IGCSE WJEC Chemistry: Key Points on Chemical Reactions | IGCSE WJEC 化学：化学反应考点精讲

📚 IGCSE WJEC Chemistry: Key Points on Chemical Reactions | IGCSE WJEC 化学：化学反应考点精讲

Chemical reactions are at the heart of WJEC IGCSE Chemistry. Understanding how substances change, how to represent those changes with equations, and how energy and speed are involved forms the basis for tackling both theory papers and practical assessments. This revision guide distils the core knowledge you need for the Chemical Reactions topic, with clear explanations matched to the WJEC specification.

化学反应是 WJEC IGCSE 化学的核心。理解物质如何变化、如何用方程式表示这些变化、以及能量和速率如何参与其中，是应对理论考试和实践评估的基础。这份复习指南凝练了化学反应专题所需的核心知识，提供与 WJEC 大纲匹配的清晰讲解。

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. Chemical bonds are broken in the reactants and new bonds are formed in the products.

化学反应是一种过程，其中一种或多种物质（反应物）转化为一种或多种具有不同性质的新物质（生成物）。反应物中的化学键被打破，生成物中形成新的化学键。

Key evidence that a chemical reaction has taken place includes: a permanent colour change, a change in temperature, the formation of a gas (effervescence), or the formation of an insoluble solid (a precipitate) when two solutions are mixed.

化学反应发生的关键证据包括：永久的颜色变化、温度变化、气体生成（气泡逸出），或两种溶液混合时生成不溶性固体（沉淀）。

• Physical change (e.g. melting ice) does not produce new substances and is easily reversible. Chemical change is usually difficult to reverse.
• 物理变化（如冰融化）不会产生新物质，且容易逆转。化学变化通常难以逆转。

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2. Writing Chemical Equations | 书写化学方程式

Word equations use the names of reactants and products. A formula equation uses chemical symbols and shows the ratio of atoms, ions or molecules involved. The arrow (→) separates reactants from products.

文字方程式使用反应物和生成物的名称。化学方程式使用化学符号，表示所涉及原子、离子或分子的比例。箭头（→）将反应物与生成物分开。

In a balanced symbol equation, the number of atoms of each element is the same on both sides of the arrow, respecting the law of conservation of mass. Never change a formula to balance an equation – only add large numbers in front (coefficients).

在配平的符号方程式中，箭头两边每种元素的原子数目相等，遵从质量守恒定律。切勿通过改变化学式来配平方程式——只可在前面添加大数字（系数）。

Example: 2Mg + O₂ → 2MgO means two magnesium atoms react with one oxygen molecule to produce two formula units of magnesium oxide.

例子：2Mg + O₂ → 2MgO 表示两个镁原子与一个氧分子反应生成两个氧化镁单元。

• Unbalanced: Na + Cl₂ → NaCl
• 未配平：Na + Cl₂ → NaCl
• Balanced: 2Na + Cl₂ → 2NaCl
• 配平后：2Na + Cl₂ → 2NaCl

Ionic equations show only the particles that actually change during a reaction. Spectator ions (unchanged in oxidation state and phase) are omitted.

离子方程式只表示反应中实际发生变化的粒子。旁观离子（氧化态和物态未变的离子）被省略。

Example: The reaction of hydrochloric acid with sodium hydroxide: H⁺(aq) + OH⁻(aq) → H₂O(l). The Na⁺ and Cl⁻ are spectator ions.

例子：盐酸与氢氧化钠反应：H⁺(aq) + OH⁻(aq) → H₂O(l)。Na⁺ 和 Cl⁻ 是旁观离子。

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3. State Symbols in Equations | 方程式中的状态符号

WJEC expects you to use and interpret state symbols in chemical equations. These appear in parentheses after each formula: (s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution (dissolved in water).

WJEC 要求你在化学方程式中使用和解释状态符号。这些符号出现在每个化学式后面的括号中：(s) 表示固体，(l) 表示液体，(g) 表示气体，(aq) 表示水溶液（溶于水）。

The state symbol (l) is used only for pure liquids, such as H₂O(l), Br₂(l) or a molten salt. A substance dissolved in water must be labelled (aq).

状态符号 (l) 只用于纯液体，例如 H₂O(l)、Br₂(l) 或熔融盐。溶于水的物质必须标注 (aq)。

2H₂(g) + O₂(g) → 2H₂O(l)

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)

Correct use of state symbols can earn marks even if the formulas are given in the question. Notice how they help distinguish a precipitate (s) from soluble ions (aq).

正确使用状态符号可以得分，即使题目提供了化学式。注意它们如何帮助区分沉淀 (s) 和可溶性离子 (aq)。

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4. Types of Chemical Reactions | 化学反应类型

WJEC candidates must recognise several important reaction types, describe their features, and predict products. The main types are synthesis, decomposition, displacement, combustion, neutralisation, and redox.

WJEC 考生必须识别几种重要的反应类型，描述其特点，并预测生成物。主要类型包括化合反应、分解反应、置换反应、燃烧反应、中和反应和氧化还原反应。

Synthesis (combination): two or more simple substances combine to form a single, more complex product. A + B → AB. Example: 2Fe + 3Cl₂ → 2FeCl₃.

化合反应：两种或多种简单物质结合形成一种更复杂的生成物。A + B → AB。例子：2Fe + 3Cl₂ → 2FeCl₃。

Decomposition: a single compound breaks down into two or more simpler substances, often with heating. AB → A + B. Example: CaCO₃(s) →(heat) CaO(s) + CO₂(g).

分解反应：一种化合物分解成两种或多种更简单的物质，通常需要加热。AB → A + B。例子：CaCO₃(s) →（加热）CaO(s) + CO₂(g)。

Displacement: a more reactive element replaces a less reactive element from its compound. Example: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). Use the reactivity series to predict if displacement occurs.

置换反应：一种更活泼的元素将较不活泼的元素从其化合物中置换出来。例子：Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)。使用金属活动性顺序预测置换是否发生。

Combustion: a substance reacts with oxygen, releasing energy as heat and light. Complete combustion of hydrocarbons produces CO₂ and H₂O: CH₄ + 2O₂ → CO₂ + 2H₂O. Incomplete combustion can produce CO or C (soot).

燃烧反应：物质与氧气反应，以热和光的形式释放能量。烃的完全燃烧生成 CO₂ 和 H₂O：CH₄ + 2O₂ → CO₂ + 2H₂O。不完全燃烧可生成 CO 或 C（炭黑）。

Neutralisation: an acid reacts with a base to produce a salt and water. Acid + base → salt + water. Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). The ionic equation always involves H⁺(aq) + OH⁻(aq) → H₂O(l).

中和反应：酸与碱反应生成盐和水。酸 + 碱 → 盐 + 水。例子：HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)。离子方程式总是包含 H⁺(aq) + OH⁻(aq) → H₂O(l)。

Redox (oxidation–reduction): a reaction involving electron transfer. Oxidation is the loss of electrons; reduction is the gain of electrons (OIL RIG). In terms of oxygen, oxidation is gain of oxygen, reduction is loss of oxygen. Example: 2Mg(s) + O₂(g) → 2MgO(s) – magnesium is oxidised, oxygen is reduced.

氧化还原反应：涉及电子转移的反应。氧化是失去电子；还原是得到电子（OIL RIG）。就氧而言，氧化是加氧，还原是脱氧。例子：2Mg(s) + O₂(g) → 2MgO(s)——镁被氧化，氧被还原。

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5. Energy Changes in Reactions | 反应中的能量变化

Chemical reactions are either exothermic or endothermic. An exothermic reaction transfers thermal energy to the surroundings, causing a temperature rise. Combustion, neutralisation and many oxidation reactions are exothermic.

化学反应要么放热，要么吸热。放热反应将热能传递给周围环境，导致温度升高。燃烧、中和及许多氧化反应都是放热的。

An endothermic reaction takes in thermal energy from the surroundings, causing a temperature drop. Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate are endothermic.

吸热反应从周围环境中吸收热能，导致温度下降。热分解以及柠檬酸与碳酸氢钠的反应是吸热的。

In a reaction profile diagram, the label ΔH shows the enthalpy change. For exothermic reactions, ΔH is negative (products have less energy than reactants). For endothermic reactions, ΔH is positive.

在反应进程图中，标签 ΔH 表示焓变。放热反应的 ΔH 为负值（生成物的能量低于反应物）。吸热反应的 ΔH 为正值。

Reaction type	Energy change	Example
Combustion of methane	Exothermic	CH₄ + 2O₂ → CO₂ + 2H₂O
Neutralisation	Exothermic	HCl + NaOH → NaCl + H₂O
Thermal decomposition of CaCO₃	Endothermic	CaCO₃ → CaO + CO₂

In an exothermic reaction, the energy released when new bonds form is greater than the energy absorbed to break old bonds. In an endothermic reaction, bond breaking absorbs more energy than is released by bond making.

在放热反应中，形成新键释放的能量大于断裂旧键吸收的能量。在吸热反应中，断键吸收的能量多于成键释放的能量。

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6. Rates of Reaction | 反应速率

The rate of a reaction is the speed at which reactants are used up or products are formed. It can be measured by monitoring the change in mass, the volume of gas produced, the formation of a precipitate, or a colour change over time.

反应速率是反应物消耗或生成物形成的速度。可以通过监测质量变化、产生气体的体积、沉淀的形成或颜色随时间的变化来测量。

Five factors affect the rate of a reaction: temperature, concentration (or pressure for gases), surface area of solids, and the presence of a catalyst. Increasing any of the first four increases the frequency of successful collisions between particles with sufficient energy.

影响反应速率的五个因素：温度、浓度（或气体的压强）、固体的表面积，以及催化剂的存在。增加前四个因素中的任何一个都会增加具有足够能量的粒子之间成功碰撞的频率。

For WJEC practical work, you are expected to plot graphs of gas volume or mass against time. The steeper the gradient at any point, the faster the rate. The graph levels off when the limiting reactant is used up.

对于 WJEC 实验要求，你应该会画气体体积或质量随时间变化的图。任意点处的斜率越陡，速率越快。当限制性反应物耗尽时，曲线趋于水平。

• Higher temperature → particles have more kinetic energy → more frequent collisions and a greater proportion have energy ≥ activation energy.
• 温度升高 → 粒子具有更多动能 → 碰撞更频繁，且有更高比例的粒子具有 ≥ 活化能的能量。
• Greater concentration or pressure → more particles per unit volume → more frequent collisions.
• 浓度或压强增大 → 单位体积内粒子数更多 → 碰撞更频繁。
• Larger surface area → more solid particles exposed → greater collision frequency.
• 表面积增大 → 更多固体粒子暴露 → 碰撞频率增加。
• Catalysts provide an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions are successful without being used up themselves.
• 催化剂提供了一条活化能更低的替代反应途径，因此更高比例的碰撞能够成功，而催化剂本身不被消耗。

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

Many chemical reactions are reversible, meaning the products can react together under certain conditions to re-form the original reactants. This is indicated by the symbol ⇌.

许多化学反应是可逆的，意味着在一定条件下生成物可以重新结合成原来的反应物。这用符号 ⇌ 表示。

In a closed system, a reversible reaction can reach dynamic equilibrium. At equilibrium, the forward and reverse reactions occur at exactly the same rate, so the concentrations of reactants and products remain constant (not necessarily equal).

在密闭系统中，可逆反应可以达到动态平衡。平衡时，正反应和逆反应的速率完全相等，因此反应物和生成物的浓度保持不变（不一定相等）。

Le Chatelier’s principle predicts the effect of changing conditions: if a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the change.

勒夏特列原理预测条件改变的影响：如果平衡系统受到扰动，平衡位置会移动以抵消这种变化。

• Increasing temperature favours the endothermic direction.
• 升温有利于吸热方向。
• Increasing pressure (for gas reactions) favours the side with fewer moles of gas.
• 增大压强（对于气体反应）有利于气体摩尔数较少的一侧。
• Adding a catalyst does not change the position of equilibrium; it only increases the rate at which equilibrium is reached.
• 加入催化剂不会改变平衡位置；它只提高达到平衡的速率。

A classic example is the hydration of copper(II) sulfate: CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g). The forward reaction (dehydration) is endothermic; the backward reaction (rehydration) is exothermic.

一个经典例子是硫酸铜的水合：CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)。正向反应（脱水）是吸热的；逆向反应（水合）是放热的。

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8. Collision Theory | 碰撞理论

Collision theory explains why reactions happen and how rate is affected. For a reaction to occur, particles must collide with the correct orientation and with kinetic energy equal to or greater than the activation energy (Eₐ) of the reaction.

碰撞理论解释了反应为什么会发生以及速率如何受到影响。要使反应发生，粒子必须以正确的取向碰撞，并且其动能必须等于或大于该反应的活化能（Eₐ）。

Activation energy is the minimum energy required for a collision to result in a reaction. It can be shown as the energy ‘hump’ on a reaction profile diagram.

活化能是碰撞导致反应所需的最低能量。它可以在反应进程图中显示为能量“山丘”。

Successful collisions are those that lead to product formation. Increasing frequency of successful collisions means a faster rate. This model links all rate-influencing factors to particle behaviour.

有效碰撞是那些导致生成物形成的碰撞。有效碰撞频率增加意味着速率更快。这个模型将所有影响速率的因素与粒子行为联系起来。

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9. Practical Investigation and Graph Skills | 实验探究与图表技能

WJEC practical assessments frequently involve measuring the rate of reaction, such as the reaction between marble chips (CaCO₃) and hydrochloric acid to produce CO₂. The volume of gas collected or the loss of mass of the flask and contents is recorded at regular time intervals.

WJEC 实验评估常常涉及测量反应速率，例如大理石碎片（CaCO₃）与盐酸反应生成 CO₂。定期记录收集到的气体体积或锥形瓶及其内容物的质量损失。

A typical method: place a known mass of marble chips in a conical flask, add a known volume of dilute HCl, and quickly attach a gas syringe or delivery tube to measure gas volume. Record the volume every 20 seconds for 2–3 minutes.

典型方法：将已知质量的大理石碎片放入锥形瓶中，加入已知体积的稀盐酸，迅速连接气体注射器或导气管以测量气体体积。每隔 20 秒记录一次体积，持续 2–3 分钟。

Plot a graph of ‘Volume of CO₂ (cm³)’ on the y-axis against ‘Time (s)’ on the x-axis. Draw a smooth curve of best fit, not dot-to-dot. Calculate the initial rate by drawing a tangent at t=0 and finding its gradient.

绘制以“CO₂ 体积 (cm³)”为纵轴、“时间 (s)”为横轴的图表。画一条最佳拟合平滑曲线，不要逐点连接。通过在 t=0 处画切线并求其斜率来计算初始速率。

When repeating the experiment with crushed marble chips (greater surface area), the curve will be steeper initially and level off at the same final volume. When using more concentrated acid, the final volume of gas may be greater if marble is in excess.

当用碾碎的大理石（更大表面积）重复实验时，曲线初始会更陡，并在相同的最终体积处趋于水平。当使用更浓的酸时，如果大理石过量，最终气体体积可能更大。

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10. Exam Tip: How to Answer Extended Questions | 考试技巧：如何回答扩展题

WJEC often asks you to describe and explain the effect of a change on reaction rate. Always use the language of collision theory: particle frequency, energy, and correct orientation. Link each factor to both collision frequency and the proportion of particles with enough energy.

WJEC 经常要求你描述和解释变化对反应速率的影响。务必使用碰撞理论的语言：粒子频率、能量和正确取向。把每个因素与碰撞频率以及具有足够能量的粒子比例联系起来。

When interpreting a reaction profile, compare the energies of reactants and products: if products are lower, it is exothermic. Identify the activation energy as the difference between the peak and the reactants’ energy.

当解释反应进程图时，比较反应物和生成物的能量：如果生成物更低，则是放热反应。将活化能识别为峰值与反应物能量之间的差值。

For balancing equations, remember to treat polyatomic ions (SO₄²⁻, NO₃⁻, CO₃²⁻) as a group if they remain intact on both sides. Count atoms carefully and adjust only the coefficients. Never alter subscripts within a formula.

对于配平方程式，如果多原子离子（SO₄²⁻、NO₃⁻、CO₃²⁻）在两边保持不变，可将它们作为一个整体处理。仔细数原子，只调整系数。切勿改变化学式中的下标。

In questions about equilibrium, use Le Chatelier’s principle to predict the shift. Always state that the system opposes the change, then say which direction (forward or backward) is favoured and what happens to the yield of a named product.

在关于平衡的问题中，使用勒夏特列原理预测移动方向。始终说明系统会抵消变化，然后指出哪个方向（正向或逆向）受到有利影响，以及对指定生成物产率的影响。

When asked to describe a practical method, include precise details: name the apparatus, give volumes and concentrations, explain how the variable is changed and the others are controlled, and state what is measured and how often. A carefully labelled diagram can help.

当被要求描述实验方法时，要包括精确的细节：命名仪器，给出体积和浓度，解释变量如何改变以及其他变量如何控制，并说明测量什么以及测量的频率。精心标注的示意图可以提供帮助。

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