IGCSE CCEA Chemistry: Mastering Redox Reactions | IGCSE CCEA 化学：氧化还原 考点精讲

📚 IGCSE CCEA Chemistry: Mastering Redox Reactions | IGCSE CCEA 化学：氧化还原 考点精讲

Redox reactions are at the heart of chemistry, connecting topics from acids and bases to electrochemistry. In the IGCSE CCEA Chemistry specification, understanding oxidation and reduction in terms of electron transfer and oxidation numbers is essential for explaining a wide range of chemical processes, including displacement reactions, electrolysis, and corrosion.

氧化还原反应是化学的核心，它将酸碱反应与电化学等主题串联起来。在 IGCSE CCEA 化学大纲中，从电子转移和氧化数变化的角度理解氧化与还原，对于解释置换反应、电解和腐蚀等多种化学过程至关重要。

1. Oxidation and Reduction: Definitions | 氧化与还原的定义

Oxidation is the loss of electrons by a substance during a chemical reaction. Reduction is the gain of electrons by a substance. These definitions are based on electron transfer and are fundamental to all redox chemistry.

氧化是指物质在化学反应中失去电子。还原是指物质获得电子。这些定义基于电子转移，是所有氧化还原化学的基础。

A useful mnemonic is ‘OIL RIG’: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

一个有用的记忆口诀是 ‘OIL RIG’：Oxidation Is Loss（氧化是失去），Reduction Is Gain（还原是获得）。

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2. Oxidation Numbers: The Basics | 氧化数的基本概念

Oxidation number (or oxidation state) is the charge an atom would have if all bonds were completely ionic. It is a book-keeping tool that helps us track electron shifts in covalent compounds as well as ionic compounds.

氧化数（或氧化态）是指假设所有化学键都是离子键时，原子所带的电荷。它是一种记录工具，帮助我们在共价化合物和离子化合物中追踪电子的偏移。

An increase in oxidation number indicates oxidation has occurred, while a decrease indicates reduction. For example, in the reaction 2Mg + O₂ → 2MgO, magnesium’s oxidation number increases from 0 to +2 (oxidation) and oxygen’s decreases from 0 to −2 (reduction).

氧化数升高表示发生了氧化，氧化数降低表示发生了还原。例如，在反应 2Mg + O₂ → 2MgO 中，镁的氧化数从 0 升高到 +2（氧化），氧的氧化数从 0 降低到 −2（还原）。

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

To identify redox reactions and calculate oxidation numbers, follow these IGCSE-level rules:

为了识别氧化还原反应并计算氧化数，请遵循以下 IGCSE 层次的规则：

• Elements in their standard state have an oxidation number of 0 (e.g. Cl₂, Na, O₂).
• 元素单质的氧化数为 0（例如 Cl₂, Na, O₂）。
• The oxidation number of a simple monatomic ion equals its charge (e.g. Na⁺ = +1, Cl⁻ = −1).
• 简单单原子离子的氧化数等于其所带电荷（例如 Na⁺ = +1, Cl⁻ = −1）。
• Oxygen usually has an oxidation number of −2 (except in peroxides where it is −1, e.g. H₂O₂, and in OF₂ where it is +2).
• 氧的氧化数通常为 −2（过氧化物中为 −1，例如 H₂O₂；在 OF₂ 中为 +2）。
• Hydrogen usually has an oxidation number of +1 (except in metal hydrides like NaH, where it is −1).
• 氢的氧化数通常为 +1（在金属氢化物如 NaH 中为 −1）。
• The sum of oxidation numbers in a neutral compound is 0; in a polyatomic ion, it equals the ion’s charge.
• 中性化合物中氧化数的总和为 0；在多原子离子中，总和等于离子所带电荷。

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4. Identifying Redox Reactions | 识别氧化还原反应

A reaction is a redox reaction if there is a change in the oxidation number of any element. Reactions like acid-base neutralisation, where no oxidation number changes occur, are not redox reactions.

如果任何元素的氧化数发生变化，则该反应就是氧化还原反应。酸碱中和反应等没有氧化数变化的反应不属于氧化还原反应。

Example: CuO + H₂ → Cu + H₂O. Copper’s oxidation number changes from +2 to 0 (reduction), and hydrogen’s changes from 0 to +1 (oxidation). Therefore, this is a redox reaction.

例如：CuO + H₂ → Cu + H₂O。铜的氧化数从 +2 变为 0（还原），氢的氧化数从 0 变为 +1（氧化）。因此，这是一个氧化还原反应。

Even combustion of fuels, rusting of iron, and respiration are all redox processes. Recognising them relies on oxidation number tracking, not just oxygen gain or loss.

甚至连燃料的燃烧、铁的生锈以及呼吸作用都是氧化还原过程。识别它们的依据是追踪氧化数的变化，而不仅仅是得氧或失氧。

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

An oxidising agent (oxidant) is the substance that accepts electrons and gets reduced. A reducing agent (reductant) is the substance that donates electrons and gets oxidised.

氧化剂是接受电子、自身被还原的物质。还原剂是提供电子、自身被氧化的物质。

In the reaction Zn + CuSO₄ → ZnSO₄ + Cu: zinc is the reducing agent (Zn → Zn²⁺ + 2e⁻), and copper(II) ions are the oxidising agent (Cu²⁺ + 2e⁻ → Cu).

在反应 Zn + CuSO₄ → ZnSO₄ + Cu 中：锌是还原剂（Zn → Zn²⁺ + 2e⁻），铜离子是氧化剂（Cu²⁺ + 2e⁻ → Cu）。

Common oxidising agents include oxygen, chlorine, hydrogen peroxide, and acidified potassium manganate(VII). Common reducing agents include carbon, hydrogen, and reactive metals like potassium or magnesium.

常见的氧化剂包括氧气、氯气、过氧化氢和酸化高锰酸钾。常见的还原剂包括碳、氢气和活泼金属如钾或镁。

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6. Half Equations: Electron Transfer | 半反应：电子转移

Redox reactions can be split into two half equations: one showing oxidation (electron loss) and the other showing reduction (electron gain). Adding the two half equations together, after balancing electrons, yields the overall ionic equation.

氧化还原反应可以拆分成两个半反应：一个表示氧化（失电子），另一个表示还原（得电子）。将配平电子后的两个半反应相加，即可得到完整的离子方程式。

For the reaction Mg + Cl₂ → MgCl₂:

对于反应 Mg + Cl₂ → MgCl₂：

• Oxidation half equation: Mg → Mg²⁺ + 2e⁻
• 氧化半反应：Mg → Mg²⁺ + 2e⁻
• Reduction half equation: Cl₂ + 2e⁻ → 2Cl⁻
• 还原半反应：Cl₂ + 2e⁻ → 2Cl⁻

Always ensure that the number of electrons lost equals the number gained before combining half equations.

在合并半反应之前，务必确保失去的电子数与获得的电子数相等。

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7. Balancing Redox Reactions | 氧化还原反应的配平

For acidified redox reactions (often tested at IGCSE level with manganate(VII) or dichromate(VI)), follow these steps: write the half equations for oxidation and reduction, balance all atoms except O and H, balance O atoms by adding H₂O, balance H atoms by adding H⁺, and balance the charge by adding electrons. Then multiply each half equation so electrons are equal and add them together.

对于酸性条件下的氧化还原反应（IGCSE 常考高锰酸根或重铬酸根），可按照以下步骤配平：写出氧化和还原的半反应，配平除 O 和 H 以外的所有原子，通过添加 H₂O 配平 O 原子，通过添加 H⁺ 配平 H 原子，通过添加电子配平电荷。然后将两个半反应乘以适当系数使电子数相等，再相加。

Example: Fe²⁺ reacting with acidified MnO₄⁻:

例子：Fe²⁺ 与酸化 MnO₄⁻ 反应：

• Oxidation: Fe²⁺ → Fe³⁺ + e⁻
• 氧化：Fe²⁺ → Fe³⁺ + e⁻
• Reduction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
• 还原：MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Multiplying the oxidation by 5 and adding gives: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺. This method is widely used in titrations.

将氧化半反应乘以 5 再相加，得到：MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺。这种方法广泛用于滴定分析。

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8. Redox in Metal Displacement Reactions | 金属置换反应中的氧化还原

A more reactive metal can displace a less reactive metal from a solution of its salt. This is a classic redox reaction where the more reactive metal loses electrons (oxidation) and the less reactive metal ions gain electrons (reduction).

更活泼的金属可以将较不活泼的金属从其盐溶液中置换出来。这是一个典型的氧化还原反应，其中更活泼的金属失去电子（氧化），较不活泼的金属离子获得电子（还原）。

Example: When an iron nail is placed in blue copper(II) sulfate solution, the nail becomes coated with reddish-brown copper and the blue colour fades. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Iron is oxidised: Fe → Fe²⁺ + 2e⁻; copper(II) ions are reduced: Cu²⁺ + 2e⁻ → Cu.

例如：将铁钉放入蓝色的硫酸铜溶液中，铁钉表面会覆盖一层红棕色铜，蓝色逐渐褪去。Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)。铁被氧化：Fe → Fe²⁺ + 2e⁻；铜离子被还原：Cu²⁺ + 2e⁻ → Cu。

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9. Redox in Electrolysis | 电解中的氧化还原

Electrolysis is the process of driving a non-spontaneous redox reaction using direct current. Oxidation occurs at the anode (positive electrode) where anions lose electrons, and reduction occurs at the cathode (negative electrode) where cations gain electrons.

电解是利用直流电驱动非自发氧化还原反应的过程。氧化发生在阳极（正极），阴离子在此失去电子；还原发生在阴极（负极），阳离子在此获得电子。

In the electrolysis of molten lead(II) bromide: at the cathode, Pb²⁺ + 2e⁻ → Pb (reduction); at the anode, 2Br⁻ → Br₂ + 2e⁻ (oxidation). This clearly illustrates the separation of oxidation and reduction in space.

在电解熔融溴化铅时：阴极，Pb²⁺ + 2e⁻ → Pb（还原）；阳极，2Br⁻ → Br₂ + 2e⁻（氧化）。这清楚地展示了氧化与还原在空间上的分离。

When aqueous solutions are electrolysed, the products depend on the reactivity of the metal cation and the concentration of the anion, but the underlying principle remains redox.

电解水溶液时，产物取决于金属阳离子的活泼性和阴离子的浓度，但其基本原理仍然是氧化还原。

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10. Corrosion and Rusting as Redox | 腐蚀与生锈的氧化还原

Rusting of iron is a spectacularly slow redox reaction that requires both oxygen and water. Iron is oxidised to iron(II) ions, which further oxidise to form hydrated iron(III) oxide (rust). Oxygen is reduced to hydroxide ions or water.

铁的生锈是一个极其缓慢的氧化还原反应，需要氧气和水同时存在。铁被氧化成亚铁离子，然后进一步氧化形成水合氧化铁（铁锈）。氧气被还原成氢氧根离子或水。

The simplified half reactions:

简化的半反应：

• Oxidation: Fe → Fe²⁺ + 2e⁻ then Fe²⁺ → Fe³⁺ + e⁻
• 氧化：Fe → Fe²⁺ + 2e⁻，然后 Fe²⁺ → Fe³⁺ + e⁻
• Reduction: O₂ + 2H₂O + 4e⁻ → 4OH⁻
• 还原：O₂ + 2H₂O + 4e⁻ → 4OH⁻

Methods of rust prevention (painting, oiling, galvanising, or sacrificial protection) work by blocking oxygen and water or by providing a more reactive metal to oxidise preferentially.

防锈方法（涂漆、涂油、镀锌或牺牲保护）通过隔绝氧气和水，或者提供更活泼的金属优先被氧化来起作用。

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Published by TutorHao | Chemistry Revision Series | aleveler.com

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