📚 Chemical Reactions: Key Concepts for IB & OCR | IB OCR 科学：化学反应 考点精讲
Chemical reactions form the heart of chemistry, driving everything from biological processes to industrial manufacturing. Whether you are following the IB syllabus or OCR specification, mastering the core ideas behind how substances interact, transform, and reach equilibrium is essential for success. This revision guide walks you through the principal concepts—from reaction types and stoichiometry to energetics and redox—presented with clear bilingual explanations to reinforce understanding.
化学反应是化学的核心，驱动着从生物过程到工业制造的方方面面。无论你学习的是IB还是OCR课程，掌握物质如何相互作用、转化以及达到平衡的核心概念都是取得好成绩的关键。这份复习指南将带你梳理主要考点——从反应类型和化学计量到能量变化与氧化还原——并用清晰的双语解释来强化理解。
1. What Is a Chemical Reaction? | 什么是化学反应？
A chemical reaction involves the rearrangement of atoms to convert one or more substances (reactants) into new substances (products) with different chemical properties. Bonds are broken in the reactants and new bonds are formed in the products.
化学反应涉及原子的重新排列，将一种或多种物质（反应物）转化为具有不同化学性质的新物质（生成物）。反应物中的化学键断裂，生成物中形成新的化学键。
Evidence for a chemical change includes colour change, gas production, precipitate formation, temperature change, or emission of light.
化学变化的证据包括颜色变化、气体生成、沉淀形成、温度变化或发光。
Chemical equations use symbols and formulas to represent reactions, with reactants on the left and products on the right separated by an arrow (→).
化学方程式使用符号和化学式表示反应，反应物在左，生成物在右，中间用箭头（→）隔开。
2. Types of Chemical Reactions | 化学反应类型
Synthesis (combination) reactions involve two or more simple reactants forming a single, more complex product: A + B → AB. An example is the formation of water from its elements: 2H₂ + O₂ → 2H₂O.
化合反应（合成反应）涉及两种或多种简单反应物生成一种更复杂的产物：A + B → AB。例如氢气与氧气化合生成水：2H₂ + O₂ → 2H₂O。
Decomposition reactions are the reverse of synthesis, where a single compound breaks down into two or more simpler substances: AB → A + B. Thermal decomposition of calcium carbonate is a classic example: CaCO₃ → CaO + CO₂.
分解反应是化合反应的逆过程，一种化合物分解成两种或多种更简单的物质：AB → A + B。碳酸钙的热分解是一个经典例子：CaCO₃ → CaO + CO₂。
Combustion reactions always involve a substance reacting rapidly with oxygen, releasing heat and light. Complete combustion of hydrocarbons yields carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O.
燃烧反应总是涉及物质与氧气迅速反应并放出热和光。碳氢化合物完全燃烧生成二氧化碳和水：CH₄ + 2O₂ → CO₂ + 2H₂O。
Displacement reactions occur when a more reactive element replaces a less reactive one in a compound. Single displacement: A + BC → AC + B. Double displacement: AB + CD → AD + CB, often producing a precipitate, gas, or water.
置换反应发生在一种较活泼的元素从化合物中取代较不活泼元素时。单置换：A + BC → AC + B。双置换：AB + CD → AD + CB，通常会生成沉淀、气体或水。
3. Balancing Chemical Equations | 化学方程式的配平
Atoms are conserved in a chemical reaction; therefore the number of atoms of each element must be the same on both sides of a balanced equation. This follows the law of conservation of mass.
原子在化学反应中守恒，因此配平后的方程式两边各元素的原子数目必须相等。这遵循质量守恒定律。
To balance an equation, adjust coefficients (whole numbers in front of formulas) rather than changing subscripts within chemical formulas. Start with elements that appear only once on each side, leaving hydrogen and oxygen until last.
配平方程式时，应调整系数（化学式前的整数）而不改动化学式中的下标。从每边只出现一次的元素开始，将氢和氧留到最后处理。
State symbols are added in parentheses to indicate the physical state: (s) solid, (l) liquid, (g) gas, and (aq) aqueous solution. Example: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g).
加入状态符号以标明物理状态：(s) 固体，(l) 液体，(g) 气体，(aq) 水溶液。例如：2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)。
4. Stoichiometry and the Mole | 化学计量学与摩尔
The mole is the SI unit for the amount of substance. One mole contains exactly 6.022 × 10²³ elementary entities (Avogadro’s constant). The molar mass (g mol⁻¹) is the mass of one mole of a substance, numerically equal to the relative atomic or formula mass.
摩尔是物质的量的国际单位。1摩尔包含精确的6.022 × 10²³个基本单元（阿伏伽德罗常数）。摩尔质量（g mol⁻¹）是1摩尔物质的质量，数值上等于相对原子质量或式量。
Balanced equations give the mole ratio of reactants and products. Using the formula n = m/M (moles = mass ÷ molar mass), you can calculate the masses, volumes, or concentrations needed or produced.
配平后的方程式给出了反应物和生成物的摩尔比例。使用公式 n = m/M（物质的量 = 质量 ÷ 摩尔质量），可以计算出所需或产生的质量、体积或浓度。
For gases at room temperature and pressure (RTP), one mole occupies 24.0 dm³ (or 24.0 L) in OCR; IB may use 22.7 dm³ at STP. Volumes can be directly related to moles for gaseous reactants and products.
对于常温常压（RTP）下的气体，OCR中1摩尔占据24.0 dm³（或24.0 L）；IB在标准状况下可能使用22.7 dm³。对于气态反应物和生成物，体积可直接与物质的量关联。
Limiting reactant problems require identifying which reactant is completely consumed first, and then using its mole amount to determine the theoretical yield of product.
限量反应物问题需要先确定哪种反应物先被完全消耗，然后用其物质的量计算理论产量。
5. Reaction Rates | 反应速率
The rate of a chemical reaction is the change in concentration of a reactant or product per unit time. It is often measured in mol dm⁻³ s⁻¹. Reaction rates decrease as reactants are used up because fewer collisions occur.
化学反应速率是指单位时间内反应物或生成物浓度的变化量，常用单位是 mol dm⁻³ s⁻¹。反应速率随反应物消耗而下降，因为碰撞频率降低。
Factors affecting rate include concentration (or pressure for gases), surface area of solids, temperature, and the presence of a catalyst. Increasing concentration or pressure raises the number of particles per unit volume, leading to more frequent collisions.
影响反应速率的因素包括浓度（或气体压强）、固体表面积、温度和催化剂的存在。增加浓度或压强会提高单位体积内的粒子数，从而增加碰撞频率。
Temperature is particularly effective because it increases both collision frequency and the proportion of particles with energy greater than or equal to the activation energy.
温度的影响尤其显著，因为它既提高了碰撞频率，又增大了能量大于或等于活化能的粒子比例。
6. Collision Theory and Activation Energy | 碰撞理论与活化能
For a reaction to occur, particles must collide with the correct orientation and with kinetic energy equal to or exceeding the activation energy (Eₐ). Only successful collisions lead to product formation.
反应发生需要粒子以正确的取向碰撞，并且动能达到或超过活化能（Eₐ）。只有有效碰撞才能生成产物。
Activation energy is the minimum energy required to break appropriate bonds in reactants so that new bonds can form. The Maxwell–Boltzmann distribution curve illustrates the distribution of kinetic energies among particles at a given temperature.
活化能是断裂反应物中相关键所需的最低能量，以便形成新键。麦克斯韦–玻尔兹曼分布曲线展示了给定温度下粒子动能分布情况。
Catalysts provide an alternative reaction pathway with a lower activation energy, increasing the rate without being consumed. Enzymes are biological catalysts crucial for metabolic reactions.
催化剂提供了活化能较低的替代反应路径，从而加快反应速率且自身不被消耗。酶是生物催化剂，对代谢反应至关重要。
7. Energy Changes: Exothermic and Endothermic | 能量变化：放热与吸热
In an exothermic reaction, energy is transferred from the system to the surroundings, usually causing a temperature rise. ΔH is negative because the products have lower enthalpy than the reactants. Combustion and neutralisation are common examples.
放热反应中，能量由系统传递到周围环境，通常导致温度升高。ΔH为负值，因为生成物的焓低于反应物。燃烧和中和反应是常见例子。
In an endothermic reaction, energy is absorbed from the surroundings, causing a temperature drop. ΔH is positive. Photosynthesis and the thermal decomposition of carbonates are endothermic processes.
吸热反应中，能量从周围环境吸收，导致温度下降。ΔH为正值。光合作用和碳酸盐的热分解是吸热过程。
Enthalpy profile diagrams show the relative energy levels of reactants and products, as well as the activation energy hump. The overall enthalpy change (ΔH) can be calculated using bond energies: ΔH = Σ(bond energies broken) − Σ(bond energies formed).
焓变示意图显示反应物与生成物的相对能级，以及活化能峰。总焓变（ΔH）可用键能计算：ΔH = Σ(断裂键的键能) − Σ(形成键的键能)。
8. Reversible Reactions and Dynamic Equilibrium | 可逆反应与动态平衡
A reversible reaction proceeds in both forward and backward directions. When the rates of the forward and reverse reactions become equal and concentrations remain constant, the system is at dynamic equilibrium. This occurs only in a closed system.
可逆反应同时向正、反两个方向进行。当正反应和逆反应速率相等且各物质浓度保持恒定时，系统处于动态平衡。这仅在封闭系统中发生。
At equilibrium, macroscopic properties (colour, pressure, concentration) remain constant, but at the microscopic level, the forward and reverse processes continue at equal rates.
在平衡态下，宏观性质（颜色、压强、浓度）保持不变，但在微观层面，正逆过程仍在以相同速率进行。
The position of equilibrium indicates whether the forward or reverse reaction is favoured. If the equilibrium mixture contains more products, the position lies to the right; if more reactants, it lies to the left.
平衡位置表示正反应或逆反应哪一个占优势。如果平衡混合物中生成物更多，平衡位置偏向右侧；如果反应物更多，则偏向左侧。
9. Le Chatelier’s Principle | 勒夏特列原理
Le Chatelier’s principle states that if an external stress is applied to a system at equilibrium, the system adjusts to partially oppose the change and re-establish equilibrium. Stresses include changes in concentration, pressure, and temperature.
勒夏特列原理指出，若对处于平衡的系统施加外部应力，系统会进行调整以部分抵消该变化并重新建立平衡。应力包括浓度、压强和温度的变化。
Increasing reactant concentration shifts the equilibrium position to the right, producing more products. Removing product also shifts equilibrium to the right.
增加反应物浓度会使平衡位置向右移动，生成更多产物。移除产物同样使平衡向右移动。
For gaseous systems, increasing pressure shifts equilibrium towards the side with fewer moles of gas. Pressure changes have no effect if the number of gas moles is equal on both sides.
对于气体系统，增加压强会使平衡向气体摩尔数较少的一侧移动。若两边气体摩尔数相等，压强变化无影响。
Increasing temperature favours the endothermic direction, while decreasing temperature favours the exothermic direction. Catalysts do not affect the position of equilibrium; they only speed up the attainment of equilibrium.
升高温度有利于吸热方向，降低温度有利于放热方向。催化剂不影响平衡位置，只加快达到平衡的速率。
10. Redox Reactions | 氧化还原反应
Redox reactions involve a transfer of electrons between species. Oxidation is the loss of electrons (or increase in oxidation number), while reduction is the gain of electrons (or decrease in oxidation number). OIL RIG: Oxidation Is Loss, Reduction Is Gain.
氧化还原反应涉及物种间的电子转移。氧化是失去电子（或氧化数升高），还原是得到电子（或氧化数降低）。记忆口诀：失升氧，得降还。
Oxidising agents accept electrons and are themselves reduced; reducing agents donate electrons and are themselves oxidised. Common oxidising agents include oxygen, halogens, and manganate(VII) ions.
氧化剂接受电子，自身被还原；还原剂提供电子，自身被氧化。常见的氧化剂有氧气、卤素和高锰酸根离子。
Oxidation numbers (or states) are assigned to atoms to track electron redistribution. Rules include: elements are 0, oxygen is typically −2, hydrogen +1 with non-metals, and the sum equals the overall charge of the species.
氧化数（或氧化态）用于追踪电子的重新分配。规则包括：单质为0，氧通常为−2，氢与非金属结合时为+1，各原子氧化数之和等于物种的总电荷。
Half-equations are used to show the electron transfer for oxidation or reduction separately, and are then combined into a full ionic equation for the redox process.
半反应方程式分别表示氧化或还原过程的电子转移，然后合并为完整的氧化还原离子方程式。
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