📚 IB WJEC Chemistry: Electrochemistry – Essential Exam Points | IB WJEC 化学：电化学 考点精讲
Electrochemistry bridges the gap between chemical reactions and electrical energy, forming a core topic in both IB and WJEC chemistry specifications. This article explores all the critical concepts you need to master, from oxidation numbers and half‑equations to the Nernst equation and electrolysis calculations, presented in a clear bilingual format to reinforce understanding.
电化学是将化学反应与电能联系起来的桥梁，也是 IB 和 WJEC 化学课程中的核心主题。本文将以清晰的双语形式，梳理从氧化数、半反应式到能斯特方程和电解计算的所有重要考点，帮助你扎实掌握。
1. Oxidation and Reduction – Definitions | 氧化与还原的定义
Oxidation is the loss of electrons, while reduction is the gain of electrons (OIL RIG). In terms of oxidation states, oxidation involves an increase in oxidation number and reduction a decrease. These definitions are fundamental for identifying redox reactions in any context.
氧化是失去电子，还原是得到电子（口诀：失升氧，得降还）。从氧化态角度看，氧化表现为氧化数升高，还原表现为氧化数降低。这两个定义是判断任何氧化还原反应的基础。
Oxidising agents accept electrons and are themselves reduced; reducing agents donate electrons and are themselves oxidised. Always connect the species that changes oxidation state to the relevant role in the reaction.
氧化剂接受电子，自身被还原；还原剂提供电子，自身被氧化。永远要把发生氧化态变化的物种与相应的角色联系起来。
2. Oxidation States and Half‑Equations | 氧化态与半反应式
An oxidation state is the imaginary charge an atom would have if all bonds were completely ionic. Rules: elements have oxidation state 0; hydrogen is +1 (except in metal hydrides, −1); oxygen is −2 (except in peroxides, −1); the sum of oxidation states equals the overall charge of the species. Use these to construct balanced half‑equations in acidic conditions by adding H+, H2O, and e−.
氧化态是假设所有化学键完全离子化时原子所带的虚拟电荷。规则：单质氧化态为 0；氢通常为 +1（金属氢化物中为 −1）；氧通常为 −2（过氧化物中为 −1）；氧化态之和等于物种的总电荷。运用这些规则可以配平酸性条件下的半反应式，必要时添加 H+、H2O 和 e−。
Practice writing half‑equations for MnO4− → Mn2+ (acidic) and Cr2O72− → Cr3+. These are common exam tasks. Balance atoms first, then charge with electrons.
练习书写酸性条件下 MnO4− 还原为 Mn2+，以及 Cr2O72− 还原为 Cr3+ 的半反应式，这是常见考题。先配平原子，再用电子配平电荷。
3. Galvanic Cells – Making Electricity | 原电池 – 产生电能
A galvanic (voltaic) cell converts chemical energy into electrical energy through a spontaneous redox reaction. It consists of two half‑cells connected by a salt bridge, and a wire completing the external circuit. Electrons flow from the more reactive metal (anode) to the less reactive metal (cathode).
原电池（伏打电池）通过自发的氧化还原反应将化学能转化为电能。它由两个半电池、盐桥和外部导线构成。电子从较活泼的金属（阳极）流向较不活泼的金属（阴极）。
The electrode potential difference is measured with a high‑resistance voltmeter. The anode is where oxidation occurs (−), and the cathode is where reduction occurs (+). In IB and WJEC, you must label a diagram of a zinc–copper cell, showing Zn → Zn2+ + 2e− at the anode and Cu2+ + 2e− → Cu at the cathode.
电极电势差用高电阻伏特计测量。阳极发生氧化（负极），阴极发生还原（正极）。在 IB 和 WJEC 考试中，你需要标注锌铜电池示意图，阳极反应为 Zn → Zn2+ + 2e−，阴极反应为 Cu2+ + 2e− → Cu。
4. Standard Electrode Potentials and the Standard Hydrogen Electrode | 标准电极电势与标准氢电极
The standard electrode potential (E°) is measured under standard conditions: 298 K, 1 mol dm−3 ion concentration, and 100 kPa pressure for gases. The reference is the standard hydrogen electrode (SHE), which is assigned an E° of exactly 0.00 V.
标准电极电势 (E°) 在标准条件下测量：298 K、离子浓度 1 mol dm−3、气体压强 100 kPa。参比电极是标准氢电极 (SHE)，规定其 E° 为 0.00 V。
The SHE consists of a platinum electrode in 1 mol dm−3 H+ solution with H2 gas at 100 kPa bubbled over. Platinum provides an inert surface for the 2H+ + 2e− ⇌ H2 equilibrium. All other electrode potentials are then recorded against this half‑cell.
SHE 由铂电极插入 1 mol dm−3 H+ 溶液中，并通入 100 kPa 的 H2 气体构成。铂为反应 2H+ + 2e− ⇌ H2 提供惰性表面。所有其他电极电势均以该半电池为基准测量。
5. The Electrochemical Series and Cell EMF | 电化学序与电池电动势
The electrochemical series lists half‑reactions in order of their standard reduction potentials. The more positive the E° value, the stronger the oxidising agent. A spontaneous cell has a positive E°cell calculated as E°cathode − E°anode.
电化学序按照标准还原电势的大小排列半反应。E° 值越正，氧化剂越强。自发电池的 E°cell 为正值，计算公式为 E°阴极 − E°阳极。
For example, in a Zn–Cu cell: E°Cu²⁺/Cu = +0.34 V, E°Zn²⁺/Zn = −0.76 V. The cell EMF is +0.34 − (−0.76) = +1.10 V. This positive value confirms the reaction is feasible under standard conditions.
例如，锌铜电池：E°Cu²⁺/Cu = +0.34 V，E°Zn²⁺/Zn = −0.76 V。电池电动势为 +0.34 − (−0.76) = +1.10 V。正数表明在标准条件下该反应可行。
6. The Nernst Equation – Non‑Standard Conditions | 能斯特方程——非标准条件
When concentrations are not 1 mol dm−3 or temperature is not 298 K, the cell potential changes. The Nernst equation for a half‑cell aA + bB + ne− ⇌ cC + dD is:
E = E° − (RT/nF) ln Q
非标准条件下的电极电势可用能斯特方程计算。对于半反应 aA + bB + ne− ⇌ cC + dD，方程为：E = E° − (RT/nF) ln Q。
At 298 K, the equation simplifies to E = E° − (0.0592/n) log Q using base‑10 logarithm, where Q is the reaction quotient. IB Higher Level students must use this, while WJEC may require qualitative understanding and calculations for concentration cells.
在 298 K 时，方程可简化为 E = E° − (0.0592/n) log Q（使用常用对数，Q 为反应商）。IB 高级水平学生需要使用此方程，而 WJEC 可能要求定性理解以及浓度差电池的计算。
A concentration cell has identical electrodes but different ion concentrations; the cell voltage arises from the tendency to equalise concentrations. The Nernst equation predicts that as Q approaches 1, the cell EMF approaches zero.
浓差电池的电极相同但离子浓度不同；电池电压源于浓度趋于相等的趋势。能斯特方程表明，当 Q 接近 1 时，电池电动势趋近于零。
7. Gibbs Free Energy and Electrochemical Work | 吉布斯自由能与电功
The relationship between cell potential and free energy is ΔG° = −nFE°cell. A positive E°cell means ΔG° is negative, confirming spontaneity. F is the Faraday constant (96 500 C mol−1).
电池电动势与吉布斯自由能的关系为 ΔG° = −nFE°cell。E°cell 为正时 ΔG° 为负，证实反应自发。F 为法拉第常数 (96 500 C mol−1)。
Under non‑standard conditions, ΔG = −nFEcell. This is powerful for predicting the direction of a reaction and for calculating equilibrium constants: at equilibrium, ΔG = 0, so Ecell = 0, giving the relation ln K = (nFE°)/(RT).
非标准条件下，ΔG = −nFEcell。这一关系可用于判断反应方向和计算平衡常数：平衡时 ΔG = 0，故 Ecell = 0，由此得出 ln K = (nFE°)/(RT)。
For IB HL and WJEC students, being able to interconvert E°, ΔG, and K is a common assessment target. Always ensure correct unit conversion: joules vs. kilojoules.
IB 高级水平和 WJEC 的学生需要熟练转换 E°、ΔG 和 K。务必注意单位换算，焦耳与千焦的差别是常见失分点。
8. Electrolytic Cells – Driving Non‑Spontaneous Reactions | 电解池——驱动非自发反应
An electrolytic cell uses an external power source to force a non‑spontaneous redox reaction to occur. The cathode is now negative (to attract cations) and the anode positive (to attract anions), opposite to a galvanic cell. Reduction still occurs at the cathode, oxidation at the anode.
电解池利用外部电源驱动非自发的氧化还原反应。此时阴极（负极）吸引阳离子，阳极（正极）吸引阴离子，极性与原电池恰好相反。但还原仍在阴极发生，氧化仍在阳极发生。
You must predict products of electrolysis of molten compounds and aqueous solutions. In aqueous solutions, water can be oxidised or reduced, often complicating the prediction. Compare electrode potentials: the species with the more positive reduction potential is preferentially reduced at the cathode.
你需要预测熔融物和水溶液电解的产物。水溶液中水也可能被氧化或还原，常常使情况复杂化。比较电极电势：在阴极，还原电势更正的物质优先被还原。
For example, in the electrolysis of aqueous NaCl, H2O is reduced at the cathode (E° = −0.83 V) rather than Na+ (E° = −2.71 V), so hydrogen gas forms. At the anode, Cl− is oxidised to Cl2 despite competition from H2O, due to overpotential effects.
例如电解 NaCl 水溶液，阴极是 H2O 被还原（E° = −0.83 V）放出氢气，而不是 Na+（E° = −2.71 V）。阳极虽存在 H2O 的竞争氧化，但因过电位影响，Cl− 被氧化成 Cl2。
9. Faraday’s Laws and Quantitative Electrolysis | 法拉第定律与电解定量计算
Faraday’s first law states that the mass of substance produced at an electrode is directly proportional to the quantity of charge passed. Quantity of charge Q (in coulombs) = current I (A) × time t (s). The second law links mass to the molar mass and number of electrons per ion.
法拉第第一定律：电极上产生的物质质量与通过的电量成正比。电量 Q（库仑）= 电流 I (A) × 时间 t (s)。第二定律将质量与摩尔质量及每个离子转移的电子数联系起来。
The key formula: n(e−) = Q/F, where F = 96 500 C mol−1. Then use the half‑equation to find moles of product and convert to mass. Always check stoichiometry: for Ag+ + e− → Ag, 1 mole electrons yields 1 mole Ag; for Cu2+ + 2e− → Cu, 1 mole Cu requires 2 moles electrons.
关键公式：n(e−) = Q/F，其中 F = 96 500 C mol−1。再根据半反应式计算产物的物质的量，进而求质量。务必核对化学计量数：Ag+ + e− → Ag 中 1 摩尔电子生成 1 摩尔 Ag；Cu2+ + 2e− → Cu 中 1 摩尔 Cu 需要 2 摩尔电子。
Calculations often involve time, current, or mass. Memorise the relationship and practice converting hours to seconds. Typical exam questions ask for the mass of zinc deposited or the current required to plate a certain amount.
题目常涉及时间、电流或质量的计算。牢记关系式，并注意将小时转换为秒。典型考题会要求计算沉积锌的质量，或电镀一定量金属所需的电流。
10. Corrosion and its Prevention | 腐蚀与防护
Rusting of iron is an electrochemical process requiring water and oxygen. Anodic sites: Fe → Fe2+ + 2e−. Cathodic sites: O2 + 2H2O + 4e− → 4OH−. The Fe2+ further oxidises and forms hydrated Fe2O3 (rust).
铁生锈是一个电化学过程，需要水和氧气。阳极区：Fe → Fe2+ + 2e−。阴极区：O2 + 2H2O + 4e− → 4OH−。Fe2+ 进一步氧化生成水合 Fe2O3（铁锈）。
Sacrificial protection uses a more reactive metal (e.g., zinc, magnesium) as a sacrificial anode, which corrodes preferentially. This connects to the electrochemical series: zinc (E° = −0.76 V) will oxidise in preference to iron (E° = −0.44 V).
牺牲阳极保护法利用更活泼的金属（如锌、镁）作为牺牲阳极，使其优先腐蚀。这与电化学序相关：锌 (E° = −0.76 V) 比铁 (E° = −0.44 V) 更易氧化。
Another method is coating with paint, oil, or alloying with chromium and nickel to form stainless steel. These topics appear in both IB and WJEC, often with a diagram to label.
其他方法包括涂漆、上油，或与铬、镍制成不锈钢。这些内容在 IB 和 WJEC 考试中均有出现，常要求标注示意图。
11. Common Mistakes and Tips for the Exam | 常见错误与考试技巧
Don’t confuse the direction of electron flow: electrons always leave the anode in both cell types, but the anode is negative in a galvanic cell and positive in an electrolytic cell. Learn to derive electrolysis products by comparing electrode potentials, not by memorising.
不要混淆电子流动方向：两种电池中电子总是从阳极流出，但原电池中阳极为负极，电解池中阳极为正极。电解产物应通过比较电极电势推导，而非死记硬背。
Always use the correct form of the Nernst equation: ensure you use ln for natural log and log for base‑10, and match with 0.0592 V at 298 K. Check that your half‑equations are balanced for atoms and charge before combining.
务必使用正确的能斯特方程形式：自然对数用 ln，常用对数用 log，298 K 时搭配 0.0592 V。合并半反应前，确保每个半反应原子与电荷均已配平。
In calculations, show units and convert times to seconds. A common slip is forgetting to square or cube concentrations in Q. Practise interpreting cell diagrams, for example: Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s).
计算时要写出单位，时间转换为秒。一个常见错误是忘记在 Q 的表达式中对浓度取相应的幂次。多练习解读电池图式，如：Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)。
12. Summary – Connecting All the Concepts | 总结——串联所有概念
Electrochemistry unites thermodynamics, kinetics, and quantitative chemistry. Whether you are an IB Higher Level candidate needing to apply the Nernst equation or a WJEC student mastering electrolysis calculations, the key is to understand the underlying redox processes rather than memorise isolated facts. Review standard electrode potentials, cell conventions, and Faraday’s laws thoroughly.
电化学融合了热力学、动力学和定量化学。无论你是需要运用能斯特方程的 IB 高等级考生，还是掌握电解计算的 WJEC 学生，理解背后的氧化还原过程比孤立记忆事实更为重要。请彻底复习标准电极电势、电池惯例和法拉第定律。
Use the electrochemical series to predict spontaneous reactions and to design cells. Connect ΔG to cell EMF, and always check for the effect of non‑standard conditions. With consistent practice, you’ll be able to tackle every electrochemistry problem confidently.
利用电化学序预测自发反应并设计电池。将 ΔG 与电池电动势相联系，时刻关注非标准条件的影响。通过持续练习，你将能自信应对任何电化学问题。
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