Redox Reactions Revisited | 氧化还原反应再探

📚 Redox Reactions Revisited | 氧化还原反应再探

Redox reactions sit at the centre of A-Level Chemistry. They link electrode potentials, transition metal chemistry, titration analysis and organic transformations. This revision guide revisits the key ideas that Cambridge examiners expect after students have met oxidation numbers, half-equations and electrochemical cells.

氧化还原反应是 A-Level 化学的核心。它们连接电极电势、过渡金属化学、滴定分析和有机转化。本复习指南重新梳理剑桥考官在学生接触氧化数、半反应式和电化学电池后期望掌握的关键概念。

1. What ‘Redox’ Really Means | 氧化还原的真实含义

Redox is short for reduction-oxidation. At A-Level the operational definition is electron transfer. Oxidation is loss of electrons; reduction is gain of electrons. Remember OIL RIG.

氧化还原是 reduction-oxidation 的缩写。在 A-Level 阶段，可操作的定义是电子转移。氧化是失去电子；还原是得到电子。记住 OIL RIG。

The oxidation number of an atom increases when it is oxidised, and decreases when it is reduced. Oxidation and reduction always occur together in a redox reaction.

原子被氧化时氧化数升高，被还原时氧化数降低。氧化和还原在氧化还原反应中总是同时发生。

2. Assigning Oxidation Numbers | 氧化数赋值规则

To decide whether redox has occurred, assign oxidation numbers to every atom. Use these rules in order.

为了判断是否发生氧化还原，要给每个原子分配氧化数。按顺序使用以下规则。

• An element in its standard state has oxidation number 0, for example O₂, Cl₂, Fe.
  单质标准态的氧化数为 0，例如 O₂、Cl₂、Fe。
• A simple ion has oxidation number equal to its charge, for example Na⁺ = +1, Cl⁻ = -1.
  简单离子的氧化数等于其电荷，例如 Na⁺ = +1、Cl⁻ = -1。
• Oxygen is usually -2, except in peroxides where it is -1 and in OF₂ where it is +2.
  氧通常为 -2，但在过氧化物中为 -1，在 OF₂ 中为 +2。
• Hydrogen is usually +1, except in metal hydrides where it is -1.
  氢通常为 +1，但在金属氢化物中为 -1。
• In a neutral compound the sum is 0; in a polyatomic ion the sum equals the ion charge.
  中性化合物中氧化数总和为 0；多原子离子中氧化数总和等于离子电荷。

3. Recognising Redox in Equations | 在方程中识别氧化还原

Compare oxidation numbers on the left and right. If at least one element increases and one decreases, the equation is redox.

比较左右两侧的氧化数。如果至少一种元素氧化数升高且一种降低，则该反应是氧化还原反应。

Example: 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂. Iron changes from +3 to +2 so it is reduced; iodine changes from -1 to 0 so it is oxidised.

例如：2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂。铁从 +3 变为 +2，被还原；碘从 -1 变为 0，被氧化。

2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂

Reactions such as acid-base neutralisation and precipitation usually show no oxidation number changes, so they are not redox reactions.

酸碱中和和沉淀反应通常没有氧化数变化，因此它们不是氧化还原反应。

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

An oxidising agent, also called an oxidant, accepts electrons and is itself reduced. A reducing agent, also called a reductant, donates electrons and is itself oxidised.

氧化剂接受电子，本身被还原；还原剂给出电子，本身被氧化。

Common oxidising agents include acidified MnO₄⁻, Cr₂O₇²⁻, H₂O₂, Cl₂, Fe³⁺ and O₂. Common reducing agents include I⁻, Fe²⁺, C₂O₄²⁻, S₂O₃²⁻, H₂ and reactive metals.

常见氧化剂包括酸化的 MnO₄⁻、Cr₂O₇²⁻、H₂O₂、Cl₂、Fe³⁺ 和 O₂。常见还原剂包括 I⁻、Fe²⁺、C₂O₄²⁻、S₂O₃²⁻、H₂ 和活泼金属。

5. Constructing Half-Equations | 构建半反应式

Half-equations show electron transfer explicitly. In acidic solution, use H⁺ and H₂O to balance hydrogen and oxygen.

半反应式明确显示电子转移。在酸性溶液中，使用 H⁺ 和 H₂O

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