Electrolysis in A-Level WJEC Chemistry: Key Points | 电解考点精讲

📚 Electrolysis in A-Level WJEC Chemistry: Key Points | 电解考点精讲

Electrolysis is a core topic in the WJEC A-Level Chemistry specification, testing your understanding of redox processes, ionic motion, and quantitative relationships. Mastery of electrolysis requires not only memorising the discharge series but also applying Faraday’s laws and understanding industrial applications. This guide breaks down the essential concepts, common pitfalls, and exam techniques you need to excel in the topic.

电解是WJEC A-Level化学大纲中的核心考点，考查你对氧化还原过程、离子运动以及定量关系的理解。要掌握电解，不仅需要记住放电顺序，还要会运用法拉第定律并理解其工业应用。本指南将为你详细拆解必备概念、常见误区以及考试技巧，帮助你在这一专题上脱颖而出。

1. What is Electrolysis? | 什么是电解？

Electrolysis is the chemical decomposition of an ionic compound brought about by passing a direct electric current through its molten or dissolved state. The external power source forces non-spontaneous redox reactions to occur, with reduction at the cathode and oxidation at the anode.

电解是指将直流电通过熔融态或溶液态的离子化合物，从而使其发生化学分解的过程。外部电源强制非自发的氧化还原反应发生，阴极发生还原反应，阳极发生氧化反应。

The key is that ions must be free to move – solid ionic compounds do not conduct electricity. The energy from the battery drives electrons around the external circuit and enables discharge of ions at the electrodes.

关键在于离子必须能自由移动——固态离子化合物无法导电。电池提供的能量驱动电子在外部电路中流动，并使离子在电极处放电。

2. The Electrolytic Cell: Components and Setup | 电解池的组成与装置

An electrolytic cell consists of a direct current supply, two electrodes (often inert graphite or platinum, but sometimes reactive metals), and an electrolyte – either a molten salt or an aqueous solution. The positive electrode is the anode, and the negative electrode is the cathode.

电解池由直流电源、两个电极（通常是惰性的石墨或铂，有时也用活性金属）以及电解质组成——电解质可以是熔融盐或水溶液。正极为阳极，负极为阴极。

Inert electrodes do not take part in the electrolysis reaction; they merely provide a surface for ion discharge. Reactive anodes, such as copper in copper refining, can oxidise and dissolve.

惰性电极不参与电解反应，仅为离子放电提供界面。活性阳极，例如铜精炼中的铜极，则会被氧化溶解。

Electron flow in the external circuit is from the negative terminal of the battery to the cathode, and from the anode to the positive terminal. Inside the electrolyte, cations move towards the cathode and anions towards the anode.

外部电路中电子从电池负极流向阴极，从阳极流向正极。在电解质内部，阳离子移向阴极，阴离子移向阳极。

3. Discharge Series and Selective Discharge | 放电顺序与选择性放电

When an electrolyte contains more than one cation or anion, the ion that is discharged at each electrode depends on the relative ease of reduction or oxidation. The ease of discharge follows a specific series that you must know for exams.

当电解质中含有不止一种阳离子或阴离子时，每种电极上放电的离子取决于其还原或氧化的相对难易程度。放电顺序遵循特定的序列，这是考试必须掌握的。

Cation discharge series (ease of reduction at cathode, from easiest to hardest): Ag⁺ > Cu²⁺ > H⁺ > Pb²⁺ > Sn²⁺ > Fe²⁺ > Zn²⁺ > Al³⁺ > Mg²⁺ > Na⁺ > Ca²⁺ > K⁺. For aqueous solutions, H⁺ ions from water compete with metal ions, and hydrogen gas is produced if the metal is more reactive than hydrogen.

阳离子放电顺序（阴极还原难易，从易到难）：Ag⁺ > Cu²⁺ > H⁺ > Pb²⁺ > Sn²⁺ > Fe²⁺ > Zn²⁺ > Al³⁺ > Mg²⁺ > Na⁺ > Ca²⁺ > K⁺。在水溶液中，来自水的H⁺会与金属离子竞争；如果金属比氢活泼，则阴极产生氢气。

Anion discharge series (ease of oxidation at anode, from easiest to hardest): I⁻ > Br⁻ > Cl⁻ > OH⁻ > SO₄²⁻ (and other oxyanions like NO₃⁻). OH⁻ ions from water are discharged instead of sulfate or nitrate; oxygen gas is evolved.

阴离子放电顺序（阳极氧化难易，从易到难）：I⁻ > Br⁻ > Cl⁻ > OH⁻ > SO₄²⁻（及其他含氧酸根如NO₃⁻）。水中的OH⁻会优先于硫酸根或硝酸根放电，析出氧气。

The position of a metal in the reactivity series determines whether hydrogen or the metal is discharged at the cathode. For metals above hydrogen, water is reduced to H₂ and OH⁻. For metals below hydrogen, the metal ion is reduced to the solid metal.

金属在活动性顺序中的位置决定了阴极是析出氢气还是金属。活动性高于氢的金属，水被还原为H₂和OH⁻；活动性低于氢的金属，则金属离子被还原成金属单质。

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

When a molten ionic compound is electrolysed, the only ions present are from the salt itself, so products are straightforward. The metal cation is reduced at the cathode, and the non-metal anion is oxidised at the anode.

当电解熔融离子化合物时，体系中只存在该盐的离子，产物十分明确。金属阳离子在阴极被还原，非金属阴离子在阳极被氧化。

Example: electrolysis of molten sodium chloride. Cathode: Na⁺ + e⁻ → Na(l) (sodium metal). Anode: 2Cl⁻ → Cl₂(g) + 2e⁻ (chlorine gas). Overall: 2NaCl(l) → 2Na(l) + Cl₂(g).

示例：电解熔融氯化钠。阴极：Na⁺ + e⁻ → Na(l)（金属钠）。阳极：2Cl⁻ → Cl₂(g) + 2e⁻（氯气）。总反应：2NaCl(l) → 2Na(l) + Cl₂(g)。

Similarly, electrolysis of molten aluminium oxide (in cryolite) is the industrial method for extracting aluminium. At the cathode: Al³⁺ + 3e⁻ → Al(l). At the carbon anode: 2O²⁻ → O₂(g) + 4e⁻, but the oxygen reacts with the carbon anode, gradually consuming it.

同样，电解熔融氧化铝（溶于冰晶石）是工业上提取铝的方法。阴极：Al³⁺ + 3e⁻ → Al(l)。在碳阳极：2O²⁻ → O₂(g) + 4e⁻，但产生的氧气会与碳阳极反应，逐渐消耗阳极。

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

In aqueous electrolysis, water provides H⁺ and OH⁻ ions that compete for discharge. This makes predicting products more complex and requires applying the discharge series carefully.

在水溶液电解中，水提供的H⁺和OH⁻会参与竞争放电。这使得产物预测更为复杂，需要仔细运用放电顺序。

Case 1: dilute NaCl(aq). Ions present: Na⁺, Cl⁻, H⁺, OH⁻. At the cathode, H⁺ is easier to discharge than Na⁺, so H₂ gas is produced. At the anode, OH⁻ is easier to discharge than Cl⁻, so O₂ gas is produced. The solution becomes alkaline due to remaining Na⁺ and OH⁻.

情况1：稀NaCl(aq)。存在的离子：Na⁺、Cl⁻、H⁺、OH⁻。阴极：H⁺比Na⁺更容易放电，产生H₂气体。阳极：OH⁻比Cl⁻更容易放电，产生O₂气体。溶液中剩余Na⁺和OH⁻，溶液呈碱性。

Case 2: concentrated NaCl(aq). High concentration of Cl⁻ means it is discharged at the anode instead of OH⁻. Anode: Cl₂ gas. Cathode: still H₂ gas. The solution becomes alkaline (NaOH formed). This is the basis of the chlor-alkali industry.

情况2：浓NaCl(aq)。Cl⁻浓度高，因此优先在阳极放电而非OH⁻。阳极：Cl₂气体。阴极：仍然是H₂气体。溶液呈碱性（生成NaOH）。这是氯碱工业的基础。

Case 3: CuSO₄(aq) with inert electrodes. Cathode: Cu²⁺ is discharged (Cu metal deposited). Anode: OH⁻ is discharged (O₂ gas), leaving acidic solution (H₂SO₄).

情况3：使用惰性电极电解CuSO₄(aq)。阴极：Cu²⁺放电（析出铜金属）。阳极：OH⁻放电（放出O₂），溶液变为酸性（生成H₂SO₄）。

6. Effect of Concentration and Electrode Type | 浓度与电极类型的影响

Concentration can alter the discharge order. A dilute solution of chloride gives oxygen at the anode, while a concentrated chloride solution gives chlorine. This is because the higher concentration lowers the discharge potential, making Cl⁻ oxidation more favourable.

浓度可以改变放电顺序。稀氯化物溶液在阳极析出氧气，而浓氯化物溶液析出氯气。这是因为高浓度降低了放电电位，使得Cl⁻的氧化更为有利。

Reactive electrodes themselves can be oxidised. If copper electrodes are used in CuSO₄ electrolysis, the anode copper dissolves: Cu(s) → Cu²⁺(aq) + 2e⁻, and the cathode becomes plated. This is used in electroplating and refining.

活性电极本身可被氧化。如在CuSO₄电解中使用铜电极，阳极铜溶解：Cu(s) → Cu²⁺(aq) + 2e⁻，阴极则镀上铜。这在电镀和精炼中得到应用。

7. Faraday’s Laws and Quantitative Electrolysis | 法拉第定律与定量电解

Faraday’s laws connect the quantity of electricity passed to the amount of substance produced at an electrode. The first law states that the mass of substance discharged is proportional to the charge passed (Q).

法拉第定律将通过的电量与电极上生成的物质的量关联起来。第一定律指出，放电物质的质量与通过的电量（Q）成正比。

The second law states that when the same charge is passed through different electrolytes, the masses of different substances deposited are proportional to their molar masses divided by the number of electrons per ion (equivalent mass).

第二定律指出，相同的电量通过不同的电解质时，析出的不同物质的质量与其摩尔质量/每离子电子数的比值（当量质量）成正比。

Q = I × t

where Q is charge in coulombs, I is current in amperes, t is time in seconds. The number of moles of electrons transferred is: n(e⁻) = Q / F, with F = 96500 C mol⁻¹ (the Faraday constant).

其中Q为电荷量（库仑），I为电流（安培），t为时间（秒）。转移电子的物质的量为：n(e⁻) = Q / F，F = 96500 C mol⁻¹（法拉第常数）。

To find the mass of product, use the stoichiometric ratio from the half-equation. For example, to deposit 1 mol of Cu (Ar = 63.5), 2 mol e⁻ are needed. So mass = (I × t × M) / (n × F), where n is the number of electrons per ion.

要计算产物质量，使用半反应式中的化学计量比。例如，析出1 mol Cu（Ar = 63.5）需要2 mol e⁻。所以质量 = (I × t × M) / (n × F)，其中n为每个离子所需的电子数。

Exam questions often ask for time to plate a certain mass, or current required. Always convert time to seconds and use the correct electrode half-equation.

考题经常要求计算电镀一定质量所需的时间，或所需的电流。一定要把时间换算成秒，并使用正确的电极半反应。

8. Industrial Applications of Electrolysis | 电解的工业应用

1. Extraction of aluminium – electrolysis of Al₂O₃ dissolved in molten cryolite (lower melting point, saves energy). Carbon anodes are consumed. Cathode: Al deposition. Overall: 2Al₂O₃ → 4Al + 3O₂.

1. 铝的提炼——电解溶于熔融冰晶石中的Al₂O₃（降低熔点，节约能源）。碳阳极被消耗。阴极：铝析出。总反应：2Al₂O₃ → 4Al + 3O₂。

2. Chlor-alkali industry – electrolysis of concentrated brine. Products: Cl₂ (anode), H₂ (cathode), and NaOH (in solution). Uses: disinfectants, PVC, paper.

2. 氯碱工业——电解浓盐水。产物：Cl₂（阳极）、H₂（阴极）及NaOH（在溶液中）。用途：消毒剂、PVC、造纸。

3. Electroplating – coating a metal with another metal for corrosion resistance or decoration. Object is cathode, anode is the plating metal. Example: silver plating uses Ag electrode and electrolyte containing Ag⁺.

3. 电镀——将一种金属镀到另一种金属上，以抗腐蚀或装饰。镀件为阴极，阳极为镀层金属。例如镀银使用银电极和含Ag⁺的电解质。

4. Electrorefining – purification of copper. Impure copper anode dissolves; pure copper deposits on cathode. Impurities (e.g., Ag, Au) fall as anode sludge.

4. 电解精炼——铜的提纯。不纯铜阳极溶解，纯铜在阴极析出。银、金等杂质以阳极泥形式沉落。

9. Common Mistakes and How to Avoid Them | 常见错误与避坑指南

Many students lose marks by confusing anode and cathode signs or failing to include the role of water in aqueous electrolysis. Remember: cation goes to cathode, anion to anode. The cathode is the negative terminal in electrolysis (but positive in a galvanic cell – do not mix them up).

许多学生因混淆阳极与阴极的正负号，或忽略了水溶液电解中水的作用而失分。记住：阳离子趋向阴极，阴离子趋向阳极。在电解池中，阴极是负极（但在原电池中阴极是正极——切勿混淆）。

Another frequent error is neglecting the competition from water ions. In aqueous NaCl, if you write Na⁺ discharge at cathode, you will lose marks; hydrogen is produced instead. Always check the reactivity series and discharge series.

另一个常见错误是忽略了水分子的竞争。在NaCl水溶液中，如果写阴极Na⁺放电，就会失分；实际是析出氢气。一定核对活动性顺序和放电顺序。

In quantitative calculations, students often forget to convert minutes to seconds or use the wrong number of electrons per ion. Practice balancing half-equations and substituting into the formula correctly.

在定量计算中，学生常忘记把分钟换算成秒，或使用了错误的每离子电子数。务必练习配平半反应式，并正确套用公式。

Also, in electroplating questions, the anode mass decreases while the cathode mass increases. The electrolyte concentration stays constant. Do not confuse with simple inert-electrode electrolysis where electrolyte composition changes.

此外，在电镀题目中，阳极质量减少，阴极质量增加，电解质浓度不变。不要与惰性电极电解（电解质组成会改变）混淆。

10. Exam Techniques and Typical Questions | 考试技巧与典型题型

WJEC A-Level electrolysis questions often come in two forms: descriptive (predicting products, explaining observations) and quantitative (Faraday calculations). Brush up on half-equation writing and balancing charges.

WJEC A-Level的电解考题通常有两种形式：描述类（预测产物、解释现象）和定量类（法拉第计算）。要熟练书写半反应式并配平电荷。

For descriptive questions, state the ions present, the ions discharged at each electrode with reasons (discharge series or concentration), the half-equations, and any observable changes (gas bubbles, colour changes, pH changes).

对于描述类题目，要写出存在的离子、各电极上放电的离子并说明理由（放电顺序或浓度影响）、半反应式以及任何可观察到的变化（气泡、颜色变化、pH变化）。

For calculations, show every step: Q = I × t, then n(e⁻) = Q / 96500, then use the ratio from the half-equation to find moles of substance, then mass. Always check units and significant figures.

对于计算题，展示每一步：Q = I × t，然后n(e⁻) = Q / 96500，再根据半反应的比例求出物质的量，最后算质量。务必检查单位和有效数字。

A typical multi-step question might ask: “Calculate the mass of copper deposited when a current of 2.0 A is passed through CuSO₄ for 30 minutes.” Step 1: t = 30 × 60 = 1800 s. Q = 2.0 × 1800 = 3600 C. n(e⁻) = 3600 / 96500 ≈ 0.0373 mol. Cu²⁺ + 2e⁻ → Cu, so 2 mol e⁻ give 1 mol Cu. n(Cu) = 0.0373 / 2 ≈ 0.01865 mol. mass = 0.01865 × 63.5 ≈ 1.18 g.

一个典型的多步计算题可能是：“计算在CuSO₄中通过2.0 A电流电解30分钟，析出的铜的质量。”步骤1：t = 30 × 60 = 1800 s。Q = 2.0 × 1800 = 3600 C。n(e⁻) = 3600 / 96500 ≈ 0.0373 mol。Cu²⁺ + 2e⁻ → Cu，因此2 mol e⁻生成1 mol Cu。n(Cu) = 0.0373 / 2 ≈ 0.01865 mol。质量 = 0.01865 × 63.5 ≈ 1.18 g。

Tables can help compare products under different conditions. For example:

Electrolyte	Cathode product	Anode product
dilute NaCl(aq)	H₂	O₂
concentrated NaCl(aq)	H₂	Cl₂
CuSO₄(aq) (inert electrodes)	Cu	O₂

表格有助于对比不同条件下的产物。例如：

电解质	阴极产物	阳极产物
稀NaCl(aq)	H₂	O₂
浓NaCl(aq)	H₂	Cl₂
CuSO₄(aq)（惰性电极）	Cu	O₂

Remember that in the exam, you may be asked to draw and label an electrolytic cell, indicate electron flow, or explain why certain products form. Using keywords like ‘cathode reduction’, ‘anode oxidation’, and ‘discharge series’ will secure marks.

记住，在考场上你可能被要求画出并标注电解池，标明电子流向，或解释为何生成某种产物。使用“阴极还原”“阳极氧化”“放电顺序”等关键词能确保得分。

11. Key Equations and Half-Equations to Memorise | 必记的关键方程式与半反应

Below are the most frequently required half-equations in WJEC electrolysis questions. Knowing them by heart will save you time and avoid errors.

以下是WJEC电解考题中最常出现的半反应式。熟记它们可以节省时间并避免错误。

Cathode: 2H⁺ + 2e⁻ → H₂(g) (reduction of hydrogen ions)
Cathode: Cu²⁺ + 2e⁻ → Cu(s) (copper deposition)
Anode: 4OH⁻ → 2H₂O + O₂(g) + 4e⁻ (oxidation of hydroxide)
Anode: 2Cl⁻ → Cl₂(g) + 2e⁻ (oxidation of chloride)
Anode: 2I⁻ → I₂(aq) + 2e⁻ (oxidation of iodide, brown colour)
Anode: 2Br⁻ → Br₂(aq) + 2e⁻ (orange colour)

阴极：2H⁺ + 2e⁻ → H₂(g)（氢离子还原）
阴极：Cu²⁺ + 2e⁻ → Cu(s)（铜析出）
阳极：4OH⁻ → 2H₂O + O₂(g) + 4e⁻（氢氧根氧化）
阳极：2Cl⁻ → Cl₂(g) + 2e⁻（氯离子氧化）
阳极：2I⁻ → I₂(aq) + 2e⁻（碘离子氧化，棕色）
阳极：2Br⁻ → Br₂(aq) + 2e⁻（橙色）

Practise writing overall equations by combining half-equations. For copper(II) sulfate electrolysis with inert electrodes: cathode: Cu²⁺ + 2e⁻ → Cu; anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻. Multiply cathode reaction by 2 and add: 2Cu²⁺ + 4OH⁻ → 2Cu + 2H₂O + O₂.

练习合并半反应写出总反应式。例如用惰性电极电解硫酸铜：阴极：Cu²⁺ + 2e⁻ → Cu；阳极：4OH⁻ → 2H₂O + O₂ + 4e⁻。将阴极反应乘以2后相加：2Cu²⁺ + 4OH⁻ → 2Cu + 2H₂O + O₂。

12. Quick Recap and Final Tips | 快速回顾与终极提点

Electrolysis boils down to: free-moving ions, external power supply, discharge series, and Faraday’s laws. Never forget water’s autodissociation in aqueous systems. Tie in the reactivity series to predict cathode products. For quantification, the Faraday constant is your bridge between electric current and chemical change.

电解可归结为：自由移动的离子、外部电源、放电顺序和法拉第定律。绝不要忘记水溶液体系中水的自解离。结合活动性顺序预测阴极产物。定量分析时，法拉第常数是电流与化学变化之间的桥梁。

A final piece of advice: always read the question carefully – check whether electrodes are inert or reactive, note the concentration of the electrolyte, and identify all ions present before predicting. With systematic practice, electrolysis can become one of your strongest topics.

最后一条建议：仔细审题——检查电极是惰性还是活性，注意电解质的浓度，预测前识别所有存在的离子。经过系统的训练，电解可以成为你最拿手的专题之一。

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Electrolysis in A-Level WJEC Chemistry: Key Points | 电解考点精讲