Using E° Values | 利用标准电极电势 (E°值)

📚 Using E° Values | 利用标准电极电势 (E°值)

Standard electrode potentials (E° values) are among the most powerful tools in A‑Level Chemistry. They allow you to predict whether a redox reaction is thermodynamically feasible, calculate the voltage of an electrochemical cell, compare the strength of oxidising and reducing agents, and even explain why some substances disproportionate. Mastering their use is essential for tackling both theoretical questions and practical applications.

标准电极电势（E°值）是A‑Level化学中最有力的工具之一。它们不仅能让你预测氧化还原反应在热力学上是否可行、计算电化学电池的电压，还能比较氧化剂和还原剂的强弱，甚至解释为什么某些物质会发生歧化。掌握E°值的应用是应对理论题和实际问题的关键。

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1. Defining Standard Electrode Potentials | 标准电极电势的定义

The standard electrode potential (E°) of a half‑cell is the potential difference measured when it is connected to a standard hydrogen electrode (SHE) under standard conditions: 298 K, all ion concentrations at 1 mol dm⁻³, and any gases at a pressure of 1 atm (101 kPa).

标准电极电势（E°）是半电池与标准氢电极（SHE）连接时，在标准条件（298 K、离子浓度均为1 mol dm⁻³、气体压强为1 atm（101 kPa））下测得的电势差。

The SHE is assigned an arbitrary E° of exactly 0.00 V, and every other half‑ his reference. Values are quoted for the reduction reaction (e.g. Cu²⁺(aq) + 2e⁻ → Cu(s)).

标准氢电极被任意赋予0.00 V的E°值，所有其他半电池的电势都以此为参照。所引用的数值均对应还原反应（例如Cu²⁺(aq) + 2e⁻ → Cu(s)）。

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2. The Electrochemical Series | 电化学系列

Arranging half‑equations in order of increasing reduction potential creates the electrochemical series. A small selection is shown below:

将半反应按还原电势升序排列就得到了电化学系列。下面列出其中一小部分：

Half‑reaction	E° / V
Li⁺(aq) + e⁻ → Li(s)	-3.04
Zn²⁺(aq) + 2e⁻ → Zn(s)	-0.76
Fe²⁺(aq) + 2e⁻ → Fe(s)	-0.44
2H⁺(aq) + 2e⁻ → H₂(g)	0.00 (by definition)
Cu²⁺(aq) + 2e⁻ → Cu(s)	+0.34
Ag⁺(aq) + e⁻ → Ag(s)	+0.80
F₂(g) + 2e⁻ → 2F⁻(aq)	+2.87

The more negative the E°, the stronger the reducing agent (it donates electrons more readily). Conversely, the more positive the E°, the stronger the oxidising agent.

E°越负，还原剂越强（越容易给出电子）；反之，E°越正，氧化剂越强。

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3. Predicting the Direction of a Redox Reaction | 预测氧化还原反应的方向

A redox reaction is thermodynamically feasible if the cell EMF (E°_cell) calculated from the half‑equations is positive. Use the relationship:

如果一个氧化还原反应根据半反应计算得到的电池电动势（E°_cell）为正值，那么该反应在热力学上就是可行的。使用以下关系式：

E°_cell = E°(reduction half‑cell) – E°(oxidation half‑cell)

It is essential to use the reduction potentials exactly as listed in the data booklet. Identify the half‑reaction with the more positive E° – this will proceed as reduction; the other will occur in reverse as oxidation.

必须严格使用数据手册中给出的还原电势。找出E°较正的半反应——它将发生还原反应；另一个半反应则逆向进行，即发生氧化。

Example: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)
Zn²⁺/Zn has E° = –76 V; Cu²⁺/Cu has E° = +0.34 V. Cu²⁺/Cu is reduced (cathode), Zn/Zn²⁺ is oxidised (anode).
E°_cell = +0.34 – (– the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) is spontaneous under standard conditions.

例子：Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)
Zn²⁺/Zn 的 E° = –0.76 V；Cu²⁺/Cu 的 E° = +0.34 V。Cu²⁺/Cu 发生还原（阴极），Zn/Zn²⁺ 发生氧化（阳极）。
E°_cell = +0.34 – (–0.76) = +1.10 V，因此反应 Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) 在标准条件下可自发进行。

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4. Calculating Cell EMF from E° Values | 由E°值计算电池电动势

Always subtract the E° of the half‑cell being oxidised from the E° of the half‑cell being reduced. A second example involving non‑metals shows the same principle:

始终用发生还原的半电池的E°减去发生氧化的半电池的E°。另一个涉及非金属的例子也体现了同一原理：

Cl₂(g) + 2e⁻ → 2Cl⁻(aq) E° = +1.36 V
Br₂(l) + 2e⁻ → 2Br⁻(aq) E° = +1.07 V
The more positive Cl₂/Cl⁻ system will undergo reduction; the Br₂/Br⁻ system will be reversed as oxidation.
E°_cell = +1.36 – (+1.07) = +0.29 V > 0, so Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ is feasible.

Cl₂(g) + 2e⁻ → 2Cl⁻(aq) E° = +1.36 V
Br₂(l) + 2e⁻ → 2Br⁻(aq) E° = +1.07 V
较正的Cl₂/Cl⁻体系将发生还原；Br₂/Br⁻体系则逆向进行氧化。
E°_cell = +1.36 – (+1.07) = +0.29 V > 0，因此 Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ 可以发生。

Never change the sign of the E° value when using the subtraction method; the data booklet always gives reduction potentials.

运用相减法时绝不要改变E°值的符号；数据手册始终给出的是还原电势。

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5. Comparing the Strength of Oxidising Agents | 比较氧化剂的强度

The most positive E° values are found at the bottom of the electrochemical series. These species are the strongest oxidising agents because they have the greatest tendency to accept electrons. For instance, F₂ (+2.87 V) is a stronger oxidising agent than MnO₄⁻ in acidic solution (+1.51 V), which in turn is stronger than Cu²⁺ (+0.34 V).

电化学系列的最底端是E°最正的物质。这些物种是最强的氧化剂，因为它们接受电子的倾向最大。例如 F₂ (+2.87 V) 的氧化性比酸性溶液中的 MnO₄⁻ (+1.51 V) 强，而 MnO₄⁻ 又比 Cu²⁺ (+0.34 V) 强。

Notice that oxidising power increases up and to the right of the series – many powerful oxidising agents contain an element in a high oxidation state (e.g. Mn in MnO₄⁻ is +7).

可以看出，氧化能力在电化序列的“右下方”最强——许多强氧化剂都含有高氧化态的元素（例如 MnO₄⁻ 中 Mn 为 +7）。

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6. Comparing the Strength of Reducing Agents | 比较还原剂的强度

The most negative E° values appear at the top of the series. These are the strongest reducing agents, readily losing electrons. Lithium (–3.04 V) is an excellent reducing agent, as are the Group 1 and Group 2 metals. Zinc (–0.76 V) is a stronger reducing agent than iron (–0.44 V), which is why zinc can protect iron sacrificially.

电化学系列的最顶端是E°最负的物质。这些是最强的还原剂，极易失去电子。锂 (–3.04 V) 是优良的还原剂，第1族和第2族金属也是如此。锌 (–0.76 V) 的还原性比铁 (–0.44 V) 强，这就是锌能作为牺牲阳极保护铁的原因。

When a reagent is placed higher in the series than the species it reacts with, it will act as a reducing agent and donate electrons to that species.

当一种试剂在电化学系列中的位置高于与之反应的物种时，它将作为还原剂把电子传递给后者。

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7. The Role of the Standard Hydrogen Electrode | 标准氢电极的角色

All E° values are measured against the standard hydrogen electrode. The SHE consists of platinum foil coated with platinum black, immersed in a solution of 1 mol dm⁻³ H⁺, with H₂ gas bubbled through at 1 atm. The half‑reaction 2H⁺(aq) + 2e⁻ ⇌ H₂(g) is given an E° of 0.00 V.

所有E°值都是相对于标准氢电极测得的。SHE由镀有铂黑的铂片构成，浸入1 mol dm⁻³ H⁺溶液中，并通入1 atm的氢气。半反应 2H⁺(aq) + 2e⁻ ⇌ H₂(g) 被赋予 0.00 V 的电势。

In practice, the SHE is not always convenient, but its fixed potential provides a consistent zero‑point that makes the whole data table comparable.

在实践中，SHE 并不总是很方便，但它固定的电势提供了一个一致的零点，使整个数据表彼此可以比较。

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8. Choosing Suitable Oxidising or Reducing Agents | 选择合适的氧化剂或还原剂

O₄⁻/Mn²⁺, +1.51 V) is widely used to oxidise Fe²⁺ to Fe³⁺ (Fe³⁺/Fe²⁺, +0.77 V). The cell EMF is +1.51 – (+0.77) = +0.74 V, so the reaction is feasible.

利用E°值可以选择一种只氧化某个物种而不影响其他物种的试剂。例如，酸化高锰酸钾（MnO₄⁻/Mn²⁺, +1.51 V）被广泛用来将 Fe²⁺ 氧化为 Fe³⁺（Fe³⁺/Fe²⁺, +0.77 V）。电池电动势为 +1.51 – (+0.77) = +0.74 V，反应可行。

You can also deduce that chlorine (+1.36 V) will oxidise iron(II) but not fluoride (–2.87 V for F₂/F⁻ is irrelevant; rather, F⁻/F₂ has an extremely positive reduction potential and is difficult to oxidise further). Always check relative positions.

你也可以推断出氯气 (+1.36 V) 能氧化亚铁离子，但无法氧化氟离子（F⁻/F₂ 具有极高的还原电势，很难被进一步氧化）。始终要核对相对位置。

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9. Disproportionation and E° Values | 歧化反应与E°值

Disproportionation is a reaction where an element in an intermediate oxidation state is simultaneously oxidised and reduced. E° values let you predict whether such a reaction is thermodynamically possible. Consider copper(I) ions:

歧化反应是同一种元素处于中间氧化态时，同时被氧化和还原的反应。利用E°值可以预测这类反应在热力学上是否可能。以铜(I)离子为例：

Cu²⁺(aq) + e⁻ → Cu⁺(aq) E° = +0.15 V
Cu⁺(aq) + e⁻ → Cu(s) E° = +0.52 V
onation: 2Cu⁺(aq) → Cu²⁺(aq) + Cu(s). The Cu⁺ being oxidised corresponds to the first half‑reaction reversed, while the Cu⁺ being reduced uses the second.
E°_cell = +0.52 – nd will disproportionate. This explains why aqueous copper(I) compounds are rarely stable.

Cu²⁺(aq) + e⁻ → Cu⁺(aq) E° = +0.15 V
Cu⁺(aq) + e⁻ → Cu(s) E° = +0.52 V
歧化反应：2Cu⁺(aq) → Cu²⁺(aq) + Cu(s)。其中被氧化的 Cu⁺ 对应第一个半反应的逆向，被还原的 Cu⁺ 对应第二个半反应。
E°_cell = +0.52 – (+0.15) = +0.37 V > 0，因此 Cu⁺ 在水溶液中不稳定，会发生歧化。这就解释了为什么水溶液中铜(I)化合物通常不稳定。

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10. Limitations of E° Values: Kinetic Factors | E°值的局限性：动力学因素

A positive E°_cell tells you a reaction is thermodynamically allowed, but it says nothing about how fast it will proceed. Some reactions with a large positive E° cell are extremely slow at room temperature because they have a high activation energy.

一个正的 E°_cell 告诉你反应在热力学上允许进行，但丝毫不涉及反应进行的快慢。有些具有很大正电势的反应在室温下极其缓慢，因为它们具有很高的活化能。

Classic examples include the reaction of magnesium with cold water (Mg²⁺/Mg –2.37 V, 2H₂O/H₂ under alkaline conditions ca.

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