Electrolysis: Key Points for GCSE WJEC Chemistry | 电解：GCSE WJEC 化学考点精讲

📚 Electrolysis: Key Points for GCSE WJEC Chemistry | 电解：GCSE WJEC 化学考点精讲

Electrolysis is a core topic in GCSE WJEC Chemistry, linking the concepts of ionic bonding, electrical conduction, and redox reactions. This article breaks down the process step by step, covering molten and aqueous electrolysis, predicting products, and key industrial applications. Use this guide to strengthen your understanding and prepare effectively for exam questions on electrolytic cells, half-equations, and the extraction of aluminium.

电解是GCSE WJEC化学的核心主题，它将离子键、导电性和氧化还原反应等概念联系起来。本文逐步解析电解过程，涵盖熔融态和水溶液电解、产物预测以及重要的工业应用。用这篇指南加深理解，有效备考电解池、半反应方程式和铝的提取等考题。

1. What is Electrolysis? | 什么是电解？

Electrolysis is the chemical decomposition of an ionic compound into its elements by passing an electric current through it. The compound must be in a molten or dissolved state so that the ions are free to move. An external power source drives a non-spontaneous redox reaction.

电解是指将电流通过离子化合物，使其发生化学分解的过程。该化合物必须处于熔融或溶解状态，使离子能够自由移动。外部电源驱动一个非自发的氧化还原反应。

In electrolysis, the electrolyte is the liquid or solution that conducts electricity and undergoes decomposition. The electrodes are solid conductors dipped into the electrolyte, connected to the power supply. The positive electrode is the anode, and the negative electrode is the cathode.

在电解中，电解质是导电并发生分解的液体或溶液。电极是浸入电解质中的固态导体，与电源相连。正极是阳极，负极是阴极。

2. Key Components of an Electrolytic Cell | 电解池的关键组成部分

An electrolytic cell contains:

电解池包含：

Power supply (battery or DC source) – provides the energy to force electrons round the circuit.
电源（电池或直流电源）——提供能量，驱使电子在回路中移动。
Electrodes – typically made of graphite (inert) or the metal being refined. Anode: positive; Cathode: negative.
电极——通常由石墨（惰性）或待精炼的金属制成。阳极：正极；阴极：负极。
Electrolyte – the molten or aqueous ionic compound containing free-moving ions.
电解质——含有自由移动离子的熔融态或水溶液中的离子化合物。
Connecting wires – complete the circuit.
导线——构成完整的回路。

At the anode, oxidation occurs: anions lose electrons. At the cathode, reduction occurs: cations gain electrons. The overall cell reaction is the sum of the two half-reactions.

在阳极发生氧化：阴离子失去电子。在阴极发生还原：阳离子得到电子。整个电池反应是两个半反应的总和。

3. Movement of Ions during Electrolysis | 电解过程中离子的移动

When the power supply is switched on, the positively charged cations are attracted to the negative cathode, and the negatively charged anions are attracted to the positive anode. This movement of ions constitutes the electric current through the electrolyte.

当电源接通时，带正电的阳离子向负极（阴极）移动，带负电的阴离子向正极（阳极）移动。这种离子的移动构成了通过电解质的电流。

In the external circuit, electrons flow from the positive terminal of the battery to the cathode, then from the anode back to the negative terminal. Inside the electrolyte, the charge is carried by the moving ions, not by electrons.

在外电路中，电子从电池的正极流向阴极，然后从阳极流回电池的负极。在电解质内部，电荷由移动的离子携带，而不是电子。

4. Electrolysis of Molten Ionic Compounds | 熔融离子化合物的电解

Heating an ionic solid until it melts frees the ions, allowing the liquid to conduct electricity. During electrolysis, the metal cation moves to the cathode and is discharged, forming the pure metal. The non-metal anion is discharged at the anode, forming the element as a gas.

将离子固体加热至熔化可以释放离子，使液体能够导电。电解时，金属阳离子移向阴极并被放电，形成纯金属。非金属阴离子在阳极被放电，形成气态单质。

Example: electrolysis of molten lead(II) bromide (PbBr₂).

示例：电解熔融溴化铅（PbBr₂）。

PbBr₂(l) → Pb²⁺(l) + 2Br⁻(l)
PbBr₂(l) → Pb²⁺(l) + 2Br⁻(l)
At the cathode: Pb²⁺ + 2e⁻ → Pb(l) (reduction). Grey lead metal forms.
阴极：Pb²⁺ + 2e⁻ → Pb(l)（还原）。生成灰色铅金属。
At the anode: 2Br⁻ → Br₂(g) + 2e⁻ (oxidation). Red-brown bromine gas is released.
阳极：2Br⁻ → Br₂(g) + 2e⁻（氧化）。放出红棕色溴蒸气。

No competing species are present in a molten salt, so products are predictable.

在熔融盐中没有竞争物种，因此产物是可以预测的。

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

When an ionic compound is dissolved in water, the solution also contains H⁺ and OH⁻ ions from the slight ionisation of water. The products at each electrode depend on the relative reactivity of the ions and their position in the electrochemical series.

当离子化合物溶于水时，溶液还含有来自水微弱电离产生的H⁺和OH⁻离子。每个电极的产物取决于离子的相对活泼性及其在电化学序中的位置。

For the cathode (negative): if the metal is less reactive than hydrogen (e.g. copper, silver), the metal ion is preferentially discharged and a metal coating forms. If the metal is more reactive than hydrogen (e.g. sodium, potassium), hydrogen gas is produced from the discharge of H⁺ ions.

对于阴极（负极）：如果金属比氢更不活泼（如铜、银），则金属离子优先放电，形成金属镀层。如果金属比氢更活泼（如钠、钾），则H⁺离子放电产生氢气。

Cathode product rule	阴极产物规则
Less reactive metal (Cu, Ag, Au) → metal deposited	不活泼金属（Cu, Ag, Au）→ 金属析出
More reactive metal (K, Na, Ca, Mg, Al) → hydrogen gas (H₂)	活泼金属（K, Na, Ca, Mg, Al）→ 氢气（H₂）

For the anode (positive): if the anion is a halide (Cl⁻, Br⁻, I⁻), the halogen gas is produced. If the anion is a sulfate (SO₄²⁻) or nitrate (NO₃⁻), oxygen gas is produced from the oxidation of OH⁻ ions. Inert electrodes (graphite) are used unless the anode itself is designed to take part.

对于阳极（正极）：如果阴离子是卤化物（Cl⁻, Br⁻, I⁻），则生成卤素气体。如果阴离子是硫酸根（SO₄²⁻）或硝酸根（NO₃⁻），则由OH⁻离子氧化生成氧气。除非阳极本身参与反应，否则使用惰性电极（石墨）。

For example, electrolysis of concentrated sodium chloride solution (brine) gives chlorine gas at the anode and hydrogen gas at the cathode, leaving sodium hydroxide in solution.

例如，电解浓氯化钠溶液（盐水）会在阳极产生氯气，在阴极产生氢气，溶液中留下氢氧化钠。

6. Predicting Products at Electrodes | 预测电极产物

A systematic approach using the electrochemical series helps predict discharge:

使用电化学序的系统方法有助于预测放电顺序：

Cations at cathode: ease of reduction increases going up the series (more reactive metal ions are harder to discharge). Anions at anode: ease of oxidation generally follows the order: halide ions > hydroxide ions > sulfate/nitrate ions.

阴极上的阳离子：还原的容易程度随活泼性增加而降低（越活泼的金属离子越难放电）。阳极上的阴离子：氧化的容易程度一般遵循顺序：卤离子 > 氢氧根离子 > 硫酸根/硝酸根离子。

If a halide ion is present, it is preferentially oxidised, unless it is very dilute (where OH⁻ might compete).
如果存在卤离子，它优先被氧化，除非浓度极稀（此时OH⁻可能竞争）。
If no halide is present, OH⁻ from water is oxidised to give oxygen gas.
如果没有卤离子，则水中的OH⁻被氧化，产生氧气。

Practice writing half-equations for each scenario to reinforce predictions.

练习写出每种情况下的半反应方程式，以巩固预测。

7. 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, and the plating metal is the anode (or added as ions in the electrolyte). A solution of a soluble salt of the plating metal is used as the electrolyte.

电镀是利用电解在金属物体表面覆盖一层薄薄的另一种金属的过程。待镀的物体作为阴极，镀层金属作为阳极（或作为离子存在于电解质中）。使用该镀层金属的可溶性盐溶液作为电解质。

Example: silver plating a spoon. The spoon is the cathode, a silver bar is the anode, and silver nitrate solution is the electrolyte. Ag⁺ ions from the solution are reduced at the cathode, forming a layer of silver on the spoon. The silver anode dissolves, replenishing Ag⁺ ions in solution.

例子：在勺子上镀银。勺子为阴极，银棒为阳极，硝酸银溶液为电解质。溶液中的Ag⁺离子在阴极被还原，在勺子上形成银层。银阳极溶解，补充溶液中的Ag⁺离子。

Electroplating is used for decorative purposes, corrosion protection, and improving wear resistance.

电镀用于装饰目的、防腐蚀以及提高耐磨性。

8. Extraction of Aluminium | 铝的提取

Aluminium is too reactive to be extracted from its ore (bauxite, Al₂O₃) by reduction with carbon. Electrolysis is used instead. The process involves dissolving purified aluminium oxide in molten cryolite (Na₃AlF₆) to lower the melting point from over 2000 °C to about 950 °C, saving energy.

铝太活泼，不能用碳还原法从矿石（铝土矿，Al₂O₃）中提取。因此使用电解。该过程将纯化的氧化铝溶解在熔融的冰晶石（Na₃AlF₆）中，将熔点从2000 °C以上降至约950 °C，节省能量。

The electrolytic cell uses carbon (graphite) anodes and a carbon-lined steel tank acting as the cathode. Aluminium ions are reduced at the cathode:

电解槽使用碳（石墨）阳极，碳衬里的钢槽作为阴极。铝离子在阴极被还原：

Al³⁺ + 3e⁻ → Al(l)

Oxygen ions are oxidised at the anode:

氧离子在阳极被氧化：

2O²⁻ → O₂(g) + 4e⁻

The oxygen reacts with the carbon anodes at high temperature, producing CO₂ and gradually wearing away the anodes. Therefore, the anodes need to be replaced regularly. Molten aluminium sinks to the bottom of the cell and is tapped off.

氧气在高温下与碳阳极反应，生成CO₂并逐渐消耗阳极。因此，阳极需要定期更换。熔融铝沉到槽底并排出。

9. Electrolysis of Brine (Chlor-alkali Process) | 食盐水的电解（氯碱工业）

The electrolysis of concentrated sodium chloride solution (brine) yields three important industrial chemicals: chlorine, hydrogen, and sodium hydroxide. A membrane cell keeps the products separate to prevent dangerous side reactions.

浓氯化钠溶液（盐水）的电解产生三种重要的工业化学品：氯气、氢气和氢氧化钠。膜电解槽将产物隔开，以防止危险的副反应。

The anode is titanium (coated), the cathode is steel. The electrolyte is concentrated brine. At the anode, chloride ions are oxidised:

阳极是钛（涂层），阴极是钢。电解质是浓盐水。在阳极，氯离子被氧化：

2Cl⁻ → Cl₂(g) + 2e⁻

At the cathode, water molecules are reduced to hydrogen and hydroxide ions (because Na⁺ is too reactive):

在阴极，水分子被还原为氢气和氢氧根离子（因为Na⁺过于活泼）：

2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)

The Na⁺ ions pass through the ion-selective membrane to balance the charge, forming sodium hydroxide solution alongside the OH⁻ ions.

Na⁺离子穿过离子选择性膜以平衡电荷，与OH⁻离子一起形成氢氧化钠溶液。

Uses: chlorine for water treatment and PVC production; hydrogen for fuel cells and margarine manufacture; sodium hydroxide for soap and paper production.

用途：氯气用于水处理和聚氯乙烯生产；氢气用于燃料电池和人造黄油制造；氢氧化钠用于肥皂和纸张生产。

10. Purification of Copper | 铜的精炼

Blister copper (about 98% pure) can be purified to 99.99% purity by electrolysis. The impure copper is the anode, a thin sheet of pure copper is the cathode, and the electrolyte is acidified copper(II) sulfate solution.

粗铜（约98%纯度）可以通过电解精炼至99.99%的纯度。不纯的铜作为阳极，纯铜薄片作为阴极，电解质是酸化硫酸铜溶液。

At the anode: Cu(s) → Cu²⁺(aq) + 2e⁻. The impure anode dissolves, and impurities such as silver, gold, and platinum fall to the bottom as ‘anode sludge’. Less reactive metal impurities stay unoxidised and collect below.

在阳极：Cu(s) → Cu²⁺(aq) + 2e⁻。不纯的阳极溶解，银、金和铂等杂质沉到槽底，成为“阳极泥”。更不活泼的金属杂质不被氧化而聚集在底部。

At the cathode: Cu²⁺(aq) + 2e⁻ → Cu(s). Pure copper is deposited on the cathode, which grows in mass.

在阴极：Cu²⁺(aq) + 2e⁻ → Cu(s)。纯铜在阴极上析出，阴极质量增加。

More reactive metal impurities (e.g. zinc, iron) remain in solution as ions. The concentration of copper ions remains approximately constant.

更活泼的金属杂质（如锌、铁）以离子形式保留在溶液中。铜离子的浓度大致保持不变。

11. Half-Equations and Oxidation-Reduction | 半反应和氧化还原

Writing half-equations is essential for describing electrode reactions. Always include state symbols where relevant. At the cathode (negative), electrons appear on the left side of the equation, signifying reduction. At the anode (positive), electrons appear on the right side, signifying oxidation.

书写半反应方程式对于描述电极反应至关重要。在相关情况下始终标注状态符号。在阴极（负极），电子出现在方程式的左边，表示还原。在阳极（正极），电子出现在方程式的右边，表示氧化。

Examples:

示例：

Cu²⁺(aq) + 2e⁻ → Cu(s) (reduction at cathode)
Cu²⁺(aq) + 2e⁻ → Cu(s)（阴极还原）
2Br⁻(l) → Br₂(g) + 2e⁻ (oxidation at anode)
2Br⁻(l) → Br₂(g) + 2e⁻（阳极氧化）
4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻ (oxidation at anode, when no halide present)
4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻（阳极氧化，当不存在卤离子时）

In any balanced half-equation, charge and mass must be conserved. Combining cathode and anode half-equations yields the overall redox equation.

在任何配平的半反应方程式中，电荷和质量必须守恒。将阴极和阳极半反应式相加即得到总氧化还原方程式。

Remember the mnemonic: ‘OIL RIG’ – Oxidation Is Loss of electrons, Reduction Is Gain of electrons. ‘RED CAT’ – Reduction at Cathode, ‘AN OX’ – Anode is Oxidation.

记住助记口诀：“OIL RIG”——氧化是失去电子，还原是得到电子。“RED CAT”——阴极发生还原，“AN OX”——阳极发生氧化。

12. Summary and Exam Tips | 总结与考试技巧

Key WJEC exam points: always check the state of the electrolyte (molten or aqueous) before predicting products. Know the role of water in aqueous electrolysis. Understand the industrial processes and their equations, especially for aluminium and brine. Practice drawing and labelling an electrolytic cell.

WJEC考试关键点：在预测产物之前，始终检查电解质的状态（熔融态还是水溶液）。了解水在电解水溶液中的作用。掌握工业过程及其方程式，尤其是铝和盐水的电解。练习绘制和标注电解池。

Common pitfalls: confusing the sign of electrodes (remember cathode is negative, anode is positive in electrolytic cells), forgetting to include water molecules when writing half-equations for OH⁻ discharge, and not considering concentration when predicting products (e.g. dilute vs concentrated NaCl).

常见误区：混淆电极的符号（记住电解池中阴极为负极，阳极为正极），在书写OH⁻放电的半反应方程时忘记加入水分子，以及在预测产物时不考虑浓度因素（例如稀NaCl与浓NaCl的区别）。

Use half-equations to explain observations: bubbles of a gas, colour changes, metal deposition. Link these to oxidation or reduction. In longer-answer questions, refer to the electrochemical series to justify why one ion is discharged in preference to another.

使用半反应方程式解释实验现象：气体气泡、颜色变化、金属沉积。将这些与氧化或还原联系起来。在长答案题中，引用电化学序来证明为什么一种离子优先于另一种离子放电。

Finally, ensure you can compare electrolytic cells with simple cells (chemical cells), noting the key difference: electrolysis uses electricity to cause a chemical change, whereas a chemical cell produces electricity from a spontaneous reaction.

最后，确保能够比较电解池和简单电池（化学电池），注意关键区别：电解利用电引起化学变化，而化学电池通过自发反应产生电。

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Electrolysis: Key Points for GCSE WJEC Chemistry | 电解：GCSE WJEC 化学考点精讲