📚 IB WJEC Science: Chemical Reactions Key Points Review | IB WJEC 科学：化学反应 考点精讲
Chemical reactions form the foundation of chemistry in both IB and WJEC specifications. Understanding how substances transform, the energy changes involved, and how to quantify these processes is essential for success in examinations. This revision guide provides a comprehensive overview of key reaction concepts, from balancing equations to equilibrium, tailored to IB and WJEC chemistry students.
化学反应是IB和WJEC化学课程的基础。理解物质如何转化、涉及的能量变化以及如何量化这些过程，对于考试成功至关重要。本复习指南全面概述了关键反应概念，从配平方程式到化学平衡，为IB和WJEC化学学生量身打造。
1. What is a Chemical Reaction? | 什么是化学反应？
A chemical reaction involves the rearrangement of atoms to form new substances with different properties. Indicators of a reaction include colour change, temperature change, gas production, or precipitate formation. Reactants are consumed and products are formed, following the law of conservation of mass.
化学反应涉及原子的重新排列，形成具有不同性质的新物质。反应的迹象包括颜色变化、温度变化、气体产生或生成沉淀。反应物被消耗，产物生成，且遵循质量守恒定律。
In both IB and WJEC courses, you must be able to distinguish between physical changes (e.g., melting) and chemical changes, where new chemical bonds are made and broken.
在IB和WJEC课程中，你必须能够区分物理变化（如熔化）和化学变化，后者涉及化学键的形成与断裂。
2. Writing and Balancing Chemical Equations | 化学方程式的书写与配平
Balanced equations reflect the conservation of atoms. Coefficients are placed before formulas to ensure the same number of each type of atom on both sides. For example, the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O.
配平后的方程式反映了原子守恒。在化学式前添加系数，以确保每种原子在方程式两侧数目相等。例如，甲烷燃烧：CH₄ + 2O₂ → CO₂ + 2H₂O。
State symbols (s), (l), (g), and (aq) are used to indicate the physical state of reactants and products. IB mark schemes often require these for full marks. Always double‑check diatomic molecules such as H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
状态符号(s)、(l)、(g)和(aq)用于表示反应物和产物的物理状态。IB评分方案常要求写出状态符号才能得满分。务必检查双原子分子，如H₂、N₂、O₂、F₂、Cl₂、Br₂、I₂。
Ionic equations simplify reactions in aqueous solution by showing only the species that actually change. Spectator ions are omitted. This is a common exam requirement in both syllabi.
离子方程式简化了水溶液中的反应，只显示实际发生变化的物种。旁观离子被省略。这是两个大纲中常见的考试要求。
3. Types of Chemical Reactions | 化学反应的类型
Reactions can be classified into several fundamental types. Recognising the pattern helps predict products.
反应可分为几种基本类型。识别反应模式有助于预测产物。
- Synthesis (Combination): A + B → AB. E.g., 2Mg + O₂ → 2MgO
合成（化合）反应：A + B → AB。例如：2Mg + O₂ → 2MgO - Decomposition: AB → A + B. E.g., CaCO₃ → CaO + CO₂
分解反应：AB → A + B。例如：CaCO₃ → CaO + CO₂ - Single Displacement: A + BC → AC + B. E.g., Zn + CuSO₄ → ZnSO₄ + Cu
置换反应：A + BC → AC + B。例如：Zn + CuSO₄ → ZnSO₄ + Cu - Double Displacement (Metathesis): AB + CD → AD + CB. Often precipitation or neutralisation. E.g., AgNO₃ + NaCl → AgCl + NaNO₃
复分解反应：AB + CD → AD + CB。常伴随沉淀或中和。例如：AgNO₃ + NaCl → AgCl + NaNO₃ - Combustion: Hydrocarbon + O₂ → CO₂ + H₂O (complete). E.g., C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
燃烧反应：烃 + O₂ → CO₂ + H₂O（完全燃烧）。例如：C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
WJEC often includes thermal decomposition of carbonates and hydrogencarbonates; IB extends to combustion analysis and redox classification.
WJEC通常涉及碳酸盐和碳酸氢盐的热分解；IB则延伸至燃烧分析和氧化还原分类。
4. Redox Reactions and Oxidation States | 氧化还原反应与氧化态
Redox reactions involve the transfer of electrons. Oxidation is loss of electrons; reduction is gain of electrons. The mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain) is widely used. Oxidation states (numbers) help track electron movement.
氧化还原反应涉及电子转移。氧化是失去电子；还原是得到电子。助记口诀OIL RIG（氧化失电子，还原得电子）被广泛使用。氧化态（氧化数）有助于追踪电子移动。
Rules for assigning oxidation states: free elements have an oxidation state of 0; oxygen is usually –2 (except in peroxides where it is –1); hydrogen is +1 with non‑metals, –1 with metals; the sum of oxidation states equals the overall charge.
确定氧化态的规则：游离元素氧化态为0；氧通常为–2（过氧化物中为–1）；氢与非金属结合时为+1，与金属结合时为–1；氧化态总和等于总电荷。
In disproportionation, a single species is both oxidised and reduced. e.g., 2H₂O₂ → 2H₂O + O₂. IB often asks to identify the oxidising and reducing agents in a reaction.
在歧化反应中，同一物种既被氧化又被还原。例如：2H₂O₂ → 2H₂O + O₂。IB常要求识别反应中的氧化剂和还原剂。
5. Acids, Bases and Salt Formation | 酸、碱与盐的生成
Acids are proton (H⁺) donors; bases are proton acceptors (Brønsted‑Lowry theory). Neutralisation reactions produce a salt and water: HCl + NaOH → NaCl + H₂O. WJEC requires knowledge of common laboratory acids and bases; IB expects the ability to write net ionic equations for neutralisation.
酸是质子(H⁺)供体；碱是质子受体（布朗斯特-劳里理论）。中和反应生成盐和水：HCl + NaOH → NaCl + H₂O。WJEC要求了解常见实验室酸碱；IB则期望能书写中和反应的净离子方程式。
Reactions of acids with metals, carbonates, and bases yield salts. For example, acid + metal carbonate → salt + water + CO₂: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂. This is frequently tested in practical contexts.
酸与金属、碳酸盐和碱的反应都会生成盐。例如，酸 + 金属碳酸盐 → 盐 + 水 + 二氧化碳：2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂。这在实验情境中经常考查。
pH scale and indicators: pH = –log₁₀[H⁺]. IB requires calculations involving pH and pOH; WJEC focuses on the use of indicators like litmus and phenolphthalein.
pH标度和指示剂：pH = –log₁₀[H⁺]。IB要求进行涉及pH和pOH的计算；WJEC则侧重于石蕊和酚酞等指示剂的使用。
6. Reaction Rates and Collision Theory | 反应速率与碰撞理论
For a reaction to occur, particles must collide with sufficient energy (activation energy, Ea) and correct orientation. The rate of reaction can be increased by raising temperature, increasing concentration/pressure, adding a catalyst, or increasing surface area.
反应发生的条件是粒子必须具有足够的能量（活化能，Ea）并以正确的取向碰撞。可通过升高温度、增大浓度/压强、加入催化剂或增大接触面积来提高反应速率。
Maxwell‑Boltzmann distribution explains the effect of temperature and catalysts. A catalyst provides an alternative pathway with lower Ea, increasing the proportion of particles exceeding the activation energy. IB explores this graph in detail.
麦克斯韦-玻尔兹曼分布解释了温度和催化剂的影响。催化剂提供了活化能较低的替代路径，增大了超过活化能的粒子比例。IB对此图有详细探究。
Rate equations (IB HL): rate = k[A]ᵐ[B]ⁿ. The order and rate constant are experimentally determined. WJEC may simply assess interpretation of rate graphs, e.g., mass loss vs time.
速率方程（IB高级）：速率 = k[A]ᵐ[B]ⁿ。反应级数和速率常数由实验确定。WJEC可能只考查解读速率图，如质量损失对时间作图。
7. Energetics: Exothermic and Endothermic Reactions | 能量学：放热与吸热反应
Exothermic reactions release heat to the surroundings (ΔH negative), e.g., combustion, neutralisation. Endothermic reactions absorb heat (ΔH positive), e.g., thermal decomposition of CaCO₃, photosynthesis.
放热反应向环境释放热量（ΔH为负），如燃烧、中和反应。吸热反应吸收热量（ΔH为正），如CaCO₃热分解、光合作用。
Enthalpy changes can be calculated using bond energies: ΔH = Σ bonds broken – Σ bonds formed. Calorimetry experiments (e.g., measuring temperature change when a fuel burns) are core practicals in both syllabi. Remember q = mcΔT, and convert J to kJ.
焓变可通过键能计算：ΔH = 断裂键能总和 – 形成键能总和。量热实验（如测量燃料燃烧时的温度变化）是两个大纲的核心实验。记住 q = mcΔT，并将焦耳转换为千焦。
Hess’s Law states that the total enthalpy change for a reaction is independent of the route taken. Energy level diagrams and enthalpy cycles (IB) are examined frequently.
盖斯定律指出，一个反应的总焓变与采取的途径无关。能级图和焓变循环（IB）经常被考查。
8. Chemical Equilibrium and Le Chatelier’s Principle | 化学平衡与勒夏特列原理
Many reactions are reversible; at equilibrium, the forward and reverse rates are equal. The equilibrium constant Kc expresses the ratio of product to reactant concentrations (each raised to their stoichiometric coefficients).
许多反应是可逆的；平衡时，正逆反应速率相等。平衡常数Kc表示产物浓度乘积与反应物浓度乘积之比（均以计量系数为指数）。
Le Chatelier’s principle: if a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the equilibrium position shifts to oppose the change. For example, increasing temperature favours
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