺ > H⁺ > Pb²⁺ > Sn²⁺ > Ni²⁺ > Fe²⁺ > Zn²⁺ > Al³⁺ > Mg²⁺ > Na⁺ > Ca²⁺ > K⁺

For anions, the discharge order depends on the oxidation potential: S₂O₈²⁻ (rarely encountered), then NO₃⁻ and SO₄²⁻ are very stable and are not discharged from aqueous solution; instead, OH⁻ (from water) is oxidised. Halide ions discharge as follows: OH⁻ > I⁻ > Br⁻ > Cl⁻, but in concentrated halide solutions, the halide ion may discharge before OH⁻ due to concentration effects.

阴离子的放电顺序取决于氧化电位：S₂O₈²⁻（少见），然后 NO₃⁻ 和 SO₄²⁻ 非常稳定，在水溶液中不放电；而是 OH⁻（来自水）被氧化。卤离子放电顺序为：OH⁻ > I⁻ > Br⁻ > Cl⁻，但在浓卤化物溶液中，由于浓度效应，卤离子可能先于 OH⁻ 放电。

6. Electrolysis of Water and Acidified Water | 水和酸化水的电解

Pure water is a very poor conductor of electricity, so dilute sulfuric acid (or another strong acid) is added to provide ions. During the electrolysis of acidified water, the reactions are:

纯水导电性很差，因此添加稀硫酸（或其他强酸）以提供离子。在酸化水电解过程中，反应为：

Cathode: 2H⁺ + 2e⁻ → H₂(g)
Anode: 4OH⁻ → O₂(g) + 2H₂O + 4e⁻

阴极：2H⁺ + 2e⁻ → H₂(g)
阳极：4OH⁻ → O₂(g) + 2H₂O + 4e⁻

(g) + O₂(g). The volume ratio of hydrogen to oxygen collected is 2 : 1. Sulfate ions (SO₄²⁻) are not discharged; they remain in solution.

总反应为 2H₂O(l) → 2H₂(g) + O₂(g)。收集到的氢气与氧气的体积比为 2 : 1。硫酸根离子 (SO₄²⁻) 不放电，它们留在溶液中。

7. Electrolysis of Sodium Chloride Solution (Brine) | 氯化钠溶液（盐水）的电解

The electrolysis of concentrated aqueous sodium chloride is a key industrial process producing chlorine, hydrogen, and sodium hydroxide. At the cathode, water is reduced instead of Na⁺ because Na⁺ is far too difficult to reduce:

浓氯化钠水溶液的电解是一项重要的工业过程，可生产氯气、氢气和氢氧化钠。在阴极，水被还原而不是 Na⁺，因为 Na⁺ 极难还原：

Cathode: 2H₂O + 2e⁻ → H₂(g) + 2OH⁻

阴极：2H₂O + 2e⁻ → H₂(g) + 2OH⁻

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At the anode, chlorine gas is produced because the solution is concentrated in chloride ions, and overpotential effects favour Cl⁻ discharge over OH⁻:

在阳极，由于溶液富含氯离子，并且过电位效应使 Cl⁻ 优先于 OH⁻ 放电，因此产生氯气：

Anode: 2Cl⁻ → Cl₂(g) + 2e⁻

阳极：2Cl⁻ → Cl₂(g) + 2e⁻

The electrolyte remaining after removal of gases becomes rich in NaOH. In a membrane cell, an ion-exchange membrane prevents Cl₂ from reacting with NaOH.

去除气体后剩余的电解质富含 NaOH。在膜电解池中，离子交换膜可防止 Cl₂ 与 NaOH 反应。

8. Purification of Copper | 铜的精炼

Impure copper (blister copper) can be purified by electrolysis. A thin sheet of pure copper acts as the cathode, and the impure copper acts as the anode. The electrolyte is a solution of copper(II) sulfate (CuSO₄) acidified with dilute sulfuric acid.

粗铜（泡铜）可通过电解精炼。薄纯铜板作阴极，粗铜作阳极。电解液是经稀硫酸酸化的硫酸铜 (CuSO₄) 溶液。

²⁺(aq) + 2e⁻ → Cu(s). The mass of the anode decreases, the cathode increases, and impurities like silver, gold, and platinum fall as 'anode sludge' are not deposited because they require more negative potentials; they remain in solution.

电解过程中，阳极铜溶解：Cu(s) → Cu²⁺(aq) + 2e⁻。在阴极，Cu²⁺ 离子被还原并以纯铜形式沉积：Cu²⁺(aq) + 2e⁻ → Cu(s)。阳极质量减小，阴极质量增加，而银、金和铂等杂质作为“阳极泥”掉落在阳极下方。较活泼金属（如 Fe、Zn）虽然溶解，但由于需要更负的电位，不会在阴极沉积，而是留在溶液中。

9. Electroplating | 电镀

Electroplating involves coating an object with a thin layer of metal (e.g., silver, chromium, nickel) using electrolysis. The object to be plated is made the cathode, the plating metal is the anode, and the electrolyte contains a salt of the plating metal. Key conditions must be carefully controlled: low current density, appropriate temperature, and clean electrode surfaces ensure a smooth, adherent coating.

电镀是通过电解在物体表面镀上一层薄金属（如银、铬、镍）。待镀物体作阴极，镀层金属作阳极，电解液含有镀层金属的盐溶液。必须严格控制关键条件：低电流密度、适宜温度以及清洁的电极表面，以确保镀层光滑、附着牢固。

For example, silver plating uses a silver anode, a cathode object, and a solution of potassium argentocyanide (K[Ag(CN)₂]) to ensure a slow, even deposit of silver.

例如，镀银使用银阳极、待镀件作阴极，以及氰合银酸钾 (K[Ag(CN)₂]) 溶液，以保证银缓慢而均匀地沉积。

10. Extraction of Aluminium | 铝的提取

Aluminium is extracted by the electrolysis of purified aluminium oxide (Al₂O₃) dissolved in molten cryolite (Na₃AlF₆) at about 950°C. Cryolite lowers the melting point of alumina from over 2000°C to around 950°C, making the process economically viable. The cell is lined with graphite, which acts as the cathode, and the anodes are large blocks of graphite.

铝是通过电解溶解在熔融冰晶石 (Na₃AlF₆) 中的纯氧化铝 (Al₂O₃) 来提取的，温度约 950°C。冰晶石可将氧化铝的熔点从超过 2000°C 降至约 950°C，使该工艺经济可行。电解槽内衬石墨作阴极，阳极是大型石墨块。

g) + 4e⁻. The oxygen reacts with the graphite anodes, forming CO and CO₂, so the anodes need periodic replacement.

在阴极：Al³⁺ + 3e⁻ → Al(l)。熔融铝沉入槽底并被取出。在阳极：2O²⁻ → O₂(g) + 4e⁻。氧气与石墨阳极反应生成 CO 和 CO₂，因此阳极需定期更换。

11. Faraday's Laws of Electrolysis | 法拉第电解定律

Faraday's first law states that the mass of a substance produced or consumed at an electrode is directly proportional to the quantity of electricity (charge) passed through the electrolyte. The second law states that when the same quantity of electricity is passed through different electrolytes, the masses of substances liberated are proportional to their equivalent weights (molar mass divided by the number of electrons transferred).

法拉第第一定律指出，电极上产生或消耗的物质的质量与通过电解质的电量（电荷）成正比。第二定律指出，当相同的电量通过不同的电解质时，析出物质的质量与其当量（摩尔质量除以转移电子数）成正比。

at an electrode is given by n = Q / (zF), where z is the number of electrons in the electrode half-equation, and F is the Faraday constant (approx. 96500 C mol⁻¹).

电荷量 Q（单位：库仑）通过 Q = I × t 计算，其中 I 为电流（安培），t 为时间（秒）。电极上产生的物质的量（摩尔）n 可由 n = Q / (zF) 得出，其中 z 是电极半反应方程式中的电子数，F 是法拉第常数（约 96500 C mol⁻¹）。

12. Quantitative Electrolysis Calculations | 定量电解计算

CCEA exam questions frequently require calculating the mass or volume of products. For example, to find the mass of silver deposited when a current of 2.00 A flows through AgNO₃ solution for 30.0 minutes:

CCEA 考试经常要求计算产物的质量或体积。例如，计算 2.00 A 电流通过 AgNO₃ 溶液 30.0 分钟所沉积的银的质量：

Q = 2.00 A × (30.0 × 60) s = 3600 C
Ag⁺ + e⁻ → Ag, z = 1
n(Ag) = 3600 C / 96500 C mol⁻¹ ≈ 0.0373 mol
mass of Ag = 0.0373 mol × 107.9 g mol⁻¹ ≈ 4.02 g

Q = 2.00 A × (30.0 × 60) s = 3600 C
Ag⁺ + e⁻ → Ag，z = 1
n(Ag) = 3600 C / 96500 C mol⁻¹ ≈ 0.0373 mol
Ag 的质量 = 0.0373 mol × 107.9 g mol⁻¹ ≈ 4.02 g

gen is produced at the anode during electrolysis of dilute H₂SO₄, involving 4 electrons per O₂ molecule, the calculations follow Q / (4F) × 24.0 dm³.

对于气体，可使用摩尔气体体积（根据 CCEA 数据手册，常温常压下为 24.0 dm³ mol⁻¹）计算体积。若在电解稀 H₂SO₄ 时阳极产生氧气，每分子 O₂ 涉及 4 个电子，计算按 Q / (4F) × 24.0 dm³ 进行。

Always check the electrode half-equation to determine the correct value of z; common errors include using the wrong number of electrons per mole of product.

务必检查电极半反应方程式以确定正确的 z 值；常见错误包括每摩尔产物使用了错误的电子数。

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