📚 Redox and Electron Transfer | 氧化还原与电子转移
Redox chemistry is built around one key idea: electron transfer. When electrons move from one species to another, oxidation and reduction always occur together, and the oxidation numbers of the atoms involved change. This article summarises the Cambridge A-Level Chemistry approach to redox, including how to assign oxidation numbers, balance half-equations, use electrode potentials, and predict whether a redox reaction is feasible.
氧化还原化学建立在一个核心思想上：电子转移。当电子从一个微粒转移到另一个微粒时，氧化和还原总是同时发生，相关原子的氧化数也发生变化。本文总结剑桥 A-Level 化学中氧化还原的内容，包括如何配给氧化数、配平半反应、使用电极电势以及判断一个氧化还原反应能否自发进行。
1. Oxidation and Reduction as Electron Transfer | 氧化与还原的电子转移定义
Oxidation is the loss of electrons; reduction is the gain of electrons. Oxidation and reduction always occur simultaneously in a redox reaction because electrons cannot exist freely in solution for long. Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain.
氧化是失去电子；还原是得到电子。氧化和还原总是同时发生，因为电子不能在溶液中长期独立存在。记住 OIL RIG：氧化是失去电子，还原是得到电子。
+ Cu(s). Zinc loses two electrons to form Zn²⁺, so Zn is oxidised. Copper(II) ions gain two electrons to form Cu, so Cu²⁺ is reduced.
例如：Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)。锌失去两个电子生成 Zn²⁺，因此 Zn 被氧化；铜离子得到两个电子生成 Cu，因此 Cu²⁺ 被还原。
2. Oxidation Numbers: Rules and Assignment | 氧化数的规则与确定
Oxidation number is the charge an atom would have if all bonds were treated as ionic. It is a book-keeping tool for tracking electron transfer.
氧化数是假设所有化学键均为离子键时原子所带的电荷。它是追踪电子转移的记账工具。
- Free element has oxidation number 0, e.g. O₂, Cl₂, Fe. / 游离态元素氧化数为 0，如 O₂、Cl₂、Fe。
- Simple monatomic ion equals its charge, e.g. Na⁺ = +1, Cl⁻ = −1. / 单原子离子的氧化数等于离子电荷，如 Na⁺ 为 +1，Cl⁻ 为 −1。
- Oxygen is usually −2, except in peroxides where it is −1 and in OF₂ where it is +2. / 氧通常为 −2，过氧化物中为 −1，OF₂ 中为 +2。
- Hydrogen is +1 with non-metals and −1 in metal hydrides. / 氢与非金属结合时为 +1，在金属氢化物中为 −1。
- The sum of oxidation numbers equals the overall charge of the species. / 氧化数之和等于微粒的总电荷。
3. Recognising Redox from Oxidation Numbers | 通过氧化数变化识别氧化还原
If any atom's oxidation number increases, oxidation has occurred; if it decreases, reduction has occurred. If no oxidation numbers change, the reaction is not redox.
若任一原子的氧化数升高，则发生氧化；若降低，则发生还原。若所有氧化数不变，则反应不是氧化还原反应。
O₂, oxygen changes from −1 in H₂O₂ to −2 in H₂O and 0 in O₂. It is both oxidised and reduced, so H₂O₂ disproportionates.
在 H₂O₂ 分解中
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