📚 Edexcel A-Level Chemistry Topic 14.2: Redox II – Electrode Potentials and Electrochemical Cells | Edexcel A-Level 化学 14.2：氧化还原 II——电极电势与电化学电池
This revision guide covers Edexcel A-Level Chemistry Topic 14.2: Redox II. It explains oxidation numbers and half-equations, then builds the ideas of the half-cell, the standard hydrogen electrode (SHE) and standard electrode potentials, before showing how to construct electrochemical cells, calculate E°cell and judge whether a reaction is feasible. Worked examples and common exam pitfalls are included throughout.
本复习指南覆盖 Edexcel A-Level 化学 14.2：氧化还原 II。内容从氧化数与半反应式讲起，逐步建立半电池、标准氢电极 (SHE) 与标准电极电势的概念，再说明如何搭建电化学电池、计算电池电动势 E°cell 以及判断反应是否可行，并配有典型例题与常见考试错误。
1. Redox Foundations and Oxidation Numbers | 氧化还原基础与氧化数
Redox reactions involve electron transfer. Oxidation is the loss of electrons and reduction is the gain of electrons; they always occur together, because the electrons lost by one species must be gained by another. Oxidation numbers are book-keeping numbers that track how electrons are redistributed in compounds and ions, and they let us identify which species is oxidised and which is reduced.
氧化还原反应涉及电子转移。氧化是失去电子，还原是得到电子；两者总是同时发生，因为某一物种失去的电子必然被另一物种得到。氧化数是用于追踪电子在化合物和离子中重新分配的记账数字，可帮助我们判断哪种物种被氧化、哪种被还原。
The key rules are: an uncombined element has oxidation number 0; a monatomic ion has an oxidation number equal to its charge; oxygen is usually −2 (except in peroxides, where it is −1); hydrogen is usually +1 (except in metal hydrides, where it is −1); and the sum of oxidation numbers equals the overall charge of the species.
关键规则为：单质中元素氧化数为 0；单原子离子的氧化数等于其所带电荷；氧通常为 −2（过氧化物中为 −1）；氢通常为 +1（金属氢化物中为 −1）；各元素氧化数之和等于该物种的总电荷。
When an element's oxidation number increases, it has been oxidised; when it decreases, it has been reduced. In a disproportionation reaction, the same element is simultaneously oxidised and reduced, which is a favourite exam context.
元素的氧化数升高即被氧化，降低即被还原。歧化反应中同一元素同时被氧化和被还原，这是考试中常考的情境。
2. Half-equations and Electron Balance | 半反应式与电子配平
⁻ → Cu. Adding the two half-equations after balancing electrons gives the overall equation Zn + Cu²⁺ → Zn²⁺ + Cu.
半反应式表示氧化还原反应的一半。例如，锌的氧化为 Zn → Zn²⁺ + 2e⁻，铜(II) 离子的还原为 Cu²⁺ + 2e⁻ → Cu。将两个半反应式配平电子后相加，即得总反应式 Zn + Cu²⁺ → Zn²⁺ + Cu。
To balance a half-equation in acidic solution: balance the atom that changes oxidation number, balance oxygen with H₂O, balance hydrogen with H⁺, then balance charge with e⁻. In alkaline solution, balance as if acidic and then add OH⁻ to both sides to neutralise the H⁺, cancelling water molecules where possible.
在酸性溶液中配平半反应式的步骤：先配平变价的原子，用 H₂O 配平氧，用 H⁺ 配平氢，最后用 e⁻ 配平电荷。在碱性溶液中，先按酸性条件配平，再在两边加 OH⁻ 中和 H⁺，并消去多余的 H₂O。
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (E° = +1.51 V)
2H⁺ + 2e⁻ ⇌ H₂ (E° = 0.00 V)
3. Half-cells and the Need for a Reference Electrode | 半电池与参比电极的必要性
To compare the tendency of different species to accept or release electrons, we build half-cells. A metal/metal-ion half-cell consists of a metal strip dipped into a solution containing its own aqueous ions, for example a zinc rod in Zn²⁺(aq), giving the couple Zn²⁺/Zn.
为了比较不同物种得失电子的倾向，我们构建半电池。金属/金属离子半电池由金属条插入含其自身离子的溶液构成，例如锌棒插入 Zn²⁺(aq) 中，构成 Zn²⁺/Zn 电对。
Where both species are in solution and neither is a metal, an inert platinum electrode is used instead, for example Pt in a mixture of Fe²⁺(aq) and Fe³⁺(aq), or Pt in Cl₂(g) over Cl⁻(aq). Platinum is used because it is chemically inert and electrically conducting.
若两个物种都在溶液中且都不是金属，则改用惰性铂电极，例如铂插入 Fe²⁺(aq) 与 Fe³⁺(aq) 的混合液，或铂插入 Cl₂(g) 与 Cl⁻(aq) 体系中。选用铂是因为它化学惰性且能导电。
The fundamental problem is that the potential of a single half-cell cannot be measured in isolation: any voltmeter needs two terminals, so we can only ever measure a difference between two half-cells. Therefore we measure every half-cell relative to an agreed standard reference, the standard hydrogen electrode.
根本问题在于：单个半电池的电势无法单独测量，因为任何电压表都需要两个接线端，我们只能测出两个半电池之间的差值。因此，所有半电池都相对一个约定的标准参比——标准氢电极——进行测量。
4. The Standard Hydrogen Electrode (SHE) | 标准氢电极 (SHE)
The standard hydrogen electrode (SHE) consists of hydrogen gas at 100 kPa bubbling over a platinised platinum foil, with the foil immersed in 1.00 mol dm⁻³ H⁺(aq) at 298 K. The platinum surface provides a conductive, inert surface on which the hydrogen/hydrogen-ion equilibrium can be established.
标准氢电极 (SHE) 由 100 kPa 的氢气在镀铂铂箔上鼓泡，铂箔浸入 298 K 的 1.00 mol dm⁻³ H⁺(aq) 中构成。铂表面提供了导电且惰性的表面，使氢/氢离子平衡得以建立。
2H⁺(aq) + 2e⁻ ⇌ H₂(g) E° = 0.00 V
The electrode potential of the SHE is defined as exactly 0.00 V by convention, and all other standard electrode potentials are quoted relative to it. This definition is arbitrary but extremely useful: it converts an unmeasurable absolute quantity into a consistent, comparable scale.
按约定，SHE 的电极电势被定义为正好 0.00 V，所有其他标准电极电势都相对它给出。这个定义是人为约定的，但极其有用：它把一个无法绝对测量的量转换成一致且可比较的标尺。
All the standard conditions must be met for the quoted E° value to apply: 298 K, 100 kPa for any gas, and 1.00 mol dm⁻³ for all aqueous species.
只有满足全部标准条件，给出的 E° 数值才适用：298 K、所有气体 100 kPa、所有水溶液物种浓度 1.00 mol dm⁻³。
5. Standard Electrode Potentials and the Electrochemical Series | 标准电极电势与电化学序
The standard electrode potential, E°, of a half-cell is the potential difference measured when that half-cell is connected to a standard hydrogen electrode under standard conditions, with the half-cell written as a reduction. A more negative E° means a greater tendency to release electrons (a stronger reducing agent); a more positive E° means a greater tendency to accept electrons (a stronger oxidising agent).
某半电池的标准电极电势 E°，是指在标准条件下该半电池与标准氢电极连接时测得的电势差，半反应式按还原方向书写。E° 越负，表示释放电子的倾向越强（还原剂越强）；E° 越正，表示接受电子的倾向越强（氧化剂越强）。
| Half-equation (reduction) | E° / V |
| Li⁺ + e⁻ ⇌ Li | −3.04 |
| Zn²⁺ + 2e⁻ ⇌ Zn | −0.76 |
| Fe²⁺ + 2e⁻ ⇌ Fe | −0.44 |
| 2H⁺ + 2e⁻ ⇌ H₂ | 0.00 |
| Cu²⁺ + 2e⁻ ⇌ Cu | +0.34 |
| Ag⁺ + e⁻ ⇌ Ag | +0.80 |
| Cl₂ + 2e⁻ ⇌ 2Cl⁻ | +1.36 |
| MnO₄⁻ + 8H⁺ + 5e⁻ ⇌ Mn²⁺ + 4H₂O | +1.51 |
The resulting list, in order of increasing E°, is the electrochemical series. It allows qualitative predictions: a metal higher in the series (more negative E°) will displace a metal lower in the series from solution, and metals with E° more negative than −0.41 V will react with dilute acid to release hydrogen, in principle.
按 E° 递增排列即得到电化学序。它可作定性预测：排在较上（E° 更负）的金属能把排在下方（E° 更正）的金属从其盐溶液中置换出来；理论上 E° 比 −0.41 V 更负的金属能与稀酸反应放出氢气。
6. Constructing Electrochemical Cells and Cell Diagrams | 构建电化学电池与电池符号
An electrochemical cell (voltaic or galvanic cell) is made by joining two different half-cells with a salt bridge and connecting the electrodes through a high-resistance voltmeter. The more negative electrode is the negative terminal, where oxidation occurs; the more positive electrode is the positive terminal, where reduction occurs.
电化学电池（伏打电池或原电池）是把两个不同半电池用盐桥连接，并通过高电阻电压表连接两电极构成。电势较负的电极为负极，发生氧化；电势较正的电极为正极，发生还原。
The salt bridge, commonly filter paper soaked in KNO₃(aq) or KCl(aq), completes the circuit by allowing ion migration, maintains electrical neutrality in each half-cell and prevents the two solutions from mixing directly. A high-resistance voltmeter is used so that negligible current flows and the measured cell potential is as close as possible to the thermodynamic value.
盐桥（常用浸有 KNO₃(aq) 或 KCl(aq) 的滤纸）通过允许离子迁移来接通电路，维持各半电池的电中性，并防止两种溶液直接混合。使用高电阻电压表是为了让电流极小，使测得的电池电势尽可能接近热力学值。
Cell diagrams use a vertical bar for a phase boundary and a double vertical bar for the salt bridge, with the most oxidised species written on the outside. For the zinc/copper cell the conventional representation is shown below.
电池符号用单竖线表示相界面，双竖线表示盐桥，氧化态最高的物种写在外侧。锌/铜电池的常规表示如下。
Zn(s) | Zn²⁺(aq) ‖ Cu²⁺(aq) | Cu(s)
Where there is no solid metal in a half-cell, platinum is inserted and shown in the diagram, for example Pt(s) | Fe²⁺(aq), Fe³⁺(aq) ‖ Cu²⁺(aq) | Cu(s).
若某半电池中没有固体金属，则加入铂并写入符号，例如 Pt(s) | Fe²⁺(aq), Fe³⁺(aq) ‖ Cu²⁺(aq) | Cu(s)。
7. Calculating E°cell and Predicting Feasibility | 计算电池电动势与判断可行性
The standard cell potential is calculated from the standard electrode potentials of the two couples involved. A reliable method is to locate the two half-equations in the data booklet and subtract the more negative E° from the more positive E°.
标准电池电势由参与的两个电对的标准电极电势计算得出。可靠做法是查数据表找到两个半反应式，用较正的 E° 减去较负的 E°。
E°cell = E°(positive electrode) − E°(negative electrode)
E°cell = E°(reduction) − E°(oxidation)
The sign is used as a test of feasibility: if E°cell is positive, the reaction as written is thermodynamically feasible; if E°cell is negative, the reaction is not feasible in that direction under standard conditions, although the reverse reaction would be feasible.
符号可用于判断可行性：若 E°cell 为正，则按该方向书写的反应在热力学上可行；若 E°cell 为负，则该方向在标准条件下不可行，但其逆反应可行。
A crucial point that examiners test repeatedly: when a half-equation is multiplied to balance electrons, the value of E° must NOT be multiplied. Electrode potential is an intensive property, like density, so it does not depend on the amount of substance.
这是一个反复考查的关键点：当为了配平电子而把半反应式乘以系数时，E° 的数值绝不能乘以该系数。电极电势是强度性质，类似密度，不随物质的量改变。
8. Worked Example: The Zinc/Copper Cell | 例题：锌/铜电池
d by a salt bridge. The copper couple has the more positive E°, so it is reduced; the zinc couple is reversed and oxidised.
将 Zn²⁺/Zn（E° = −0.76 V）与 Cu²⁺/Cu（E° = +0.34 V）两个半电池用盐桥连接。铜电对的 E° 更正，故被还原；锌电对反向书写，被氧化。
Zn(s) → Zn²⁺(aq) + 2e⁻ (oxidation, at the negative electrode)
Cu²⁺(aq) + 2e⁻ → Cu(s) (reduction, at the positive electrode)
E°cell = (+0.34) − (−0.76) = +1.10 V
lectrons flow through the external circuit from zinc to copper, conventional current flows the opposite way, zinc dissolves and copper is deposited, and the blue colour of Cu²⁺(aq) fades.
由于 E°cell 为正，反应 Zn + Cu²⁺ → Zn²⁺ + Cu 可行。电子经外电路由锌流向铜，常规电流方向相反；锌溶解、铜析出，Cu²⁺(aq) 的蓝色逐渐变浅。
A second typical example: can Fe³⁺(aq) oxidise I⁻(aq)? +0.54) = +0.23 V, which is positive, so the reaction 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ is feasible.
第二个典型例子：Fe³⁺(aq) 能否氧化 I⁻(aq)？相关电对为 Fe³⁺/Fe²⁺（E° = +0.77 V）与 I₂/I⁻（E° = +0.54 V），E°cell = +0.77 − (+0.54) = +0.23 V，为正值，因此反应 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂ 可行。
9. Non-Standard Conditions and the Limits of E° Predictions | 非标准条件与 E° 预测的局限
Standard electrode potentials apply only under standard conditions. If concentrations differ from 1.00 mol dm⁻³, or if gas pressures or temperature differ from the standard values, the actual cell potential changes. Qualitatively, the potential of a half-cell becomes more positive when the concentration of the oxidised species increases or that of the reduced species decreases.
标准电极电势只在标准条件下适用。若浓度不是 1.00 mol dm⁻³，或气体压强、温度偏离标准值，实际电池电势就会改变。定性来看，当氧化态物种浓度升高或还原态物种浓度降低时，该半电池电势会变得更正。
This matters for redox systems containing H⁺, such as MnO₄⁻/Mn²⁺ or Cr₂O₇²⁻/Cr³⁺: raising the H⁺ concentration (lowering pH) makes E more positive and increases oxidising power. Changing the pH can therefore switch a reaction from feasible to non-feasible.
这对含 H⁺ 的氧化还原体系（如 MnO₄⁻/Mn²⁺ 或 Cr₂O₇²⁻/Cr³⁺）尤为重要：提高 H⁺ 浓度（降低 pH）会使 E 更正，氧化能力增强。因此改变 pH 可以使反应由可行变为不可行。
Even when E°cell is positive, a reaction may not happen at a measurable rate, because a large activation energy or kinetic barrier can block it. Conversely, a slightly negative E°cell does not absolutely forbid reaction if concentrations are far from standard. Thermodynamic feasibility and kinetic rate are separate questions.
即使 E°cell 为正，反应也可能因活化能高或存在动力学障碍而以可观察速率不发生。反之，若浓度远离标准值，E°cell 略为负也不能绝对禁止反应发生。热力学可行性与动力学速率是两个独立问题。
10. Applications: Fuel Cells, Batteries and Corrosion | 应用：燃料电池、电池与腐蚀
culated from the O₂/H₂O and H⁺/H₂ couples. Fuel cells convert chemical energy directly into electrical energy and produce only water, so they are attractive for clean transport.
电化学电池支撑着现代技术。氢氧燃料电池的总反应为 2H₂ + O₂ → 2H₂O，由 O₂/H₂O 与 H⁺/H₂ 电对算得 E°cell = +1.23 V。燃料电池把化学能直接转化为电能，产物只有水，因此对清洁交通很有吸引力。
Rechargeable cells such as the lithium-ion cell and the lead-acid accumulator are non-equilibrium systems whose direction of reaction can be reversed by an external supply. In the lead-acid cell, the overall cell reaction involves Pb, PbO₂ and H₂SO₄, and the cell is recharged by reversing both electrode reactions.
锂离子电池和铅酸蓄电池等可充电电池属于非平衡体系，其反应方向可通过外加电源逆转。铅酸电池的总反应涉及 Pb、PbO₂ 与 H₂SO₄，充电时两个电极反应均反向进行。
The electrochemical series also explains sacrificial protection: a more negative E° metal such as zinc or magnesium is attached to iron, so it is oxidised preferentially and the iron is protected from rusting. A less negative metal such as tin does not protect iron and can even accelerate corrosion once the coating is broken.
电化学序也可解释牺牲阳极保护法：将 E° 更负的金属（如锌或镁）与铁连接，它会优先被氧化，从而保护铁不被锈蚀。而 E° 较正的金属（如锡）不能保护铁，镀层一旦破损甚至会加速铁的腐蚀。
11. Exam Technique and Common Pitfalls | 考试技巧与常见错误
In written answers, always state E° values with their sign and unit, quote standard conditions explicitly when asked why a value applies, and identify which electrode is positive and which is negative. Many marks are lost simply by omitting the word standard or by writing the sign incorrectly.
书面作答时，务必写出 E° 的符号与单位；被问及数值适用原因时明确写出标准条件；并指出哪个电极是正极、哪个是负极。很多失分仅仅是因为漏写 standard 一词或符号写错。
- Do not multiply E° when multiplying a half-equation to balance electrons.
- Remember that E°(SHE) = 0.00 V is a definition, not a measurement.
- Include Pt(s) in the cell diagram when neither species in a half-cell is a solid metal.
- Do not confuse the salt bridge (ion migration, neutrality) with the external circuit (electron flow).
- Quote E°cell to an appropriate number of significant figures, usually two decimal places in volts.
- 为配平电子而乘以半反应式系数时，E° 不可同乘。
- 牢记 E°(SHE) = 0.00 V 是定义值，而非测量值。
- 当半电池中两物种都不是固体金属时，电池符号中必须写出 Pt(s)。
- 不要把盐桥（离子迁移、维持电中性）与外电路（电子流动）混为一谈。
- E°cell 应保留合理有效数字，通常为两位小数，单位为伏特。
A further pitfall concerns language: electrons flow through the external wire, whereas ions move through the salt bridge and the solutions. Statements such as "electrons flow through the salt bridge" are incorrect and are penalised in mark schemes.
另一个易错点在于表述：电子通过外电路导线流动，而离子通过盐桥和溶液迁移。"电子通过盐桥流动"这类说法是错误的，会被评分标准扣分。
12. Summary Checklist | 总结清单
Before the exam, check that you can: assign oxidation numbers correctly; construct and balance half-equations in acidic and alkaline media; describe the SHE precisely, including all standard conditions; explain why a reference electrode is required; convert between half-cell descriptions and cell diagrams; calculate E°cell and use its sign to judge feasibility; and explain the limitations of E° predictions in terms of non-standard conditions and kinetics.
考试前，请确认自己能够：正确判断氧化数；在酸碱介质中书写并配平半反应式；准确描述 SHE 及其全部标准条件；解释为何需要参比电极；在半电池描述与电池符号之间相互转化；计算 E°cell 并用其符号判断可行性；并从非标准条件与动力学角度解释 E° 预测的局限。
The central logic to remember is simple: potentials are always measured as differences relative to the SHE; a more positive E° means a stronger oxidising agent; and a positive E°cell means the reaction is thermodynamically feasible under standard conditions.
需要记住的核心逻辑很简单：电势总是相对 SHE 的差值；E° 越正代表氧化剂越强；E°cell 为正表示该反应在标准条件下热力学可行。
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