Electrode Potentials and Electrochemical Cells | 电极电势与电化学电池

📚 Electrode Potentials and Electrochemical Cells | 电极电势与电化学电池

Electrochemical cells are devices that convert chemical energy into electrical energy (or vice versa) through redox reactions. In A-Level chemistry, understanding electrode potentials is essential for predicting the direction of electron flow, calculating cell potentials, and designing practical batteries and fuel cells.

电化学电池是通过氧化还原反应将化学能转化为电能（或反之）的装置。在A-Level化学中，理解电极电势对于预测电子流动方向、计算电池电势以及设计实用电池和燃料电池至关重要。

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1. What Is an Electrochemical Cell? | 什么是电化学电池？

An electrochemical cell consists of two half-cells, each containing an electrode immersed in an electrolyte solution. Each half-cell undergoes either oxidation or reduction. The two half-cells are connected by a salt bridge to complete the electrical circuit, allowing ions to flow and maintain charge neutrality.

电化学电池由两个半电池组成，每个半电池包含一个浸在电解质溶液中的电极。每个半电池分别发生氧化反应或还原反应。两个半电池通过盐桥连接以形成完整的电路，使离子能够流动并维持电荷中性。

The anode is the electrode where oxidation occurs (electrons are lost), while the cathode is where reduction occurs (electrons are gained). Electrons flow from the anode to the cathode through the external circuit.

阳极是发生氧化反应的电极（失去电子），而阴极是发生还原反应的电极（获得电子）。电子通过外电路从阳极流向阴极。

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2. Standard Electrode Potential (E⦵) | 标准电极电势（E⦵）

The standard electrode potential is the voltage measured when a half-cell is connected to the standard hydrogen electrode under standard conditions. Standard conditions include: 1.00 mol dm⁻³ concentration for solutions, 100 kPa pressure for gases, and a temperature of 298 K (25°C).

标准电极电势是在标准条件下将半电池与标准氢电极连接时测得的电压。标准条件包括：溶液浓度为1.00 mol dm⁻³，气体压力为100 kPa，温度为298 K（25°C）。

The more positive the E⦵ value, the greater the tendency of the species to be reduced. Conversely, the more negative the E⦵ value, the greater the tendency to undergo oxidation. These values are measured relative to the standard hydrogen electrode, which is assigned a potential of 0.00 V.

E⦵值越正，该物种被还原的趋势越大。反之，E⦵值越负，发生氧化的趋势越大。这些值相对于标准氢电极（被赋予0.00 V的电势）进行测量。

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3. The Standard Hydrogen Electrode (SHE) | 标准氢电极（SHE）

The standard hydrogen electrode is the reference electrode against which all other electrode potentials are measured. It consists of a platinum electrode (inert) immersed in a 1.00 mol dm⁻³ solution of H⁺ ions, with hydrogen gas at 100 kPa pressure bubbled over the platinum surface.

标准氢电极是用来测定所有其他电极电势的参比电极。它由浸在1.00 mol dm⁻³ H⁺溶液中的铂电极（惰性）组成，氢气以100 kPa的压力在铂表面冒泡。

The half-reaction for the SHE is:

2H⁺(aq) + 2e⁻ ⇌ H₂(g)    E⦵ = 0.00 V

The platinum electrode serves two purposes: it provides a surface for the redox reaction to occur, and it conducts electrons to and from the external circuit. Platinum is used because it is chemically inert and conducts electricity well.

铂电极有两个作用：为氧化还原反应提供反应表面，并传导电子进出外电路。使用铂是因为它化学性质惰性且导电性能良好。

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4. Measuring Electrode Potentials | 测量电极电势

To measure the standard electrode potential of a half-cell, it is connected to a standard hydrogen electrode via a salt bridge and a high-resistance voltmeter. The voltmeter measures the potential difference between the two half-cells, which gives the E⦵ of the test half-cell.

要测量某半电池的标准电极电势，需要通过盐桥和高电阻电压表将其与标准氢电极连接。电压表测量两个半电池之间的电势差，即得到被测半电池的E⦵。

For example, to measure E⦵ of Zn²⁺/Zn, zinc metal is placed in 1.00 mol dm⁻³ Zn²⁺ solution and connected to the SHE. Since zinc has a greater tendency to lose electrons than hydrogen, electrons flow from the zinc half-cell to the SHE, giving an E⦵ of −0.76 V.

例如，测量Zn²⁺/Zn的E⦵时，将锌金属置于1.00 mol dm⁻³ Zn²⁺溶液中并与SHE连接。由于锌比氢更容易失去电子，电子从锌半电池流向SHE，测得E⦵为−0.76 V。

When the zinc electrode is the negative terminal (anode) and SHE is the cathode, the cell representation is written as:

当锌电极为负极端（阳极）、SHE为阴极时，电池表示式写作：

Zn(s) | Zn²⁺(aq) ∥ H⁺(aq) | H₂(g) | Pt(s)    E⦵cell = +0.76 V

By convention, the half-cell with the more negative potential is written on the left. The single vertical line (|) represents a phase boundary, and the double vertical line (∥) represents the salt bridge.

按惯例，电势更负的半电池写在左侧。单竖线（|）表示相界面，双竖线（∥）表示盐桥。

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5. Calculating Cell Potentials | 计算电池电势

The overall cell potential (E⦵cell) is calculated using the equation:

电池总电势（E⦵cell）使用以下方程计算：

E⦵cell = E⦵(reduced species) − E⦵(oxidised species)

Alternatively, it can be written as E⦵cell = E⦵cathode − E⦵anode. The species with the more positive E⦵ undergoes reduction at the cathode, while the species with the more negative E⦵ undergoes oxidation at the anode.

或者，可写为E⦵cell = E⦵阴极 − E⦵阳极。E⦵更正的物种在阴极被还原，而E⦵更负的物种在阳极被氧化。

Consider a cell made from Zn²⁺/Zn (E⦵ = −0.76 V) and Cu²⁺/Cu (E⦵ = +0.34 V). Copper has the more positive potential, so Cu²⁺ is reduced. Zinc has the more negative potential, so Zn is oxidised:

考虑一个由Zn²⁺/Zn（E⦵ = −0.76 V）和Cu²⁺/Cu（E⦵ = +0.34 V）组成的电池。铜的电势更正，因此Cu²⁺被还原。锌的电势更负，因此Zn被氧化：

E⦵cell = (+0.34) − (−0.76) = +1.10 V

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

A positive E⦵cell indicates that the reaction is spontaneous under standard conditions. The larger the positive value, the more favourable the reaction.

正的E⦵cell表示反应在标准条件下是自发的。正值越大，反应越有利。

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6. Predicting Feasibility Using Electrode Potentials | 利用电极电势预测反应可行性

Electrode potentials can predict whether a redox reaction will occur spontaneously. For a reaction to proceed, the species with the more positive E⦵ must accept electrons from the species with the more negative E⦵.

电极电势可以预测氧化还原反应是否会自发进行。反应要发生，E⦵更正的物种必须从E⦵更负的物种处接受电子。

For example, can chlorine gas oxidise bromide ions? The relevant half-equations are:

例如，氯气能否氧化溴离子？相关半反应为：

Cl₂(g) + 2e⁻ ⇌ 2Cl⁻(aq)   E⦵ = +1.36 V

Br₂(l) + 2e⁻ ⇌ 2Br⁻(aq)   E⦵ = +1.09 V

Since Cl₂ has a more positive E⦵ (+1.36 V) than Br₂ (+1.09 V), Cl₂ is the stronger oxidising agent. It will accept electrons from Br⁻ ions, oxidising them to Br₂:

由于Cl₂的E⦵（+1.36 V）比Br₂（+1.09 V）更正，Cl₂是更强的氧化剂。它将从Br⁻离子处接受电子，将其氧化为Br₂：

Cl₂(g) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)   E⦵cell = +0.27 V

The positive E⦵cell confirms this reaction is feasible. However, electrode potentials predict thermodynamic feasibility — they do not tell us about the rate of reaction, which may be kinetically slow.

正E⦵cell证实该反应可行。然而，电极电势预测的是热力学可行性——它不告诉我们反应的速率，反应在动力学上可能是缓慢的。

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7. Storage Cells and Fuel Cells | 蓄电池与燃料电池

A primary cell is a non-rechargeable battery in which the redox reaction is not easily reversible. Once the reactants are consumed, the cell is dead. An example is the alkaline battery.

原电池（一次电池）是不可充电电池，其氧化还原反应不易逆转。一旦反应物耗尽，电池即报废。例如碱性电池。

A secondary cell (rechargeable battery) involves redox reactions that can be reversed by applying an external voltage. A common example is the lead-acid battery used in cars, where:

二次电池（可充电电池）涉及可通过施加外部电压而逆转的氧化还原反应。一个常见例子是汽车中使用的铅酸电池，其反应为：

Pb(s) + PbO₂(s) + 4H⁺(aq) + 2SO₄²⁻(aq) ⇌ 2PbSO₄(s) + 2H₂O(l)

During discharge, the cell produces electricity; during recharging, an external power source drives the reaction backwards, regenerating Pb and PbO₂.

放电时，电池产生电能；充电时，外部电源驱动反应反向进行，重新生成Pb和PbO₂。

In rechargeable lithium-ion batteries, lithium ions move between the anode and cathode during charge and discharge cycles, providing high energy density and low self-discharge rates.

在可充电的锂离子电池中，锂离子在充放电循环期间在阳极和阴极之间移动，提供高能量密度和低自放电率。

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8. Fuel Cells | 燃料电池

A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (such as hydrogen) directly into electrical energy. Unlike batteries, fuel cells do not store chemical energy internally — they require a continuous supply of fuel and oxidant.

燃料电池是将燃料（如氢气）的化学能直接转化为电能的电化学电池。与电池不同，燃料电池不在内部储存化学能——它们需要持续供给燃料和氧化剂。

The hydrogen-oxygen fuel cell is the most common example. Its half-reactions under alkaline conditions are:

氢氧燃料电池是最常见的例子。在碱性条件下其半反应为：

Anode: 2H₂(g) + 4OH⁻(aq) → 4H₂O(l) + 4e⁻

Cathode: O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)

Overall:

总反应：

2H₂(g) + O₂(g) → 2H₂O(l)    E⦵cell = +1.23 V

Fuel cells are highly efficient and produce only water as the product when hydrogen is used, making them an environmentally clean energy source. Applications include spacecraft, electric vehicles, and backup power systems.

燃料电池效率高，使用氢气时仅产生水作为产物，是环保的清洁能源。应用包括航天器、电动汽车和备用电源系统。

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9. Electrochemical Series and Oxidising Agents | 电化序与氧化剂

The electrochemical series arranges half-cells in order of decreasing E⦵ values. The most negative values appear at the top (strongest reducing agents), and the most positive values appear at the bottom (strongest oxidising agents).

电化序按E⦵值递减的顺序排列各半电池。最负的值出现在顶部（最强的还原剂），最正的值出现在底部（最强的氧化剂）。

Key trends to remember:

需要记住的关键趋势：

• Species on the left of a half-equation (oxidised form) act as oxidising agents; the higher the E⦵, the stronger the oxidising agent.
• 物种位于半方程左侧（氧化态）作为氧化剂；E⦵越高，氧化剂越强。
• Species on the right of a half-equation (reduced form) act as reducing agents; the lower the E⦵, the stronger the reducing agent.
• 物种位于半方程右侧（还原态）作为还原剂；E⦵越低，还原剂越强。
• Any species on the right of a half-cell can reduce any species higher up the series (with a more positive E⦵).
• 任何位于半电池右侧的物种都可以还原序列中更靠下（E⦵更正）的物种。

For instance, among the halogens, F₂ has the highest E⦵ (+2.87 V), making it the most powerful oxidising agent known. It can oxidise chloride, bromide, and iodide ions.

例如，在卤素中，F₂的E⦵最高（+2.87 V），使其成为已知最强的氧化剂。它可以氧化氯离子、溴离子和碘离子。

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10. Exam Tips and Common Mistakes | 考试要点与常见错误

Students frequently make errors in electrode potential calculations. Click through the table below for the most common pitfalls to avoid.

学生在电极电势计算中经常犯错。请查看下表，了解需要避免的常见陷阱。

Common Mistake | 常见错误	Correct Approach | 正确方法
Using E⦵ = E⦵(more positive) + E⦵(less positive) | 用相加计算电池电势	Always subtract: E⦵cell = E⦵cathode − E⦵anode | 总是相减：E⦵cell = E⦵阴极 − E⦵阳极
Reversing the sign of the half-cell when the equation is reversed | 将半方程反转时也反转电势符号	E⦵ values are independent of the direction of the equation — do not change the sign | E⦵值与方程方向无关——不要改变符号
Forgetting to multiply E⦵ by stoichiometric coefficients | 忘记将E⦵乘以化学计量系数	E⦵ is an intensive property — never multiply it by coefficients | E⦵是强度性质——绝不乘以系数
Confusing anode and cathode assignments | 混淆阳极和阴极的判定	The more negative E⦵ is the anode (oxidation); more positive is the cathode (reduction) | E⦵更负的是阳极（氧化）；更正的是阴极（还原）

Additionally, always specify the conditions when quoting E⦵ values: concentrations of 1.00 mol dm⁻³, temperature of 298 K, and gas pressures of 100 kPa. When conditions deviate from standard, the actual potential changes according to the Nernst equation.

此外，引用E⦵值时始终要说明条件：浓度1.00 mol dm⁻³、温度298 K、气体压力100 kPa。当条件偏离标准时，实际电势根据能斯特方程变化。

For non-standard conditions, the Nernst equation is:

对于非标准条件，能斯特方程为：

E = E⦵ − (0.0592/n) × log₁₀Q   (at 298 K)

where n is the number of electrons transferred and Q is the reaction quotient. An increased concentration of the reduced form (or decreased concentration of the oxidised form) makes E more negative, and vice versa.

其中n是转移的电子数，Q是反应商。还原形态浓度增加（或氧化形态浓度减少）使E变得更负，反之亦然。

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Understanding electrode potentials and electrochemical cells is a core skill in A-Level chemistry. Master the conventions, practise the calculations, and always interpret E⦵ values in terms of the relative strengths of oxidising and reducing agents.

理解电极电势与电化学电池是A-Level化学的核心技能。掌握惯例、练习计算，并始终从氧化剂和还原剂相对强弱的视角解释E⦵值。

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