📚 Electrode Potentials | 电极电势与电动势
Electrode potentials form the foundation of electrochemistry, explaining why some reactions occur spontaneously while others require an external energy input. By measuring the tendency of a half-cell to gain or lose electrons relative to a standard hydrogen electrode, we can predict the feasibility of redox reactions, calculate cell EMF values, and understand the principles behind batteries, fuel cells, and corrosion prevention. This guide covers the complete A-Level Chemistry syllabus for electrode potentials, from half-cell construction and the standard hydrogen electrode through to the Nernst equation and commercial applications.
电极电势是电化学的基础，解释了为什么某些反应会自发发生，而另一些则需要外部能量输入。通过测量半电池相对于标准氢电极获得或失去电子的趋势，我们可以预测氧化还原反应的可行性，计算电池电动势值，并理解电池、燃料电池和防腐蚀背后的原理。本指南涵盖A-Level化学电极电势的完整大纲，从半电池构建和标准氢电极到能斯特方程及商业应用。
1. Oxidation States & Redox Fundamentals | 氧化态与氧化还原基础
Oxidation is the loss of electrons, and reduction is the gain of electrons ： remembered by the mnemonic OIL RIG. Every redox reaction involves both processes occurring simultaneously: when zinc metal is placed in copper(II) sulfate solution, zinc atoms lose electrons to form Zn²⁺ ions (oxidation), while Cu²⁺ ions gain those electrons to form copper metal (reduction). The species that donates electrons is the reducing agent, and the species that accepts electrons is the oxidising agent. Oxidation states (or oxidation numbers) are a bookkeeping system that tracks electron distribution: the oxidation state of an atom in its elemental form is 0, for a monatomic ion it equals the ion charge, oxygen is typically −2 (except in peroxides where it is −1), hydrogen is +1 (except in metal hydrides where it is −1), and the sum of oxidation states in a neutral compound is 0.
氧化是失去电子，还原是获得电子：记住口诀OIL RIG（氧化即失电子，还原即得电子）。每个氧化还原反应都涉及这两个过程同时发生：当锌金属放入硫酸铜(II)溶液中时，锌原子失去电子形成Zn²⁺离子（氧化），而Cu²⁺离子获得这些电子形成铜金属（还原）。提供电子的物质是还原剂，接受电子的物质是氧化剂。氧化态（或氧化数）是一种跟踪电子分布的簿记系统：元素形式中原子的氧化态为0，单原子离子的氧化态等于离子电荷，氧通常为−2（过氧化物中为−1），氢为+1（金属氢化物中为−1），中性化合物中氧化态的总和为0。
2. Half-Cells & the Standard Hydrogen Electrode | 半电池与标准氢电极
A half-cell consists of an element in two oxidation states that can interconvert by electron transfer. The simplest type is a metal/metal-ion half-cell, where a metal rod (the electrode) dips into a solution of its own ions (the electrolyte). For example, a Zn²⁺/Zn half-cell uses a zinc rod in ZnSO₄(aq). A non-metal half-cell, such as the Fe²⁺/Fe³⁺ system, uses an inert platinum electrode to provide a surface for electron transfer between ions in solution. To measure the electrode potential of any half-cell, we need a reference point ： this is the standard hydrogen electrode (SHE). The SHE consists of hydrogen gas at 100 kPa bubbling over a platinum electrode immersed in 1.00 mol dm⁻³ H⁺(aq) at 298 K, and by convention its electrode potential is defined as exactly 0.00 V.
半电池由一种元素的两个氧化态组成，它们可以通过电子转移相互转化。最简单的类型是金属/金属离子半电池，其中金属棒（电极）浸入其自身离子的溶液（电解质）中。例如，Zn²⁺/Zn半电池使用锌棒置于ZnSO₄(aq)中。非金属半电池，如Fe²⁺/Fe³⁺体系，使用惰性铂电极提供溶液中离子间电子转移的表面。要测量任何半电池的电极电势，我们需要一个参比点：这就是标准氢电极（SHE）。SHE由100 kPa的氢气在浸入1.00 mol dm⁻³ H⁺(aq)、298 K的铂电极上鼓泡组成，按照惯例其电极电势被精确定义为0.00 V。
3. Standard Electrode Potential (E°) | 标准电极电势 (E°)
The standard electrode potential, E°, is the EMF of a cell composed of the half-cell under investigation connected to the standard hydrogen electrode, measured under standard conditions: 298 K, 100 kPa for gases, and 1.00 mol dm⁻³ for all solutions. E° values are listed in the electrochemical series, arranged from the most negative (strongest reducing agents, most easily oxidised) to the most positive (strongest oxidising agents, most easily reduced). A negative E° means the half-cell has a greater tendency to undergo oxidation than the SHE, while a positive E° means it has a greater tendency to undergo reduction. For example, Zn²⁺/Zn has E° = −0.76 V (zinc is a stronger reducing agent than H₂), and Cu²⁺/Cu has E° = +0.34 V (Cu²⁺ is a stronger oxidising agent than H⁺).
标准电极电势E°是由待研究半电池与标准氢电极连接组成的电池在标准条件下的电动势：298 K，气体100 kPa，所有溶液1.00 mol dm⁻³。E°值列在电化学系列中，从最负（最强还原剂，最易被氧化）到最正（最强氧化剂，最易被还原）排列。负的E°值表示该半电池比SHE有更大的氧化趋势，而正的E°值表示其有更大的还原趋势。例如，Zn²⁺/Zn的E° = −0.76 V（锌是比H₂更强的还原剂），Cu²⁺/Cu的E° = +0.34 V（Cu²⁺是比H⁺更强的氧化剂）。
4. Calculating Cell EMF | 计算电池电动势
The EMF (electromotive force) of a complete electrochemical cell is calculated by taking the difference between the two half-cell potentials. The convention is E_cell = E°(right-hand electrode) − E°(left-hand electrode), where the right-hand electrode is the one undergoing reduction (cathode) and the left-hand electrode undergoes oxidation (anode). A more practical approach is E_cell = E°(reduction half-cell) − E°(oxidation half-cell), which yields the same result. For a Daniell cell (Zn|Zn²⁺ || Cu²⁺|Cu), E_cell = (+0.34) − (−0.76) = +1.10 V. A positive EMF indicates the reaction is thermodynamically feasible under standard conditions. The magnitude of the cell EMF reflects the driving force of the reaction ： the larger the difference in E° values, the greater the tendency for the reaction to proceed.
完整电化学电池的电动势（EMF）通过计算两个半电池电势之差求得。惯例是E_cell = E°(右侧电极) − E°(左侧电极)，其中右侧电极发生还原（阴极），左侧电极发生氧化（阳极）。更实用的方法是E_cell = E°(还原半电池) − E°(氧化半电池)，结果相同。对于丹尼尔电池（Zn|Zn²⁺ || Cu²⁺|Cu），E_cell = (+0.34) − (−0.76) = +1.10 V。正的电动势表明该反应在标准条件下是热力学可行的。电池电动势的大小反映了反应的驱动力：E°值差异越大，反应进行的趋势越强。
5. Predicting Redox Reaction Feasibility | 预测氧化还原反应可行性
A redox reaction is feasible when the species with the more positive E° undergoes reduction (gains electrons) and the species with the more negative E° undergoes oxidation (loses electrons). This rule can be extended to predict whether one species can displace another. For instance, can chlorine oxidise bromide ions to bromine? Cl₂/Cl⁻ has E° = +1.36 V and Br₂/Br⁻ has E° = +1.07 V, so Cl₂ (+1.36) will be reduced to Cl⁻ while Br⁻ (+1.07) will be oxidised to Br₂. The cell EMF is (+1.36) − (+1.07) = +0.29 V, confirming the reaction is feasible. Conversely, can bromine oxidise chloride ions? The cell EMF would be (+1.07) − (+1.36) = −0.29 V, meaning the reaction is not feasible under standard conditions ： a negative EMF tells us the reaction proceeds in the reverse direction.
当E°更正的物种发生还原（获得电子）而E°更负的物种发生氧化（失去电子）时，氧化还原反应是可行的。这一规则可以扩展到预测一种物质是否能置换另一种物质。例如，氯能否将溴离子氧化为溴？Cl₂/Cl⁻的E° = +1.36 V，Br₂/Br⁻的E° = +1.07 V，因此Cl₂（+1.36）将被还原为Cl⁻，而Br⁻（+1.07）将被氧化为Br₂。电池电动势为(+1.36) − (+1.07) = +0.29 V，确认反应可行。反之，溴能否氧化氯离子？电池电动势为(+1.07) − (+1.36) = −0.29 V，表明该反应在标准条件下不可行：负的电动势告诉我们反应向相反方向进行。
6. Limitations of E° Values | E° 值的局限性
While E° values reliably predict thermodynamic feasibility, they have important limitations. First, they apply only under standard conditions (298 K, 1.00 mol dm⁻³, 100 kPa): changing concentration, temperature, or pressure alters the electrode potential according to the Nernst equation. A reaction predicted to be feasible under standard conditions may become infeasible when concentrations differ significantly from 1.00 mol dm⁻³. Second, E° tells us nothing about the rate of reaction ： a reaction with a large positive E_cell may be kinetically inert due to high activation energy. For example, the reaction between magnesium and water has E_cell = +2.37 − (−0.83) = +3.20 V, yet magnesium reacts only slowly with cold water because of the oxide layer and kinetic barriers. Third, competing reactions can occur: when aqueous solutions are used, the electrolysis of water (2H₂O + 2e⁻ ⇌ H₂ + 2OH⁻) may take precedence over the predicted half-reaction.
虽然E°值可靠地预测热力学可行性，但它们有重要的局限性。首先，它们仅在标准条件下适用（298 K，1.00 mol dm⁻³，100 kPa）：改变浓度、温度或压力会根据能斯特方程改变电极电势。在标准条件下预测为可行的反应，当浓度与1.00 mol dm⁻³显著不同时可能变得不可行。其次，E°不告诉我们任何关于反应速率的信息：具有大正E_cell的反应可能因高活化能而动力学惰性。例如，镁与水的反应E_cell = +2.37 − (−0.83) = +3.20 V，但镁与冷水仅缓慢反应，因为氧化层和动力学障碍。第三，可能发生竞争反应：当使用水溶液时，水的电解（2H₂O + 2e⁻ ⇌ H₂ + 2OH⁻）可能优先于预测的半反应。
7. The Nernst Equation | 能斯特方程
The Nernst equation quantifies how electrode potential varies with concentration and temperature, extending E° beyond standard conditions. For a half-cell reaction aA + bB + ne⁻ ⇌ cC + dD, the Nernst equation is E = E° − (RT/nF)lnQ, where R is the gas constant (8.314 J K⁻¹ mol⁻¹), T is temperature in kelvin, n is the number of electrons transferred, F is the Faraday constant (96485 C mol⁻¹), and Q is the reaction quotient. At 298 K, the simplified form becomes E = E° − (0.0592/n)log₁₀Q. For a metal/metal-ion half-cell M^n⁺/M, this reduces to E = E° + (0.0592/n)log₁₀[M^n⁺]. A tenfold increase in metal ion concentration raises the electrode potential by approximately 0.0592/n V, making reduction more favourable, while a tenfold decrease lowers it by the same amount.
能斯特方程量化了电极电势如何随浓度和温度变化，将E°扩展到非标准条件。对于半电池反应aA + bB + ne⁻ ⇌ cC + dD，能斯特方程为E = E° − (RT/nF)lnQ，其中R是气体常数（8.314 J K⁻¹ mol⁻¹），T是开尔文温度，n是转移电子数，F是法拉第常数（96485 C mol⁻¹），Q是反应商。在298 K下，简化形式为E = E° − (0.0592/n)log₁₀Q。对于金属/金属离子半电池M^n⁺/M，这简化为E = E° + (0.0592/n)log₁₀[M^n⁺]。金属离子浓度增加十倍使电极电势升高约0.0592/n V，使还原更有利，而减少十倍则降低相同幅度。
8. Electrochemical Cells & Cell Notation | 电化学电池与电池符号
An electrochemical cell consists of two half-cells connected by a salt bridge (often a strip of filter paper soaked in saturated KNO₃ or KCl solution). The salt bridge completes the circuit by allowing ions to flow between the two solutions, maintaining electrical neutrality without allowing the solutions to mix. The standard cell notation uses vertical lines to represent phase boundaries and double vertical lines for the salt bridge. The left-hand electrode is always the anode (oxidation), and the right-hand electrode is the cathode (reduction). For example, the Daniell cell is written as Zn(s)|Zn²⁺(aq, 1 M)||Cu²⁺(aq, 1 M)|Cu(s). When both species in a half-cell are in solution, a platinum electrode is indicated with a comma separating the two oxidation states: Pt(s)|Fe²⁺(aq),Fe³⁺(aq). The electrode with the more positive E° always appears on the right.
电化学电池由两个半电池通过盐桥（通常是浸泡在饱和KNO₃或KCl溶液中的滤纸条）连接组成。盐桥通过允许离子在两个溶液之间流动来完善电路，保持电中性而不让溶液混合。标准电池符号使用垂直线表示相界面，双垂直线表示盐桥。左侧电极始终是阳极（氧化），右侧电极是阴极（还原）。例如，丹尼尔电池写作Zn(s)|Zn²⁺(aq, 1 M)||Cu²⁺(aq, 1 M)|Cu(s)。当半电池中两种物质都在溶液中时，用铂电极表示，逗号分隔两种氧化态：Pt(s)|Fe²⁺(aq),Fe³⁺(aq)。E°更正的电极始终出现在右侧。
9. Types of Commercial Cells | 商业电池类型
Primary cells (non-rechargeable) produce electricity from irreversible chemical reactions. The common alkaline battery uses a zinc anode and manganese dioxide cathode in a KOH electrolyte, delivering about 1.5 V. Secondary cells (rechargeable) allow the cell reaction to be reversed by applying an external voltage larger than the cell EMF. The lithium-ion battery, with a graphite anode and lithium metal oxide cathode (e.g., LiCoO₂), operates at 3.6-3.7 V and dominates portable electronics due to its high energy density. The lead-acid battery (12 V car battery) uses Pb and PbO₂ electrodes in H₂SO₄, producing 2 V per cell with six cells connected in series. Fuel cells convert chemical energy directly into electrical energy with higher efficiency than combustion: the hydrogen-oxygen fuel cell produces only water as its product, with the half-reactions 2H₂ + 4OH⁻ = 4H₂O + 4e⁻ (anode, alkaline) and O₂ + 2H₂O + 4e⁻ = 4OH⁻ (cathode).
一次电池（不可充电）通过不可逆的化学反应产生电能。常见的碱性电池使用锌阳极和二氧化锰阴极在KOH电解液中，提供约1.5 V。二次电池（可充电）允许通过施加大于电池电动势的外部电压来逆转电池反应。锂离子电池以石墨为阳极，锂金属氧化物（如LiCoO₂）为阴极，工作电压为3.6-3.7 V，因其高能量密度主导便携电子设备。铅酸电池（12 V汽车电池）使用Pb和PbO₂电极在H₂SO₄中，每个电池产生2 V，六个电池串联。燃料电池以比燃烧更高的效率将化学能直接转化为电能：氢氧燃料电池仅产生水作为产物，半反应为2H₂ + 4OH⁻ = 4H₂O + 4e⁻（阳极，碱性）和O₂ + 2H₂O + 4e⁻ = 4OH⁻（阴极）。
10. Measurement Techniques & Practical Considerations | 测量技术与实验要点
To measure the EMF of a cell accurately, a high-resistance voltmeter must be used ： a low-resistance voltmeter would draw current from the cell, causing the concentrations at the electrode surfaces to change and the measured voltage to drop. The salt bridge should contain ions that do not react with either half-cell solution; KNO₃ and KCl are common choices because K⁺, NO₃⁻, and Cl⁻ are generally inert. Standard conditions must be carefully controlled: 298 K (thermostated water bath), 1.00 mol dm⁻³ solutions (made accurately from standard solutions), and 100 kPa for gases. For gas electrodes, the electrode surface must be platinised (coated with finely divided platinum black) to increase the surface area and catalyse the establishment of equilibrium. The measured EMF should be recorded when the reading stabilises, and at least three readings should be taken to ensure reproducibility.
要准确测量电池的电动势，必须使用高电阻电压表：低电阻电压表会从电池中抽取电流，导致电极表面浓度变化和测量电压下降。盐桥应包含不与任一半电池溶液反应的离子；KNO₃和KCl是常见选择，因为K⁺、NO₃⁻和Cl⁻通常是惰性的。标准条件必须仔细控制：298 K（恒温水浴），1.00 mol dm⁻³溶液（从标准溶液精确配制），气体100 kPa。对于气体电极，电极表面必须镀铂（涂有细分散的铂黑）以增加表面积并催化平衡的建立。当读数稳定后记录测量的电动势，并至少读取三次以确保重现性。
Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com
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