📚 AQA International A2 Chemistry CH03: Redox Equilibria, Electrode Potentials and Electrochemical Cells | AQA 国际 A2 化学 CH03：氧化还原平衡、电极电势与电化学电池
Redox equilibria and electrode potentials form a core part of AQA International A2 Chemistry Unit 3 (CH03). This article explains how standard electrode potentials are measured, how they are used to calculate cell EMF, and how E° values help predict the feasibility of redox reactions. We also highlight common exam pitfalls and work through an AQA-style question.
氧化还原平衡和电极电势是 AQA 国际 A2 化学第三单元（CH03）的核心内容。本文将解释标准电极电势的测量方法、如何用其计算电池电动势，以及如何利用 E° 判断氧化还原反应的可行性；同时强调常见考试误区，并演练一道 AQA 风格题目。
1. Why Redox Equilibria Matter in CH03 | 为什么氧化还原平衡在 CH03 中很重要
In AQA International Chemistry A2, Unit 3 brings together ideas from kinetics, equilibria, acids and bases, and electrochemistry. Redox equilibria are the basis for electrochemical cells, batteries, fuel cells, and many analytical techniques. A common exam task is to use E° values to predict whether a reaction will occur under standard conditions.
在 AQA 国际化学 A2 中，第三单元综合了动力学、平衡、酸碱和电化学等内容。氧化还原平衡是电化学电池、电池、燃料电池以及许多分析技术的基础。考试中常见任务是利用 E° 判断一个反应在标准条件下能否发生。
Questions on the June 2023 CH03 paper often expect you to combine half-equations, calculate E°cell, and then justify a conclusion about feasibility. You may also need to link electrode potentials to redox titrations or to explain why a thermodynamically feasible reaction may not actually be observed.
2023 年 6 月 CH03 试卷中的题目通常要求你合并半反应、计算 E°cell，然后对可行性得出结论。你还可能需要将电极电势与氧化还原滴定联系起来，或解释为什么热力学上可行的反应实际上不一定被观察到。
2. Oxidation States and Half-Equations | 氧化数与半反应式
Redox reactions involve electron transfer. Oxidation is an increase in oxidation state (loss of electrons); reduction is a decrease in oxidation state (gain of electrons). Writing balanced half-equations is the first step in any electrode potential calculation.
氧化还原反应涉及电子转移。氧化指氧化数升高（失去电子）；还原指氧化数降低（得到电子）。书写配平的半反应式是任何电极电势计算的第一步。
For example, the reduction of manganate(VII) in acidic solution is written as:
例如，高锰酸根在酸性溶液中的还原半反应写作：
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
The oxidation of iron(II) is much simpler:
铁(II)的氧化半反应则简单得多：
Fe²⁺ → Fe³⁺ + e⁻
Electrode potentials are always quoted as reduction half-equations by convention. If you need the oxidation version, you reverse the half-equation and change the sign of E°.
电极电势按惯例总是写成还原半反应。如果需要氧化方向的半反应，只需反转半反应并改变 E° 的符号。
3. Standard Electrode Potentials and the SHE | 标准电极电势与标准氢电极
tandard hydrogen electrode (SHE) as the zero reference. The SHE half-cell is written as Pt | H₂(g) | 2H⁺(aq), and it is assigned E° = 0.00 V.
标准电极电势 E° 在 298 K、100 kPa、1.0 mol dm⁻³ 溶液条件下测量，并以标准氢电极为零点参考。标准氢电极半电池写作 Pt | H₂(g) | 2H⁺(aq)，规定 E° = 0.00 V。
Other half-cells are measured relative to this reference. A more positive E° means the oxidised form is more likely to be reduced; a more negative E° means the reduced form is more likely to be oxidised. You should remember the following common values:
其他半电池的电势均相对此参考电极测定。E° 越正，氧化态越容易被还原；E° 越负，还原态越容易被氧化。你应记住以下常见数值：
| Reduction half-equation | E° / V |
| Zn²⁺ + 2e⁻ ⇌ Zn | −0.76 |
| 2H⁺ + 2e⁻ ⇌ H₂ | 0.00 |
| Cu²⁺ + 2e⁻ ⇌ Cu | +0.34 |
| Fe³⁺ + e⁻ ⇌ Fe²⁺ | +0.77 |
| MnO₄⁻ + 8H⁺ + 5e⁻ ⇌ Mn²⁺ + 4H₂O | +1.51 |
4. Constructing an Electrochemical Cell | 构建电化学电池
In a standard electrochemical cell, two half-cells are connected by a salt bridge, and the electrodes are joined through a high-resistance voltmeter. The salt bridge contains a concentrated solution of an ionic salt, such as KNO₃, to complete the circuit by allowing ions to flow without mixing the half-cell solutions.
在标准电化学电池中，两个半电池通过盐桥连接，电极则通过高电阻伏特计相连。盐桥中含有浓的离子盐溶液，例如 KNO₃，通过离子迁移完成电路，同时避免两种半电池溶液混合。
For a gas or ion-based half-cell, an inert platinum electrode is used. A standard cell diagram follows the convention:
对于气体或离子型半电池，使用惰性铂电极。标准电池图遵循以下惯例：
Pt | H₂ | 2H⁺ || Fe³⁺, Fe²⁺ | Pt
e reduction half-cell when the cell is drawn conventionally, so E°cell = E°right − E°left.
双竖线表示盐桥。按照惯例，右侧电极是还原半电池，因此 E°cell = E°right − E°left。
5. Calculating Cell EMF | 计算电池电动势
The standard cell EMF is calculated from the standard electrode potentials of the two half-cells:
标准电池电动势由两个半电池的标准电极电势计算得到：
E°cell = E°(reduction) − E°(oxidation)
is the positive electrode where reduction occurs, and zinc is the negative electrode where oxidation occurs:
还原半电池具有较正的 E° 值。例如，锌-铜电池使用 Zn²⁺/Zn（E° = −0.76 V）和 Cu²⁺/Cu（E° = +0.34 V）。铜为正极发生还原，锌为负极发生氧化：
E°cell = 0.34 V − (−0.76 V) = 1.10 V
Remember that E° values are not multiplied by stoichiometric coefficients. The cell EMF depends only on the identity of the half-cells, not on the number of moles in the equation.
记住，E° 值不随化学计量系数变化。电池电动势只取决于半电池的种类，与方程中的摩尔数无关。
6. Predicting Redox Feasibility Using E° Values | 用 E° 预测氧化还原可行性
A redox reaction is thermodynamically feasible under standard conditions if E°cell is positive. If E°cell is negative, the forward reaction is not feasible; the reverse reaction would be favoured instead.
在标准条件下，如果 E°cell 为正，则氧化还原反应在热力学上可行。如果 E°cell 为负，正向反应不可行；反向反应反而更有利。
Consider whether Fe³⁺ can oxidise iodide ions to iodine:
考虑 Fe³⁺ 能否将碘离子氧化为碘：
Fe³⁺ + e⁻ ⇌ Fe²⁺ E° = +0.77 V
I₂ + 2e⁻ ⇌ 2I⁻ E° = +0.54 V
idation half-reaction is 2I⁻ → I₂ + 2e⁻, so we use the I₂/I⁻ E° with oxidation sign. Then:
氧化半反应为 2I⁻ → I₂ + 2e⁻，因此 I₂/I⁻ 的 E° 以氧化方向参与计算。于是：
