📚 Edexcel A-Level Chemistry Topic 14.3: Standard Electrode Potentials and Feasibility | Edexcel A-Level化学14.3：标准电极电势与反应可行性
Standard electrode potentials are at the heart of Edexcel A-Level Chemistry Topic 14.3, helping you predict whether a redox reaction is thermodynamically feasible. This article covers half-cells, the standard hydrogen electrode, cell diagrams, E°cell calculations, feasibility predictions, and common exam traps.
标准电极电势是Edexcel A-Level化学14.3的核心内容，它帮助你判断一个氧化还原反应在热力学上是否可行。本文涵盖半电池、标准氢电极、电池图示、E°cell计算、可行性预测以及常见考试陷阱。
1. What Are Half-Cells and Electrode Potentials? | 什么是半电池和电极电势？
A half-cell contains an element in two oxidation states in contact with each other, often a metal rod dipped into a solution of its own ions, such as Zn(s) in Zn²⁺(aq). At the metal-solution boundary, a potential difference develops because the metal atom may lose electrons to form ions, or metal ions may gain electrons to form atoms.
半电池含有两种氧化态相互接触的同种元素，通常是一根金属棒浸入其自身离子的溶液中，例如Zn(s)浸在Zn²⁺(aq)中。在金属与溶液的界面会形成电势差，因为金属原子可能失去电子变成离子，或金属离子可能得到电子变成原子。
The equilibrium position of the half-reaction determines the value of the electrode potential. A more negative value means the equilibrium lies further to the left, and the reduced form is a stronger reducing agent; a more positive value means the oxidised form is a stronger oxidising agent.
半反应平衡的位置决定了电极电势的数值。数值越负，平衡越偏向左方，还原型物质的还原性越强；数值越正，氧化型物质的氧化性越强。
2. The Standard Hydrogen Electrode (SHE) | 标准氢电极
All standard electrode potentials are measured relative to the standard hydrogen electrode, which is assigned a potential of exactly 0.00 V. The SHE uses hydrogen gas at 100 kPa bubbling over a platinum electrode immersed in 1.00 mol dm⁻³ H⁺(aq) at 298 K.
所有标准电极电势都是相对于标准氢电极测得的，标准氢电极的电势被规定为0.00 V。标准氢电极使用氢气在100 kPa压力下通过铂电极，铂电极浸在298 K、1.00 mol dm⁻³ H⁺(aq)溶液中。
The platinum electrode is inert, conducts electrons, and provides a surface for the half-reaction: 2H⁺(aq) + 2e⁻ ⇌ H₂(g). By comparing another half-cell with the SHE, we obtain its standard electrode potential E°.
铂电极是惰性的，能够导电，并为半反应2H⁺(aq) + 2e⁻ ⇌ H₂(g)提供表面。将另一个半电池与标准氢电极进行比较，即可得到它的标准电极电势E°。
3. Standard Conditions for E° Values | 标准电极电势的标准条件
To compare electrode potentials fairly, E° values are measured under standard conditions: temperature 298 K, gas pressure 100 kPa, and ion concentration 1.00 mol dm⁻³. A high-resistance voltmeter is used so that no significant current flows during measurement.
为了公平比较电极电势，E°值在标准条件下测定：温度298 K、气体压强100 kPa、离子浓度1.00 mol dm⁻³。使用高电阻电压表，使测量过程中几乎没有电流通过。
If these conditions change, the electrode potential changes. For example, increasing the concentration of metal ions usually makes the electrode potential more positive, while increasing the concentration of the reduced form usually makes it more negative.
如果这些条件改变，电极电势也会改变。例如，增大金属离子浓度通常使电极电势更正，而增大还原型物质浓度通常使电极电势更负。
4. Writing Cell Diagrams | 书写电池图示
A cell diagram shows the oxidation half-cell on the left and the reduction half-cell on the right. A single vertical line | represents a phase boundary, while a double vertical line || represents the salt bridge. For example: Zn(s)|Zn²⁺(aq)||Cu²⁺(aq)|Cu(s).
电池图示把氧化半电池写在左边，还原半电池写在右边。单竖线|表示相界面，双竖线||表示盐桥。例如：Zn(s)|Zn²⁺(aq)||Cu²⁺(aq)|Cu(s)。
The more negative half-cell is always placed on the left, and the more positive half-cell is placed on the right. If a half-cell contains no solid metal, an inert platinum electrode is included, such as Pt(s)|Fe²⁺(aq),Fe³⁺(aq)||MnO₄⁻(aq),Mn²⁺(aq)|Pt(s).
电极电势较负的半电池总是写在左边，较正的写在右边。如果半电池中没有固体金属，就加入惰性铂电极，例如Pt(s)|Fe²⁺(aq),Fe³⁺(aq)||MnO₄⁻(aq),Mn²⁺(aq)|Pt(s)。
5. Calculating E°cell | 计算标准电池电动势
The standard cell potential is calculated using the equation:
标准电池电动势用以下公式计算：
E°cell = E°(right-hand electrode) − E°(left-hand electrode)
Alternatively, E°cell = E°(reduction half-cell) − E°(oxidation half-cell). A positive E°cell means the reaction is expected to proceed in the direction written.
也可以写成：E°cell = E°(还原半电池) − E°(氧化半电池)。E°cell为正，说明该方向书写的反应预计可以自发进行。
| Half-reaction | E° / V |
|---|---|
| Zn²⁺(aq) + 2e⁻ ⇌ Zn(s) | −0.76 |
| Cu²⁺(aq) + 2e⁻ ⇌ Cu(s) | +0.34 |
| Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq) | +0.77 |
| MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ ⇌ Mn²⁺(aq) + 4H₂O(l) | +1.51 |
For a zinc-copper cell with Cu²⁺/Cu on the right and Zn²⁺/Zn on the left:
对于铜在右、锌在左的锌铜电池：
E°cell = (+0.34 V) − (−0.76 V) = +1.10 V
6. Predicting Feasibility from E°cell | 用电势差预测反应可行性
If E°cell is positive under standard conditions, the redox reaction is thermodynamically feasible. The half-cell with the more negative E° will be oxidised and supplies electrons, while the half-cell with the more positive E° will be reduced and accepts electrons.
在标准条件下，如果E°cell为正，则该氧化还原反应在热力学上可行。E°较负的半电池发生氧化反应，提供电子；E°较正的半电池发生还原反应，接受电子。
For example, chlorine gas should oxidise bromide ions because the E° for Cl₂/Cl⁻ is +1.36 V and for Br₂/Br⁻ is +1.09 V, giving E°cell = +0.27 V, which is positive.
例如，氯气应当能氧化溴离子，因为Cl₂/Cl⁻的E°为+1.36 V，Br₂/Br⁻的E°为+1.09 V，E°cell = +0.27 V，为正值。
7. Limitations of Feasibility Predictions | 电极电势预测的局限性
E° values only tell you whether a reaction is thermodynamically possible; they do not tell you whether it will happen at a measurable rate. A positive E°cell does not guarantee an observable reaction because the activation energy may be too high.
E°值只能说明反应在热力学上是否可能，而不能说明反应是否会以可测量的速率进行。E°cell为正值并不能保证反应可观察到，因为活化能可能过高。
Non-standard conditions also change the electrode potential. If concentrations, temperature, or gas pressure differ from standard values, the actual cell potential will be different, so the predicted feasibility may no longer apply.
非标准条件也会改变电极电势。如果浓度、温度或气体压强偏离标准值，实际电池电势就会不同，因此所预测的可行性可能不再适用。
8. Measuring Electrode Potentials Experimentally | 实验测定电极电势
To measure a standard electrode potential, connect the half-cell to a standard hydrogen electrode using a salt bridge and a high-resistance voltmeter. The voltmeter reading gives the cell emf, which is the difference between the two electrode potentials.
要测量标准电极电势，将待测半电池通过盐桥和高电阻电压表与标准氢电极相连。电压表读数就是电池电动势，即两个电极电势之差。
The salt bridge contains a concentrated solution of an ionic salt, often KNO₃, and allows ions to move between half-cells to maintain charge balance without mixing the solutions.
盐桥中含有高浓度的离子盐溶液，常用KNO₃，允许离子在半电池之间移动以维持电荷平衡，同时不让两侧溶液混合。
9. Redox Titrations in Topic 14.3 | 氧化还原滴定
Standard electrode potentials help explain the use of redox titrations such as manganate(VII) titrations. In acid solution, MnO₄⁻ is reduced to Mn²⁺ while Fe²⁺ is oxidised to Fe³⁺:
标准电极电势可以帮助解释氧化还原滴定的原理，例如高锰酸根滴定。在酸性溶液中，MnO₄⁻被还原为Mn²⁺，Fe²⁺被氧化为Fe³⁺：
MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺
E° for MnO₄⁻/Mn²⁺ is +1.51 V and for Fe³⁺/Fe²⁺ is +0.77 V. Since E°cell is positive, the titration reaction is feasible and reaches a sharp endpoint.
MnO₄⁻/Mn²⁺的E°为+1.51 V，Fe³⁺/Fe²⁺的E°为+0.77 V。由于E°cell为正值，该滴定反应可行，并具有敏锐的终点。
10. Common Exam Mistakes | 常见考试错误
Many students lose marks by confusing the left and right half-cells when calculating E°cell. Always remember: E°cell = E°(right-hand electrode) − E°(left-hand electrode), and the more negative half-cell is on the left.
许多学生在计算E°cell时混淆左右半电池而失分。务必记住：E°cell = E°(右侧电极) − E°(左侧电极)，且较负的半电池在左侧。
Other common errors include forgetting standard conditions, writing the salt bridge as a single line, or claiming that a positive E°cell proves a reaction is fast. E° values indicate thermodynamic feasibility, not reaction rate.
其他常见错误包括忘记标准条件、把盐桥写成单竖线，或声称E°cell为正值即可证明反应速率快。E°值反映的是热力学可行性，而不是反应速率。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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