Reaction Rates: Key Exam Points for IB & Edexcel Chemistry | IB Edexcel 化学：反应速率 考点精讲

📚 Reaction Rates: Key Exam Points for IB & Edexcel Chemistry | IB Edexcel 化学：反应速率 考点精讲

Reaction rates lie at the heart of physical chemistry, bridging thermodynamics and kinetics. Both IB and Edexcel specifications demand a solid grasp of how and why reactions proceed at different speeds, the mathematical tools to describe them, and the experimental skills to measure them. This revision guide consolidates every core concept – from collision theory to the Arrhenius equation – with clear bilingual explanations, targeted at top marks in your exams.

反应速率是物理化学的核心知识，连接着热力学与动力学两大板块。IB 和 Edexcel 考试大纲都要求考生透彻理解反应为何以不同速率进行、如何用数学工具描述速率，以及怎样通过实验测量速率。本文以中英双语精讲碰撞理论、速率方程、阿伦尼乌斯方程等全部核心考点，助力你在考场上稳拿高分。

1. Definition of Reaction Rate | 反应速率的定义

Reaction rate is the change in concentration of a reactant or product per unit time. It is typically given in mol dm⁻³ s⁻¹. For a reaction A → B, the rate can be expressed as −Δ[A]/Δt or Δ[B]/Δt. The negative sign indicates consumption of the reactant.

反应速率定义为单位时间内反应物或产物浓度的变化量，常用单位为 mol dm⁻³ s⁻¹。对于反应 A → B，速率可表示为 −Δ[A]/Δt 或 Δ[B]/Δt，其中负号代表反应物浓度在减少。

Average rate is calculated over a time interval, whereas instantaneous rate is the slope of the concentration–time graph at a specific moment. Exam questions often ask you to draw a tangent or calculate the gradient.

平均速率基于一段时间计算，瞬时速率则是浓度–时间曲线在某时刻的切线斜率。考试常要求画切线或计算梯度。

2. Collision Theory | 碰撞理论

For a reaction to occur, reactant particles must collide with sufficient energy (activation energy, E_a) and correct orientation. Collision theory explains why increasing concentration, temperature, or surface area speeds up a reaction – all these factors raise the frequency of successful collisions.

反应发生的先决条件是反应物粒子发生碰撞，且必须具备足够的能量（即活化能 E_a）以及正确的取向。碰撞理论解释了为何增大浓度、升高温度或增大表面积会加快反应速率——这些因素都提高了有效碰撞的频率。

Solvent effects and steric factors also influence the effective collision frequency. In IB HL and Edexcel papers, you may need to relate molecular geometry to the probability of fruitful collisions.

溶剂效应与空间位阻因素同样影响有效碰撞的频率。在 IB HL 和 Edexcel 试卷中，你可能需要将分子几何构型与有效碰撞概率关联起来。

3. Concentration & Pressure | 浓度与压强

Higher concentration (or higher pressure for gases) means more particles per unit volume. This directly increases the collision frequency, leading to a faster reaction rate, provided E_a remains unchanged. Rate vs. concentration graphs often show a directly proportional relationship for zero- or first-order, but not always.

浓度越高（或气体压强越大），单位体积内的粒子数越多。这直接提高了碰撞频率，在活化能保持不变的条件下反应速率加快。速率对浓度的图形常呈现正比关系（对零级或一级而言），但并非总是线性。

In Edexcel questions, you may be asked to explain the effect of doubling the pressure of a gaseous reactant on the rate using kinetic theory. IB Paper 2 experiments often involve varying concentrations and monitoring initial rates.

在 Edexcel 考题中，可能要求利用分子动理论解释将气态反应物压强加倍对速率的影响。IB 卷二实验常见通过改变浓度并监测初始速率来研究速率方程。

4. Temperature & The Boltzmann Distribution | 温度与玻尔兹曼分布

Increasing temperature gives particles greater average kinetic energy. Crucially, the Boltzmann distribution curve flattens and shifts to the right, so a much larger fraction of particles possesses energy ≥ E_a. This exponential increase in successful collisions results in a dramatic rate enhancement.

升高温度使粒子平均动能增大。关键是玻尔兹曼分布曲线变平且右移，因此能量 ≥ E_a 的粒子比例显著增大。有效碰撞呈指数级增加，从而使速率急剧提升。

Be precise: temperature changes the proportion of particles with E ≥ E_a, not the activation energy itself. A common trick in exams is to claim that E_a decreases with temperature – this is incorrect.

务必精确：温度改变的是能量 ≥ E_a 的粒子比例，而非改变活化能本身。考试常见陷阱是声称活化能随温度升高而下降——这是错误的。

5. Surface Area & Catalysts | 表面积与催化剂

For solid reactants, increasing surface area exposes more particles to collisions. Crushing a solid into a powder dramatically speeds up heterogeneous reactions, such as the reaction of marble chips with acid.

对于固体反应物，增大表面积使更多粒子可参与碰撞。将固体研磨成粉末能显著加快非均相反应，例如大理石碎片与酸的反应。

A catalyst provides an alternative reaction pathway with lower activation energy. It is chemically unchanged at the end of the reaction. Catalysts increase both the forward and backward rates equally, so the equilibrium yield remains unaffected – only the speed to reach equilibrium changes.

催化剂通过提供较低活化能的替代路径来加快反应，反应前后自身化学性质不变。催化剂同等程度地加快正、逆反应速率，因此平衡产率不受影响——改变的只是到达平衡的快慢。

6. Rate Equations & Order of Reaction | 速率方程与反应级数

The rate equation links the reaction rate to the concentrations of reactants raised to some power. For a reaction aA + bB → products, the rate law is often of the form: rate = k [A]^m [B]ⁿ, where k is the rate constant, and m and n are the orders with respect to A and B. The overall order is m + n.

速率方程将反应速率与各反应物浓度的某次方关联起来。对于反应 aA + bB → 产物，速率定律通常形如 rate = k [A]^m [B]ⁿ，其中 k 为速率常数，m 和 n 分别为对 A 和 B 的分级数，总反应级数为 m + n。

Orders are not necessarily the stoichiometric coefficients. They must be determined experimentally. Edexcel and IB HL require you to deduce orders from concentration–time or rate–concentration data.

分级数不一定等于化学计量系数，必须通过实验确定。Edexcel 和 IB HL 均要求你从浓度–时间或速率–浓度数据推断反应级数。

Order	Effect on Rate when [A] doubles	级数	[A] 加倍时速率变化
0	Unchanged	0	不变
1	×2	1	速率×2
2	×4	2	速率×4

7. Determining Orders: Initial Rates Method | 确定级数：初始速率法

The initial rates method involves measuring the rate at time zero for different starting concentrations. By comparing two experiments where only one reactant’s concentration is changed, you can deduce the order with respect to that reactant. For first order, rate ∝ [A]; for second order, rate ∝ [A]².

初始速率法在时间零点测量不同起始浓度下的速率。通过比较仅变动一种反应物浓度的两次实验，即可推断该反应物的反应级数。一级反应中速率 ∝ [A]，二级反应中速率 ∝ [A]²。

Continuous monitoring methods (volume of gas evolved, mass loss, colorimetry) can also generate concentration–time graphs from which orders are inferred via half-life or tangent analysis. IB requires students to design such experiments and evaluate uncertainties.

连续监测法（气体体积变化、质量减少、比色法）可生成浓度–时间曲线，再利用半衰期或切线分析推断级数。IB 要求考生能设计此类实验并评估不确定度。

8. Rate-Determining Step & Mechanisms | 速率决定步骤与反应机理

Many reactions proceed via a series of elementary steps. The slowest step – the rate-determining step (RDS) – controls the overall rate. The rate equation reflects only the species involved in or before the RDS, up to and including the RDS.

许多反应通过一系列基元步骤进行。最慢的一步——速率决定步骤（RDS）——控制着总反应速率。速率方程仅反映 RDS 及其之前涉及的物质，包含 RDS 本身。

For example, if the rate law is rate = k [NO₂][CO], a possible mechanism is: Step 1 (slow): NO₂ + NO₂ → NO₃ + NO; Step 2 (fast): NO₃ + CO → NO₂ + CO₂. The rate law matches the molecularity of the slow step.

例如，若速率定律为 rate = k [NO₂][CO]，一种可能的机理是：第一步（慢）NO₂ + NO₂ → NO₃ + NO；第二步（快）NO₃ + CO → NO₂ + CO₂。速率定律与慢步骤的分子数相符。

Both IB HL and Edexcel require you to propose a mechanism that is consistent with the experimental rate equation and to explain the role of intermediates.

IB HL 和 Edexcel 均要求你提出与实验速率方程相吻合的反应机理，并解释中间体的角色。

9. The Arrhenius Equation | 阿伦尼乌斯方程

The quantitative link between rate constant, temperature, and activation energy is given by the Arrhenius equation:

速率常数、温度与活化能之间的定量关系由阿伦尼乌斯方程表示：

k = A e^−E_a/RT

where A is the pre-exponential factor, E_a is activation energy (J mol⁻¹), R = 8.31 J K⁻¹ mol⁻¹, and T is absolute temperature in Kelvin. The logarithmic form is often more useful for calculations:

其中 A 为指前因子，E_a 为活化能（单位 J mol⁻¹），R = 8.31 J K⁻¹ mol⁻¹，T 为开氏温度。对数形式更便于计算：

ln k = ln A − E_a/(RT)

Plotting ln k against 1/T yields a straight line with slope = −E_a/R. Edexcel papers frequently feature this graph; IB data-based questions expect you to calculate E_a from given rate constants at different temperatures.

绘制 ln k 对 1/T 的图形可得一直线，其斜率为 −E_a/R。Edexcel 试卷常考此图像；IB 数据分析题要求你通过不同温度下的速率常数计算活化能。

A larger E_a means the rate is more sensitive to temperature changes. This explains why many reactions have a very slow rate at room temperature but become rapid upon heating.

活化能越大，速率对温度变化越敏感。这解释了为何许多反应在室温下速率极慢，一经加热便迅速进行。

10. Catalysis: Homogeneous & Heterogeneous | 催化作用：均相与非均相

Homogeneous catalysts exist in the same phase as the reactants. They typically form an intermediate species that reacts further to regenerate the catalyst. For example, the iodide ion catalyses the decomposition of hydrogen peroxide: H₂O₂ + I⁻ → H₂O + IO⁻, then H₂O₂ + IO⁻ → H₂O + O₂ + I⁻.

均相催化剂与反应物处于同一相中。它们通常先形成中间体，中间体进一步反应并再生催化剂。例如，碘离子催化过氧化氢分解：H₂O₂ + I⁻ → H₂O + IO⁻，随后 H₂O₂ + IO⁻ → H₂O + O₂ + I⁻。

Heterogeneous catalysts are in a different phase, often a solid with a gas or liquid reactant. Reactant molecules adsorb onto active sites, bonds weaken, and products desorb. Industrial examples include the Haber process (iron catalyst) and catalytic converters (platinum/rhodium).

非均相催化剂处于不同相，常为固体，与气态或液态反应物作用。反应物分子吸附在活性位点上，化学键减弱，产物随后脱附。工业实例包括哈伯法（铁催化剂）和催化转化器（铂/铑）。

Examiners may ask you to sketch energy profiles with and without a catalyst, showing the lower activation energy pathway. Both IB and Edexcel value the ability to explain catalytic action at the molecular level.

考官可能要求你绘制有催化剂和无催化剂的能量曲线，显示活化能更低的路径。IB 和 Edexcel 都看重从分子层面阐释催化作用的能力。

11. Experimental Techniques for Measuring Rates | 测量速率的实验技术

Common methods include collecting gas over water or in a gas syringe to monitor volume change, mass loss using a balance, colorimetry for coloured reactants/products, and conductivity for reactions that change the number of ions. Choosing the right technique depends on the reaction’s signature property.

常见方法包括排水集气或用气体注射器监测气体体积变化、用天平测量质量减少、用比色法跟踪有色物质、以及利用电导率测量离子数目变化的反应。选择合适技术取决于反应的特征性质。

For the iodine clock experiment (Edexcel Core Practical / IB IA), the sudden colour change is timed for varying concentrations, giving initial rate data. Good data logging and error analysis are essential for high marks in internal assessments.

碘钟实验（Edexcel 核心实验 / IB IA）中，通过记录不同浓度下溶液突然变色的时间来获取初始速率数据。扎实的数据记录和误差分析对于在内部评估中取得高分至关重要。

12. Common Exam Pitfalls & Tips | 常见考试误区与提分技巧

Mistake 1: confusing the rate constant k with the rate itself. k is independent of concentration but depends on temperature and activation energy. Rate changes with concentration.

常见误区一：混淆速率常数 k 与反应速率。k 与浓度无关，但随温度和活化能而变；速率则随浓度改变。

Mistake 2: assuming reaction orders equal stoichiometric coefficients. They must be experimentally determined. Always check data.

常见误区二：认为反应级数等于化学计量系数。级数必须通过实验确定，务必核查数据。

Mistake 3: misreading the units of k. The units depend on the overall order: e.g., for first order, s⁻¹; for second order, mol⁻¹ dm³ s⁻¹.

常见误区三：记错速率常数 k 的单位。单位取决于总级数：一级反应为 s⁻¹，二级反应为 mol⁻¹ dm³ s⁻¹。

Exam tip: always label axes clearly, draw tangents with a ruler, and show all conversion steps when dealing with the Arrhenius equation. Precision in language – such as ‘larger fraction of particles with E ≥ E_a‘ instead of ‘more energy’ – gains marks.

提分技巧：作图时清晰标注坐标轴，用直尺画切线，处理阿伦尼乌斯方程时写出所有换算步骤。用词精准——比方说“能量 ≥ E_a 的粒子比例更大”而非“能量更多”——能有效得分。

---

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）