📚 Edexcel A-Level Chemistry Topic 14: Redox II | Edexcel A-Level 化学 第14讲：氧化还原 II
Redox II extends your understanding of oxidation and reduction from simple electron transfer to quantitative electrochemical measurements. This topic covers standard electrode potentials, electrochemical cells, feasibility predictions, and redox titrations such as manganate(VII) and iodine-thiosulfate reactions.
氧化还原 II 将你对氧化和还原的理解从简单的电子转移延伸到定量的电化学测量。本专题涵盖标准电极电势、电化学电池、反应可行性预测以及高锰酸盐(VII)和碘-硫代硫酸盐等氧化还原滴定。
1. Redox Fundamentals and Oxidation States | 氧化还原基础与氧化数
Oxidation is the loss of electrons, and reduction is the gain of electrons. In a redox reaction, one species is oxidised while another is reduced, so electrons are transferred from the reducing agent to the oxidising agent.
氧化是失去电子，还原是得到电子。在氧化还原反应中，一种物质被氧化，另一种物质被还原，因此电子从还原剂转移到氧化剂。
Oxidation states are bookkeeping numbers assigned to atoms using agreed rules. They allow you to identify what has been oxidised and what has been reduced even when no obvious electron transfer is shown.
氧化数是按照约定规则分配给原子的记数。它们可以让你在即使没有明显电子转移的情况下识别什么被氧化、什么被还原。
- Uncombined elements have oxidation state 0, e.g. Fe, O₂, Cl₂.
- 单质氧化数为 0，例如 Fe、O₂、Cl₂。
- Hydrogen is usually +1, oxygen is usually −2, and group 1 metals are +1.
- 氢通常为 +1，氧通常为 −2，第 1 族金属为 +1。
- The sum of oxidation states equals the charge on the ion or zero for a neutral compound.
- 氧化数之和等于离子的电荷，或中性化合物为零。
2. Balancing Redox Half-Equations | 配平氧化还原半反应方程式
Half-equations separate oxidation and reduction processes. They must be balanced for atoms and charge using H⁺ ions and H₂O in acidic solution.
半反应方程式将氧化和还原过程分开。在酸性溶液中，必须使用 H⁺ 离子和 H₂O 配平原子和电荷。
A common manganate(VII) half-equation is:
常见的高锰酸盐(VII)半反应方程式为：
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
To combine half-equations, multiply each so that the number of electrons is equal, then add and cancel electrons.
要合并半反应方程式，需要对每个式子乘以适当系数使电子数相等，然后相加并消去电子。
3. Standard Electrode Potential and the Standard Hydrogen Electrode | 标准电极电势与标准氢电极
The standard electrode potential E° of a half-cell measures the tendency of a species to gain electrons relative to the standard hydrogen electrode under standard conditions.
半电池的标准电极电势 E° 是相对于标准氢电极，在标准条件下物质获得电子趋势的量度。
1.00 mol dm⁻³ ion concentrations. The standard hydrogen electrode is assigned E° = 0.00 V.
标准条件为 298 K、100 kPa 压力和 1.00 mol dm⁻³ 离子浓度。标准氢电极被指定为 E° = 0.00 V。
More positive E° values mean the species is more likely to be reduced, so it is a stronger oxidising agent.
E° 值越正，说明该物质越容易被还原，因此它是更强的氧化剂。
4. Measuring Standard Electrode Potentials | 测量标准电极电势
A half-cell is connected to the standard hydrogen electrode by a salt bridge and a high-resistance voltmeter. The measured voltage is the standard electrode potential of the half-cell.
半电池通过盐桥和高电阻电压表与标准氢电极相连。测得的电压就是该半电池的标准电极电势。
If the half-cell has a greater tendency to gain electrons than H⁺, the potential is positive. If it has a lower tendency, the potential is negative.
如果半电池比 H⁺ 更容易得到电子，则电势为正。如果倾向性较低，则电势为负。
| Half-reaction | E° / V |
|---|---|
| Zn²⁺ + 2e⁻ ⇌ Zn | −0.76 |
| Fe²⁺ + 2e⁻ ⇌ Fe | −0.44 |
| 2H⁺ + 2e⁻ ⇌ H₂ | 0.00 |
| Cu²⁺ + 2e⁻ ⇌ Cu | +0.34 |
| Fe³⁺ + e⁻ ⇌ Fe²⁺ | +0.77 |
| MnO₄⁻ + 8H⁺ + 5e⁻ ⇌ Mn²⁺ + 4H₂O | +1.51 |
5. Electrochemical Cells and Cell Diagrams | 电化学电池与电池图示
An electrochemical cell is formed by connecting two half-cells with a salt bridge and an external wire. The cell potential is the difference between the two half-cell potentials.
电化学电池由两个半电池通过盐桥和外部导线连接而成。电池电动势是两个半电池电势之差。
The standard cell EMF is calculated using:
标准电池电动势计算公式为：
E°cell = E°(reduction half-cell) − E°(oxidation half-cell)
Cell diagrams use a vertical line for a phase boundary and a double vertical line for the salt bridge. The more positive half-cell is placed on the right.
电池图示使用单竖线表示相界面，双竖线表示盐桥。电势更正的半电池放在右侧。
For example, a zinc-copper cell is written as:
例如，锌-铜电池表示为：
Zn | Zn²⁺ ∥ Cu²⁺ | Cu
6. Predicting Feasibility of Redox Reactions | 预测氧化还原反应的可行性
A redox reaction is thermodynamically feasible if the calculated cell EMF is positive. This means the reaction can occur under standard conditions.
如果计算出的电池电动势为正，则氧化还原反应在热力学上是可行的。这意味着在标准条件下该反应可以发生。
metal can reduce Cu²⁺ ions because E°cell = +0.34 − (−0.76) = +1.10 V, which is positive.
例如，锌金属可以还原 Cu²⁺ 离子，因为 E°cell = +0.34 − (−0.76) = +1.10 V，为正值。
The relationship between cell EMF and Gibbs free energy is:
电池电动势与吉布斯自由能的关系为：
ΔG° = −nFE°cell
A positive E°cell gives a negative ΔG°, confirming thermodynamic feasibility.
正的 E°cell 给出负的 ΔG°，从而确认热力学可行性。
7. Limitations of Standard Electrode Potential Predictions | 标准电极电势预测的局限性
E° values refer to standard conditions. If concentration, temperature, or pressure changes, the actual electrode potential changes and feasibility may be different.
E° 值是指在标准条件下。如果浓度、温度或压力发生变化，实际电极电势就会改变，可行性也可能不同。
Thermodynamic feasibility also does not guarantee a reaction will proceed at a measurable rate. A reaction may have a positive E°cell but be kinetically slow.
热力学可行性也不能保证反应以可测量的速率进行。反应可能具有正的 E°cell，但动力学上很慢。
For example, MnO₄⁻ and Fe²⁺ react rapidly, but some reactions with positive E°cell are slow due to high activation energy.
例如，MnO₄⁻ 与 Fe²⁺ 反应很快，但一些 E°cell 为正的反应由于活化能高而反应缓慢。
8. Redox Titrations: Manganate(VII) | 氧化还原滴定：高锰酸盐(VII)法
Potassium manganate(VII) is a powerful oxidising agent used in redox titrations under acidic conditions. It acts as its own indicator because MnO₄⁻ is purple and Mn²⁺ is almost colourless.
高锰酸钾是一种强氧化剂，在酸性条件下用于氧化还原滴定。它本身可作指示剂，因为 MnO₄⁻ 为紫色，而 Mn²⁺ 几乎无色。
The titration is carried out with dilute sulfuric acid. The end point is the first permanent pale pink colour due to excess MnO₄⁻.
滴定使用稀硫酸进行。终点是过量的 MnO₄⁻ 呈现的第一个持久淡粉红色。
MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
This equation shows that 1 mol of MnO₄⁻ reacts with 5 mol of Fe²⁺, so the mole ratio is 1:5.
该方程式表明 1 mol MnO₄⁻ 与 5 mol Fe²⁺ 反应，因此摩尔比为 1:5。
9. Redox Titrations: Iodine and Thiosulfate | 氧化还原滴定：碘与硫代硫酸盐
Iodine-sodium thiosulfate titration is a common method for determining oxidising agents. Iodine oxidises thiosulfate ions to tetrathionate ions.
碘-硫代硫酸钠滴定是测定氧化剂的常用方法。碘将硫代硫酸根离子氧化为连四硫酸根离子。
The key half-equations are:
关键半反应方程式为：
I₂ + 2e⁻ ⇌ 2I⁻
2S₂O₃²⁻ → S₄O₆²⁻ + 2e⁻
The overall reaction is:
总反应为：
I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
Starch is added near the end point, when the iodine solution has become pale yellow. The blue-black starch-iodine complex disappears sharply at the end point.
淀粉在接近终点时加入，此时碘溶液已变为浅黄色。蓝黑色的淀粉-碘络合物在终点时急剧消失。
10. Disproportionation Reactions | 歧化反应
Disproportionation is a redox reaction in which the same element is both oxidised and reduced. The oxidation state of the element splits into one higher and one lower state.
歧化反应是同一元素既被氧化又被还原的氧化还原反应。该元素的氧化数分裂为一个较高和一个较低的状态。
A classic example is the reaction of chlorine with water:
一个经典例子是氯与水的反应：
Cl₂ + H₂O ⇌ HCl + HClO
Chlorine changes from 0 in Cl₂ to −1 in HCl and +1 in HClO, so it disproportionates.
氯从 Cl₂ 中的 0 变为 HCl 中的 −1 和 HClO 中的 +1，因此发生歧化。
11. Redox in Organic Chemistry | 有机化学中的氧化还原
Redox concepts also apply in organic chemistry. Oxidation is often seen as gain of oxygen or loss of hydrogen, while reduction is loss of oxygen or gain of hydrogen.
氧化还原概念也适用于有机化学。氧化通常表现为加氧或失氢，还原则为失氧或加氢。
For example, primary alcohols are oxidised to aldehydes then carboxylic acids using acidified potassium dichromate(VI).
例如，伯醇用酸化重铬酸钾(VI)氧化成醛，然后氧化成羧酸。
The chromium species changes from orange Cr₂O₇²⁻ to green Cr³⁺, showing reduction of chromium while the alcohol is oxidised.
铬物种从橙色的 Cr₂O₇²⁻ 变为绿色的 Cr³⁺，表明铬被还原，而醇被氧化。
12. Exam Technique and Common Pitfalls | 考试技巧与常见错误
When constructing a cell diagram, remember that the more negative half-cell is the oxidation site and is placed on the left. The electrons flow from the negative electrode to the positive electrode.
在构建电池图示时，记住较负的半电池是氧化位点，应放在左侧。电子从负极流向正极。
Always show state symbols and charges in half-equations unless the question says otherwise. Balance oxygen using H₂O and hydrogen using H⁺ in acidic solution.
除非题目另有说明，半反应方程式中始终标明状态符号和电荷。在酸性溶液中用 H₂O 配平氧，用 H⁺ 配平氢。
ut when using E°cell = E°(right) − E°(left), do not reverse the values before subtraction.
不要混淆 E°cell 的符号规则。如果反向书写半反应，E° 的符号会反转，但使用 E°cell = E°(右) − E°(左) 时，不要先反转数值再相减。
Finally, link redox predictions to both thermodynamics and kinetics. A positive E°cell indicates feasibility, but only experimental observation can confirm the rate.
最后，将氧化还原预测与热力学和动力学联系起来。正 E°cell 表示反应可行，但只有实验观察才能确认反应速率。
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