📚 Redox II: Electrode Potentials and Cells (Edexcel A-Level Chemistry Topic 14) | 氧化还原II：电极电势与电池（爱德思A-Level化学第14单元）
Edexcel A-Level Chemistry Topic 14 Redox II extends the electron-transfer ideas from Topic 3 Redox I into quantitative electrochemistry. You must be able to measure standard electrode potentials, build electrochemical cells, calculate cell potentials, predict feasibility, and apply redox equations in titrations and commercial cells.
爱德思A-Level化学第14单元氧化还原II将第3单元氧化还原I中的电子转移概念延伸到定量电化学。你需要掌握标准电极电势的测量、电化学电池的构建、电池电动势的计算、反应可行性的判断，以及氧化还原方程式在滴定和商品电池中的应用。
1. Redox Recap and Oxidation Numbers | 氧化还原复习与氧化数
Redox reactions involve electron transfer. Oxidation is loss of electrons and reduction is gain of electrons. Oxidation numbers give a useful bookkeeping method: oxidation increases the oxidation number, while reduction decreases it. The oxidising agent is reduced and the reducing agent is oxidised.
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。氧化数是一种有用的记账方法：氧化使氧化数升高，还原使氧化数降低。氧化剂本身被还原，还原剂本身被氧化。
Common oxidation-number rules are: elements are 0; hydrogen is +1 except in metal hydrides where it is −1; oxygen is −2 except in peroxides where it is −1; the sum of oxidation numbers equals the overall charge.
常见的氧化数规则是：单质为0；氢通常为+1，但在金属氢化物中为−1；氧通常为−2，但在过氧化物中为−1；所有氧化数之和等于粒子所带电荷。
2. Standard Electrode Potentials and the Hydrogen Electrode | 标准电极电势与氢电极
The standard electrode potential, E°, of a half-cell is measured relative to the standard hydrogen electrode (SHE), which is assigned a value of 0.00 V. Standard conditions are 298 K, 100 kPa pressure for gases, and 1.00 mol dm⁻³ ion concentration.
半电池的标准电极电势 E° 是相对于标准氢电极（SHE）测得的，标准氢电极的电势被规定为 0.00 V。标准条件为 298 K、气体压强 100 kPa、离子浓度 1.00 mol dm⁻³。
The SHE uses hydrogen gas bubbled over a platinum electrode in contact with H⁺ ions. The platinum is inert and provides a surface for electron transfer.
标准氢电极使用铂电极，氢气通入并与 H⁺ 接触。铂是惰性电极，为电子转移提供表面。
| Half-equation | E° / V |
|---|---|
| Zn²⁺ + 2e⁻ ⇌ Zn | −0.76 |
| Fe²⁺ + 2e⁻ ⇌ Fe | −0.44 |
| 2H⁺ + 2e⁻ ⇌ H₂ | 0.00 |
| Cu²⁺ + 2e⁻ ⇌ Cu | +0.34 |
| I₂ + 2e⁻ ⇌ 2I⁻ | +0.54 |
| Fe³⁺ + e⁻ ⇌ Fe²⁺ | +0.77 |
| Cl₂ + 2e⁻ ⇌ 2Cl⁻ | +1.36 |
| MnO₄⁻ + 8H⁺ + 5e⁻ ⇌ Mn²⁺ + 4H₂O | +1.51 |
3. Electrochemical Cells and Cell Notation | 电化学电池与电池表示法
An electrochemical cell combines two half-cells connected by a salt bridge. The more negative half-cell pushes electrons out and is oxidised; the more positive half-cell takes electrons in and is reduced. Electrons flow through the external wire from the oxidation half-cell to the reduction half-cell.
电化学电池由两个半电池通过盐桥连接而成。电极电势较负的半电池向外提供电子，发生氧化；电极电势较正的半电池接受电子，发生还原。电子通过外电路从氧化半电池流向还原半电池。
Cell notation always places the oxidation half-cell on the left and the reduction half-cell on the right. A single vertical line shows a phase boundary and a double vertical line represents the salt bridge.
电池表示法总是把氧化半电池写在左侧，还原半电池写在右侧。单竖线表示相界面，双竖线表示盐桥。
Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)
In this cell, zinc is oxidised at the negative anode and copper(II) ions are reduced at the positive cathode.
在这个电池中，锌在负极被氧化，铜(II)离子在正极被还原。
4. Calculating Cell Potentials and Predicting Feasibility | 计算电池电动势并判断可行性
For a standard cell, the cell potential is calculated from the standard reduction potentials using the equation below. The right-hand half-cell is the reduction and the left-hand half-cell is the oxidation.
对于标准电池，电池电动势可由以下公式用标准还原电势计算。右侧半电池发生还原，左侧半电池发生氧化。
E°cell = E°(reduction) − E°(oxidation)
For the zinc-copper cell:
对于锌-铜电池：
E°cell = +0.34 − (−0.76) = +1.10 V
A positive E°cell means the overall redox reaction is thermodynamically feasible under standard conditions. A negative E°cell means the reaction is not feasible in the forward direction under standard conditions.
E°cell 为正值，说明总氧化还原反应在标准条件下热力学可行。E°cell 为负值，说明在标准条件下该反应正向不可行。
5. Limitations of E° Predictions | E° 预测的局限性
E° values only tell you about thermodynamic feasibility, not rate. A reaction may have a positive E°cell but still not occur at a measurable rate because its activation energy is too high.
E° 值只能说明热力学可行性，不能说明反应速率。一个反应的 E°cell 可能为正值，但如果活化能过高，反应仍可能无法以可测速率进行。
Non-standard conditions also change electrode potentials. If concentrations, pressure or temperature differ from standard values, the cell potential shifts and a reaction predicted feasible may become non-feasible.
非标准条件也会改变电极电势。如果浓度、压强或温度偏离标准值，电池电动势会发生移动，原本被预测为可行的反应可能变得不可行。
6. Storage Cells: Primary and Secondary | 蓄电池：一次电池与二次电池
Primary cells are non-rechargeable because the redox reaction is not easily reversed. Secondary cells are rechargeable because the redox reaction can be reversed by applying an external electric current.
一次电池不可充电，因为其氧化还原反应不易逆转。二次电池可充电，因为通过外加电流可以逆转其氧化还原反应。
A common secondary cell is the lead-acid battery used in vehicles. During discharge, lead and lead(IV) oxide react with sulfuric acid to form lead(II) sulfate.
常见的二次电池是汽车使用的铅酸电池。放电时，铅和二氧化铅与硫酸反应生成硫酸铅。
Pb(s) + PbO₂(s) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l)
During recharging, the reaction is reversed. Lithium-ion cells are also secondary cells and have a high energy density, making them important in portable electronics and electric vehicles.
充电时，该反应逆转。锂离子电池也是二次电池，具有高能量密度，因此在便携电子设备和电动汽车中非常重要。
7. Fuel Cells and the Hydrogen Economy | 燃料电池与氢经济
A fuel cell converts chemical energy directly into electrical energy. In a hydrogen-oxygen fuel cell, hydrogen is oxidised and oxygen is reduced, producing water as the only product in the acidic version.
燃料电池将化学能直接转化为电能。在氢氧燃料电池中，氢气被氧化，氧气被还原，酸性条件下唯一产物是水。
In acidic solution:
在酸性溶液中：
Anode: H₂ → 2H⁺ + 2e⁻
Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O
In alkaline solution, the half-equations are different because hydroxide ions take part.
在碱性溶液中，由于氢氧根离子参与反应，半方程式不同。
Anode: H₂ + 2OH⁻ → 2H₂O + 2e⁻
Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻
Fuel cells are more efficient than combustion and produce no direct CO₂ at the point of use, but hydrogen storage, distribution, safety and cost remain major challenges.
燃料电池比燃烧效率更高，在使用时不会直接产生二氧化碳，但氢气的储存、运输、安全性和成本仍然是主要挑战。
8. Redox Titrations: Manganate(VII) and Iodine-Thiosulfate | 氧化还原滴定：高锰酸盐与碘-硫代硫酸盐
Redox titrations use oxidising or reducing agents with accurately known concentrations. Potassium manganate(VII) is a common oxidising agent in acidic titration because it is self-indicating: MnO₄⁻ is deep purple and Mn²⁺ is almost colourless.
氧化还原滴定使用浓度精确已知的氧化剂或还原剂。高锰酸钾是酸性滴定中常见的氧化剂，因为它可以自身指示终点：MnO₄⁻ 为深紫色，而 Mn²⁺ 几乎无色。
MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺
This equation shows that 1 mol of manganate(VII) reacts exactly with 5 mol of iron(II). Endpoint is the first permanent pale pink colour.
该方程式表明 1 mol 高锰酸根与 5 mol 铁(II)恰好反应。滴定终点是第一次出现稳定的浅粉红色。
In iodine-thiosulfate titrations, iodine is reduced by thiosulfate ions. Starch is added near the endpoint and gives a blue-black colour with iodine that disappears at the endpoint.
在碘-硫代硫酸盐滴定中，碘被硫代硫酸根还原。接近终点时加入淀粉，淀粉与碘形成蓝黑色，终点时颜色消失。
I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻
Example calculation: 25.0 cm³ of Fe²⁺ solution required 18.7 cm³ of 0.0200 mol dm⁻³ KMnO₄. Calculate the concentration of Fe²⁺.
计算示例：25.0 cm³ 的 Fe²⁺ 溶液需要 18.7 cm³ 的 0.0200 mol dm⁻³ KMnO₄。计算 Fe²⁺
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