Redox Reactions | IGCSE Edexcel Chemistry: Oxidation & Reduction Essential Revision

📚 Redox Reactions | IGCSE Edexcel Chemistry: Oxidation & Reduction Essential Revision | 氧化还原考点精讲

Redox reactions are at the heart of chemistry, linking electron transfer, oxidation numbers, and energy changes. In the IGCSE Edexcel specification, you need to confidently identify what is oxidised and reduced, write half equations, analyse displacement and electrolysis, and apply the concept of oxidation states. This revision guide walks you through every core idea, from the simplest memory aid ‘OIL RIG’ to balancing full ionic equations, and gives you real-exam style explanations to build your understanding.

氧化还原反应是化学的核心，它将电子转移、氧化数和能量变化联系在一起。在 IGCSE Edexcel 考试大纲中，你需要自信地判断什么被氧化、什么被还原，会写半反应方程式，分析置换反应和电解，并能应用氧化态的概念。这份复习指南带你梳理每一个核心知识点，从最简单的记忆口诀 ‘OIL RIG’ 到配平完整的离子方程式，并提供贴近真题的讲解，帮助你扎实掌握。

1. What Is a Redox Reaction? | 什么是氧化还原反应？

A redox reaction is any chemical change in which one species is oxidised and another is reduced. Oxidation and reduction always happen together; you cannot have one without the other. The term ‘redox’ comes from reduction and oxidation.

氧化还原反应是指一种物质被氧化、同时另一种物质被还原的化学变化。氧化和还原总是同时发生，不能单独存在。“氧化还原”这个词来源于还原和氧化。

Oxidation originally meant gaining oxygen, and reduction meant losing oxygen. For example, in the reaction 2Mg + O₂ → 2MgO, magnesium is oxidised (gains oxygen) and oxygen is reduced (gains magnesium, i.e., ‘loses’ oxygen in the sense of combining with another element). However, the modern definition is based on electrons.

氧化最初指得到氧，还原指失去氧。例如，反应 2Mg + O₂ → 2MgO 中，镁被氧化（得到氧），氧气被还原（与镁结合，即“失去”氧）。不过现代定义是基于电子的。

2. The Electron Definition: OIL RIG | 电子定义：OIL RIG 口诀

The modern, universal definition of oxidation and reduction is about electron transfer. Remember OIL RIG:

氧化和还原的现代通用定义是关于电子转移的。记住 OIL RIG：

Oxidation Is Loss of electrons (氧化是失去电子)
Reduction Is Gain of electrons (还原是得到电子)

When a substance loses electrons, it is oxidised; when it gains electrons, it is reduced. This definition works for reactions that do not involve oxygen, such as reactions between metals and acids or displacement reactions. For instance, when zinc reacts with copper(II) ions, Zn loses two electrons to become Zn²⁺ (oxidation), and Cu²⁺ gains two electrons to become Cu (reduction): Zn + Cu²⁺ → Zn²⁺ + Cu.

当物质失去电子时，它被氧化；当它得到电子时，它被还原。这个定义适用于不涉及氧气的反应，比如金属与酸的反应或置换反应。例如，当锌与铜离子反应时，Zn 失去两个电子变成 Zn²⁺（氧化），Cu²⁺ 得到两个电子变成 Cu（还原）：Zn + Cu²⁺ → Zn²⁺ + Cu。

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

An oxidising agent (oxidant) is the species that causes oxidation by accepting electrons; it is itself reduced. A reducing agent (reductant) is the species that causes reduction by donating electrons; it is itself oxidised. You must be able to identify the agent from a given equation.

氧化剂（氧化剂）是接受电子而导致氧化发生的物质，它自身被还原。还原剂是给出电子而导致还原发生的物质，它自身被氧化。你必须能从给出的方程式中识别出这些试剂。

For example, in the reaction Mg + CuO → MgO + Cu, magnesium is the reducing agent because it reduces copper(II) oxide to copper while itself being oxidised to magnesium oxide. Copper(II) oxide is the oxidising agent because it oxidises magnesium to magnesium oxide while itself being reduced to copper. Always check what happens to each reactant.

例如，在反应 Mg + CuO → MgO + Cu 中，镁是还原剂，因为它将氧化铜还原成铜，同时自身被氧化成氧化镁。氧化铜是氧化剂，因为它将镁氧化成氧化镁，同时自身被还原成铜。一定要检查每一种反应物的变化。

4. Oxidation Numbers Made Simple | 氧化数轻松学

Oxidation number (or oxidation state) is a bookkeeping method to count electrons in a compound or ion. You must know the rules for IGCSE Edexcel:

氧化数（或氧化态）是一种记录化合物或离子中电子的方法。你需要掌握 IGCSE Edexcel 要求的规则：

The oxidation number of any uncombined element is 0 (e.g., Na, O₂, Cl₂). 单质的氧化数为 0。
For a simple ion, the oxidation number equals the charge on the ion (e.g., Na⁺ is +1, Cl⁻ is –1). 对于简单离子，氧化数等于离子所带电荷。
Oxygen usually has oxidation number –2 in compounds (except in peroxides like H₂O₂ where it is –1). 氧在化合物中通常为 –2（过氧化物如 H₂O₂ 中为 –1）。
Hydrogen usually has oxidation number +1 in compounds (except in metal hydrides where it is –1). 氢在化合物中通常为 +1（金属氢化物中为 –1）。
The sum of oxidation numbers in a neutral compound is 0; in a polyatomic ion, the sum equals the charge on the ion. 中性化合物中所有原子氧化数之和为 0；多原子离子中等于离子所带电荷。

Use these rules to work out an unknown oxidation number. For example, in SO₄²⁻, total charge = –2. Oxygen is –2 × 4 = –8, so S + (–8) = –2 ⇒ S = +6. Oxidation numbers help you recognise redox: if the oxidation number of an element increases, it is oxidised; if it decreases, it is reduced.

利用这些规则可以推算出未知的氧化数。例如，在 SO₄²⁻ 中，总电荷为 –2。氧的氧化数为 –2×4 = –8，所以 S + (–8) = –2 ⇒ S = +6。氧化数可以帮助你识别氧化还原：某元素的氧化数升高，则该元素被氧化；氧化数降低，则该元素被还原。

5. Using Oxidation Numbers to Identify Redox | 用氧化数判断氧化还原

Not all reactions are redox. For a reaction to be redox, there must be a change in oxidation numbers of some elements. If no oxidation numbers change, the reaction is not redox (e.g., precipitation, acid-base neutralisation).

并非所有反应都是氧化还原反应。要成为氧化还原反应，某些元素的氧化数必须发生变化。如果没有氧化数变化，就不是氧化还原反应（例如沉淀反应、酸碱中和）。

Example: HCl + NaOH → NaCl + H₂O. Assign oxidation numbers: H is +1, Cl is –1, Na is +1, O is –2 all remain unchanged. No electron transfer, so this is not a redox reaction. In contrast, 2Na + Cl₂ → 2NaCl shows Na going from 0 to +1 (oxidation) and Cl going from 0 to –1 (reduction) — definitely redox.

例子：HCl + NaOH → NaCl + H₂O。标出氧化数：H 为 +1，Cl 为 –1，Na 为 +1，O 为 –2，均未变化。没有电子转移，所以这不是氧化还原反应。相反，2Na + Cl₂ → 2NaCl 中 Na 从 0 变成 +1（氧化），Cl 从 0 变成 –1（还原）——是氧化还原反应。

This method is crucial for complex reactions like combustion: CH₄ + 2O₂ → CO₂ + 2H₂O. C goes from –4 to +4 (oxidised), O goes from 0 to –2 (reduced).

这种方法对燃烧等复杂反应很关键：CH₄ + 2O₂ → CO₂ + 2H₂O。C 从 –4 变成 +4（被氧化），O 从 0 变成 –2（被还原）。

6. Half Equations: Electron Accounting | 半反应方程式：电子的账本

A half equation shows either oxidation or reduction separately, with the electrons explicitly written. Being able to construct and combine half equations is a key skill for IGCSE.

半反应方程式单独表示氧化或还原过程，并明确写出电子。能够构建并合并半反应方程式是 IGCSE 的关键技能。

Steps to write a half equation: (i) Write the species before and after. (ii) Balance atoms of the element being oxidised/reduced. (iii) Balance oxygen by adding H₂O. (iv) Balance hydrogen by adding H⁺ (in acidic solution). (v) Balance charge by adding electrons. For IGCSE, you often deal with simpler half equations without H₂O and H⁺, but you might encounter them in electrolysis.

书写半反应方程式的步骤：(i) 写出反应前后的物质。(ii) 配平被氧化/被还原元素的原子。(iii) 通过添加 H₂O 配平氧。(iv) 通过添加 H⁺ 配平氢（酸性溶液中）。(v) 通过添加电子配平电荷。在 IGCSE 中，经常遇到的是更简单的半反应方程式，不需要 H₂O 和 H⁺，但在电解中可能会遇到。

Example: Oxidation of magnesium: Mg → Mg²⁺ + 2e⁻. Reduction of chlorine: Cl₂ + 2e⁻ → 2Cl⁻. Combining them: Mg + Cl₂ → Mg²⁺ + 2Cl⁻ → MgCl₂. Ionic equations often come from combining half equations.

例子：镁的氧化：Mg → Mg²⁺ + 2e⁻。氯的还原：Cl₂ + 2e⁻ → 2Cl⁻。合并得到：Mg + Cl₂ → Mg²⁺ + 2Cl⁻ → MgCl₂。离子方程式往往由半反应方程式合并而成。

7. Displacement Reactions as Redox | 置换反应中的氧化还原

Metal displacement reactions are classic redox examples at IGCSE. A more reactive metal displaces a less reactive metal from a solution of its salt. For instance, placing an iron nail in copper(II) sulfate solution: Fe + CuSO₄ → FeSO₄ + Cu. Ionic equation: Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s).

金属置换反应是 IGCSE 中典型的氧化还原实例。较活泼的金属能把较不活泼的金属从其盐溶液中置换出来。例如，将铁钉放入硫酸铜溶液中：Fe + CuSO₄ → FeSO₄ + Cu。离子方程式：Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)。

Identify: Fe is oxidised (0 → +2), Cu²⁺ is reduced (+2 → 0). The blue colour fades because Cu²⁺ ions are reduced and removed from solution, and a brown solid (copper) coats the nail.

判断：Fe 被氧化 (0 → +2)，Cu²⁺ 被还原 (+2 → 0)。溶液蓝色变浅，因为 Cu²⁺ 被还原并从溶液中被移除，铁钉表面覆盖一层棕色固体（铜）。

Halogen displacement is also redox. A more reactive halogen displaces a less reactive halogen from a halide solution: Cl₂ + 2KBr → 2KCl + Br₂. Chlorine is reduced (0 → –1), bromide ions are oxidised (–1 → 0). Colour changes indicate the redox reaction has occurred.

卤素置换也是氧化还原。较活泼的卤素能把较不活泼的卤素从其卤化物溶液中置换出来：Cl₂ + 2KBr → 2KCl + Br₂。氯被还原 (0 → –1)，溴离子被氧化 (–1 → 0)。颜色变化表明发生了氧化还原反应。

8. Redox in Electrolysis | 电解中的氧化还原

Electrolysis is a process that uses a direct electric current to drive a non-spontaneous redox reaction. Oxidation always occurs at the anode (positive electrode), where anions lose electrons. Reduction always occurs at the cathode (negative electrode), where cations gain electrons.

电解是利用直流电驱动非自发的氧化还原反应的过程。氧化总是发生在阳极（正极），阴离子在此失去电子。还原总是发生在阴极（负极），阳离子在此得到电子。

For the electrolysis of molten lead(II) bromide: Cathode: Pb²⁺ + 2e⁻ → Pb (reduction). Anode: 2Br⁻ → Br₂ + 2e⁻ (oxidation). Overall: PbBr₂ → Pb + Br₂. The same redox pattern applies to aqueous electrolysis, where competing reduction/oxidation of water ions may occur.

熔融溴化铅的电解：阴极：Pb²⁺ + 2e⁻ → Pb（还原）。阳极：2Br⁻ → Br₂ + 2e⁻（氧化）。总反应：PbBr₂ → Pb + Br₂。同样的氧化还原模式也适用于水溶液电解，此时可能存在水的电离离子的竞争还原/氧化。

The mnemonic OIL RIG still applies: Anode Oxidation, Cathode Reduction (AN OX, RED CAT). Electrolysis questions frequently ask you to state what is produced at each electrode and write the half equations.

口诀 OIL RIG 仍然适用：阳极氧化，阴极还原 (AN OX, RED CAT)。电解题目经常要求说明每个电极的产物并写出半反应方程式。

9. Testing for Oxidising and Reducing Agents | 氧化剂和还原剂的检验

The IGCSE course expects you to recall simple laboratory tests for oxidising and reducing agents using potassium iodide and potassium manganate(VII).

IGCSE 课程要求你记住使用碘化钾和高锰酸钾检验氧化剂和还原剂的简单实验室方法。

Test for an oxidising agent	Add potassium iodide (KI) solution. If a brown colour of iodine appears, the substance is an oxidising agent. The iodide ions, I⁻, are oxidised to I₂. Equation: 2I⁻ → I₂ + 2e⁻ (colourless to brown).
氧化剂检验	加入碘化钾溶液。若出现碘的棕色，说明该物质是氧化剂。碘离子 I⁻ 被氧化成 I₂。方程式：2I⁻ → I₂ + 2e⁻（无色变棕色）。
Test for a reducing agent	Add acidified potassium manganate(VII) (KMnO₄) solution. If the purple colour fades or disappears, the substance is a reducing agent. MnO₄⁻ is reduced to Mn²⁺ (purple to colourless).
还原剂检验	加入酸化高锰酸钾溶液。若紫色褪去或消失，说明该物质是还原剂。MnO₄⁻ 被还原成 Mn²⁺（紫色变无色）。

These colour changes are direct evidence of electron transfer and are often examined in practical-based questions.

这些颜色变化是电子转移的直接证据，经常在实验类考题中出现。

10. Common Redox Reactions in the Specification | 考纲中的常见氧化还原反应

Make sure you can recognise and explain the following redox reactions that appear frequently in Edexcel IGCSE Chemistry:

务必能识别并解释以下 Edexcel IGCSE 化学中常见的氧化还原反应：

Combustion of fuels: e.g., C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Carbon is oxidised, oxygen is reduced.
燃料燃烧：例如 C₃H₈ + 5O₂ → 3CO₂ + 4H₂O。碳被氧化，氧被还原。
Reactions of metals with acids: Mg + 2HCl → MgCl₂ + H₂. Mg is oxidised, H⁺ is reduced.
金属与酸的反应：Mg + 2HCl → MgCl₂ + H₂。Mg 被氧化，H⁺ 被还原。
Extraction of metals (blast furnace): Fe₂O₃ + 3CO → 2Fe + 3CO₂. Iron(III) oxide is reduced, carbon monoxide is oxidised.
金属的提取（高炉）：Fe₂O₃ + 3CO → 2Fe + 3CO₂。氧化铁被还原，一氧化碳被氧化。
Rusting of iron: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃•xH₂O. Iron is oxidised.
铁的锈蚀：4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃•xH₂O。铁被氧化。

In each case, you should be able to write the overall equation, note the oxidation number changes, and describe what acts as the oxidising and reducing agent.

在每种情况下，你都应该能写出总方程式，标出氧化数变化，并描述什么物质充当氧化剂和还原剂。

11. Common Mistakes and Exam Tips | 常见错误与考试技巧

Confusing oxidation and reduction in terms of oxygen vs electrons: Many students still think oxidation only means gaining oxygen. Remember, the electron definition overrides the old one; if no oxygen is involved, analyse changes in oxidation numbers.
混淆氧定义和电子定义：许多学生仍认为氧化只是得到氧。记住，电子定义优于旧定义；如果没有氧参与，就分析氧化数的变化。

Forgetting that oxidation and reduction occur simultaneously: Whenever you identify oxidation, there must also be reduction. Label both.
忘记氧化和还原同时发生：每当你指出氧化时，必然同时存在还原。两者都要标出。

Incorrectly assigning oxidation numbers: Double-check the rules, especially for H and O. Never forget that H is +1 with non-metals and –1 with metals (hydrides).
氧化数标错：仔细核对规则，特别是 H 和 O。切勿忘记 H 与非金属结合为 +1，与金属（氢化物）结合为 –1。

Balancing half equations without checking charge: After adding electrons, the total charge on both sides of the arrow must be equal. Always verify.
配平半反应方程式时未检查电荷：添加电子后，箭头两边的总电荷必须相等。一定要验证。

Electrolysis confusion: Practise stating which electrode attracts which type of ion and writing the correct half reactions. Remember cation → cathode (negative electrode), anion → anode (positive electrode).
电解混淆：练习说出哪个电极吸引哪种离子，并写出正确的半反应。记住阳离子 → 阴极（负极），阴离子 → 阳极（正极）。

Practice with past paper questions, especially those asking to ‘explain in terms of electrons’ or ‘using oxidation numbers’. Precision in language is key.

用真题练习，特别是那些要求“用电子解释”或“用氧化数解释”的题目。准确用词是关键。

12. Summary: The Redox Checklist | 总结：氧化还原检查清单

Below is a quick checklist of what you must be able to do for the IGCSE Edexcel exam:

下面是一份快速检查清单，列出了你为 IGCSE Edexcel 考试必须掌握的内容：

Define oxidation and reduction in terms of both oxygen and electrons. 从氧和电子两方面定义氧化和还原。
Apply OIL RIG to any reaction. 对任何反应运用 OIL RIG 口诀。
Identify oxidising and reducing agents. 识别氧化剂和还原剂。
Calculate and interpret oxidation numbers to decide if a reaction is redox. 计算并解释氧化数以判断反应是否为氧化还原反应。
Construct balanced half equations and combine them into full ionic equations. 构建配平的半反应方程式并合并成完整离子方程式。
Explain displacement and electrolysis as redox processes with half equations. 用半反应方程式解释置换和电解为氧化还原过程。
Describe the colour changes in tests for oxidising/reducing agents and write the relevant half equations. 描述氧化剂/还原剂检验中的颜色变化并写出相关半反应方程式。

Keep this guide handy as you revise, and test yourself by explaining each concept aloud. Redox is a unifying theme that will appear across topics — from reactivity series to electrochemistry. Master it, and you gain a powerful tool for tackling many types of questions.

复习时把这份指南放在手边，并尝试大声解释每个概念来自我检测。氧化还原是一个贯穿各主题的统一主题——从活动性顺序到电化学。掌握它，你就拥有了应对多种题型的有力工具。

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Redox Reactions | IGCSE Edexcel Chemistry: Oxidation & Reduction Essential Revision | 氧化还原 考点精讲