A Level Chemistry Electrode Potentials

Introduction to Electrochemistry

Electrochemistry is the branch of chemistry that studies the relationship between electrical energy and chemical reactions. At A-Level, you need to understand how chemical reactions can produce electricity (in galvanic cells) and how electricity can drive chemical reactions (in electrolytic cells). The central concept is the electrode potential, which measures the tendency of a chemical species to gain or lose electrons.

电化学是研究电能与化学反应之间关系的化学分支。在A-Level阶段，你需要理解化学反应如何产生电（在原电池中）以及电如何驱动化学反应（在电解池中）。核心概念是电极电势，它衡量化学物种获得或失去电子的倾向。

Redox Reactions and Half-Equations

Every electrochemical process involves oxidation (loss of electrons) and reduction (gain of electrons). These two processes always occur together, which is why we call them redox reactions. A redox reaction can be split into two half-equations: one showing oxidation and one showing reduction.

每个电化学过程都涉及氧化（失去电子）和还原（获得电子）。这两个过程总是一起发生，因此我们称之为氧化还原反应。氧化还原反应可以拆分为两个半反应方程式：一个表示氧化，一个表示还原。

For example, when zinc metal is placed in copper(II) sulfate solution:

例如，当锌金属放入硫酸铜(II)溶液中时：

Oxidation (Zn): Zn(s) → Zn²⁺(aq) + 2e⁻
Reduction (Cu): Cu²⁺(aq) + 2e⁻ → Cu(s)
Overall: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

The zinc loses electrons (is oxidised) while the copper ions gain electrons (are reduced). The zinc acts as a reducing agent and the copper ions act as an oxidising agent.

锌失去电子（被氧化），而铜离子获得电子（被还原）。锌充当还原剂，铜离子充当氧化剂。

What Are Electrode Potentials?

An electrode potential is the voltage measured when a metal (or other conducting material) is dipped into a solution of its own ions. It represents the tendency of that half-cell to undergo reduction — i.e., to gain electrons. A more positive electrode potential means a greater tendency to be reduced; a more negative electrode potential means a greater tendency to be oxidised.

电极电势是当金属（或其他导电材料）浸入其自身离子溶液中时测得的电压。它表示该半电池发生还原反应（即获得电子）的倾向。越正的电极电势意味着越强的还原倾向；越负的电极电势意味着越强的氧化倾向。

The electrode potential is not an absolute value that can be measured directly. Instead, it is always measured relative to a reference electrode. Think of it like measuring the height of a mountain: you can only measure it relative to sea level, not from the centre of the Earth.

电极电势不是一个可以直接测量的绝对值。相反，它总是相对于参比电极来测量的。可以想象成测量一座山的高度：你只能相对于海平面来测量，而不是从地心测量。

The Standard Hydrogen Electrode (SHE)

The Standard Hydrogen Electrode (SHE) is the universal reference electrode. By international convention, its electrode potential is defined as exactly 0.00 V under standard conditions.

标准氢电极（SHE）是通用参比电极。根据国际惯例，在标准条件下，其电极电势被定义为恰好0.00 V。

The SHE consists of:

SHE的组成包括：

Platinum electrode coated with finely divided platinum (platinum black) — hydrogen gas is adsorbed onto this surface
氢气通入1 mol dm⁻³的H⁺溶液（通常是盐酸）
Hydrogen gas bubbled through at 100 kPa pressure
温度保持在298 K (25°C)
Temperature maintained at 298 K (25°C)
镀有铂黑的铂电极——氢气吸附在此表面上
H⁺ solution at 1 mol dm⁻³ (usually hydrochloric acid)
氢气在100 kPa压力下通入

The half-equation for the SHE is:
2H⁺(aq) + 2e⁻ ⇌ H₂(g) E⦵ = 0.00 V

Because the SHE is awkward to use in practice (it requires a constant supply of hydrogen gas and careful maintenance of the platinum electrode), secondary reference electrodes such as the silver/silver chloride electrode or the calomel electrode are often used in the laboratory. Their potentials are calibrated against the SHE.

由于SHE在实际使用中很麻烦（需要持续供应氢气和精心维护铂电极），实验室中常使用二级参比电极，如银/氯化银电极或甘汞电极。它们的电势已针对SHE进行了校准。

Standard Conditions

Electrode potentials are only meaningful when measured under standard conditions. These are:

电极电势只有在标准条件下测量时才有意义。标准条件如下：

Temperature: 298 K (25°C)
Pressure: 100 kPa (for any gases involved)
Concentration: 1 mol dm⁻³ for all solutions
温度：298 K (25°C)
压力：100 kPa（涉及的所有气体）
浓度：所有溶液均为1 mol dm⁻³

When these conditions are met, the measured potential is called the standard electrode potential and is given the symbol E⦵. The plimsoll sign (⦵) indicates standard conditions.

当满足这些条件时，测得的电势称为标准电极电势，符号为E⦵。标准符号（⦵）表示标准条件。

It is crucial to remember that changing any of these conditions (e.g., temperature or concentration) will change the measured electrode potential. This is why exam questions often ask you to state the standard conditions before predicting cell EMF values.

记住，改变任何这些条件（如温度或浓度）都会改变测得的电极电势。这就是为什么考试题目经常要求你在预测电池电动势之前说明标准条件。

Measuring Standard Electrode Potentials

To measure the standard electrode potential of a half-cell, you construct a cell in which one half-cell is the SHE (E⦵ = 0.00 V) and the other half-cell is the one you want to measure. The two half-cells are connected by a salt bridge (usually a strip of filter paper soaked in saturated KNO₃ or KCl), and the voltage is measured using a high-resistance voltmeter.

要测量半电池的标准电极电势，需要构建一个电池，其中一个半电池是SHE（E⦵ = 0.00 V），另一个是待测量的半电池。两个半电池通过盐桥（通常是一条浸泡在饱和KNO₃或KCl溶液中的滤纸条）连接，并使用高电阻电压表测量电压。

The salt bridge serves two purposes: it completes the electrical circuit by allowing ions to flow, and it prevents the two solutions from mixing directly (which would cause a direct reaction, bypassing the external circuit).

盐桥有两个作用：通过允许离子流动来完成电路，以及防止两种溶液直接混合（这将导致绕过外部电路的直接反应）。

The high-resistance voltmeter is important because it ensures that negligible current flows through the circuit. If current were allowed to flow, the concentrations of ions at each electrode would change, and the measured potential would drift away from the standard value.

高电阻电压表很重要，因为它确保几乎没有电流流过电路。如果允许电流流动，各电极处的离子浓度会发生变化，测得的电势会偏离标准值。

The Electrochemical Series

When you arrange half-equations in order of their standard electrode potentials (from most negative to most positive), you get the electrochemical series. This table is one of the most powerful tools in A-Level chemistry — it allows you to predict whether redox reactions are feasible.

当你将半反应方程式按其标准电极电势（从最负到最正）排列时，就得到了电化学序。这张表是A-Level化学中最强大的工具之一——它可以预测氧化还原反应是否可行。

Here is a selected portion of the electrochemical series:

以下是电化学序的一部分：

Li⁺ + e⁻ ⇌ Li(s) E⦵ = -3.04 V
K⁺ + e⁻ ⇌ K(s) E⦵ = -2.92 V
Zn²⁺ + 2e⁻ ⇌ Zn(s) E⦵ = -0.76 V
Fe²⁺ + 2e⁻ ⇌ Fe(s) E⦵ = -0.44 V
2H⁺ + 2e⁻ ⇌ H₂(g) E⦵ = 0.00 V
Cu²⁺ + 2e⁻ ⇌ Cu(s) E⦵ = +0.34 V
I₂(s) + 2e⁻ ⇌ 2I⁻(aq) E⦵ = +0.54 V
Fe³⁺ + e⁻ ⇌ Fe²⁺(aq) E⦵ = +0.77 V
Ag⁺ + e⁻ ⇌ Ag(s) E⦵ = +0.80 V
Cl₂(g) + 2e⁻ ⇌ 2Cl⁻(aq) E⦵ = +1.36 V
F₂(g) + 2e⁻ ⇌ F⁻(aq) E⦵ = +2.87 V

Key interpretation rules:

关键解读规则：

Species on the left of a half-equation with a more positive E⦵ are stronger oxidising agents (they are more easily reduced). For example, F₂ is the strongest oxidising agent in the series above.
Species on the right of a half-equation with a more negative E⦵ are stronger reducing agents (they are more easily oxidised). For example, Li(s) is the strongest reducing agent.
在半反应方程式左边、E⦵越正的物种，是越强的氧化剂（越容易被还原）。例如，F₂是上表中最强的氧化剂。
在半反应方程式右边、E⦵越负的物种，是越强的还原剂（越容易被氧化）。例如，Li(s)是最强的还原剂。

Predicting Reaction Feasibility

The most common A-Level exam application of electrode potentials is predicting whether a redox reaction is thermodynamically feasible. The rule is deceptively simple:

A-Level考试中电极电势最常见的应用是预测氧化还原反应在热力学上是否可行。规则看似简单：

A redox reaction is feasible if the EMF of the cell is positive.

如果电池的电动势（EMF）为正，则氧化还原反应是可行的。

To work this out, you need to identify which species is being oxidised and which is being reduced, write the two half-equations with their E⦵ values, and then calculate:

要计算这一点，需要确定哪种物种被氧化、哪种被还原，写出两个半反应方程式及其E⦵值，然后计算：

E⦵(cell) = E⦵(reduction half-cell) − E⦵(oxidation half-cell)

Alternatively, many students find this easier:

另外，许多学生觉得以下方法更简单：

E⦵(cell) = E⦵(more positive) − E⦵(more negative)

If the result is positive, the reaction is feasible under standard conditions.

如果结果为正，则该反应在标准条件下是可行的。

Worked Example: Will zinc metal reduce copper(II) ions?

例题：锌金属能否还原铜(II)离子？

Zn²⁺ + 2e⁻ ⇌ Zn(s) E⦵ = -0.76 V
Cu²⁺ + 2e⁻ ⇌ Cu(s) E⦵ = +0.34 V

Zinc is oxidised (Zn → Zn²⁺ + 2e⁻), so it is the oxidation half-cell. Copper ions are reduced (Cu²⁺ + 2e⁻ → Cu), so it is the reduction half-cell.

锌被氧化（Zn → Zn²⁺ + 2e⁻），因此它是氧化半电池。铜离子被还原（Cu²⁺ + 2e⁻ → Cu），因此它是还原半电池。

E⦵(cell) = E⦵(Cu²⁺/Cu) − E⦵(Zn²⁺/Zn) = +0.34 V − (-0.76 V) = +1.10 V

Since E⦵(cell) is positive, the reaction is feasible. Zinc will reduce copper(II) ions to copper metal. This is exactly what happens when you put zinc in copper sulfate solution.

由于E⦵(cell)为正，该反应是可行的。锌会将铜(II)离子还原为铜金属。这正是将锌放入硫酸铜溶液时发生的现象。

Common Pitfall: Feasibility vs Rate

A positive cell EMF tells you that a reaction is thermodynamically feasible, but it says nothing about the rate of reaction. Many reactions with positive EMF values are so slow that they effectively do not happen at room temperature. This is because E⦵ is a thermodynamic quantity (related to ΔG), not a kinetic one.

正的电池电动势告诉你反应在热力学上是可行的，但它与反应速率无关。许多具有正EMF值的反应非常缓慢，以至于在室温下实际上不会发生。这是因为E⦵是一个热力学量（与ΔG相关），而不是动力学量。

For example, consider mixing aqueous solutions of MnO₄⁻ and Cl⁻ under acidic conditions. The relevant half-equations are:

例如，考虑在酸性条件下混合MnO₄⁻和Cl⁻的水溶液。相关的半反应方程式为：

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O E⦵ = +1.51 V
Cl₂ + 2e⁻ → 2Cl⁻ E⦵ = +1.36 V

E⦵(cell) = +1.51 − (+1.36) = +0.15 V. The positive value means MnO₄⁻ should oxidise Cl⁻ to Cl₂. However, this reaction is kinetically very slow at room temperature. In practice, concentrated HCl must be heated with KMnO₄ to observe chlorine gas evolution.

E⦵(cell) = +1.51 − (+1.36) = +0.15 V。正值意味着MnO₄⁻应该会将Cl⁻氧化为Cl₂。然而，这个反应在室温下动力学上非常缓慢。实际上，需要用浓HCl加热KMnO₄才能观察到氯气的产生。

This is a classic exam trap. Always remember: a positive E⦵(cell) indicates thermodynamic feasibility, not kinetic reality.

这是经典的考试陷阱。永远记住：正的E⦵(cell)表示热力学可行性，而非动力学现实。

Concentration Effects and the Nernst Equation

Standard electrode potentials are measured under standard conditions (1 mol dm⁻³). But what happens when concentrations are not standard? Changing the concentration of ions shifts the equilibrium position of the half-reaction, which changes the electrode potential. This is described by the Nernst equation.

标准电极电势是在标准条件（1 mol dm⁻³）下测量的。但当浓度不标准时会发生什么？改变离子浓度会移动半反应式的平衡位置，从而改变电极电势。这由能斯特方程描述。

For a general half-reaction: Oxidised form + ne⁻ ⇌ Reduced form

对于一般的半反应：氧化态 + ne⁻ ⇌ 还原态

E = E⦵ − (RT/nF) ln([Reduced]/[Oxidised])

At 298 K, this simplifies to:

在298 K时，这简化为：

E = E⦵ − (0.059/n) log₁₀([Reduced]/[Oxidised])

where n is the number of electrons transferred.

其中n是转移的电子数。

Key consequence: If you increase the concentration of the oxidised form (the species on the left), the electrode potential becomes more positive (greater tendency to be reduced). If you increase the concentration of the reduced form (species on the right), the electrode potential becomes more negative (greater tendency to be oxidised). This follows from Le Chatelier’s principle.

关键推论：如果增加氧化态（方程式左边的物种）的浓度，电极电势变得更正（更强的还原倾向）。如果增加还原态（方程式右边的物种）的浓度，电极电势变得更负（更强的氧化倾向）。这遵循勒沙特列原理。

Types of Half-Cells

A-Level specifications typically cover three main types of half-cells:

A-Level大纲通常涵盖三种主要类型的半电池：

1. Metal / Metal Ion Half-Cells

The simplest type. A metal rod is dipped into a solution of its own ions. Example: Zn(s) | Zn²⁺(aq). The vertical line represents a phase boundary between the solid metal and the aqueous solution.

最简单的类型。将金属棒浸入其自身离子的溶液中。例如：Zn(s) | Zn²⁺(aq)。竖线表示固体金属与水溶液之间的相界面。

2. Gas / Ion Half-Cells

A gas is bubbled over an inert platinum electrode immersed in a solution containing the corresponding ion. The platinum provides a surface for the gas to adsorb and for electron transfer to occur, without participating in the reaction itself. The SHE is the most important example. Another is the chlorine half-cell: Pt(s) | Cl₂(g) | Cl⁻(aq).

将气体通入浸在含相应离子溶液中的惰性铂电极上方。铂为气体吸附和电子转移提供表面，而本身不参与反应。SHE是最重要的例子。另一个是氯半电池：Pt(s) | Cl₂(g) | Cl⁻(aq)。

3. Ion / Ion Half-Cells (Redox Electrodes)

Both the oxidised and reduced forms are ions in solution, so an inert platinum electrode is used. Example: Fe³⁺(aq), Fe²⁺(aq) | Pt(s). The half-equation is Fe³⁺ + e⁻ ⇌ Fe²⁺ with E⦵ = +0.77 V.

氧化态和还原态都是溶液中的离子，因此使用惰性铂电极。例如：Fe³⁺(aq), Fe²⁺(aq) | Pt(s)。半反应方程式为Fe³⁺ + e⁻ ⇌ Fe²⁺，E⦵ = +0.77 V。

When writing cell diagrams in exam answers, the convention is:

在考试答案中书写电池图示时，惯例是：

The half-cell with the more negative E⦵ goes on the left (oxidation occurs here)
The half-cell with the more positive E⦵ goes on the right (reduction occurs here)
A double vertical line (||) represents the salt bridge
E⦵更负的半电池在左边（此处发生氧化）
E⦵更正的半电池在右边（此处发生还原）
双竖线（||）表示盐桥

For the zinc-copper cell: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s) E⦵(cell) = +1.10 V

Practical Electrochemical Cells

While the SHE-based setups are used for measuring standard potentials, practical electrochemical cells (batteries) use different designs to maximise voltage, current, and longevity. Three types worth knowing for A-Level:

虽然基于SHE的装置用于测量标准电势，但实际的电化学电池（电池）使用不同的设计来最大化电压、电流和寿命。以下三种类型值得在A-Level中了解：

Non-Rechargeable (Primary) Cells

These produce electricity from irreversible chemical reactions. The most familiar example is the zinc-carbon dry cell. Once the reactants are used up, the battery is dead and must be discarded.

这些通过不可逆的化学反应产生电。最熟悉的例子是锌碳干电池。一旦反应物耗尽，电池就没电了，必须丢弃。

The zinc casing acts as the anode (oxidation): Zn(s) → Zn²⁺ + 2e⁻. The cathode is a carbon rod surrounded by MnO₂ paste (reduction): 2MnO₂ + 2NH₄⁺ + 2e⁻ → Mn₂O₃ + 2NH₃ + H₂O.

锌壳作为阳极（氧化）：Zn(s) → Zn²⁺ + 2e⁻。阴极是碳棒，周围是MnO₂糊状物（还原）：2MnO₂ + 2NH₄⁺ + 2e⁻ → Mn₂O₃ + 2NH₃ + H₂O。

Rechargeable (Secondary) Cells

These cells can be recharged by applying an external current that reverses the discharge reactions. The lithium-ion cell is the most important modern example, used in phones, laptops, and electric vehicles.

这些电池可以通过施加外部电流使放电反应逆转来充电。锂离子电池是最重要的现代例子，用于手机、笔记本电脑和电动汽车。

During discharge, lithium ions move from the graphite anode to the metal oxide cathode through an organic electrolyte. During charging, an external power source forces the ions back. The key to lithium-ion technology is that the electrode materials can intercalate (absorb and release) lithium ions without significant structural damage over many cycles.

放电时，锂离子通过有机电解液从石墨阳极移动到金属氧化物阴极。充电时，外部电源迫使离子返回。锂离子技术的关键在于电极材料可以在多次循环中嵌入（吸收和释放）锂离子而不会出现显著的结构损伤。

Fuel Cells

Fuel cells convert chemical energy directly into electrical energy by reacting a fuel (usually hydrogen) with oxygen. Unlike batteries, they require a continuous supply of fuel and oxidant. The hydrogen-oxygen fuel cell is the most common type and is increasingly used in vehicles.

燃料电池通过燃料（通常是氢气）与氧气反应，将化学能直接转化为电能。与电池不同，它们需要持续供应燃料和氧化剂。氢氧燃料电池是最常见的类型，并越来越多地用于车辆中。

In an alkaline hydrogen fuel cell:

在碱性氢燃料电池中：

Anode (oxidation): H₂ + 2OH⁻ → 2H₂O + 2e⁻
Cathode (reduction): O₂ + 2H₂O + 4e⁻ → 4OH⁻
Overall: 2H₂ + O₂ → 2H₂O

The only product is water, making fuel cells an attractive clean energy technology. The main challenges are hydrogen storage and the cost of the platinum catalysts used in the electrodes.

唯一的产物是水，使燃料电池成为一种有吸引力的清洁能源技术。主要挑战是氢气的储存和电极中使用的铂催化剂的成本。

Common Exam Questions and How to Answer Them

Exam questions on electrode potentials follow predictable patterns. Here are the most frequent types and how to approach them:

关于电极电势的考试题目遵循可预测的模式。以下是最常见的类型及应对方法：

Type 1: Calculate E⦵(cell) and state whether the reaction is feasible

This is a straightforward application of E⦵(cell) = E⦵(right) − E⦵(left) or E⦵(cell) = E⦵(more positive) − E⦵(more negative). Always show your working and give the sign. A positive answer = feasible; a negative answer = not feasible.

这是E⦵(cell) = E⦵(右) − E⦵(左)或E⦵(cell) = E⦵(更正) − E⦵(更负)的直接应用。始终展示计算过程并给出符号。正答案 = 可行；负答案 = 不可行。

Type 2: Explain why a reaction does not occur despite a positive E⦵(cell)

This tests the thermodynamic vs kinetic distinction. The answer always involves high activation energy, slow kinetics, or non-standard conditions. Remember to mention that E⦵ values only apply under standard conditions.

这测试热力学与动力学的区别。答案总是涉及高活化能、缓慢的动力学或非标准条件。记得提到E⦵值仅适用于标准条件。

Type 3: Predict the effect of changing concentration on E(cell)

Use the Nernst equation or Le Chatelier’s principle. Increasing [reactants] makes the potential more positive; increasing [products] makes it more negative. A common question asks how E(cell) changes as a cell discharges: as reactants are consumed and products build up, E(cell) decreases.

使用能斯特方程或勒沙特列原理。增加[反应物]使电势更正；增加[产物]使电势更负。一个常见的问题是电池放电时E(cell)如何变化：随着反应物被消耗和产物积累，E(cell)减小。

Type 4: Write cell diagrams and identify the direction of electron flow

Electrons always flow from the more negative half-cell (where oxidation occurs) to the more positive half-cell (where reduction occurs) through the external wire. In the cell diagram, the more negative half-cell is on the left.

电子总是通过外部导线从较负的半电池（发生氧化）流向较正的半电池（发生还原）。在电池图示中，较负的半电池在左边。

Type 5: Explain the purpose of the salt bridge and high-resistance voltmeter

Salt bridge: completes the circuit via ion flow; prevents direct mixing of solutions. High-resistance voltmeter: prevents current flow, so concentrations remain constant and the measured EMF equals the standard value.

盐桥：通过离子流动完成电路；防止溶液直接混合。高电阻电压表：阻止电流流动，使浓度保持恒定，测得的电动势等于标准值。

Summary and Key Takeaways

Electrode potentials are a fundamental concept in A-Level chemistry that bridge thermodynamics and practical electrochemistry. Here are the essential points to remember:

电极电势是A-Level化学中连接热力学和实用电化学的基本概念。以下是需要记住的要点：

The Standard Hydrogen Electrode (SHE) is the reference point: E⦵ = 0.00 V by definition.
电极电势总是在标准条件下测量：298 K、100 kPa、1 mol dm⁻³。
Electrode potentials are always measured under standard conditions: 298 K, 100 kPa, 1 mol dm⁻³.
标准氢电极（SHE）是参考点：按定义E⦵ = 0.00 V。
E⦵(cell) = E⦵(more positive) − E⦵(more negative). A positive value means the reaction is thermodynamically feasible.
A more positive E⦵ means a stronger oxidising agent (more easily reduced). A more negative E⦵ means a stronger reducing agent (more easily oxidised).
E⦵(cell) = E⦵(更正) − E⦵(更负)。正值意味着反应在热力学上是可行的。
越正的E⦵意味着越强的氧化剂（越容易被还原）。越负的E⦵意味着越强的还原剂（越容易被氧化）。
Feasibility does not guarantee a reaction will occur at an observable rate — kinetics also matter.
The salt bridge completes the circuit; the high-resistance voltmeter prevents current flow during measurement.
可行性并不保证反应会以可观察的速率发生——动力学也很重要。
盐桥完成电路；高电阻电压表在测量过程中阻止电流流动。
The Nernst equation tells you how concentration changes affect electrode potential away from standard conditions.
能斯特方程告诉你浓度变化如何影响非标准条件下的电极电势。

Master these concepts, practise calculating E⦵(cell) values from given data, and be ready to explain the difference between thermodynamic feasibility and kinetic rate. With a solid understanding of electrode potentials, you will find questions on electrochemical cells, batteries, and fuel cells much more approachable.

掌握这些概念，练习从给定数据计算E⦵(cell)值，并准备解释热力学可行性与动力学速率之间的区别。有了对电极电势的扎实理解，你会发现关于电化学电池、电池和燃料电池的题目更加容易应对。