📚 Combined Science 146: Redox Equilibria and Electrochemical Cells | Combined Science 146: 氧化还原平衡与电化学电池
Electrochemical cells turn chemical energy into electrical energy by separating oxidation and reduction processes. In A-Level Edexcel Combined Science, understanding how to measure, calculate and apply standard electrode potentials is essential for predicting the feasibility of redox reactions. This article breaks down the key concepts, from half-cell construction to fuel cells.
电化学电池通过分离氧化与还原过程，将化学能转化为电能。在 A-Level Edexcel Combined Science 中，掌握如何测量、计算并应用标准电极电势是预测氧化还原反应是否可行的关键。本文将从半电池的构建一直讲解到燃料电池。
1. Introduction to Redox and Electron Transfer | 氧化还原与电子传递导论
Redox reactions involve the simultaneous transfer of electrons. Oxidation is loss of electrons, and reduction is gain of electrons. When these two half-reactions occur in separate containers connected by a wire and a salt bridge, the electron flow can be harnessed as an electric current.
氧化还原反应涉及电子的同时转移。氧化是失电子，还原是得电子。当这两个半反应在由导线和盐桥连接的不同容器中发生时，电子的流动就可以被利用为电流。
2. Understanding Half Cells and Electrode Potentials | 理解半电池与电极电势
A half-cell consists of a metal electrode dipped in a solution of its own ions, for example Zn(s) in Zn²⁺(aq). An equilibrium is established at the metal surface: Zn²⁺(aq) + 2e⁻ ⇌ Zn(s). The tendency of the electrode to gain or lose electrons creates a potential difference relative to the solution.
半电池由浸在自身离子溶液中的金属电极构成，例如 Zn(s) 放入 Zn²⁺(aq) 中。在金属表面会建立一个平衡：Zn²⁺(aq) + 2e⁻ ⇌ Zn(s)。电极得失电子的倾向形成了一个相对于溶液的电势差。
It is impossible to measure the absolute electrode potential of a single half-cell. Hence, a reference electrode is needed. Electrode potentials are always quoted as standard electrode potentials, E°, measured under standard conditions: 298 K, 1.0 mol dm⁻³ ion concentration and a pressure of 100 kPa for gases.
不可能测量单个半电池的绝对电极电势，因此需要一个参比电极。电极电势总是以标准状态下的标准电极电势 E° 给出，标准条件为：298 K、1.0 mol dm⁻³ 离子浓度，气体压强为 100 kPa。
3. The Standard Hydrogen Electrode: A Reference Point | 标准氢电极：参比基准
The standard hydrogen electrode (SHE) is assigned an E° value of 0.00 V. It consists of platinum black electrode immersed in a solution of H⁺(aq) at 1 mol dm⁻³, with hydrogen gas at 100 kPa bubbled over the surface. The equilibrium is 2H⁺(aq) + 2e⁻ ⇌ H₂(g).
标准氢电极 (SHE) 被赋予 0.00 V 的 E° 值。它由铂黑电极浸泡在 1 mol dm⁻³ 的 H⁺(aq) 溶液中，并向电极表面通入 100 kPa 的氢气。对应的平衡为 2H⁺(aq) + 2e⁻ ⇌ H₂(g)。
All other standard electrode potentials are measured by connecting the half-cell under test to a SHE and recording the cell EMF. A voltmeter with very high resistance is used to prevent current flow.
所有其他标准电极电势都是通过将待测半电池与 SHE 连接并测量电池电动势而得到的。测量时使用高阻抗电压表以防止有电流通过。
4. Measuring Standard Electrode Potentials | 测量标准电极电势
To measure E° for Zn²⁺|Zn, set up the cell:
Pt(s)|H₂(g)|2H⁺(aq) || Zn²⁺(aq)|Zn(s)
测量 Zn²⁺|Zn 的 E° 时，构建如下电池：
Pt(s)|H₂(g)|2H⁺(aq) || Zn²⁺(aq)|Zn(s)
The voltmeter reading is the E° of the zinc half-cell. In practice, the Zn²⁺|Zn half-cell shows a negative potential (–0.76 V), meaning electrons flow from the zinc electrode towards the hydrogen electrode.
电压表的读数即为锌半电池的 E°。实际上，Zn²⁺|Zn 半电池显示负电势（–0.76 V），意味着电子从锌电极向氢电极流动。
5. Cell Potential Calculations and Direction | 电池电势的计算与方向
For any two half-cells, the standard cell potential E°cell is calculated using the reduction potentials from a data table:
E°cell = E°(right-hand electrode) – E°(left-hand electrode)
对任意两个半电池，标准电池电势 E°cell 通过数据表中的还原电势计算得到：
E°cell = E°(right-hand electrode) – E°(left-hand electrode)
Using the Daniell cell as an example: Cu²⁺|Cu has E° = +0.34 V, Zn²⁺|Zn has E° = –0.76 V. If Cu is placed on the right, E°cell = +0.34 – (– s reaction; electrons will flow from the more negative electrode to the more positive one.
以丹尼尔电池为例：Cu²⁺|Cu 的 E° = +0.34 V，Zn²⁺|Zn 的 E° = –0.76 V。若将 Cu 放在右侧，E°cell = +0.34 – (–0.76) = +1.10 V。正的 E°cell 表示反应可自发进行；电子将从电势更负的电极流向更正的电极。
6. Interpreting Cell Notation | 解读电池符号
Cell notation follows the convention: the left half-cell is the anode (where oxidation occurs) and the right half-cell is the cathode (where reduction occurs). A single vertical line '|' represents a phase boundary, and a double vertical line '||' represents the salt bridge.
电池符号遵循以下惯例：左侧半电池为阳极（发生氧化），右侧半电池为阴极（发生还原）。单竖线 '|' 表示相界面，双竖线 '||' 表示盐桥。
For example, Zn(s)|Zn²⁺(aq)||Cu²⁺(aq)|Cu(s) implies Zn is oxidised to Zn²⁺ and Cu²⁺ is reduced to Cu. The EMF of this cell is +1.10 V under standard conditions.
例如，Zn(s)|Zn²⁺(aq)||Cu²⁺(aq)|Cu(s) 意味着 Zn 被氧化为 Zn²⁺，Cu²⁺ 被还原为 Cu。在标准条件下，该电池的电动势为 +1.10 V。
7. Using E° Values to Predict Reaction Feasibility | 用 E° 值预测反应可行性
A redox reaction is thermodynamically feasible if the calculated E°cell is positive. For a reaction between two redox couples, reverse the half-equation of the species being oxidised and add the two E° values. If the sum is positive, the reaction should proceed in the forward direction.
若计算得出的 E°cell 为正值，则该氧化还原反应在热力学上可行。对于两个氧化还原电对之间的反应，将被氧化的物种的半反应式反转后，将两个 E° 值相加。若相加结果为正，反应应能正向进行。
Will Cu²⁺ oxidise Zn? E°(Cu²⁺/Cu) = +0.34 V, E°(Zn²⁺/Zn) = –76 V. Zn (oxidised) couple is reversed: E°cell = +0.34 + 0.76 = +1.10 V, so yes, feasible.
Cu²⁺ 能否氧化 Zn？ E°(Cu²⁺/Cu) = +0.34 V，E°(Zn²⁺/Zn) = –0.76 V。Zn（被氧化）电对取反号：E°cell = +0.34 + 0.76 = +1.10 V，因此可行。
8. Limitations and Kinetic Factors | 局限性与动力学因素
A positive E°cell only indicates thermodynamic feasibility; it says nothing about the rate of reaction. Some reactions with positive cell potentials are so slow that no observable change occurs — for example, the oxidation of water by dichromate(VI) has a positive E°cell but proceeds imperceptibly without heating.
正的 E°cell 只表明热力学可行性，并不说明反应速率。有些具有正电池电势的反应速率极慢，观察不到变化——例如，重铬酸根氧化水具有正的 E°cell，但在不加热的条件下基本不反应。
Moreover, standard potentials apply only under standard conditions. Changing temperature, concentration or pressure can alter the actual cell potential, which may reverse the feasibility prediction. Exam questions often ask you to comment on 'non-standard conditions'.
此外，标准电势仅在标准条件下适用。改变温度、浓度或压力会改变实际电池电势，从而可能颠倒可行性预测。考卷中常要求评论“非标准条件”的影响。
9. Effect of Concentration: The Nernst Equation | 浓度的影响：能斯特方程
When concentrations are not 1 mol dm⁻³, the cell potential deviates from E° according to the Nernst equation. At 298 K the equation simplifies to:
E = E° – (0.0592/n) log Q
当浓度不为 1 mol dm⁻³ 时，电池电势会根据能斯特方程偏离 E°。在 298 K 时，方程简化为：
E = E° – (0.0592/n) log Q
Here 'n' is the number of electrons transferred and Q is the reaction quotient. Although Nernst calculations are not always required, understanding that decreasing the product ion concentration increases the cell potential is important for concentration cells and pH meters.
其中 'n' 为转移电子数，Q 为反应商。虽然能斯特计算并不总是必考，但理解“降低产物离子浓度会增大电池电势”对于浓差电池和 pH 计的原理非常重要。
10. Electrochemical Cells in Practice: Batteries | 实际中的电化学电池：蓄电池
Primary batteries, such as alkaline cells, are not rechargeable because the reactions go to completion. Secondary batteries, like lithium-ion cells, are rechargeable because applying a reverse voltage can drive the reaction backwards.
原电池（如碱性电池）不可充电，因为反应会进行到底；二次电池（如锂离子电池）可充电，因为施加反向电压可以使反应逆向进行。
In an Edexcel paper, you may be asked to explain why the e.m.f. of a cell falls during use. This is due to reactant concentration decreasing and product concentration increasing, which reduces the cell potential according to the Nernst equation, as well as build-up of polarisation products.
在 Edexcel 试卷中，可能要求解释为何电池在使用过程中电动势会下降。这是由于反应物浓度下降、产物浓度上升，根据能斯特方程使得电池电势减小，同时还会因极化产物累积而下降。
11. Fuel Cells and Energy Conversion | 燃料电池与能量转换
A hydrogen– uel cell generates electricity from the reaction 2H₂ + O₂ → 2H₂O, with the half-reactions:
Anode: 2H₂ + 4OH⁻ → 4H₂O + 4e⁻
Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻
氢氧燃料电池通过 2H₂ + O₂ → 2H₂O 反应产生电能，半反应为：
阳极：2H₂ + 4OH⁻ → 4H₂O + 4e⁻
阴极：O₂ + 2H₂O + 4e⁻ → 4OH⁻
Fuel cells offer higher efficiency than internal combustion engines and produce only water as a waste product. However, the production of hydrogen often relies on fossil fuels, and storage remains a challenge. Understanding the electrode reactions and the role of the alkaline electrolyte is a common combined science exam topic.
燃料电池比内燃机效率更高，且仅产生水这一种废物。然而，氢气生产常依赖化石燃料，储存也是一大挑战。理解电极反应和碱性电解质的作用是 combined science 试卷中的常见课题。
12. Exam Strategy for Edexcel Combined Science | Edexcel Combined Science 备考策略
ays underline the standard conditions and note which electrode is the right-hand side. Use 'E°cell = E°right – E°left' to give the overall equation. Remember to balance electrons but do not multiply E° values.
回答电极电势问题时，务必标出标准条件，并注意哪个是右侧电极。系统使用“E°cell = E°right – E°left”，若需要预测反应电池的电动势，应选择两个能组合成总方程的半反应式。记住配平电子，但不要将 E° 值乘以系数。
Practice drawing cell diagrams, labelling the cathode and anode, and explaining the movement of ions in the salt bridge. Strong answers will also discuss why a reaction may not occur despite a positive E°cell, referencing kinetics or non-standard concentrations.
练习绘制电池示意图，标注阴极和阳极，并解释盐桥中离子的移动。高分答案还会讨论为何在 E°cell 为正的情况下反应仍
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