Electrolysis Key Points Review | 电解考点精讲

📚 Electrolysis Key Points Review | 电解考点精讲

Electrolysis is a fundamental topic in IB and CIE Chemistry, bridging redox theory and quantitative analysis. This guide breaks down the core concepts, from ion discharge to Faraday’s laws, helping you master every exam question.

电解是IB和CIE化学的基础主题，连接了氧化还原理论与定量分析。本指南分解了从离子放电到法拉第定律的核心概念，帮助你掌握每一道考题。

1. Introduction to Electrolysis | 电解简介

Electrolysis is the process of using direct current (DC) to drive a non-spontaneous redox reaction. Electrical energy is converted into chemical energy, causing the decomposition of an electrolyte.

电解是利用直流电驱使非自发氧化还原反应发生的过程。电能转化为化学能，导致电解质分解。

Unlike a galvanic cell where reactions produce electricity, electrolysis requires an external power source. The electrolyte contains mobile ions which are discharged at the electrodes.

与原电池中反应产生电流不同，电解需要外部电源。电解质含有可移动的离子，它们在电极上放电。

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2. Components of an Electrolytic Cell | 电解池的组成

The key components are the anode (positive electrode), cathode (negative electrode), electrolyte, and an external DC supply.

关键组成部分包括阳极（正极）、阴极（负极）、电解质和外部直流电源。

Oxidation occurs at the anode, where anions lose electrons. Reduction occurs at the cathode, where cations gain electrons. Remember: An Ox and Red Cat (Anode Oxidation, Reduction Cathode).

氧化发生在阳极，阴离子失去电子。还原发生在阴极，阳离子得到电子。记住口诀：阳氧阴还（阳极氧化，阴极还原）。

Electrodes can be inert (graphite, platinum) or reactive (copper, silver). Inert electrodes do not participate in the reaction; reactive electrodes may be consumed or plated.

电极可以是惰性（石墨、铂）或活泼（铜、银）。惰性电极不参与反应；活泼电极本身可能被消耗或镀上金属。

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3. Electrolyte: Molten vs. Aqueous | 熔融电解质与水溶液

In molten electrolytes, only the ions of the compound are present. Example: molten NaCl dissociates into Na⁺ and Cl⁻ ions. At the cathode, Na⁺ is reduced to Na metal; at the anode, Cl⁻ is oxidized to Cl₂ gas.

在熔融电解质中，只有化合物本身的离子存在。例如：熔融NaCl解离为Na⁺和Cl⁻。在阴极，Na⁺被还原为金属钠；在阳极，Cl⁻被氧化为氯气。

In aqueous solutions, water is also present and can be oxidized or reduced. The competing reactions from water molecules must be considered. For example, in aqueous NaCl, the cathode discharge depends on the relative ease of reduction: Na⁺ vs. H₂O.

在水溶液中，水也存在并可被氧化或还原。必须考虑水分子的竞争反应。例如，在NaCl水溶液中，阴极放电取决于哪种物质更易还原：Na⁺还是H₂O。

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4. Discharge of Ions: The Reactivity Series and Position | 离子放电顺序：活动性顺序与位置

The selective discharge of ions is governed by three main factors: the position in the electrochemical series (reactivity), the concentration of ions, and the nature of the electrode.

离子的选择性放电由三个主要因素决定：在电化学系列（活动性顺序）中的位置、离子浓度和电极的性质。

For cations: the lower the reactivity of the metal, the easier it is to discharge. The order (easiest to hardest): Ag⁺ > Cu²⁺ > H⁺ > Zn²⁺ > Al³⁺ > Na⁺ > K⁺. In aqueous solutions, H⁺ ions from water are discharged in preference to very reactive metals like Na⁺ or K⁺.

对于阳离子：金属的活泼性越低，越容易放电。顺序（从易到难）：Ag⁺ > Cu²⁺ > H⁺ > Zn²⁺ > Al³⁺ > Na⁺ > K⁺。在水溶液中，来自水的H⁺比非常活泼的金属如Na⁺或K⁺优先放电。

For anions: the general order is SO₄²⁻ < NO₃⁻ < Cl⁻ < Br⁻ < I⁻ < OH⁻. Hydroxide ions are often discharged in preference to other anions unless the solution is concentrated halide. For example, in dilute NaCl solution, OH⁻ discharges at anode giving O₂, whereas in concentrated NaCl, Cl⁻ discharges giving Cl₂.

对于阴离子：一般顺序为SO₄²⁻ < NO₃⁻ < Cl⁻ < Br⁻ < I⁻ < OH⁻。氢氧根离子通常比其他阴离子优先放电，除非是浓卤化物溶液。例如，在稀NaCl溶液中，OH⁻在阳极放电产生O₂；而在浓NaCl中，Cl⁻放电产生Cl₂。

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5. Half-Equations at Electrodes | 电极半反应式

Writing half-equations is essential. At the cathode, reduction: Xⁿ⁺ + n e⁻ → X. At the anode, oxidation: Yⁿ⁻ → Y + n e⁻, or for water: 2H₂O → O₂ + 4H⁺ + 4e⁻ (acidic) or 4OH⁻ → O₂ + 2H₂O + 4e⁻ (alkaline).

书写半反应式至关重要。在阴极，还原反应：Xⁿ⁺ + n e⁻ → X。在阳极，氧化反应：Yⁿ⁻ → Y + n e⁻，或水的氧化：2H₂O → O₂ + 4H⁺ + 4e⁻（酸性）或4OH⁻ → O₂ + 2H₂O + 4e⁻（碱性）。

For example, electrolysis of aqueous copper(II) sulfate with inert electrodes: Cathode: Cu²⁺ + 2e⁻ → Cu; Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (or 2H₂O → O₂ + 4H⁺ + 4e⁻). The blue color fades as Cu²⁺ ions are reduced.

例如，使用惰性电极电解硫酸铜水溶液：阴极：Cu²⁺ + 2e⁻ → Cu；阳极：4OH⁻ → O₂ + 2H₂O + 4e⁻（或2H₂O → O₂ + 4H⁺ + 4e⁻）。随着Cu²⁺被还原，蓝色逐渐褪去。

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6. Predicting Products in Aqueous Solutions | 水溶液中的产物预测

When predicting products, follow these steps: (1) Identify all ions present, including H⁺ and OH⁻ from water. (2) Determine which ion is discharged at each electrode based on the series. (3) Write half-equations and overall redox equation. (4) State observations.

预测产物时，遵循以下步骤：（1）确定所有存在的离子，包括来自水的H⁺和OH⁻。（2）根据顺序确定每个电极上放电的离子。（3）写出半反应式和总氧化还原方程式。（4）描述观察结果。

The table below summarises products for common electrolytes in aqueous solution with inert electrodes:

Electrolyte	Cathode Product	Anode Product	Notes
Dilute NaCl	H₂ (g)	O₂ (g)	OH⁻ discharged, pH increases
Concentrated NaCl	H₂ (g)	Cl₂ (g)	Cl⁻ discharged, NaOH formed
CuSO₄ (inert electrodes)	Cu (s)	O₂ (g)	Solution becomes acidic (H₂SO₄)
CuSO₄ (copper electrodes)	Cu (s)	Cu²⁺ enters	Anode dissolves, cathode plates
Dilute H₂SO₄	H₂ (g)	O₂ (g)	Volume ratio H₂:O₂ = 2:1

中文总结：稀释NaCl电解产物为阴极氢气、阳极氧气；浓NaCl为阴极氢气、阳极氯气，同时生成NaOH。CuSO₄用惰性电极得阴极铜、阳极氧气，溶液变酸性；若用铜电极则阳极溶解，阴极镀铜。稀硫酸电解得到体积比2:1的氢气和氧气。

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

Faraday’s First Law: The mass of substance deposited or liberated at an electrode is directly proportional to the quantity of electricity (charge) passed. m ∝ Q.

法拉第第一定律：在电极上沉积或释放的物质质量与通过的电量（电荷）成正比。m ∝ Q。

Q = I × t, where I is current in amperes (A) and t is time in seconds (s). Unit of charge is coulomb (C).

Q = I × t，其中I是电流，单位为安培（A），t是时间，单位为秒（s）。电荷单位是库仑（C）。

Faraday’s Second Law: 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 number of electrons transferred, M/n).Published by TutorHao | IB Chemistry Revision Series | aleveler.com

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