Reaction Rates | 反应速率考点精讲

📚 Reaction Rates | 反应速率考点精讲

Reaction rates are a fundamental topic in A-Level WJEC Chemistry, examining how quickly chemical reactions occur and the factors that influence them. Understanding kinetics is essential for predicting reaction behaviour and for applications in industrial and biological systems.

反应速率是 A-Level WJEC 化学的核心内容，考察化学反应进行的快慢以及影响速率的因素。掌握动力学对于预测反应行为和工业、生物系统中的应用至关重要。

1. Defining Reaction Rate | 定义反应速率

The reaction rate is the change in concentration of a reactant or product per unit time. It is usually expressed in mol dm⁻³ s⁻¹. For a general reaction aA + bB → cC + dD, the rate can be measured in terms of any participating species:

反应速率是指单位时间内反应物或产物浓度的变化，通常用 mol dm⁻³ s⁻¹ 表示。对于一般反应 aA + bB → cC + dD，速率可以用任一参与物质来度量：

Rate = – 1/a × Δ[A]/Δt = – 1/b × Δ[B]/Δt = 1/c × Δ[C]/Δt = 1/d × Δ[D]/Δt

The negative sign for reactants indicates their concentration decreases, while the positive sign for products shows an increase. The stoichiometric coefficients a, b, c, d ensure the rate is numerically the same regardless of which species is monitored.

反应物的负号表示其浓度减少，而产物的正号表示浓度增加。化学计量系数 a、b、c、d 确保了无论监测哪种物质，速率的数值都相同。

2. Collision Theory and Activation Energy | 碰撞理论与活化能

For a reaction to occur, reactant particles must collide with sufficient energy and the correct orientation. Only a fraction of collisions possess energy equal to or greater than the activation energy (Eₐ), leading to a successful reaction.

反应发生的条件：反应物粒子必须发生碰撞，且具备足够的能量和正确的取向。只有一小部分碰撞拥有不低于活化能 (Eₐ) 的能量，才能引发成功的反应。

Activation energy is the minimum energy required for colliding particles to break existing bonds and form new ones. The higher the Eₐ, the slower the reaction at a given temperature, because fewer molecules possess the necessary energy.

活化能是碰撞粒子断裂原有化学键并形成新键所需的最低能量。Eₐ 越高，在相同温度下反应越慢，因为具有必要能量的分子越少。

3. Effect of Concentration on Rate | 浓度对速率的影响

Increasing the concentration of a reactant in solution increases the number of particles per unit volume. This leads to a higher collision frequency, and therefore a greater rate of reaction, provided the activation energy barrier remains unchanged.

提高溶液中反应物的浓度会增加单位体积内的粒子数量，从而提高碰撞频率，使反应速率增大。前提是活化能垒保持不变。

For many reactions, the rate is proportional to the concentration of one or more reactants raised to some power. Experimentally, this relationship is revealed through initial rates measurements.

对许多反应而言，速率与一种或多种反应物浓度的某次幂成正比。通过初始速率法实验可以揭示这种关系。

4. Effect of Pressure on Gaseous Reactions | 压力对气体反应的影响

For gases, increasing the pressure (at constant temperature) reduces the volume, which increases the concentration of gas molecules. The collision frequency rises, and so does the rate of reaction. This effect is analogous to increasing concentration in solutions.

对于气体反应，在恒温下增大压强会缩小体积，从而提高气体分子的浓度。碰撞频率上升，反应速率随之加快。这一影响类似于溶液中提高浓度的效应。

If the reaction involves a different number of moles on each side, pressure changes may also affect the equilibrium position, but the instantaneous rate is primarily influenced by collision frequency.

如果反应前后气体分子数不同，压强变化还会影响平衡位置，但瞬时速率主要受碰撞频率的影响。

5. Effect of Temperature | 温度的影响

Raising the temperature increases the average kinetic energy of particles. More importantly, the fraction of particles with energy ≥ Eₐ increases dramatically. The frequency of collisions also rises, but it is the significant growth in successful collisions that dominates the rate enhancement.

升高温度增加了粒子的平均动能。更重要的是，能量 ≥ Eₐ 的粒子比例急剧增大。碰撞频率也有提升，但主导速率提高的是成功碰撞次数的显著增长。

A rough rule of thumb is that for many reactions near room temperature, the rate approximately doubles for every 10 °C rise. This is explained quantitatively by the Boltzmann distribution and the Arrhenius equation.

一个粗略的经验规律是：室温附近的许多反应，温度每升高 10 °C，速率大致加倍。这可以通过玻尔兹曼分布和阿伦尼乌斯方程定量解释。

6. Effect of Surface Area | 表面积的影响

For heterogeneous reactions involving solids, the reaction only occurs at the surface. Breaking a solid into smaller pieces increases the total surface area. This exposes more reactant particles to collisions, resulting in a higher reaction rate.

对于涉及固体的多相反应，反应只在表面发生。将固体分解成更小的颗粒能增大总表面积，使更多反应物粒子暴露出来参与碰撞，从而提高反应速率。

Powders react faster than lumps; this is critical in industrial processes such as combustion of pulverised coal or catalytic converters with high-surface-area supports.

粉末比块状反应更快；这在诸如煤粉燃烧或使用大表面积载体的催化转换器等工业过程中尤为关键。

7. Catalysts and Reaction Pathways | 催化剂与反应路径

A catalyst provides an alternative reaction pathway with a lower activation energy. It does not get consumed in the overall reaction. The same reactants are transformed into the same products, but the energy barrier is reduced, so a larger fraction of collisions become effective at a given temperature.

催化剂提供了一条活化能更低的替代反应路径，自身在整体反应中不被消耗。相同的反应物转化为相同的产物，但由于能垒降低，在给定温度下有效碰撞的比例大幅增加。

Catalysts can be homogeneous (same phase as reactants) or heterogeneous (different phase). WJEC candidates should be able to draw and interpret energy profile diagrams showing the effect of a catalyst on Eₐ.

催化剂可以是均相（与反应物同相）或非均相（不同相）。WJEC 考生需要能够绘制并解释能级图，展示催化剂对 Eₐ 的影响。

8. Rate Equations and the Rate Constant | 速率方程与速率常数

For a reaction A + B → products, the rate law may be written as:

对于反应 A + B → 产物，速率方程可写作：

Rate = k [A]ᵐ [B]ⁿ

where k is the rate constant, and m and n are the orders of reaction with respect to A and B. The overall order is m + n. The units of k depend on the overall order.

其中 k 为速率常数，m 和 n 分别是相对于 A 和 B 的反应级数。总反应级数为 m + n。k 的单位取决于总级数。

WJEC expects students to determine orders of reaction from experimental data, understand zero, first, and second order behaviour, and calculate k with appropriate units, e.g., s⁻¹ for first order, dm³ mol⁻¹ s⁻¹ for second order.

WJEC 要求考生通过实验数据确定反应级数，理解零级、一级和二级行为，并计算带有正确单位的 k，如一级反应的单位是 s⁻¹，二级反应为 dm³ mol⁻¹ s⁻¹。

9. Determining Reaction Order | 确定反应级数

The order can be found using the initial rates method: measuring the initial rate while systematically varying the concentration of one reactant and keeping others constant. If doubling [A] doubles the rate, the order with respect to A is 1; if the rate quadruples, the order is 2; if the rate is unchanged, the order is 0.

级数可通过初始速率法确定：系统改变一种反应物的浓度、保持其余不变，测量初始速率。若 [A] 加倍导致速率加倍，则对 A 的级数为 1；若速率变为四倍，级数为 2；若速率不变，级数为 0。

Continuous monitoring methods, such as following gas volume, mass loss, or colour change over time, allow concentration-time graphs to be plotted. The shape can also indicate the order: linear decay of concentration with time suggests zero order; constant half-life suggests first order.

连续监测法（如跟踪气体体积、质量损失或颜色变化）可绘制浓度-时间图。曲线的形状也能指示级数：浓度随时间的线性下降暗示零级反应；恒定的半衰期暗示一级反应。

10. The Rate-Determining Step | 决速步骤

In a multi-step reaction, the slowest step governs the overall rate and is called the rate-determining step. The rate equation is determined solely by the species involved in this step (or steps leading up to it).

在多步反应中，最慢的步骤控制着总反应速率，该步骤称为决速步骤。速率方程仅由参与该步骤（或之前步骤）的物质所决定。

If a reactant does not appear in the rate equation, it is not involved in the rate-determining step, but reacts in a later fast step. Proposing a mechanism consistent with the rate equation is a common exam task.

如果某种反应物未出现在速率方程中，说明它不参与决速步骤，而是在后续的快反应中才参与反应。提出与速率方程相符的反应机理是常见的考试题目。

11. Concentration-Time Graphs and Half-Life | 浓度-时间图与半衰期

Plots of [reactant] versus time are essential for analysing kinetics:

反应物浓度对时间作图是分析动力学的重要手段：

Zero order: A straight line with constant negative gradient; rate is independent of concentration.
零级：一条斜率为负常数的直线；速率与浓度无关。
First order: Constant half-life (t₁/₂ = ln 2 / k); the concentration falls by equal fractions in equal time intervals.
一级：恒定的半衰期 (t₁/₂ = ln 2 / k)；浓度在相等的时间间隔内以相同比例减少。
Second order: Half-life increases as concentration falls; a plot of 1/[A] vs time gives a straight line.
二级：半衰期随浓度下降而增大；1/[A] 对时间作图呈直线。

The concept of half-life is particularly useful in first-order processes such as radioactive decay and many drug elimination profiles.

半衰期的概念在一级过程中尤为有用，例如放射性衰变和许多药物消除曲线。

12. The Arrhenius Equation | 阿伦尼乌斯方程

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

温度、活化能与速率常数之间的定量关系由阿伦尼乌斯方程给出：

k = A e⁻ᴱᵃ/ᴿᵀ

Taking natural logarithms:

取自然对数：

ln k = ln A – Eₐ / RT

A plot of ln k against 1/T gives a straight line with gradient –Eₐ/R and intercept ln A. This allows Eₐ to be determined experimentally. R is the gas constant (8.31 J K⁻¹ mol⁻¹) and T is the temperature in Kelvin.

以 ln k 对 1/T 作图可得一条直线，斜率为 –Eₐ/R，截距为 ln A，从而可实验测定 Eₐ。R 为摩尔气体常数（8.31 J K⁻¹ mol⁻¹），T 为开尔文温度。

WJEC expects students to use this equation to calculate activation energy from given data and to understand the significance of the pre-exponential factor A, which relates to collision frequency and orientation.

WJEC 要求考生运用该方程从给定数据计算活化能，并理解指前因子 A 的意义，它与碰撞频率和取向有关。

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Reaction Rates | 反应速率 考点精讲