Redox Reactions | 氧化还原反应

📚 Redox Reactions | 氧化还原反应

Redox reactions form one of the most important pillars of A-Level Chemistry. The term ‘redox’ combines ‘reduction’ and ‘oxidation’ — two processes that always occur together. A solid grasp of redox chemistry is essential for topics ranging from electrochemistry to transition metal chemistry, and Cambridge examiners frequently test redox concepts across all three A-Level papers.

氧化还原反应是 A-Level 化学最重要的基石之一。”Redox” 一词结合了 “reduction”（还原）与 “oxidation”（氧化）——这两个过程总是同时发生。扎实掌握氧化还原化学，对于从电化学到过渡金属化学等众多主题都至关重要，剑桥考官也经常在三份 A-Level 试卷中考查氧化还原概念。

1. What Are Redox Reactions? | 什么是氧化还原反应？

Oxidation is the loss of electrons, while reduction is the gain of electrons. A useful mnemonic is OIL RIG: Oxidation Is Loss, Reduction Is Gain. In any redox reaction, one species loses electrons (is oxidised) while another gains them (is reduced). These two processes always occur simultaneously — you cannot have oxidation without reduction.

氧化是指失去电子，还原是指获得电子。一个有用的助记口诀是 OIL RIG：Oxidation Is Loss（氧化即失去），Reduction Is Gain（还原即获得）。在任何氧化还原反应中，一种物质失去电子（被氧化），同时另一种物质获得电子（被还原）。这两个过程总是同时发生——不可能只有氧化而没有还原。

For example, when zinc metal reacts with copper(II) ions:

例如，当锌金属与铜(II)离子反应时：

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

Zinc atoms lose two electrons to form Zn²⁺, and Cu²⁺ ions gain two electrons to form copper metal. Electrons are transferred directly from the reductant to the oxidant. In this reaction, zinc is the reducing agent (it donates electrons) while Cu²⁺ is the oxidising agent (it accepts electrons).

锌原子失去两个电子形成 Zn²⁺，而 Cu²⁺ 离子获得两个电子形成铜金属。电子从还原剂直接转移到氧化剂。在该反应中，锌是还原剂（提供电子），而 Cu²⁺ 是氧化剂（接受电子）。

2. Oxidation Numbers | 氧化数

Oxidation number (or oxidation state) is a bookkeeping device used to track electron transfer, even in reactions where no formal electron transfer occurs. It represents the charge an atom would have if all shared electrons in a bond were assigned to the more electronegative element. Unlike ionic charge, oxidation numbers can be fractional and do not necessarily correspond to real charges.

氧化数（或称氧化态）是一种用于追踪电子转移的”记账”工具，即使在并无真正电子转移的反应中也同样适用。它表示若将成键中的所有共用电子都分配给电负性更强的元素时，该原子所带的电荷。与实际离子电荷不同，氧化数可以是分数，且不一定对应真实电荷。

Oxidation numbers are crucial for three main purposes: identifying what is oxidised and what is reduced in a reaction, balancing redox equations, and predicting whether a reaction is thermodynamically feasible.

氧化数有三个主要用途：判断反应中何者被氧化、何者被还原；配平氧化还原方程式；预测反应在热力学上是否可行。

3. Rules for Assigning Oxidation Numbers | 氧化数的确定规则

Cambridge A-Level questions require students to calculate oxidation numbers confidently and quickly. The following rules apply, in order of priority:

剑桥 A-Level 考题要求学生能熟练且快速地计算氧化数。以下规则按优先级排列：

• For an uncombined element, the oxidation number is 0. (e.g. Na, O₂, P₄)

  未化合的单质，氧化数为 0。（如 Na、O₂、P₄）

• For a monatomic ion, the oxidation number equals its charge. (e.g. Na⁺ = +1, Cl⁻ = −1)

  单原子离子的氧化数等于其电荷。（如 Na⁺ = +1，Cl⁻ = −1）

• Fluorine always has an oxidation number of −1 in its compounds.

  氟在其化合物中氧化数始终为 −1。

• Oxygen normally has an oxidation number of −2, except in peroxides where it is −1 (e.g. H₂O₂) and in OF₂ where it is +2.

  氧通常为 −2，但在过氧化物中为 −1（如 H₂O₂），在 OF₂ 中为 +2。

• Hydrogen is +1 in most compounds, but −1 in metal hydrides (e.g. NaH).

  氢在大多数化合物中为 +1，但在金属氢化物中为 −1（如 NaH）。

• The sum of oxidation numbers in a neutral compound is 0; for a polyatomic ion, it equals the ionic charge.

  中性化合物中各原子氧化数之和为 0；多原子离子的氧化数之和等于该离子的电荷。

Worked example: Determine the oxidation number of manganese in MnO₄⁻. Let the oxidation number of Mn be x. Since oxygen is −2 and the total charge is −1: x + 4(−2) = −1, so x = +7.

实例：求 MnO₄⁻ 中锰的氧化数。设 Mn 的氧化数为 x。由于氧为 −2 且总电荷为 −1：x + 4(−2) = −1，因此 x = +7。

Worked example 2: Determine the oxidation number of sulfur in S₂O₃²⁻. Let the oxidation number of S be x. Then 2x + 3(−2) = −2, so 2x = +4, giving x = +2. Note that this is an average oxidation state; the two sulfur atoms actually have different oxidation states in the thiosulfate ion.

实例 2：求 S₂O₃²⁻ 中硫的氧化数。设 S 的氧化数为 x，则 2x + 3(−2) = −2，因此 2x = +4，x = +2。注意这是平均氧化态；硫代硫酸根中两个硫原子实际具有不同的氧化数。

4. Oxidising and Reducing Agents | 氧化剂与还原剂

An oxidising agent (oxidant) accepts electrons and is itself reduced in the process. A reducing agent (reductant) donates electrons and is

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