📚 Electrochemistry | 电化学
Electrochemistry deals with the interconversion of electrical energy and chemical energy through redox reactions. In these reactions one species is oxidized and another is reduced, and the electron transfer can be harnessed to generate an electric current in a cell or forced by an external voltage in electrolysis. At Cambridge A-Level, you need to understand cell potentials, the electrochemical series, electrolysis, Faraday’s laws and practical applications.
电化学研究通过氧化还原反应实现电能与化学能的相互转化。在反应中一种物质被氧化，另一种被还原，电子转移可在电池中产生电流，也可通过外加电压在电解中强制进行。在剑桥A-Level课程中，你需要掌握电池电势、电化学序、电解、法拉第定律以及实际应用。
1. Oxidation-Reduction Basics | 氧化还原基础
Oxidation is defined as the loss of electrons from a species, while reduction is the gain of electrons. An oxidising agent gains electrons and is itself reduced; a reducing agent loses electrons and is itself oxidised. Oxidation numbers (states) are used to keep track of electron shifts: an increase in oxidation number signals oxidation, a decrease signals reduction.
氧化是指物质失去电子，还原则是得到电子。氧化剂得到电子自身被还原；还原剂失去电子自身被氧化。氧化数（化合价）用于追踪电子转移：氧化数升高表示氧化，降低表示还原。
²⁺ + Cu, zinc is oxidised from 0 to +2 and is the reducing agent; Cu²⁺ is reduced to Cu and acts as the oxidising agent.
例如：在反应 Zn + Cu²⁺ → Zn²⁺ + Cu 中，锌从0被氧化至+2，是还原剂；Cu²⁺被还原成Cu，是氧化剂。
2. Half-Cells and Electrochemical Cells | 半电池与电化学电池
A half-cell consists of an element in two oxidation states, typically a metal electrode immersed in a solution of its ions. The half-cell where reduction occurs is the cathode; the half-cell where oxidation occurs is the anode. Two half-cells connected by a wire and a salt bridge form an electrochemical cell. The salt bridge maintains charge balance by allowing ion migration without mixing the solutions.
半电池由两种氧化态的物质构成，通常是将金属电极浸入其离子的溶液中。发生还原的半电池为阴极，发生氧化的半电池为阳极。两个半电池通过导线和盐桥连接即构成电化学电池。盐桥允许离子迁移以维持电荷平衡，同时防止溶液直接混合。
For a Zn/Cu cell: Zn(s)|Zn²⁺(aq) ∥ Cu²⁺(aq)|Cu(s). Electrons flow from the zinc anode to the copper cathode through the external circuit.
以Zn/Cu电池为例：Zn(s)|Zn²⁺(aq) ∥ Cu²⁺(aq)|Cu(s)。电子通过外电路从锌阳极流向铜阴极。
3. Standard Electrode Potential and the SHE | 标准电极电势与标准氢电极
The standard electrode potential, E°, is the tendency of a half-cell to be reduced relative to the standard hydrogen electrode (SHE), measured under standard conditions: 298 K, 1.00 mol dm⁻³ ion concentration and 100 kPa gas pressure. The SHE is assigned a value of 0.00 V exactly and the half-reaction is 2H⁺(aq) + 2e⁻ ⇌ H₂(g). A more positive E° means a greater tendency to accept electrons (stronger oxidising agent).
标准电极电势 E° 是指半电池在标准条件下（298 K、离子浓度1.00 mol dm⁻³、气体压强100 kPa）相对于标准氢电极（SHE）的还原趋势。SHE 被精确指定为 0.00 V，其半反应为 2H⁺(aq) + 2e⁻ ⇌ H₂(g)。E° 越正，得电子趋势越强（氧化剂越强）。
If the measured E° is negative, the half‑cell has a greater tendency to oxidise than the SHE.
如果测得的 E° 为负值，则该半电池比 SHE 更容易发生氧化。
4. The Electrochemical Series | 电化学序
Arranging half‑cells in order of their standard reduction potentials gives the electrochemical series. The table below lists some common E° values versus SHE.
将半电池按标准还原电势排序得到电化学序。下表列出了一些常见电对的 E° 值（相对于 SHE）。
| Half-reaction | E° / V |
|---|---|
| Li⁺ + e⁻ ⇌ Li | −3.04 |
| Zn²⁺ + 2e⁻ ⇌ Zn | −0.76 |
| Fe²⁺ + 2e⁻ ⇌ Fe | −0.44 |
| 2H⁺ + 2e⁻ ⇌ H₂ | 0.00 |
| Cu²⁺ + 2e⁻ ⇌ Cu | +0.34 |
| Ag⁺ + e⁻ ⇌ Ag | +0.80 |
Species higher in the series (more negative E°) are stronger reducing agents; those lower down (more positive E°) are stronger oxidising agents.
位置越靠上（E° 越负）的物质还原性越强；越靠下（E° 越正）的物质氧化性越强。
5. Predicting Feasibility of Redox Reactions | 预测氧化还原反应可行性
thode) − E°(anode), where both E° values are taken as reduction potentials. A reaction is thermodynamically feasible if E°(cell) is positive, indicating a negative ΔG.
反应的电池电动势可用 E°(cell) = E°(阴极) − E°(阳极) 计算，其中两个 E° 都是还原电势。若 E°(cell) 为正值，则热力学上反应可行，对应 ΔG 为负。
le, to see if zinc can displace copper: cathode Cu²⁺/Cu (+0.34 V), anode Zn²⁺/Zn (−0.76 V) ⇒ E°(cell) = +0.34 − (−0.76) = +1.10 V, so the reaction is spontaneous.
例如，判断锌能否置换铜：阴极 Cu²⁺/Cu (+0.34 V)，阳极 Zn²⁺/Zn (−0.76 V) ⇒ E°(cell) = +0.34 − (−0.76) = +1.10 V，反应自发进行。
Limitations: E° values only predict thermodynamic feasibility; they say nothing about the rate of reaction. A reaction with a positive E°(cell) may be kinetically very slow under standard conditions.
局限性：E° 仅预测热力学可行性，不涉及反应速率。即使 E°(cell) 为正，某些反应在标准条件下动力学上可能非常缓慢。
Also, non‑standard concentrations or complex formation can alter the actual potential, so the prediction may not hold in practice.
此外，非标准浓度或配位作用会改变实际电势，因此单纯根据 E° 的预测在实验中未必成立。
6. Nernst Equation and Concentration Effects | 能斯特方程与浓度效应
The Nernst equation relates the electrode potential to concentrations (or pressures) of the species involved. For a reduction half‑reaction Ox + n e⁻ ⇌ Red, at 298 K the equation is often written as:
能斯特方程将电极电势与有关物质的浓度（或压强）联系起来。对于还原半反应 Ox + n e⁻ ⇌ Red，298 K 下常写出如下形式：
E = E° + (0.059 V / n) log₁₀ ( [Ox] / [Red] )
An increase in the concentration of the oxidised form makes the potential more positive; an increase in the concentration of the reduced form makes it more negative.
增大氧化型物质的浓度使电势更正；增大还原型物质的浓度则使电势更负。
This explains why the voltage of a cell drops during discharge: the reactant concentrations change over time, shifting each electrode potential until equilibrium is reached.
这就解释了电池放电过程中电压下降的原因：随着时间推移，反应物浓度改变，引起各电极电势移动，直到平衡。
7. Electrolysis: Principles and Faraday's Laws | 电解原理与法拉第定律
In electrolysis, a direct current is passed through an electrolyte (molten or in solution), forcing a non‑spontaneous redox reaction. The cathode attracts cations and reduction takes place; the anode attracts anions and oxidation takes place.
电解时直流电通过电解质（熔融或溶液），驱动非自发的氧化还原反应。阴极吸引阳离子并发生还原，阳极吸引阴离子并发生氧化。
Faraday's first law: the mass of substance discharged is directly proportional to the quantity of charge passed. Second law: the masses of different substances discharged by the same quantity of charge are proportional to their equivalent masses.
法拉第第一定律：析出物质的质量与通过的电量成正比。第二定律：相同电量下不同物质析出的质量与其当量成正比。
The combined relationship is:
综合关系式为：
m = (M × I × t) / (n × F)
where m is mass (g), M is molar mass (g mol⁻¹), I is current (A), t is time (s), n is the number of electrons transferred per ion, and F is the Faraday constant (96 500 C mol⁻¹).
其中 m 为质量（克），M 为摩尔质量（g mol⁻¹），I 为电流（安培），t 为时间（秒），n 为每离子转移的电子数，F 为法拉第常数（96 500 C mol⁻¹）。
8. Electrolysis of Aqueous Solutions and Selective Discharge | 水溶液的电解与选择性放电
When an aqueous solution is electrolysed, more than one cation or anion may be present and the product at each electrode depends on the relative ease of discharge. For cations, reactivity dictates the order: Ag⁺ > Cu²⁺ > H⁺ > Zn²⁺ > Na⁺ etc. For anions, the order is typically OH⁻ > Cl⁻ > Br⁻ > I⁻ > SO₄²⁻ (and NO₃⁻, F⁻ are not discharged from aqueous solution).
电解水溶液时，可能存在多种阳离子和阴离子，各电极产物取决于放电竞争能力。阳离子顺序一般为：Ag⁺ > Cu²⁺ > H⁺ > Zn²⁺ > Na⁺ 等。阴离子顺序通常为：OH⁻ > Cl⁻ > Br⁻ > I⁻ > SO₄²⁻（NO₃⁻ 和 F⁻ 在水溶液中不放电）。
Example: Electrolysis of concentrated NaCl(aq) yields chlorine gas at the anode and hydrogen gas at the cathode, leaving Na⁺ and OH⁻ in solution.
例如：电解浓氯化钠水溶液，阳极产生氯气，阴极产生氢气，Na⁺ 和 OH⁻ 留在溶液中。
9. Industrial Applications: Aluminium Extraction and Electroplating | 工业应用：铝提取与电镀
Aluminium is extracted by the Hall‑Hé (Na₃AlF₆). The cryolite lowers the melting point and enhances conductivity. At the carbon‑ the carbon anode: 2O²⁻ → O₂ + 4e⁻, and the liberated oxygen gradually consumes the anode.
铝通过 Hall‑Héroult 法提取，将纯化后的氧化铝（Al₂O₃）溶于熔融冰晶石（Na₃AlF₆）中进行电解。冰晶石降低熔点并提高导电性。在碳衬里阴极上：Al³⁺ + 3e⁻ → Al(l)；碳阳极上：2O²⁻ → O₂ + 4e⁻，生成的氧气逐渐消耗阳极。
Electroplating uses electrolysis to coat a metal object with a thin layer of another metal (e.g., silver plating). The object is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal.
电镀是利用电解在金属物件表面覆盖一层薄薄的另一种金属（如镀银）。被镀件为阴极，镀层金属为阳极，电解液含镀层金属离子。
10. Fuel Cells | 燃料电池
chemical energy of a fuel directly into electrical energy. The most common is the hydrogen‑oxygen fuel cell. At the platinum‑ OH⁻ (alkaline) or O₂ + 4H⁺ + 4e⁻ → 2H₂O (acidic). The overall reaction is 2H₂ + O₂ → 2H₂O.
燃料电池将燃料的化学能直接转化为电能。最常见的是氢氧燃料电池。在铂催化的阳极上 H₂ 被氧化：碱性介质中 2H₂ + 4OH⁻ → 4H₂O + 4e⁻；酸性介质 H₂ → 2H⁺ + 2e⁻。在阴极，O₂ 被还原：碱性 O₂ + 2H₂O + 4e⁻ → 4OH⁻；酸性 O₂ + 4H⁺ + 4e⁻ → 2H₂O。总反应为 2H₂ + O₂ → 2H₂O。
Fuel cells are more efficient than heat engines because they bypass the thermal steps and directly produce electricity, with water as the only exhaust (in H₂/O₂ systems).
燃料电池比热机更高效，因为它绕过热转换步骤直接输出电能，H₂/O₂ 系统的唯一产物是水。
11. Rusting of Iron and Its Prevention | 铁的锈蚀与防护
al to cathodic regions where oxygen and water are reduced: O₂ + 2H₂O + 4e⁻ → 4OH⁻. Iron(II) ions further oxidise to Fe³⁺ and form hydrated iron(III) oxide (rust). Water and oxygen are both essential for rusting.
铁锈是一种电化学过程。在铁表面，阳极区发生氧化：Fe → Fe²⁺ + 2e⁻。电子经金属传导至阴极区，氧气与水被还原：O₂ + 2H₂O + 4e⁻ → 4OH⁻。亚铁离子进一步氧化成 Fe³⁺，生成水合氧化铁（铁锈）。水和氧气两者缺一不可。
Prevention methods include painting, oiling, alloying to make stainless steel, and cathodic protection. In sacrificial protection, a more reactive metal (e.g., zinc) is attached; it corrodes preferentially, safeguarding the iron.
防锈方法包括涂漆、涂油、制成不锈钢合金和阴极保护。牺牲性保护中，连接更活泼的金属（如锌），该金属优先腐蚀，从而保护铁。
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