📚 AS Chemistry: Electrochemistry Key Concepts Explained | AS 化学：电化学 考点精讲
Electrochemistry bridges the gap between chemical reactions and electricity, a fundamental topic in AS Chemistry. This guide consolidates essential concepts—from oxidation states and half-equations to electrochemical cells, standard electrode potentials, and electrolysis—equipping you with the skills to predict reaction spontaneity and understand real-world applications like batteries and corrosion prevention.
电化学连接了化学反应与电能，是 AS 化学的基础主题。本文汇总核心概念——从氧化态和半反应方程式到电化学电池、标准电极电势与电解——帮助您掌握预测反应自发性的技能，并理解电池与防腐等实际应用。
1. Oxidation, Reduction and Oxidation States | 氧化、还原与氧化态
Oxidation is the loss of electrons and an increase in oxidation state; reduction is the gain of electrons and a decrease in oxidation state. An oxidising agent accepts electrons and is itself reduced, while a reducing agent donates electrons and is oxidised.
氧化是失去电子、氧化数升高的过程；还原是得到电子、氧化数降低的过程。氧化剂接受电子，自身被还原；还原剂提供电子，自身被氧化。
To assign oxidation numbers, follow these rules: free elements have 0, monatomic ions equal their charge, oxygen is –2 (except –1 in peroxides), hydrogen is +1 (except –1 in metal hydrides), and the sum in a neutral compound is 0. For example, in KMnO₄, K = +1, each O = –2, so Mn = +7.
确定氧化数的规则：单质为 0，单原子离子等于其电荷，氧通常为 –2（过氧化物中为 –1），氢为 +1（金属氢化物中 –1），中性分子中各原子氧化数之和为 0。例如在 KMnO₄ 中，K = +1，每个 O = –2，因此 Mn = +7。
A redox reaction always involves simultaneous oxidation and reduction. The substance oxidised is the reducing agent, and the substance reduced is the oxidising agent. Monitoring oxidation states makes it easy to identify both.
氧化还原反应总同时包含氧化和还原。被氧化的物质是还原剂，被还原的物质是氧化剂。观察氧化数的变化能迅速鉴别二者。
2. Half-equations and Ion-electron Method | 半反应与离子-电子法配平
Half-equations show either the oxidation or reduction part of a redox process. For example, the oxidation of zinc: Zn(s) → Zn²⁺(aq) + 2e⁻, and the reduction of copper ions: Cu²⁺(aq) + 2e⁻ → Cu(s). Balancing ensures that the number of electrons lost equals the number gained.
半反应方程式展示氧化还原过程中的氧化或还原部分。例如锌的氧化：Zn(s) → Zn²⁺(aq) + 2e⁻；铜离子的还原：Cu²⁺(aq) + 2e⁻ → Cu(s)。配平半反应确保失去的电子数等于得到的电子数。
The ion-electron method in acidic medium involves: 1) write the skeleton half-equation; 2) balance atoms other than O and H; 3) balance O using H₂O; 4) balance H using H⁺; 5) balance charge with e⁻. Combine half-equations after multiplying to equalise electrons.
酸性介质中的离子-电子法步骤：1）写出骨架半反应；2）配平除 O 和 H 以外的原子；3）用 H₂O 配平 O；4）用 H⁺ 配平 H；5）用 e⁻ 配平电荷。将两个半反应乘以适当系数使电子数相等后相加。
Example: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O. The two half-equations are:
示例：MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O。两个半反应为：
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Fe²⁺ → Fe³⁺ + e⁻
Multiply the iron half-equation by 5, then add them together to cancel the electrons, giving the overall ionic equation.
将铁的氧化半反应乘以 5，然后与还原半反应相加消去电子，即得总离子方程式。
3. Electrochemical Cells and Cell Diagrams | 电化学电池与电池图示
A voltaic (galvanic) cell converts chemical energy into electrical energy. It consists of two half-cells connected by a salt bridge and an external wire. Oxidation occurs at the anode (negative electrode) and reduction at the cathode (positive electrode). Electrons flow from anode to cathode through the external circuit.
原电池（伏打电池）将化学能转变为电能。它由盐桥和外导线连接的两个半电池构成。阳极（负极）发生氧化，阴极（正极）发生还原。电子通过外电路从阳极流向阴极。
The cell diagram summarises the cell using phase-boundary lines. Convention: anode half-cell is written on the left, cathode on the right: Zn(s)|Zn²⁺(aq) || Cu²⁺(aq)|Cu(s). The double vertical line represents the salt bridge.
电池图示用相界面线概括电池结构。惯例：阳极半电池写在左侧，阴极在右侧：Zn(s)|Zn²⁺(aq) || Cu²⁺(aq)|Cu(s)。双竖线代表盐桥。
The salt bridge completes the circuit by allowing ion migration without mixing the solutions, maintaining electrical neutrality.
盐桥允许离子迁移而不使溶液混合，维持电中性，从而构成完整电路。
4. Standard Hydrogen Electrode and Standard Electrode Potentials | 标准氢电极与标准电极电势
The standard hydrogen electrode (SHE) is the universal reference for measuring electrode potentials. It consists of platinum black immersed in 1 mol dm⁻³ H⁺ with H₂ gas bubbled at 100 kPa, all at 298 K. Its potential is defined as 0.00 V.
标准氢电极（SHE）是测量电极电势的通用基准。它由铂黑浸于 1 mol dm⁻³ H⁺溶液中，并通入 100 kPa 氢气，温度 298 K。其电势定为 0.00 V。
A standard electrode potential (E°) is measured by connecting the half-cell under standard conditions to the SHE. It gives the tendency of a species to be reduced; more positive E° means a stronger oxidising agent.
标准电极电势（E°）是将标准条件下的待测半电池与标准氢电极相连测得的电势差。它反映物种被还原的倾向；E° 越正，氧化剂越强。
Selected standard reduction potentials are shown below:
一些标准还原电极电势列举如下：
| Half-reaction | E° / V |
|---|---|
| F₂(g) + 2e⁻ ⇌ 2F⁻(aq) | +2.87 |
| MnO₄⁻(aq) + 8H⁺ + 5e⁻ ⇌ Mn²⁺(aq) + 4H₂O(l) | +1.52 |
| Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq) | +0.77 |
| Cu²⁺(aq) + 2e⁻ ⇌ Cu(s) | +0.34 |
| 2H⁺(aq) + 2e⁻ ⇌ H₂(g) | 0.00 |
| Zn²⁺(aq) + 2e⁻ ⇌ Zn(s) | –0.76 |
| Li⁺(aq) + e⁻ ⇌ Li(s) | –3.04 |
5. The Electrochemical Series and Cell EMF | 电化序与电池电动势
The electrochemical series arranges half-reactions by their standard reduction potentials from most positive (strongest oxidising agent) to most negative (strongest reducing agent). It allows quick prediction of redox behaviour.
电化序将半反应按标准还原电势从最正（最强氧化剂）排到最负（最强还原剂）。它可快速预测氧化还原行为。
The electromotive force (EMF) of a cell, E°cell, is calculated as:
电池的电动势（EMF）E°cell 的计算式为：
E°cell = E°(cathode) – E°(anode)
where E°(cathode) is the reduction potential of the half-cell where reduction occurs, and E°(anode) is that of the half-cell where oxidation occurs. For a spontaneous cell, E°cell must be positive.
其中 E°(cathode) 是发生还原的半电池还原电势，E°(anode) 是发生氧化的半电池还原电势。自发电池的 E°cell 必须为正值。
For a Daniell cell: Cu²⁺/Cu has E° = +0.34 V, Zn²⁺/Zn has E° = –0.76 V. Copper is the cathode (reduction), zinc is the anode (oxidation), so:
对于丹尼尔电池：Cu²⁺/Cu 的 E° = +0.34 V，Zn²⁺/Zn 的 E° = –0.76 V。铜为阴极（还原），锌为阳极（氧化），因此：
E°cell = +0.34 V – (–0.76 V) = 1.10 V
6. Predicting Spontaneous Redox Reactions | 预测自发氧化还原反应
A redox reaction is thermodynamically feasible if the calculated cell EMF is positive. The species with the more positive E° (stronger oxidising agent) will be reduced, while the one with the more negative E° (stronger reducing agent) will be oxidised.
若计算的电池电动势为正，氧化还原反应在热力学上可行。E° 较正的物种（较强氧化剂）被还原，E° 较负的物种（较强还原剂）被氧化。
Using the diagonal rule in the electrochemical series, a reaction between a top-left oxidant and bottom-right reductant is spontaneous. For example, will Fe³⁺ oxidise I⁻? Relevant potentials: Fe³⁺/Fe²⁺ = +0.77 V, I₂/I⁻ = +0.54 V. Fe³⁺ is a stronger oxidising agent than I₂, and I⁻ is a stronger reducing agent than Fe²⁺.
用电化序对角线法则，左上角的氧化剂与右下角的还原剂之间的反应是自发的。例如，Fe³⁺ 能否氧化 I⁻？相关电势：Fe³⁺/Fe²⁺ = +0.77 V，I₂/I⁻ = +0.54 V。Fe³⁺ 是比 I₂ 更强的氧化剂，I⁻ 是比 Fe²⁺ 更强的还原剂。
E°cell = 0.77 V – 0.54 V = +0.23 V > 0
The reaction 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ is therefore spontaneous under standard conditions. If E°cell were negative, the reaction would be non-spontaneous.
因此，在标准条件下反应 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ 是自发的。如果 E°cell 为负，则该反应不能自发进行。
7. Electrolysis: Principles and Products | 电解原理与产物
Electrolysis uses a direct current to drive a non-spontaneous chemical reaction. The electrolytic cell has two electrodes in contact with an electrolyte; the anode is connected to the positive terminal, the cathode to the negative terminal. Oxidation still occurs at the anode and reduction at the cathode, but now external power determines the flow.
电解利用直流电驱动非自发反应。电解池中两个电极与电解质接触；阳极接电源正极，阴极接电源负极。阳极仍发生氧化，阴极发生还原，但此时由外部电源决定反应方向。
When electrolysing molten salts, the ions are free to move. Electrolysis of molten NaCl yields sodium metal at the cathode and chlorine gas at the anode. For aqueous solutions, the discharge order of ions depends on their E° values and concentration effects.
电解熔融盐时离子可自由移动。电解熔融 NaCl 在阴极生成金属钠，阳极生成氯气。对于水溶液，离子放电次序取决于其标准电极电势及浓度效应。
At the cathode, the ion with the more positive E° is preferentially discharged. Generally, metal ions less reactive than hydrogen (e.g. Cu²⁺, Ag⁺) are reduced, but for reactive metals like Na⁺, H₂O is reduced to H₂ and OH⁻ instead. At the anode, the ease of oxidation typically follows: I⁻ > Br⁻ > Cl⁻ > OH⁻ > SO₄²⁻. If a halide is present in sufficient concentration, it is oxidised; otherwise, water is oxidised to O₂.
在阴极，E° 较正的离子优先放电。通常，活泼性低于氢的金属离子（如 Cu²⁺、Ag⁺）被还原；而对于活泼金属如 Na⁺，则是水被还原生成 H₂ 和 OH⁻。在阳极，氧化难易顺序一般为：I⁻ > Br⁻ > Cl⁻ > OH⁻ > SO₄²⁻。若卤离子浓度足够，则优先放电；否则水被氧化放出 O₂。
Example: electrolysis of concentrated aqueous NaCl produces H₂ at cathode, Cl₂ at anode, and NaOH in solution. Electrolysis of CuSO₄ with inert electrodes gives Cu at cathode and O₂ at anode; with a copper anode, the anode dissolves and Cu is deposited at cathode (electrorefining).
示例：电解浓食盐水在阴极产生 H₂，阳极产生 Cl₂，溶液中生成 NaOH。用惰性电极电解 CuSO₄ 溶液，阴极析出 Cu，阳极产生 O₂；用铜作阳极时，阳极溶解而阴极上沉积铜（电解精炼）。
8. Factors Affecting Electrode Potentials | 影响电极电势的因素
Electrode potentials are not fixed; they shift with changes in concentration, pressure, and temperature. Le Chatelier’s principle explains the direction of change: increasing the concentration of the oxidised form favours reduction and makes the potential more positive; increasing the concentration of the reduced form makes it more negative.
电极电势并非恒定不变，它会随浓度、压力和温度而移动。勒夏特列原理可解释变化方向：增大氧化型物质浓度有利于还原，电势变得更正；增大还原型物质浓度则使电势变负。
For a half-cell Zn²⁺/Zn, raising [Zn²⁺] shifts the equilibrium Zn²⁺ + 2e⁻ ⇌ Zn to the right, making the electrode potential less negative (more positive). Raising temperature alters the potential according to the enthalpy change of the half-reaction.
对 Zn²⁺/Zn 半电池，增加 [Zn²⁺] 使平衡 Zn²⁺ + 2e⁻ ⇌ Zn 右移，电极电势向负值减小的方向移动（变得更正）。温度变化则根据半反应焓变影响电极电势。
The Nernst equation quantitatively links E to concentration, but at AS level a qualitative understanding suffices. Non‑standard conditions can alter the cell EMF enough to make some predicted reactions non‑spontaneous.
能斯特方程定量联系电势与浓度，但 AS 阶段只需定性了解。非标准条件可能导致电池电动势变化，使某些预测的自发反应变为非自发。
9. Applications: Corrosion and Prevention | 应用：腐蚀与防护
Corrosion, especially rusting of iron, is an electrochemical process. When iron is in contact with water and oxygen, tiny voltaic cells form on its surface. At anodic regions, Fe is oxidised to Fe²⁺; at cathodic regions, O₂ is reduced in the presence of water to OH⁻. The Fe²⁺ further oxidises and hydrolyses to form rust (hydrated iron(III) oxide).
腐蚀，特别是铁的生锈，是一个电化学过程。当铁与水和氧气接触时，表面形成微小的原电池。在阳极区，Fe 被氧化为 Fe²⁺；在阴极区，O₂ 在有水存在时被还原为 OH⁻。Fe²⁺ 进一步氧化和水解生成铁锈（水合氧化铁）。
Prevention methods include: barrier protection (paint, oil, plastic), sacrificial protection (coating with a more reactive metal like zinc – galvanising), and cathodic protection (impressed current). Zinc acts as the anode and corrodes instead of iron, hence its name ‘sacrificial anode’.
防护方法有：屏蔽保护（涂漆、涂油、塑料覆盖）、牺牲保护（镀上更活泼金属如锌——镀锌），以及外加电流阴极保护。锌作为优先腐蚀的阳极，保护了铁，因此得名“牺牲阳极”。
Galvanising is particularly effective because even if the coating is scratched, the zinc remains reactive and continues to protect the exposed iron electrochemically.
镀锌尤其有效，因为即使涂层被刮伤，锌仍然活泼，继续电化学保护裸露的铁。
10. Common Electrochemical Cells in Use | 常用电化学电池
The Daniell cell (Cu–Zn) is a classic example of a primary cell, but real commercial cells are designed for efficiency and practicality. A lead‑acid battery (secondary cell) has PbO₂ as cathode, Pb as anode, and H₂SO₄ as electrolyte, giving about 2 V per cell. During discharge, both electrodes become coated with PbSO₄; recharging reverses the reaction.
丹尼尔电池（铜‑锌）是经典原电池，但实际商业电池更注重效率和实用性。铅酸蓄电池（二次电池）以 PbO₂ 为阴极，Pb 为阳极，H₂SO₄ 为电解液，每单格约 2 V。放电时两极均生成 PbSO₄；充电时反应逆转。
Fuel cells, such as the hydrogen–oxygen fuel cell, convert chemical energy directly into electricity with high efficiency. The electrode reactions are:
燃料电池，如氢氧燃料电池，直接将化学能转化为电能，效率很高。电极反应为：
An
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导