📚 AS Chemistry: Redox Reactions | 氧化还原 考点精讲
Redox chemistry lies at the heart of AS-level chemistry, linking electron transfer, oxidation states, and energy changes. Mastering redox concepts is essential for understanding everything from electrolysis to titrations.
氧化还原是AS化学的核心，连接电子转移、氧化态和能量变化。掌握氧化还原概念是从电解到滴定等理解的关键。
1. Defining Oxidation and Reduction | 氧化与还原的定义
Oxidation is the loss of electrons; reduction is the gain of electrons. This is the fundamental electron-transfer definition.
氧化是失去电子；还原是获得电子。这是电子转移的基本定义。
Oxidation can also be defined as an increase in oxidation number; reduction as a decrease in oxidation number.
氧化也可以定义为氧化数增加；还原为氧化数减少。
A helpful mnemonic: OIL RIG – Oxidation Is Loss, Reduction Is Gain.
记忆口诀：OIL RIG – 氧化是失电子，还原是得电子。
2. Oxidation Number Rules | 氧化数规则
| Rule | 规则 |
| Free elements have oxidation number 0 (e.g., O₂, Na). | 游离态单质氧化数为0（如O₂， Na）。 |
| For monatomic ions, oxidation number equals the charge (e.g., Na⁺ = +1, Cl⁻ = -1). | 单原子离子氧化数等于电荷（如Na⁺=+1, Cl⁻=-1）。 |
| Oxygen usually has oxidation number -2, except in peroxides (-1) and with fluorine. | 氧的氧化数通常为-2，过氧化物中为-1，与氟结合时例外。 |
| Hydrogen is +1 when bonded to non-metals, -1 when bonded to metals (hydrides). | 氢与非金属结合时为+1，与金属结合时为-1（氢化物）。 |
| The sum of oxidation numbers in a neutral compound is 0; in a polyatomic ion equals the ion charge. | 中性化合物中氧化数之和为0；多原子离子中等离子电荷。 |
3. Calculating Oxidation Numbers | 氧化数的计算
Work out the oxidation number of Mn in KMnO₄. K is +1, O is -2 each, so let Mn be x: +1 + x + 4(-2) = 0 → x = +7.
计算KMnO₄中Mn的氧化数。K为+1，O为-2，设Mn为x：+1 + x + 4(-2) = 0 → x = +7。
In the ion Cr₂O₇²⁻, O is -2, let Cr be y: 2y + 7(-2) = -2 → 2y = +12 → y = +6.
在Cr₂O₇²⁻离子中，O为-2，设Cr为y：2y + 7(-2) = -2 → 2y = +12 → y = +6。
Practice determining oxidation numbers in unfamiliar compounds – this is a frequent exam skill.
练习确定陌生化合物中的氧化数——这是常见的考试技能。
4. Oxidising and Reducing Agents | 氧化剂与还原剂
An oxidising agent (oxidant) accepts electrons and is itself reduced; its oxidation number decreases.
氧化剂接受电子，自身被还原；其氧化数降低。
A reducing agent (reductant) donates electrons and is itself oxidised; its oxidation number increases.
还原剂提供电子，自身被氧化；其氧化数升高。
For example, in Zn + Cu²⁺ → Zn²⁺ + Cu, Zn is the reducing agent and Cu²⁺ is the oxidising agent.
例如，Zn + Cu²⁺ → Zn²⁺ + Cu，Zn是还原剂，Cu²⁺是氧化剂。
5. Half-Equations | 半反应方程式
A half-equation shows either the oxidation or reduction process separately, with electrons explicitly shown.
半反应方程式分别显示氧化或还原过程，并明确写出电子。
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