📚 Electrolysis | 电解
Electrolysis is the process of using direct electric current to drive a non-spontaneous chemical reaction. It is a key topic in IGCSE Edexcel Science, linking chemistry and physics through the decomposition of ionic compounds. Understanding electrolysis helps explain industrial processes like metal extraction, electroplating, and the production of chemicals such as chlorine and sodium hydroxide.
电解是利用直流电驱动非自发化学反应的过程。它是IGCSE Edexcel科学中的一个核心主题，通过离子化合物的分解将化学和物理联系起来。理解电解有助于解释金属提取、电镀以及氯气和氢氧化钠等化学品的工业生产过程。
1. What is Electrolysis? | 什么是电解？
Electrolysis involves passing an electric current through an electrolyte, causing it to decompose. In ionic compounds, the ions must be free to move – this is possible in the molten state or in aqueous solution. The electrical energy forces chemical changes at the electrodes.
电解是通过电流通过电解质使其分解的过程。在离子化合物中，离子必须能够自由移动——在熔融状态或水溶液中可以实现这一点。电能迫使电极上发生化学变化。
2. Key Terms in Electrolysis | 电解关键术语
Several important terms are essential: electrolyte – an ionic substance that conducts electricity when molten or dissolved; electrodes – conductors through which current enters or leaves the electrolyte (usually graphite or metal); anode – the positive electrode where oxidation occurs; cathode – the negative electrode where reduction occurs; anion – negatively charged ion attracted to the anode; cation – positively charged ion attracted to the cathode.
几个重要的术语必须掌握：电解质——在熔融或溶解时能导电的离子物质；电极——电流进入或离开电解质的导体（通常是石墨或金属）；阳极——发生氧化反应的正极；阴极——发生还原反应的负极；阴离子——被阳极吸引的带负电荷的离子；阳离子——被阴极吸引的带正电荷的离子。
3. The Electrolytic Cell | 电解池
An electrolytic cell consists of a container with the electrolyte, two electrodes connected to a direct current (DC) power supply, and wires. The anode is connected to the positive terminal, and the cathode to the negative terminal. Electrons flow from the power supply to the cathode, reducing cations. At the anode, anions give up electrons, which flow back to the power supply.
电解池包括装有电解质的容器、连接直流电源的两个电极和导线。阳极接正极，阴极接负极。电子从电源流向阴极，还原阳离子。在阳极，阴离子失去电子，电子流回电源。
4. Electrolysis of Molten Ionic Compounds | 熔融离子化合物的电解
When an ionic compound is melted, the ions become mobile. During electrolysis, the cation is reduced at the cathode to form the metal, and the anion is oxidized at the anode to form a non-metal. For example, electrolysis of molten sodium chloride (NaCl) produces sodium metal at the cathode and chlorine gas at the anode:
当离子化合物熔化时，离子变得可移动。在电解过程中，阳离子在阴极被还原生成金属，阴离子在阳极被氧化生成非金属。例如，熔融氯化钠 (NaCl) 的电解在阴极生成金属钠，在阳极生成氯气：
At cathode: Na⁺ + e⁻ → Na (reduction)
At anode: 2Cl⁻ → Cl₂ + 2e⁻ (oxidation)
Similarly, electrolysis of molten lead(II) bromide yields lead metal and bromine gas.
类似地，熔融溴化铅电解生成金属铅和溴气。
5. Electrolysis of Aqueous Solutions | 水溶液的电解
In aqueous solutions, water molecules are also present, and they can be oxidized or reduced. This makes the products more complex. At the cathode, if the metal is more reactive than hydrogen, hydrogen gas is produced; otherwise, the metal is deposited. At the anode, hydroxide ions or halide ions can be oxidized. The concentration of the solution also affects the product.
在水溶液中，水分子也存在，它们可以被氧化或还原。这使得产物更加复杂。在阴极，如果金属比氢更活泼，则产生氢气；否则金属会析出。在阳极，氢氧根离子或卤离子可以被氧化。溶液浓度也会影响产物。
6. Factors Affecting Products of Aqueous Electrolysis | 影响水溶液电解产物的因素
Reactivity series: A cation of a metal less reactive than hydrogen (e.g., Cu²⁺, Ag⁺) will be reduced to metal at the cathode. Ions of metals more reactive than hydrogen (e.g., K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺) stay in solution, and hydrogen gas is produced from water.
金属活动性顺序：比氢不活泼的金属阳离子（如 Cu²⁺, Ag⁺）会在阴极被还原成金属。比氢活泼的金属离子（如 K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺）留在溶液中，由水产生氢气。
Anion type: Halide ions (Cl⁻, Br⁻, I⁻) are oxidized to the halogen at the anode if their concentration is high. If they are dilute, or if the anion is sulfate or nitrate, oxygen gas is produced from hydroxide ions (OH⁻) in water.
阴离子类型：卤离子（Cl⁻, Br⁻, I⁻）在浓度高时在阳极被氧化成卤素。如果浓度低，或阴离子是硫酸根或硝酸根，则由水中的氢氧根离子 (OH⁻) 产生氧气。
Concentration: In concentrated sodium chloride solution, chlorine is produced at the anode; in very dilute NaCl, oxygen is more likely.
浓度：在浓氯化钠溶液中，阳极产生氯气；在很稀的 NaCl 溶液中，更可能产生氧气。
7. Electrolysis of Brine (Concentrated NaCl Solution) | 盐水电解（浓氯化钠溶液）
The electrolysis of concentrated aqueous sodium chloride (brine) is industrially important. The products are hydrogen gas at the cathode, chlorine gas at the anode, and sodium hydroxide left in the solution. The reactions are:
电解浓氯化钠水溶液（盐水）具有重要的工业价值。产物是阴极的氢气、阳极的氯气以及留在溶液中的氢氧化钠。反应如下：
Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻
Anode: 2Cl⁻ → Cl₂ + 2e⁻
The overall reaction shows that NaCl and water produce hydrogen, chlorine, and sodium hydroxide. This is the chlor-alkali process.
总反应表明 NaCl 和水生成氢气、氯气和氢氧化钠。这就是氯碱工业过程。
8. Electrolysis of Acidified Water | 酸化水的电解
Pure water is a poor conductor, so a small amount of sulfuric acid or an ionic compound is added. At the cathode, hydrogen gas is formed; at the anode, oxygen gas is formed. The volume ratio of hydrogen to oxygen produced is 2:1. This confirms the composition of water as H₂O.
纯水导电性差，因此加入少量硫酸或离子化合物。阴极生成氢气，阳极生成氧气。产生的氢气与氧气的体积比为 2:1。这证实了水的组成为 H₂O。
Cathode: 4H⁺ + 4e⁻ → 2H₂
Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻
9. Electrolysis of Copper(II) Sulfate with Inert Electrodes | 用惰性电极电解硫酸铜
Using inert carbon or platinum electrodes, copper(II) ions are reduced at the cathode to form a reddish-brown copper metal deposit. At the anode, sulfate ions are not oxidized; instead, oxygen gas is produced from water. The blue colour of the solution fades as Cu²⁺ ions are removed.
使用惰性碳或铂电极时，铜(II)离子在阴极被还原，形成红棕色的金属铜沉积。在阳极，硫酸根离子不被氧化，而是由水产生氧气。随着 Cu²⁺ 离子被移除，溶液的蓝色逐渐褪去。
Cathode: Cu²⁺ + 2e⁻ → Cu
Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻
10. Electrolysis with Copper Electrodes – Purification of Copper | 用铜电极电解——精炼铜
A pure copper cathode and an impure copper anode are placed in copper(II) sulfate solution. During electrolysis, the impure anode loses mass as copper atoms are oxidized to Cu²⁺ ions. These ions migrate to the cathode, where they are reduced and deposited as pure copper. Impurities like silver and gold fall as anode sludge.
将纯铜阴极和杂质铜阳极放入硫酸铜溶液中。电解时，不纯的阳极失去质量，因为铜原子被氧化成 Cu²⁺ 离子。这些离子移到阴极，在那里被还原并以纯铜形式沉积。银和金等杂质以阳极泥的形式掉落。
Anode (oxidation): Cu → Cu²⁺ + 2e⁻
Cathode (reduction): Cu²⁺ + 2e⁻ → Cu
11. Electroplating | 电镀
Electroplating involves coating an object with a thin layer of metal using electrolysis. The object to be plated is made the cathode, the anode is a bar of the plating metal, and the electrolyte contains ions of that metal. For silver plating, a silver anode and an object as cathode are placed in a silver nitrate solution. This process improves appearance and resists corrosion.
电镀是利用电解在物体表面覆盖一层薄金属。被镀的物体作为阴极，阳极是镀层金属，电解液含有该金属的离子。例如镀银时，银阳极和物体阴极放入硝酸银溶液。这一过程改善外观并抗腐蚀。
12. Calculations in Electrolysis – Faraday's Laws | 电解计算——法拉第定律
The quantity of product formed during electrolysis depends on the amount of charge passed. The relevant equations are:
电解过程中生成产物的量取决于通过的电量。相关的公式如下：
Charge (Q) = Current (I) × Time (t) — Q = I × t
Number of moles of electrons: n(e⁻) = Q / F, where F = 96 500 C mol⁻¹
copper (Cu²⁺ + 2e⁻ → Cu), 2 moles of electrons are needed. Then mass = moles × molar mass.
质量计算：先求转移电子的物质的量，再根据半反应的电子比求出产物的物质的量，最后乘以摩尔质量。例如沉积 1 摩尔铜需要 2 摩尔电子。
mass of copper is deposited if a current of 2.0 A passes through CuSO₄ solution for 1 hour? 96500 ≈ 0.0746 mol, moles of Cu = 0.0746 / 2 = 0.0373 mol, mass = 0.0373 × 63.5 ≈ 2.37 g.
典型题目：如果 2.0 A 的电流通过硫酸铜溶液 1 小时，沉积铜的质量是多少？计算：Q = 2.0 × 3600 = 7200 C，n(e⁻) = 7200 / 96500 ≈ 0.0746 mol，Cu 的物质的量 = 0.0746 / 2 = 0.0373 mol，质量 = 0.0373 × 63.5 ≈ 2.37 g。
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