AQA A-Level Chemistry: Electrochemical Cells and Standard Electrode Potentials | AQA A-Level 化学：电化学电池与标准电极电势

什么是电化学电池？

电化学电池是一种能够将化学能转化为电能（原电池），或者将电能转化为化学能（电解池）的装置。在 A-Level 化学中，我们主要关注原电池（galvanic/voltaic cell）——它利用自发的氧化还原反应产生电流。每一个电化学电池都由两个半电池（half-cell）组成，每个半电池包含一个电极浸在含有该金属离子的电解质溶液中。两个半电池通过盐桥（salt bridge）连接，盐桥里含有惰性电解质（如 KNO₃），作用是维持电荷平衡，让离子在两个半电池之间自由移动，从而构成一个完整的电路。

An electrochemical cell is a device that can either convert chemical energy into electrical energy (galvanic/voltaic cell) or electrical energy into chemical energy (electrolytic cell). In A-Level Chemistry, our focus is on galvanic cells — they harness a spontaneous redox reaction to generate an electric current. Every electrochemical cell consists of two half-cells. Each half-cell contains an electrode immersed in an electrolyte solution of its own metal ions. The two half-cells are connected by a salt bridge containing an inert electrolyte (such as KNO₃), whose purpose is to maintain charge neutrality by allowing ions to migrate freely between the two compartments, thereby completing the circuit.

标准电极电势 E° — 核心概念

标准电极电势（standard electrode potential, E°）是衡量一个半电池相对于标准氢电极（SHE）获得电子的倾向（即被还原的能力）的物理量。测量必须在标准条件下进行：298 K（25°C）、所有离子的浓度为 1 mol dm⁻³、气体压强为 100 kPa（1 bar）。标准氢电极被定义为零点，E°(H⁺/H₂) = 0.00 V。所有其他电极的电势都是相对于这个参考点来测量的。E° 数值越正，说明该物种越容易被还原（氧化性越强）；E° 数值越负，说明该物种越容易被氧化（还原性越强）。

The standard electrode potential (E°) quantifies a half-cell’s tendency to gain electrons — in other words, its ability to be reduced — relative to the standard hydrogen electrode (SHE). Measurements must be carried out under standard conditions: 298 K (25°C), all ion concentrations at 1 mol dm⁻³, and gas pressure at 100 kPa (1 bar). The standard hydrogen electrode is assigned as the zero point: E°(H⁺/H₂) = 0.00 V. All other electrode potentials are measured against this reference. A more positive E° value means the species is more easily reduced (stronger oxidising agent); a more negative E° value means the species is more easily oxidised (stronger reducing agent).

测量电极电势：实验装置

要测量一个半电池的标准电极电势，我们需要将它和标准氢电极组成一个完整的电池。标准氢电极的构造如下：一根铂电极（镀有铂黑以增大表面积）浸在 H⁺ 浓度为 1 mol dm⁻³ 的酸溶液中，氢气以 100 kPa 的压强不断通入。铂本身不参与反应，只是作为电子传递的惰性平台。然后将待测半电池（比如 Cu²⁺/Cu）通过盐桥与标准氢电极连接，用高阻抗电压表测量两个电极之间的电势差。由于标准氢电极的电势定义为零，电压表的读数就直接等于待测半电池的标准电极电势。

To measure the standard electrode potential of a half-cell, we construct a complete cell by pairing it with the standard hydrogen electrode. The SHE is built by inserting a platinum electrode (coated with platinum black to increase surface area) into an acid solution with H⁺ concentration of 1 mol dm⁻³, while hydrogen gas is bubbled through at 100 kPa. Platinum itself does not participate in the reaction — it merely serves as an inert platform for electron transfer. The half-cell under investigation (e.g., Cu²⁺/Cu) is connected to the SHE via a salt bridge, and a high-resistance voltmeter measures the potential difference between the two electrodes. Because the SHE potential is defined as zero, the voltmeter reading directly gives the standard electrode potential of the test half-cell.

标准电极电势表及其应用

标准电极电势表按 E° 值从最负到最正排列。以下是 AQA 考试大纲中一些关键的电势值（单位：V）：

• Li⁺(aq) + e⁻ ⇌ Li(s)：-3.04（最负，最强的还原剂）
• Zn²⁺(aq) + 2e⁻ ⇌ Zn(s)：-0.76
• Fe²⁺(aq) + 2e⁻ ⇌ Fe(s)：-0.44
• 2H⁺(aq) + 2e⁻ ⇌ H₂(g)：0.00（参考点）
• Cu²⁺(aq) + 2e⁻ ⇌ Cu(s)：+0.34
• I₂(s) + 2e⁻ ⇌ 2I⁻(aq)：+0.54
• Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq)：+0.77
• Ag⁺(aq) + e⁻ ⇌ Ag(s)：+0.80
• Cl₂(g) + 2e⁻ ⇌ 2Cl⁻(aq)：+1.36
• F₂(g) + 2e⁻ ⇌ 2F⁻(aq)：+2.87（最正，最强的氧化剂）

记住：所有半电池方程式都按还原方向书写（氧化态 + ne⁻ ⇌ 还原态）。

The electrochemical series, or standard electrode potential table, lists half-equations in order of E° from most negative to most positive. Here are key values from the AQA specification (in V):

• Li⁺(aq) + e⁻ ⇌ Li(s): −3.04 (most negative, strongest reducing agent)
• Zn²⁺(aq) + 2e⁻ ⇌ Zn(s): −0.76
• Fe²⁺(aq) + 2e⁻ ⇌ Fe(s): −0.44
• 2H⁺(aq) + 2e⁻ ⇌ H₂(g): 0.00 (reference)
• Cu²⁺(aq) + 2e⁻ ⇌ Cu(s): +0.34
• I₂(s) + 2e⁻ ⇌ 2I⁻(aq): +0.54
• Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq): +0.77
• Ag⁺(aq) + e⁻ ⇌ Ag(s): +0.80
• Cl₂(g) + 2e⁻ ⇌ 2Cl⁻(aq): +1.36
• F₂(g) + 2e⁻ ⇌ 2F⁻(aq): +2.87 (most positive, strongest oxidising agent)

Remember: all half-equations are written as reduction (oxidised form + ne⁻ ⇌ reduced form).

计算电池电动势 E°_cell

对于一个完整的电化学电池，其标准电动势（E°_cell 或 EMF）的计算公式非常简单：

E°_cell = E°(正极) − E°(负极)

正极（cathode）是发生还原反应的一侧，是 E° 较正的那个半电池；负极（anode）发生氧化反应，是 E° 较负的那个半电池。另一种记忆方式是：E°_cell = E°(右) − E°(左)，如果你按照电池图（cell diagram）画出了电池的布局。注意：在计算中你永远不应该改变 E° 的符号——公式里的减号已经帮你处理好了。

For a complete electrochemical cell, the standard cell potential (E°_cell or EMF) is given by a straightforward formula:

E°_cell = E°(cathode) − E°(anode)

The cathode is the site of reduction and is the half-cell with the more positive E°; the anode is the site of oxidation and is the half-cell with the more negative E°. Another way to remember this is: E°_cell = E°(right) − E°(left), following the layout of the cell diagram. Crucially, you should never flip the sign of E° manually — the subtraction in the formula already accounts for the direction of the reaction.

实例计算

例题 1：锌-铜电池

一个原电池由 Zn²⁺/Zn 半电池和 Cu²⁺/Cu 半电池构成。已知 E°(Zn²⁺/Zn) = −0.76 V，E°(Cu²⁺/Cu) = +0.34 V。求该电池的 E°_cell。

解：正极是铜（+0.34 V 更正），负极是锌（−0.76 V 更负）。
E°_cell = (+0.34) − (−0.76) = +1.10 V
正极反应（还原）：Cu²⁺ + 2e⁻ → Cu
负极反应（氧化）：Zn → Zn²⁺ + 2e⁻
总反应：Zn + Cu²⁺ → Zn²⁺ + Cu

因为 E°_cell 为正值，这个反应是自发的。

Example 1: The Zinc-Copper Cell

A galvanic cell is constructed from a Zn²⁺/Zn half-cell and a Cu²⁺/Cu half-cell. Given E°(Zn²⁺/Zn) = −0.76 V and E°(Cu²⁺/Cu) = +0.34 V, calculate E°_cell.

Solution: The cathode is copper (+0.34 V, more positive); the anode is zinc (−0.76 V, more negative).
E°_cell = (+0.34) − (−0.76) = +1.10 V
Cathode half-reaction (reduction): Cu²⁺ + 2e⁻ → Cu
Anode half-reaction (oxidation): Zn → Zn²⁺ + 2e⁻
Overall reaction: Zn + Cu²⁺ → Zn²⁺ + Cu

Since E°_cell is positive, this reaction is spontaneous.

预测氧化还原反应的自发性

这是 AQA 考试中最常见的考题类型之一。给定一个氧化剂-还原剂组合，我们需要判断它们之间能否发生自发的氧化还原反应。规则很简单：

1. 从电极电势表中找出两种半反应的标准电极电势。
2. E° 较正的那个物种作为氧化剂发生还原（获得电子），E° 较负的那个物种作为还原剂发生氧化（失去电子）。
3. 用公式 E°_cell = E°(oxidising agent) − E°(reducing agent) 计算。
4. 如果 E°_cell > 0，反应自发进行；如果 E°_cell < 0，反应不自发。

典型陷阱：不要简单地说”E° 较正的会氧化 E° 较负的”。实际上电势差需要足够大——通常如果 E°_cell < +0.3 V，反应的动力学因素可能使反应在室温下进行得非常缓慢。

This is one of the most common AQA exam question types. Given a combination of an oxidising agent and a reducing agent, we need to determine whether a spontaneous redox reaction occurs. The rule is simple:

1. Look up the standard electrode potentials of both half-reactions from the electrochemical series.
2. The species with the more positive E° acts as the oxidising agent (gets reduced, gains electrons); the species with the more negative E° acts as the reducing agent (gets oxidised, loses electrons).
3. Calculate using E°_cell = E°(oxidising agent) − E°(reducing agent).
4. If E°_cell > 0, the reaction is spontaneous; if E°_cell < 0, it is not.

Common pitfall: Do not simply say “the more positive E° will oxidise the more negative E°”. In practice, kinetics can make thermodynamically feasible reactions very slow at room temperature — typically if E°_cell < +0.3 V, the reaction may appear not to occur without heating or a catalyst.

电池图示法（Cell Diagrams）

AQA 要求你能够用标准符号表示电化学电池。电池图遵循固定的格式：

负极 | 负极溶液 || 正极溶液 | 正极

具体规则：

• 负极（氧化侧）写在左边，物种之间用单竖线 | 分隔，代表相界面。
• 盐桥用双竖线 || 表示。
• 正极（还原侧）写在右边。
• 如果电极是惰性的（如铂 Pt），需要明确写出。
• 每种溶液中各种离子的状态符号 (aq) 也可以标注。

例题：写出锌-铜电池的电池图。
答：Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)

对于包含 Fe²⁺/Fe³⁺ 这种没有金属电极的半电池，需要用到铂电极：
Pt(s) | Fe²⁺(aq), Fe³⁺(aq) || …

AQA requires you to represent electrochemical cells using standard cell diagram notation. The format follows a fixed convention:

anode | anodic solution || cathodic solution | cathode

Key rules:

• The anode (oxidation side) is on the left; a single vertical line | separates different phases.
• The salt bridge is represented by a double vertical line ||.
• The cathode (reduction side) is on the right.
• If the electrode is inert (such as platinum Pt), it must be shown explicitly.
• State symbols (aq, s, g) may be included for clarity.

Example: Write the cell diagram for the zinc-copper cell.
Answer: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)

For half-cells involving ions only (e.g., Fe²⁺/Fe³⁺) with no solid metal electrode, a platinum electrode must be included: Pt(s) | Fe²⁺(aq), Fe³⁺(aq) || …

标准氢电极的局限性与替代方案

虽然标准氢电极是参考标准，但在实际实验中使用它有很多不便：需要持续通入氢气（有爆炸风险）、铂电极容易中毒失去活性、装置复杂。因此，在实际测量中通常使用二级参考电极，比如银-氯化银电极（Ag/AgCl）或甘汞电极（calomel electrode）。这些电极的电势已经被精确测定并与 SHE 校准过，使用更方便。考试中，AQA 可能给出一个用其他参考电极测得的电势值，然后要求你将它与标准值进行比较或换算。

Although the standard hydrogen electrode serves as the universal reference, it is inconvenient for practical work: hydrogen gas must be continuously supplied (posing an explosion hazard), the platinum electrode is susceptible to poisoning and deactivation, and the setup is cumbersome. Consequently, secondary reference electrodes such as the silver–silver chloride electrode (Ag/AgCl) or the calomel electrode are commonly used in laboratory measurements. Their potentials have been precisely determined and calibrated against the SHE, making them far more practical. In exams, AQA may provide a potential measured against a different reference electrode and ask you to compare or convert it to the SHE scale.

非标准条件下的电动势：Nernst 方程简介

当浓度或温度偏离标准条件时，用 Nernst 方程可以对 E° 进行修正：

E = E° − (RT/nF) × ln Q

其中 R 是气体常数（8.314 J mol⁻¹ K⁻¹），T 是温度（K），n 是转移的电子数，F 是法拉第常数（96,500 C mol⁻¹），Q 是反应商。在 298 K 时，这个方程简化为：

E = E° − (0.0592/n) × log₁₀ Q

例如，如果锌离子的浓度从 1.0 mol dm⁻³ 降到 0.01 mol dm⁻³，Zn²⁺/Zn 半电池的电势会变得更负，有利于氧化方向（即锌更倾向于失去电子）。虽然 AQA 不要求你完整使用 Nernst 方程进行计算，但理解浓度变化会影响电池电动势是一个重要的概念点。

When concentrations or temperature deviate from standard conditions, the Nernst equation corrects E° accordingly:

E = E° − (RT/nF) × ln Q

where R is the gas constant (8.314 J mol⁻¹ K⁻¹), T is temperature in Kelvin, n is the number of electrons transferred, F is the Faraday constant (96,500 C mol⁻¹), and Q is the reaction quotient. At 298 K, this simplifies to:

E = E° − (0.0592/n) × log₁₀ Q

For example, if the zinc ion concentration drops from 1.0 mol dm⁻³ to 0.01 mol dm⁻³, the Zn²⁺/Zn half-cell potential becomes more negative, favouring the oxidation direction — zinc is more inclined to lose electrons. Although AQA does not require full Nernst equation calculations, understanding that concentration changes affect cell EMF is an important conceptual point.

AQA 考试常见题型与答题技巧

题型一：计算 E°_cell
直接给出两个半电池的 E° 值，要求计算电动势。记住公式 E°_cell = E°(正极) − E°(负极)，答案带单位 V。最好也写出哪个是正极哪个是负极，并写出总反应方程式。

题型二：判断反应是否自发
给出一个化学方程式，要求用标准电极电势判断该反应在标准条件下能否自发进行。分三步：确定哪个是氧化剂哪个是还原剂、查找各自的 E°、计算 E°_cell 并判断符号。

题型三：解释为什么实际电势偏离理论值
可能是由于非标准浓度、非标准温度、或者电极表面形成氧化层导致动力学阻碍。要明确指出”标准条件不满足”。

题型四：电池图与电极识别
画出或补齐电池图，识别正极和负极。注意区分”正极是还原发生的场所”与”电子流入正极”这两个等价的表述。

Exam Question Type 1: Calculate E°_cell
Two E° values are given directly. Apply E°_cell = E°(cathode) − E°(anode). Always include the unit V. It is good practice to also identify which electrode is the cathode and which is the anode, and write the overall redox equation.

Exam Question Type 2: Determine spontaneity
Given a chemical equation, use standard electrode potentials to predict whether the reaction is spontaneous under standard conditions. Three steps: identify the oxidising and reducing agents, look up their respective E° values, and calculate E°_cell — a positive value confirms spontaneity.

Exam Question Type 3: Explain deviation from theoretical EMF
Possible causes include non-standard concentrations, non-standard temperature, or kinetic barriers such as an oxide layer forming on an electrode surface. Always state explicitly that “standard conditions are not met”.

Exam Question Type 4: Cell diagrams and electrode identification
Draw or complete a cell diagram, and identify the cathode and anode. Remember that “reduction occurs at the cathode” and “electrons flow into the cathode” are equivalent statements.

总结

电化学电池和标准电极电势是 AQA A-Level 化学中连接热力学与实际应用的关键章节。掌握以下核心要点是成功的关键：理解标准氢电极作为参考点的作用、熟练使用 E° 表比较不同物质的氧化还原能力、正确使用 E°_cell 公式进行计算、能够书写和解读电池图、以及理解非标准条件对电动势的影响。多练习历年真题中的计算和推理题，你会发现这个章节其实比初看时要简单得多。

Electrochemical cells and standard electrode potentials form a crucial bridge between thermodynamics and real-world applications in AQA A-Level Chemistry. Mastering the following core points is key to success: understanding the role of the standard hydrogen electrode as the reference point, confidently using the electrochemical series to compare the oxidising and reducing power of different species, correctly applying the E°_cell formula, being able to write and interpret cell diagrams, and understanding how non-standard conditions affect cell EMF. With plenty of practice on past-paper calculations and reasoning questions, you will find this topic far more manageable than it first appears.