📚 IB Chemistry: Chemical Reactions Revision | IB化学：化学反应考点精讲
Chemical reactions form the core of IB Chemistry, bridging atomic theory, energetics, kinetics, and equilibrium. This revision guide distills the key concepts you need to master for papers 1, 2, and the internal assessment. From balancing equations and calculating yields to exploring reaction mechanisms and predicting spontaneity, every topic is explained with precision and linked directly to the IB syllabus statements.
化学反应是 IB 化学的核心，连接着原子理论、能量学、动力学和平衡。本精讲提炼了你在试卷一、二和内部评估中必须掌握的关键概念。从配平方程、计算产率到探索反应机理与预测自发性，每个专题都严格对应 IB 教学大纲，并提供精确解释。
1. Balancing Chemical Equations | 化学方程式的配平
A balanced chemical equation obeys the law of conservation of mass: the number of atoms of each element must be the same on both sides. IB exam questions often require you to balance equations for unfamiliar reactions including redox processes and combustion of organic compounds.
平衡的化学方程式遵循质量守恒定律：每种元素的原子数目在两边必须相等。IB 考试常要求你配平陌生反应的方程式，包括氧化还原过程和有机物的燃烧。
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Begin with compounds containing the most atoms, leave single elements for last, and always double-check diatomic gases (H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂). In ionic equations, balance both mass and charge simultaneously.
从含原子数最多的化合物开始，将单质留到最后，并始终检查双原子气体（H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂）。在离子方程中，需同时平衡质量与电荷。
- State symbols: (s), (l), (g), (aq) are required to gain full marks.
- 物态符号：(s)、(l)、(g)、(aq) 是获得满分所必需的。
2. Types of Chemical Reactions | 化学反应类型
IB Chemistry expects you to classify reactions into synthesis, decomposition, single displacement, double displacement, combustion, neutralization, and redox. Recognising patterns helps predict products and write equations faster.
IB 化学要求你将反应分类为化合、分解、置换、复分解、燃烧、中和与氧化还原。识别模式有助于更快预测产物和书写方程式。
- Synthesis: A + B → AB
- 化合反应：A + B → AB
- Decomposition: AB → A + B
- 分解反应：AB → A + B
- Single displacement: A + BC → AC + B
- 置换反应：A + BC → AC + B
- Double displacement: AB + CD → AD + CB
- 复分解反应：AB + CD → AD + CB
- Combustion: hydrocarbon + O₂ → CO₂ + H₂O
- 燃烧：烃 + O₂ → CO₂ + H₂O
Neutralisation is a special double displacement producing a salt and water. Proton transfer theories (Bronsted-Lowry) refine acid-base reactions, which are crucial for Topic 8.
中和反应是生成盐和水的特殊复分解反应。质子转移理论（Bronsted-Lowry）完善了酸碱反应，对第八专题至关重要。
3. The Mole Concept and Stoichiometry | 摩尔概念与化学计量
The mole (mol) is the SI unit for amount of substance, containing 6.02 x 10²³ entities. Stoichiometry uses the coefficients in a balanced equation to relate moles of reactants and products. The three key formulas are:
摩尔 (mol) 是物质的量的国际单位，包含 6.02 × 10²³ 个实体。化学计量利用平衡方程中的系数来关联反应物和产物的摩尔数。三个关键公式为：
n = m / M , n = N / L , n = V (gas at STP) / 22.7 dm³ mol⁻¹
where n = amount (mol), m = mass (g), M = molar mass (g mol⁻¹), N = number of particles, L = Avogadro’s constant, V = volume (dm³). Remember STP for IB is 0 °C, 100 kPa, and molar volume is 22.7 dm³ mol⁻¹.
其中 n = 物质的量 (mol)，m = 质量 (g)，M = 摩尔质量 (g mol⁻¹)，N = 粒子数，L = 阿伏伽德罗常数，V = 体积 (dm³)。注意 IB 中 STP 是 0 °C、100 kPa，摩尔体积为 22.7 dm³ mol⁻¹。
Always convert masses to moles first, use the mole ratio from the equation, then convert back to mass, volume, or concentration as required.
总是先将质量换算为摩尔，利用方程式中的摩尔比，再按要求换回质量、体积或浓度。
4. Limiting Reagent and Percentage Yield | 限量试剂与产率
The limiting reagent is completely consumed first, determining the maximum amount of product. Compare the mole ratios of reactants to find which one runs out. Theoretical yield is calculated from the limiting reagent; actual yield is given by experiment.
限量试剂最先耗尽，决定产物的最大量。比较反应物的摩尔比以确定哪种用尽。理论产率根据限量试剂计算；实际产率由实验给出。
% yield = (actual yield / theoretical yield) × 100%
Low yields can be caused by incomplete reactions, side reactions, or losses during purification. IB questions may ask you to identify the limiting reagent, calculate theoretical yield, and comment on atom economy.
低产率可能源于反应不完全、副反应或纯化损失。IB 题目可能要求你识别限量试剂、计算理论产率并评价原子经济性。
5. Reaction Kinetics: Rate of Reaction | 反应动力学：反应速率
Reaction rate is defined as the change in concentration of a reactant or product per unit time. Common units are mol dm⁻³ s⁻¹. The rate can be determined from the gradient of a concentration vs time graph, or by measuring the time for a certain amount of product to form.
反应速率定义为单位时间内反应物或产物浓度的变化。常见单位为 mol dm⁻³ s⁻¹。速率可通过浓度–时间图的斜率测定，或通过测量生成一定量产物所需的时间来确定。
Methods for following reaction progress include: change in mass, volume of gas evolved, colour change, pH change, and conductivity. IB data-based questions often present raw data and ask you to calculate rates and analyze trends.
跟踪反应进程的方法包括：质量变化、放出气体的体积、颜色变化、pH 变化以及电导率。IB 数据分析题常给出原始数据，要求你计算速率并分析趋势。
6. Collision Theory and Activation Energy | 碰撞理论与活化能
For a reaction to occur, particles must collide with proper orientation and with energy equal to or greater than the activation energy (Eₐ). The rate depends on collision frequency and the fraction of successful collisions.
反应发生需要粒子以正确取向碰撞，且能量等于或超过活化能 (Eₐ)。速率取决于碰撞频率和有效碰撞的比例。
k = A e⁻ᴱᵃ⁽ᴿᵀ⁾
Increasing temperature increases the average kinetic energy and, more importantly, the proportion of particles exceeding Eₐ, significantly boosting the rate. Catalysts provide an alternative pathway with a lower Eₐ, increasing the rate without being consumed.
升高温度增加了平均动能，更重要的是增大了超过 Eₐ 的粒子比例，从而显著提高速率。催化剂提供一条 Eₐ 更低的替代路径，在反应中不被消耗而提高速率。
Enzymes are biological catalysts whose activity depends on temperature and pH due to their protein structure; denaturation leads to a loss of function.
酶是生物催化剂，因其蛋白质结构，活性受温度和 pH 影响；变性会导致功能丧失。
7. Chemical Equilibrium | 化学平衡
In a closed system, a reversible reaction reaches dynamic equilibrium when the rates of the forward and reverse reactions become equal, and the concentrations of all species remain constant. The equilibrium constant Kc is given by:
在封闭体系中，可逆反应达到动态平衡时，正逆反应速率相等，且所有物质的浓度保持不变。平衡常数 Kc 的表达式为：
Kc = [C]ᶜ [D]ᵈ / [A]ᵃ [B]ᵇ
Only gases and aqueous species appear in the expression; solids and pure liquids are omitted. Kc is temperature-dependent; a change in concentration or pressure does not alter Kc but causes a shift to restore equilibrium (Le Chatelier’s principle).
仅气态和溶液物种出现在表达式里；固体和纯液体不写入。Kc 随温度变化；浓度或压强的改变不会改变 Kc 但会引起平衡移动以恢复平衡（勒夏特列原理）。
For exothermic reactions, increasing temperature shifts equilibrium left (decreasing Kc); for endothermic reactions, temperature increase shifts equilibrium right (increasing Kc). Always analyse the effect of temperature on Kc using ΔH.
对放热反应，升温使平衡左移（Kc 减小）；对吸热反应，升温使平衡右移（Kc 增大）。始终利用 ΔH 分析温度对 Kc 的影响。
8. Acids and Bases | 酸与碱
Bronsted-Lowry theory defines an acid as a proton (H⁺) donor and a base as a proton acceptor. Conjugate acid-base pairs differ by one proton. Strong acids (HCl, HNO₃, H₂SO₄) fully dissociate, while weak acids (CH₃COOH, H₂CO₃) partially dissociate, establishing an equilibrium.
Bronsted-Lowry 理论定义酸为质子 (H⁺) 给予体，碱为质子接受体。共轭酸碱对相差一个质子。强酸 (HCl, HNO₃, H₂SO₄) 完全解离，弱酸 (CH₃COOH, H₂CO₃) 部分解离，建立平衡。
pH = -log[H⁺] , [H⁺] = 10⁻ᴾᴴ
Mol dm⁻³ (molar concentration) is used. Neutralization reactions form a salt and water: H⁺ + OH⁻ → H₂O. The ionic equation shows the net change. Titration curves exhibit characteristic shapes for strong acid–strong base, weak acid–strong base, etc.
使用 mol dm⁻³（摩尔浓度）。中和反应生成盐和水：H⁺ + OH⁻ → H₂O。离子方程式显示净变化。滴定曲线对强酸–强碱、弱酸–强碱等表现出特征形状。
Buffer solutions resist pH change and consist of a weak acid and its conjugate base, or a weak base and its conjugate acid. They are vital in biological systems.
缓冲溶液能抵抗 pH 变化，由弱酸及其共轭碱或弱碱及其共轭酸组成。它们在生物体系中至关重要。
9. Redox Reactions | 氧化还原反应
Oxidation is loss of electrons, reduction is gain of electrons (OIL RIG). Oxidation states are assigned to track electron transfer. The sum of oxidation states in a neutral compound is zero; in a polyatomic ion, it equals the charge.
氧化是失电子，还原是得电子（OIL RIG）。氧化数用于追踪电子转移。中性化合物中氧化数之和为零；多原子离子中等于离子电荷。
Redox reactions can be split into half-equations. Balancing redox equations requires equalizing electrons lost and gained. In acidic solutions, add H⁺ and H₂O; in basic solutions, add OH⁻ and H₂O.
氧化还原反应可拆分为半反应式。配平氧化还原方程式需使失电子数与得电子数相等。在酸性溶液中，加入 H⁺ 和 H₂O；在碱性溶液中，加入 OH⁻ 和 H₂O。
Displacement reactions and combustion are common redox processes. Electrochemical cells (voltaic and electrolytic) are applications of redox chemistry; the reactivity series helps predict spontaneous redox reactions.
置换反应和燃烧是常见的氧化还原过程。电化学池（原电池和电解池）是氧化还原化学的应用；活动性顺序有助于预测自发的氧化还原反应。
10. Energetics: Enthalpy Changes | 能量学：焓变
Chemical reactions involve energy changes; enthalpy (H) is the heat content of a system at constant pressure. The enthalpy change ΔH is measured in kJ mol⁻¹. Exothermic reactions release heat (ΔH negative); endothermic reactions absorb heat (ΔH positive).
化学反应伴随能量变化；焓 (H) 是体系在恒压下的热含量。焓变 ΔH 以 kJ mol⁻¹ 为单位。放热反应释放热量 (ΔH 为负)；吸热反应吸收热量 (ΔH 为正)。
q = mcΔT
Calorimetry experiments determine ΔH by measuring temperature change (ΔT) when a known mass (m) of solution with specific heat capacity (c) absorbs heat. The enthalpy change per mole is then calculated from the limiting reagent.
量热实验通过测量已知质量 (m) 和比热容 (c) 的溶液吸收热量时的温度变化 (ΔT) 来测定 ΔH。然后根据限量试剂计算出每摩尔的焓变。
Hess’s law states that the total enthalpy change for a reaction is independent of the pathway. It allows ΔH to be calculated using enthalpies of formation, combustion, or by combining known reactions. Bond enthalpies (average values) can also estimate ΔH, though they are less accurate.
赫斯定律指出，反应的总焓变与路径无关。可利用生成焓、燃烧焓或组合已知反应来计算 ΔH。键焓（平均值）也可估算 ΔH，但准确性稍低。
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