Electrochemistry for A-Level OCR Chemistry | A-Level OCR 化学：电化学考点精讲

📚 Electrochemistry for A-Level OCR Chemistry | A-Level OCR 化学：电化学考点精讲

Electrochemistry is the branch of chemistry that deals with the interconversion of chemical energy and electrical energy. In OCR A-Level Chemistry, this topic covers how redox reactions can be used to generate electricity in electrochemical cells, and how external electrical sources can drive non‑spontaneous reactions. Understanding electrode potentials, cell notation, and the standard hydrogen electrode is essential for predicting reaction feasibility and comprehending modern applications such as batteries and fuel cells.

电化学是研究化学能与电能相互转化的化学分支。在 OCR A-Level 化学中，该专题涉及如何利用氧化还原反应在原电池中产生电能，以及如何通过外加电源促使非自发反应发生。掌握电极电势、电池符号和标准氢电极是预测反应可行性和理解电池、燃料电池等现代应用的关键。

1. Electrochemical Cells | 电化学电池

An electrochemical cell consists of two half‑cells. Each half‑cell contains an electrode (a solid conductor) dipped into an electrolyte solution containing ions of the same element. The two half‑cells are connected by a salt bridge, which allows ions to flow and completes the circuit without mixing the solutions. Electrons flow from the more negative electrode to the more positive electrode through an external wire, generating an electric current.

电化学电池由两个半电池组成。每个半电池包含一个电极（固体导体）和一种含有同种元素离子的电解质溶液。两个半电池通过盐桥连接，盐桥允许离子迁移并构成完整回路，同时防止溶液混合。电子通过外导线从较负的电极流向较正的电极，从而产生电流。

A common example is the Daniell cell: Zn | Zn²⁺ ∥ Cu²⁺ | Cu. At the zinc electrode, oxidation occurs: Zn(s) → Zn²⁺(aq) + 2e⁻. At the copper electrode, reduction occurs: Cu²⁺(aq) + 2e⁻ → Cu(s). The salt bridge typically contains KNO₃ or NH₄NO₃.

一个常见实例是丹尼尔电池：Zn | Zn²⁺ ∥ Cu²⁺ | Cu。在锌电极上发生氧化反应：Zn(s) → Zn²⁺(aq) + 2e⁻；在铜电极上发生还原反应：Cu²⁺(aq) + 2e⁻ → Cu(s)。盐桥通常装有 KNO₃ 或 NH₄NO₃。

2. Standard Electrode Potential (E°) | 标准电极电势 (E°)

The standard electrode potential, E°, is the voltage produced by a half‑cell relative to the standard hydrogen electrode, measured under standard conditions: 298 K, 100 kPa, and all ion concentrations at 1.00 mol dm⁻³. It is a measure of the tendency of a species to gain electrons (be reduced). More positive E° values indicate a stronger oxidising agent; more negative values indicate a stronger reducing agent.

标准电极电势 E° 是半电池相对于标准氢电极的电压值，测量条件为标准状况：298 K、100 kPa 以及所有离子浓度为 1.00 mol dm⁻³。它衡量物质得电子（被还原）的趋势。E° 值越正，氧化性越强；E° 值越负，还原性越强。

By convention, half‑cell potentials are always written as reduction potentials: Oxidised form + ne⁻ → Reduced form. For zinc: Zn²⁺ + 2e⁻ → Zn(s) E° = –0.76 V. For copper: Cu²⁺ + 2e⁻ → Cu(s) E° = +0.34 V.

根据约定，半电池电势均以还原电势书写：氧化态 + ne⁻ → 还原态。锌：Zn²⁺ + 2e⁻ → Zn(s) E° = –0.76 V；铜：Cu²⁺ + 2e⁻ → Cu(s) E° = +0.34 V。

3. The Standard Hydrogen Electrode (SHE) | 标准氢电极

The SHE is the reference electrode against which all other electrode potentials are measured. It consists of a platinum electrode coated with porous platinum black, immersed in a solution of H⁺ ions (usually 1 mol dm⁻³ HCl). Hydrogen gas at 100 kPa is bubbled over the electrode. The half‑reaction is: 2H⁺(aq) + 2e⁻ ⇌ H₂(g). By definition, its E° is exactly 0.00 V at all temperatures.

标准氢电极是测定其他电极电势的参比电极。它由镀有铂黑的铂电极浸入 H⁺ 离子溶液（通常为 1 mol dm⁻³ HCl）组成，氢气在 100 kPa 下鼓入该电极。其半反应为：2H⁺(aq) + 2e⁻ ⇌ H₂(g)。根据定义，其 E° 在任何温度下均精确为 0.00 V。

The SHE can act as either an anode or a cathode depending on the other half‑cell connected. When paired with a metal that has a negative E°, the hydrogen electrode is the positive electrode (cathode) where reduction occurs; with a positive E°, it becomes the negative electrode (anode).

SHE 可以作为阳极或阴极，取决于所连接的另一半电池。当与具有负 E° 的金属配对时，氢电极为正极（阴极），发生还原反应；与正 E° 金属配对时，则变为负极（阳极）。

4. Measuring Standard Electrode Potentials | 测量标准电极电势

To measure the E° of a half‑cell, it is connected to the SHE via a high‑resistance voltmeter and a salt bridge. The reading on the voltmeter gives the cell potential E°_cell. Since E° of SHE is zero, the measured voltage equals the standard electrode potential of the half‑cell, including its sign if the leads are connected correctly. The complete cell can be represented with cell notation.

测量半电池的 E° 时，将其通过高阻抗电压表和盐桥与 SHE 相连。电压表的读数即为电池电势 E°_cell。因为 SHE 的 E° 为零，若接线正确，测得的电压即为该半电池的标准电极电势（并带有正负号）。整个电池可用电池符号表示。

Example: To measure E° for Zn²⁺/Zn, set up Pt | H₂ | H⁺ ∥ Zn²⁺ | Zn. The voltmeter reads –0.76 V, confirming that Zn²⁺/Zn has E° = –0.76 V. The negative sign indicates that the zinc electrode is the negative pole relative to SHE.

示例：测量 Zn²⁺/Zn 的 E°，构建 Pt | H₂ | H⁺ ∥ Zn²⁺ | Zn。电压表读数为 –0.76 V，证实 Zn²⁺/Zn 的 E° = –0.76 V。负号表明锌电极相对于 SHE 是负极。

5. Cell Potential and EMF | 电池电势与电动势

The electromotive force (EMF) of a cell is the maximum potential difference between its two electrodes when no current flows. Under standard conditions, the standard cell potential E°_cell is calculated as E°_cell = E°(right‑hand electrode) – E°(left‑hand electrode) or, more generally, E°_cell = E°(reduction half‑cell) – E°(oxidation half‑cell), always using reduction potentials.

电池的电动势（EMF）是指无电流通过时两电极之间的最大电势差。在标准状况下，标准电池电势 E°_cell 计算公式为：E°_cell = E°(右侧电极) – E°(左侧电极)，或者更一般地，E°_cell = E°(还原半电池) – E°(氧化半电池)，均使用还原电势。

A positive E°_cell indicates the reaction is thermodynamically feasible as written. For the Daniell cell: E°_cell = (+0.34 V) – (–0.76 V) = +1.10 V. The corresponding reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) is spontaneous under standard conditions.

E°_cell 为正值，表明所书写的反应在热力学上可行。以丹尼尔电池为例：E°_cell = (+0.34 V) – (–0.76 V) = +1.10 V。相应的反应 Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) 在标准条件下是自发的。

6. Predicting Feasibility of Redox Reactions | 预测氧化还原反应可行性

A redox reaction is feasible if the calculated E°_cell is positive. To predict, identify the two relevant half‑equations, reverse the one with the more negative E° to represent oxidation, and add the potentials algebraically. For example, will Fe³⁺ oxidise I⁻ to I₂? Half‑reactions: Fe³⁺ + e⁻ → Fe²⁺ E°=+0.77 V; I₂ + 2e⁻ → 2I⁻ E°=+0.54 V. Since I₂/I⁻ is less positive, it is reversed: 2I⁻ → I₂ + 2e⁻. E°_cell = +0.77 – (+0.54) = +0.23 V > 0, so reaction is feasible.

如果计算得到的 E°_cell 为正值，则该氧化还原反应可行。预测时，识别两个相关半反应方程式，将 E° 更负的那个反转以代表氧化反应，再将电势代数相加。例如，Fe³⁺ 能否将 I⁻ 氧化为 I₂？半反应：Fe³⁺ + e⁻ → Fe²⁺ E°=+0.77 V；I₂ + 2e⁻ → 2I⁻ E°=+0.54 V。因为 I₂/I⁻ 较不正，将其反转：2I⁻ → I₂ + 2e⁻。E°_cell = +0.77 – (+0.54) = +0.23 V > 0，所以反应可行。

However, a positive E°_cell only indicates thermodynamic feasibility. Kinetic factors may make the reaction extremely slow (e.g. activation energy barrier) or a large overpotential may be required, meaning some “feasible” reactions are not observed in practice.

但 E°_cell 为正仅表示热力学可行性。动力学因素可能导致反应极其缓慢（如活化能势垒），或需要很大的超电势，因此某些“可行”反应在实际中观察不到。

7. The Electrochemical Series | 电化学序列

The electrochemical series lists half‑reactions in order of decreasing E° values. Species at the top (most positive) are strong oxidising agents (e.g. F₂, MnO₄⁻); species at the bottom (most negative) are strong reducing agents (e.g. Li⁺/Li, K⁺/K). The series allows quick predictions of redox spontaneity: any metal above hydrogen can displace H⁺ from acid; any metal higher than another metal can displace it from its salt solution.

电化学序列是按 E° 值递减顺序排列的半反应表。位于顶端的物质（最正）是强氧化剂（如 F₂、MnO₄⁻）；底端的物质（最负）是强还原剂（如 Li⁺/Li、K⁺/K）。该序列可快速判断氧化还原的自发性：任何在氢以下的金属可从酸中置换出 H⁺；任何排在另一金属之上的金属可从其盐溶液中将其置换出来。

Half‑reaction	E° / V
F₂(g) + 2e⁻ → 2F⁻(aq)	+2.87
MnO₄⁻(aq) + 8H⁺ + 5e⁻ → Mn²⁺(aq) + 4H₂O(l)	+1.51
Cl₂(g) + 2e⁻ → 2Cl⁻(aq)	+1.36
O₂(g) + 4H⁺ + 4e⁻ → 2H₂O(l)	+1.23
Br₂(l) + 2e⁻ → 2Br⁻(aq)	+1.09
Ag⁺(aq) + e⁻ → Ag(s)	+0.80
Fe³⁺(aq) + e⁻ → Fe²⁺(aq)	+0.77
Cu²⁺(aq) + 2e⁻ → Cu(s)	+0.34
2H⁺(aq) + 2e⁻ → H₂(g)	0.00
Pb²⁺(aq) + 2e⁻ → Pb(s)	–0.13
Sn²⁺(aq) + 2e⁻ → Sn(s)	–0.14
Fe²⁺(aq) + 2e⁻ → Fe(s)	–0.44
Zn²⁺(aq) + 2e⁻ → Zn(s)	–0.76
Al³⁺(aq) + 3e⁻ → Al(s)	–1.66
Li⁺(aq) + e⁻ → Li(s)	–3.04

This table is used in OCR exams to determine reaction feasibility and relative oxidising/reducing power.

上表用于 OCR 考试中判断反应可行性和比较氧化/还原能力的强弱。

8. Cell Notation and Conventions | 电池符号与书写惯例

Cells are represented by a standard notation to avoid drawing diagrams. The format is: electrode | oxidised form, reduced form (or solution) || reduced form, oxidised form | electrode. A single vertical line | represents a phase boundary; a double line || represents the salt bridge. The more negative electrode (oxidation) is written on the left. Species in the same phase are separated by commas; platinum is used as an inert electrode when there is no solid metal, e.g. Pt | Fe²⁺, Fe³⁺.

电池可用标准符号表示，无需画图。格式为：电极 | 氧化态，还原态（或溶液） || 还原态，氧化态 | 电极。单竖线 | 表示相界面；双竖线 || 表示盐桥。较负的电极（发生氧化）写在左侧。同相的物质用逗号分隔；当没有固体金属电极时，用铂作为惰性电极，如 Pt | Fe²⁺, Fe³⁺。

9. Limitations of Standard Electrode Potentials | 标准电极电势的局限性

E° values are measured under standard conditions. If concentrations change (e.g. not 1.00 mol dm⁻³), the electrode potential shifts according to the Nernst equation (not required for OCR, but the qualitative effect matters). For a half‑cell like Cu²⁺/Cu, if [Cu²⁺] decreases, the equilibrium Cu²⁺ + 2e⁻ ⇌ Cu shifts left, making the potential more negative than the standard value.

E° 值是在标准条件下测定的。若浓度改变（如不是 1.00 mol dm⁻³），电极电势将根据 Nernst 方程移动（OCR 不要求，但定性效应很重要）。对于 Cu²⁺/Cu 半电池，如果 [Cu²⁺] 降低，平衡 Cu²⁺ + 2e⁻ ⇌ Cu 向左移动，电势会比标准值更负。

Also, E° values cannot predict the rate of reaction. A reaction with a positive E°_cell may be kinetically inert, such as the reaction between Mg and cold water. Similarly, side reactions or passivation can prevent the predicted reaction from occurring.

此外，E° 值不能预测反应速率。E°_cell 为正的反应可能在动力学上是惰性的，如镁与冷水的反应。同样，副反应或钝化效应也会阻止预期反应的发生。

Lastly, E° is measured at pH = 0 for acidic half‑reactions. In neutral or alkaline solutions, different potentials apply, which explains why oxidizing power of permanganate depends on pH.

最后，酸性半反应的 E° 是在 pH = 0 条件下测得的。在中性或碱性溶液中，电势会不同，这就解释了为什么高锰酸盐的氧化能力取决于 pH。

10. Fuel Cells | 燃料电池

A fuel cell converts chemical energy from a fuel (usually hydrogen) and an oxidant directly into electrical energy, with high efficiency and low pollution. In an alkaline hydrogen‑oxygen fuel cell, the reactions are:

燃料电池将燃料（通常为氢气）和氧化剂的化学能直接转化为电能，具有高效率和低污染的特点。在碱性氢氧燃料电池中，反应如下：

Cathode (reduction): O₂ + 2H₂O + 4e⁻ → 4OH⁻

阳极（还原）：O₂ + 2H₂O + 4e⁻ → 4OH⁻

Anode (oxidation): 2H₂ + 4OH⁻ → 4H₂O + 4e⁻

阴极（氧化）：2H₂ + 4OH⁻ → 4H₂O + 4e⁻

Overall: 2H₂ + O₂ → 2H₂O. This is essentially the reverse of electrolysis of water. Fuel cells require a porous electrode and an electrolyte (e.g. KOH). They differ from batteries because they do not store chemical energy inside; fuels are supplied externally.

总反应：2H₂ + O₂ → 2H₂O。这本质上是水电解的逆过程。燃料电池需要多孔电极和电解质（如 KOH）。它们与电池不同，因为内部不储存化学能；燃料由外部持续供给。

Advantages: higher efficiency than heat engines, direct energy conversion, only water as exhaust. Disadvantages: hydrogen storage and safety, cost of catalysts (platinum), sensitivity to impurities.

优点：比热机效率高、直接转换、唯一产物为水。缺点：氢气的储存与安全、催化剂成本（铂）、对杂质敏感。

11. Corrosion as an Electrochemical Process | 腐蚀作为电化学过程

Rusting of iron is an electrochemical process involving the formation of anodic and cathodic regions on the metal surface. In the presence of water and oxygen, iron oxidises at anodic sites: Fe(s) → Fe²⁺(aq) + 2e⁻. Electrons travel through the metal to cathodic regions where oxygen is reduced: O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq). Fe²⁺ reacts with OH⁻ to form Fe(OH)₂, which further oxidises to hydrated Fe₂O₃ (rust).

铁生锈是一种电化学过程，涉及金属表面形成阳极区和阴极区。在有水和氧气存在下，铁在阳极位置被氧化：Fe(s) → Fe²⁺(aq) + 2e⁻。电子通过金属流向阴极区，氧气在此被还原：O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)。Fe²⁺ 与 OH⁻ 反应生成 Fe(OH)₂，再进一步被氧化为水合 Fe₂O₃（铁锈）。

Prevention methods include barrier protection (painting, coating), sacrificial protection (attaching a more reactive metal such as zinc or magnesium, which corrodes instead), or impressed current cathodic protection.

防护方法包括隔离保护（涂漆、镀层）、牺牲阳极保护（连接如锌或镁等更活泼的金属，使其优先腐蚀），或外加电流阴极保护。

12. Exam Tips for Electrochemistry | 电化学考试技巧

Always write half‑equations as reductions unless asked for oxidation. When calculating E°_cell, subtract the more negative E° from the more positive E°; do not change the sign of the reduction potential before subtraction if you use the formula E°_cell = E°(cathode) – E°(anode). Remember that E°_cell must be positive for feasibility.

除非题目要求，否则半反应均以还原形式书写。计算 E°_cell 时，用更正的电势减去较负的电势；若使用公式 E°_cell = E°(阴极) – E°(阳极)，切勿在相减前更改还原电势的符号。切记 E°_cell 必须为正反应才可行。

In cell diagrams, the oxidised form is placed near the salt bridge, but in practice, follow OCR conventions. State the direction of electron flow: from the more negative electrode to the more positive electrode. Label polarities: the electrode where oxidation occurs is the anode (negative), and where reduction occurs is the cathode (positive).

在电池符号图中，氧化态靠近盐桥，但实践中应遵循 OCR 规范。指明电子流动方向：从较负的电极端流向较正的电极端。标注极性：发生氧化的电极为阳极（负极），发生还原的电极为阴极（正极）。

When explaining corrosion, link to electrochemical concepts: formation of a local cell, anodic and cathodic processes, and the necessity of both water and oxygen. Use standard potentials to justify the choice of sacrificial metals.

解释腐蚀时，应与电化学概念联系：局部电池的形成、阴阳极过程以及水和氧气均为必需条件。利用标准电极电势说明选择牺牲金属的依据。

Finally, memorise key E° values for common species like Fe³⁺/Fe²⁺, Zn²⁺/Zn, and halogen/halide couples, as they are frequently examined. Practice constructing cell notation for unfamiliar redox pairs, including those with two aqueous ions requiring a Pt electrode.

最后，熟记常见物质如 Fe³⁺/Fe²⁺、Zn²⁺/Zn 以及卤素/卤离子对等关键 E° 值，它们经常作为考点。练习为陌生氧化还原对构建电池符号，包括需要 Pt 电极的两离子溶液情况。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Electrochemistry for A-Level OCR Chemistry | A-Level OCR 化学：电化学 考点精讲