Chemical Reactions: IGCSE Edexcel Chemistry Revision Guide | 化学反应考点精讲

📚 Chemical Reactions: IGCSE Edexcel Chemistry Revision Guide | 化学反应考点精讲

Chemical reactions are the heart of chemistry. From the rusting of iron to the combustion of fuels, understanding how substances interact and transform is essential for IGCSE Edexcel Chemistry. This guide systematically breaks down every key concept – including equation writing, reaction types, energy changes, rates, and equilibrium – so you can approach exams with confidence.

化学反应是化学的核心。从铁的生锈到燃料的燃烧，理解物质如何相互作用和转化是 IGCSE Edexcel 化学的关键。本指南系统梳理每一个核心概念——包括方程式的书写、反应类型、能量变化、速率和平衡，帮助你自信应对考试。

1. What is a Chemical Reaction? | 什么是化学反应？

A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with different properties. Bonds between atoms are broken and new bonds are formed. Signs of a chemical reaction include colour change, temperature change, formation of a gas (effervescence), appearance of a precipitate, and sometimes a change in smell.

化学反应是将一种或多种物质（反应物）转化为一种或多种具有不同性质的新物质（产物）的过程。原子间的化学键断裂并形成新的化学键。化学反应常见的迹象有颜色变化、温度变化、产生气体（冒泡）、沉淀生成，有时还有气味变化。

Importantly, a chemical reaction is not just a change of state; a new substance must be produced. For example, melting ice is not a chemical reaction because the water molecules remain unchanged. However, electrolysis of water to produce hydrogen and oxygen is a chemical reaction.

重要的是，化学反应不仅仅是状态变化，必须产生新物质。例如，冰融化不是化学反应，因为水分子没有改变。然而，电解水生成氢气和氧气则是化学反应。

2. Physical vs Chemical Changes | 物理变化与化学变化的区别

A physical change alters the form or appearance of a substance but not its chemical composition. Examples include melting, boiling, dissolving, and cutting. These changes are usually reversible using physical methods. In contrast, a chemical change produces at least one new substance and involves energy transfer. The products have different chemical properties from the reactants. Many chemical changes are difficult or impossible to reverse simply.

物理变化改变物质的形式或外观，但不改变其化学组成。例子包括熔化、沸腾、溶解、切割。这些变化通常可以通过物理方法逆转。相反，化学变化至少生成一种新物质，并涉及能量转移。产物具有与反应物不同的化学性质。许多化学变化很难或不可能简单地逆转。

Key distinction for exams: mass is conserved in both changes, but only chemical changes alter the arrangement of atoms to form new substances.

考试关键区分：两种变化中质量都守恒，但只有化学变化通过重新排列原子形成新物质。

3. Word Equations and Chemical Equations | 文字方程式与化学方程式

A word equation summarises a reaction using the names of reactants and products, separated by an arrow. For example: sodium + water → sodium hydroxide + hydrogen. It shows what reacts and what is produced but does not reveal the number of atoms involved or the chemical formulas.

文字方程式用反应物和产物的名称总结反应，用箭头分隔。例如：钠 + 水 → 氢氧化钠 + 氢气。它显示了什么物质参与反应以及生成什么，但不揭示参与原子的数目或化学式。

A chemical equation uses symbols and formulas to represent the reaction. State symbols are added in brackets: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution (dissolved in water). E.g.: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). Learning to write correct state symbols is crucial for IGCSE Edexcel.

化学方程式使用符号和化学式表示反应，并加上状态符号： (s) 表示固体，(l) 表示液体，(g) 表示气体，(aq) 表示水溶液（溶于水）。例如：2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)。学会正确书写状态符号对 IGCSE Edexcel 至关重要。

4. Balancing Chemical Equations | 化学方程式的配平

Atoms cannot be created or destroyed in a chemical reaction. Therefore, a balanced equation must have the same number of each type of atom on both sides. Balancing is done by placing large numbers (coefficients) in front of formulas – never by changing the subscript in a formula.

化学反应中原子不能被创造或消灭，因此配平后的方程式两边每种原子的数目必须相同。配平通过在各化学式前放置大数字（系数）来实现——绝不能通过改变化学式中的下标来配平。

Example: H₂ + O₂ → H₂O. Count atoms: left has 2 H, 2 O; right has 2 H, 1 O. To balance O, put coefficient 2 in front of H₂O: H₂ + O₂ → 2H₂O. Now left has 2 H, 2 O; right has 4 H, 2 O. Balance H by placing 2 in front of H₂: 2H₂ + O₂ → 2H₂O. Always check the final atom counts.

例如：H₂ + O₂ → H₂O。数一数原子：左边有2个H、2个O；右边有2个H、1个O。为配平O，在H₂O前加系数2：H₂ + O₂ → 2H₂O。此时左边2H、2O；右边4H、2O。再配平H，在H₂前加2：2H₂ + O₂ → 2H₂O。最后务必检查原子数目。

5. Types of Chemical Reactions – Overview | 化学反应类型概览

IGCSE Edexcel Chemistry classifies reactions into several major types: combination (synthesis), decomposition, displacement, combustion, and redox (oxidation-reduction). Some reactions fit more than one category. Understanding these patterns helps predict products and write equations efficiently.

IGCSE Edexcel 化学将反应分为几大类型：化合（合成）、分解、置换、燃烧和氧化还原。有些反应属于不止一个类别。理解这些模式有助于预测产物并高效书写方程式。

Combination: A + B → AB

化合反应：A + B → AB
Decomposition: AB → A + B

分解反应：AB → A + B
Displacement: A + BC → AC + B

置换反应：A + BC → AC + B
Combustion: fuel + O₂ → CO₂ + H₂O (for hydrocarbons)

燃烧反应：燃料 + O₂ → CO₂ + H₂O（针对碳氢化合物）
Redox: one species is oxidised (loses electrons), another is reduced (gains electrons)

氧化还原：一种物质被氧化（失去电子），另一种被还原（得到电子）

6. Combination and Decomposition Reactions | 化合反应与分解反应

In a combination reaction, two or more substances combine to form a single product. A classic example is the formation of magnesium oxide: 2Mg(s) + O₂(g) → 2MgO(s). Another is the synthesis of ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). These reactions are often exothermic.

化合反应中，两种或多种物质结合生成单一产物。经典例子是氧化镁的生成：2Mg(s) + O₂(g) → 2MgO(s)。另一个是氨的合成：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)。这类反应通常放热。

Decomposition is the breakdown of a compound into simpler substances, typically requiring heat, light, or electricity. Thermal decomposition of calcium carbonate: CaCO₃(s) → CaO(s) + CO₂(g). Another common IGCSE example is the decomposition of hydrogen peroxide: 2H₂O₂(aq) → 2H₂O(l) + O₂(g), often catalysed by manganese(IV) oxide.

分解反应是化合物分解成更简单物质的过程，通常需要热、光或电的作用。碳酸钙的热分解：CaCO₃(s) → CaO(s) + CO₂(g)。另一个常见的 IGCSE 例子是过氧化氢的分解：2H₂O₂(aq) → 2H₂O(l) + O₂(g)，常以二氧化锰作催化剂。

7. Displacement Reactions and the Reactivity Series | 置换反应与金属活动性顺序

A displacement reaction occurs when a more reactive element pushes out a less reactive element from its compound. For metals, a more reactive metal displaces a less reactive metal from a solution of its salt. The reactivity series must be memorised: potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), aluminium (Al), zinc (Zn), iron (Fe), copper (Cu), silver (Ag), gold (Au).

置换反应发生在较活泼的元素从化合物中取代出较不活泼的元素时。对于金属，较活泼的金属能从较不活泼金属的盐溶液中将其置换出来。必须记住金属活动性顺序：钾(K)、钠(Na)、钙(Ca)、镁(Mg)、铝(Al)、锌(Zn)、铁(Fe)、铜(Cu)、银(Ag)、金(Au)。

Example: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Iron is above copper in the series, so displacement occurs. The blue colour of copper(II) sulfate solution fades, and a brown copper coating forms. A reactivity series also allows prediction of reactions with water and acids – metals above hydrogen can displace hydrogen from acids.

例子：Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)。铁在活动性顺序中位于铜之上，因此发生置换。蓝色硫酸铜溶液颜色变浅，生成棕色铜镀层。利用活动性顺序还能预测金属与水、酸的反应——位于氢之上的金属能从酸中置换出氢气。

8. Combustion and Oxidation | 燃烧与氧化反应

Combustion is the rapid reaction of a fuel with oxygen, releasing heat and light. Complete combustion of hydrocarbons produces carbon dioxide and water: e.g., CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). Incomplete combustion, due to limited oxygen, forms carbon monoxide (CO) or carbon (soot), which is hazardous.

燃烧是燃料与氧气发生的快速反应，放出热和光。碳氢化合物的完全燃烧生成二氧化碳和水：例如 CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)。氧气不足时发生不完全燃烧，生成有毒的一氧化碳（CO）或碳（烟黑），非常危险。

Oxidation originally meant adding oxygen, but in IGCSE Chemistry it is defined as the loss of electrons. Reduction is the gain of electrons. A redox reaction involves both processes simultaneously. For example, in the reaction 2Mg(s) + O₂(g) → 2MgO(s), magnesium is oxidised (Mg → Mg²⁺ + 2e⁻) and oxygen is reduced (O₂ + 4e⁻ → 2O²⁻). Use of half equations and recognising oxidising and reducing agents is expected.

氧化最初指与氧结合，但在 IGCSE 化学中被定义为电子失去。还原则是电子获得。氧化还原反应同时涉及这两个过程。例如反应 2Mg(s) + O₂(g) → 2MgO(s) 中，镁被氧化（Mg → Mg²⁺ + 2e⁻），氧气被还原（O₂ + 4e⁻ → 2O²⁻）。要求会写半反应式并识别氧化剂和还原剂。

9. Exothermic and Endothermic Reactions | 放热反应与吸热反应

Chemical reactions involve energy transfer between the system and the surroundings. In an exothermic reaction, energy is released to the surroundings, causing a temperature rise. Typical examples: combustion, neutralisation, and the reaction of acids with metals. In an endothermic reaction, energy is absorbed from the surroundings, so the temperature drops. Examples: thermal decompositions, photosynthesis, and dissolving some salts like ammonium nitrate in water.

化学反应涉及体系与环境的能量转移。放热反应中，能量释放到环境中，导致温度升高。典型例子包括燃烧、中和反应以及酸与金属的反应。吸热反应从环境吸收能量，因此温度下降。例子有热分解、光合作用以及某些盐如硝酸铵溶于水。

IGCSE Edexcel requires interpretation of reaction profile diagrams (energy level diagrams). For an exothermic reaction, the products have lower energy than reactants; the energy change ΔH is negative. For endothermic, products have higher energy; ΔH is positive. The activation energy (Eₐ) is the minimum energy needed for a reaction to start.

IGCSE Edexcel 要求能够解读反应进程图（能级图）。放热反应中，产物能量低于反应物，能量变化 ΔH 为负。吸热反应中，产物能量高于反应物，ΔH 为正。活化能 (Eₐ) 是引发反应所需的最低能量。

10. Rate of Reaction – Key Factors | 反应速率的影响因素

The rate of a chemical reaction measures how quickly reactants are used up or products are formed. According to collision theory, for a reaction to occur, particles must collide with sufficient energy (greater than or equal to the activation energy) and with the correct orientation.

化学反应速率衡量反应物被消耗或产物生成的速度。根据碰撞理论，反应发生需要粒子以足够的能量（大于或等于活化能）和正确的取向发生碰撞。

Factors affecting rate:

Concentration: higher concentration increases the number of particles per unit volume, leading to more frequent collisions.

浓度：浓度越高，单位体积内粒子数越多，碰撞频率增加。
Temperature: higher temperature increases particle kinetic energy, so collisions are more frequent and more particles have energy ≥ Eₐ.

温度：温度升高，粒子动能增大，碰撞更频繁且更多粒子具有 ≥ Eₐ 的能量。
Surface area: smaller pieces of solid expose more area for collisions, increasing rate.

表面积：固体颗粒越小，暴露出更多碰撞面积，速率提高。
Catalyst: a catalyst provides an alternative reaction pathway with lower activation energy, increasing rate without being used up.

催化剂：催化剂提供活化能较低的替代反应路径，加快反应而自身不消耗。

Practical investigations include measuring gas volume produced or mass loss over time, often using marble chips and hydrochloric acid (CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂) or the ‘disappearing cross’ method with sodium thiosulfate and hydrochloric acid.

实验探究包括测量随时间变化的气体体积或质量损失，常用大理石碎片与盐酸反应 (CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂) 或硫代硫酸钠与盐酸的“十字消失”法。

11. Reversible Reactions and Equilibrium | 可逆反应与平衡

In a reversible reaction, products can react together to re-form the original reactants. The arrow ⇌ is used. A classic IGCSE example is the thermal decomposition of hydrated copper(II) sulfate: CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g). Heating drives the forward reaction (blue to white), adding water reverses it (white to blue).

可逆反应中,产物可以再反应重新生成原来的反应物，使用 ⇌ 箭头。经典的 IGCSE 例子是硫酸铜晶体的热分解：CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)。加热促进正反应（蓝色变白色），加水则逆反应（白色变蓝色）。

Dynamic equilibrium is reached in a closed system when the forward and reverse reactions occur at the same rate. At equilibrium, concentrations of reactants and products remain constant, but reactions are still proceeding. The position of equilibrium tells us whether reactants or products are favoured. Changing conditions (temperature, pressure, concentration) shifts the equilibrium according to Le Chatelier’s principle.

动态平衡是在封闭体系中，正逆反应速率相等时达到的状态。平衡时，反应物和产物的浓度保持恒定，但反应仍在进行。平衡位置告诉我们反应物或产物哪一方占优势。改变条件（温度、压力、浓度）会根据勒夏特列原理导致平衡移动。

12. Conservation of Mass | 质量守恒定律

The law of conservation of mass states that the total mass of reactants equals the total mass of products in a chemical reaction. This is because atoms are not created or destroyed; they are only rearranged. A balanced equation reflects this law. In an open system that produces a gas, mass may appear to decrease; but measuring the total mass in a sealed container confirms conservation.

质量守恒定律指出，化学反应中反应物的总质量等于生成物的总质量。因为原子既不被创造也不被消灭，只是重新排列。配平的方程式体现了这一定律。在生成气体的开放体系中，质量似乎减少；但在封闭容器中测量总质量就能验证守恒。

Example: When 5.3 g of sodium carbonate reacts with excess hydrochloric acid, the expected mass loss due to CO₂ gas should equate to the theoretical yield. Students must be able to calculate masses of products using balanced equations and relative atomic/formula masses (stoichiometry). This links directly to the mole concept and quantitative chemistry, a vital skill for IGCSE Edexcel Paper 1 and Paper 2.

例子：5.3 g 碳酸钠与过量盐酸反应时，因 CO₂ 气体造成的质量损失应等于理论产量。学生必须能利用配平方程式和相对原子质量/式量计算产物的质量（化学计量学）。这与摩尔概念和定量化学直接相关，是 IGCSE Edexcel 卷一卷二的重要技能。

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Chemical Reactions: IGCSE Edexcel Chemistry Revision Guide | 化学反应考点精讲