Reaction Kinetics | 反应动力学

📚 Reaction Kinetics | 反应动力学

Reaction kinetics is the branch of chemistry that studies the speed of chemical reactions, the factors that control that speed, and the step-by-step pathway from reactants to products.

反应动力学是化学的一个分支，研究化学反应进行的快慢、控制该快慢的因素以及从反应物到产物的逐步路径。

Thermodynamics can predict whether a reaction is energetically feasible, but kinetics tells us whether it will actually happen at an observable rate.

热力学可以预测一个反应在能量上是否可行，而动力学则告诉我们该反应是否会以可观察的速率实际发生。

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1. What Is Reaction Kinetics? | 什么是反应动力学

Reaction kinetics focuses on rates rather than equilibrium positions or energy changes.

反应动力学关注的是反应速率，而不是平衡位置或能量变化。

For example, diamond converting to graphite is thermodynamically favourable, yet it does not occur on a human timescale because the kinetics are extremely slow.

例如，金刚石转化为石墨在热力学上是有利的，但由于动力学上极其缓慢，在人类时间尺度上不会发生。

Kinetics therefore bridges the gap between theoretical feasibility and practical observation by introducing the concept of reaction rate.

因此，动力学通过引入反应速率的概念，把理论可行性与实际观察联系了起来。

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2. Rate of Reaction and Measurement | 反应速率及其测量

The rate of a reaction is defined as the change in concentration of a reactant or product per unit time.

反应速率定义为反应物或产物浓度在单位时间内的变化。

For the general reaction aA + bB → cC + dD, the rate can be expressed as:

对于一般反应 aA + bB → cC + dD，速率可表示为：

rate = −(1/a)(Δ[A]/Δt) = +(1/c)(Δ[C]/Δt)

The negative sign is used for reactants because their concentrations decrease over time.

反应物使用负号，因为其浓度随时间减小。

The usual unit of rate is mol dm⁻³ s⁻¹, but gas pressure or other measurable quantities may also be used.

速率的常用单位是 mol dm⁻³ s⁻¹，但也可以使用气体压强或其他可测量物理量。

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3. Rate Equation and Order of Reaction | 速率方程与反应级数

The rate equation links the rate of reaction to the concentrations of the reactants raised to some powers.

速率方程将反应速率与各反应物浓度的某次幂联系起来。

For a reaction involving A and B, the general rate equation is:

对于涉及 A 和 B 的反应，一般速率方程为：

rate = k[A]ᵐ[B]ⁿ

Here m is the order with respect to A, n is the order with respect to B, and k is the rate constant.

其中 m 是反应对 A 的级数，n 是反应对 B 的级数，k 是速率常数。

The overall order of reaction is m + n.

反应的总级数为 m + n。

At A-Level, orders are usually 0, 1 or 2, and they must be found by experiment; they are not simply the stoichiometric coefficients in the balanced equation.

在 A-Level 阶段，级数通常为 0、1 或 2，必须通过实验测定；它们不一定是配平方程中的化学计量系数。

For example, the reaction 2NO + O₂ → 2NO₂ is second order with respect to NO and first order with respect to O₂, giving a third order overall.

例如，反应 2NO + O₂ → 2NO₂ 对 NO 为二级，对 O₂ 为一级，总级数为三级。

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4. Experimental Determination of Orders | 实验测定反应级数

Common methods for determining reaction orders include the initial rates method, continuous monitoring, and the use of half-life data.

测定反应级数的常用方法包括初速率法、连续监测法和半衰期数据法。

In the initial rates method, the initial concentration of one reactant is changed while the others are kept constant, and the initial rate is measured each time.

在初速率法中，改变一种反应物的初始浓度，同时保持其他反应物不变，并每次测量初始速率。

The following table summarises the deduction rules:

下表总结了推断规则：

Change in [A] | [A] 的变化	Effect on initial rate | 对初速率的影响	Order | 级数
Doubled | 加倍	No change | 不变	0
Doubled | 加倍	Rate doubles | 速率加倍	1
Doubled | 加倍	Rate quadruples | 速率变为四倍	2

Continuous monitoring involves recording concentration at regular time intervals and plotting a concentration-time graph.

连续监测法包括每隔一定时间记录浓度，并绘制浓度-时间图。

The gradient of the concentration-time graph at any point gives the rate at that instant.

浓度-时间图上任意一点的斜率即为该时刻的瞬时速率。

Clock reactions are also useful because the time taken to reach a fixed observable endpoint can be used as a relative measure of initial rate.

时钟反应也很有用，因为达到固定可观察终点所需的时间可以作为初始速率的相对量度。

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5. The Rate Constant k | 速率常数 k

The rate constant k is the proportionality constant in the rate equation.

速率常数 k 是速率方程中的比例常数。

Its value is constant for a given reaction at a fixed temperature, but its units depend on the overall order of reaction.

对于给定反应，在固定温度下 k 的值是常数，但其单位取决于反应的总级数。

For zero order, the units of k are mol dm⁻³ s⁻¹.

对于零级反应，k 的单位是 mol dm⁻³ s⁻¹。

For first order, the units are s⁻¹.

对于一级反应，单位是 s⁻¹。

For second order, the units are mol⁻¹ dm³ s⁻¹.

对于二级反应，单位是 mol⁻¹ dm³ s⁻¹。

A larger value of k indicates a faster reaction under the same concentration conditions.

在相同浓度条件下，k 值越大，反应越快。

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6. Half-Life and Reaction Orders | 半衰期与反应级数

Half-life is the time taken for the concentration of a reactant to decrease to half of its initial value.

半衰期是指反应物浓度降低到初始值一半所需的时间。

The relationship between half-life and concentration depends on reaction order.

半衰期与浓度之间的关系取决于反应级数。

Zero order: t₁/₂ = [A]₀ / (2k)

First order: t₁/₂ = 0.693 / k

Second order: t₁/₂ = 1 / (k[A]₀)

For a first-order reaction, the half-life is constant and does not depend on the initial concentration.

对于一级反应，半衰期是常数，与初始浓度无关。

For a second-order reaction, the half-life becomes longer as the concentration decreases.

对于二级反应，随着浓度降低，半衰期会变长。

This pattern is often used to identify the order from experimental concentration-time data.

这一规律常用于通过实验浓度-时间数据来确定反应级数。

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

According to collision theory, particles must collide with sufficient energy and in the correct orientation for a reaction to occur.

根据碰撞理论，粒子必须以足够的能量和正确的取向碰撞，反应才能发生。

The minimum energy needed for a successful collision is called the activation energy Eₐ.

成功碰撞所需的最低能量称为活化能 Eₐ。

Even if the energy is sufficient, an incorrect orientation may still prevent the collision from producing products.

即使能量足够，取向不正确也可能使碰撞无法生成产物。

Increasing concentration increases the frequency of collisions, while increasing temperature increases both collision frequency and the fraction of particles with enough energy to overcome Eₐ.

增大浓度会增加碰撞频率，而升高温度则同时增加碰撞频率和具有足够能量克服 Eₐ 的粒子比例。

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8. Boltzmann Distribution | 玻尔兹曼分布

The Boltzmann distribution shows the distribution of kinetic energy among molecules at a particular temperature.

玻尔兹曼分布显示了在某一温度下分子动能分布的情况。

The curve starts at the origin, rises to a peak, and then decreases towards high energy; it never touches the energy axis.

曲线从原点开始，上升到峰值，然后向高能端逐渐降低；它永远不会与能量轴相交。

When temperature is increased, the peak moves to the right and becomes lower, but the area under the curve beyond Eₐ increases greatly.

温度升高时，峰值向右移动并降低，但超过 Eₐ 的曲线下面积会大幅增加。

This means a much larger proportion of molecules can react, so the rate increases sharply with temperature.

这意味着更多比例的分子能够发生反应，因此反应速率随温度急剧上升。

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9. The Arrhenius Equation | 阿伦尼乌斯方程

The temperature dependence of the rate constant is described by the Arrhenius equation.

速率常数与温度的关系可以用阿伦尼乌斯方程描述。

k = A exp(−Eₐ/RT)

A is the pre-exponential factor, Eₐ is the activation energy, R is the gas constant, and T is the absolute temperature in kelvin.

A 是指前因子，Eₐ 是活化能，R 是气体常数，T 是开尔文绝对温度。

Taking natural logarithms gives a linear form:

取自然对数可得线性形式：

ln k = ln A − Eₐ/(RT)

A graph of ln k against 1/T gives a straight line with gradient −Eₐ/R.

以 ln k 对 1/T 作图，可得斜率为 −Eₐ/R 的直线。

This is a common A-Level calculation in which the activation energy is found from experimental rate constants at several temperatures.

这是 A-Level 中常见的计算类型：利用不同温度下的实验速率常数求出活化能。

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10. Catalysts | 催化剂

A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy.

催化剂通过提供活化能较低的替代路径来加快反应。

It is chemically unchanged at the end of the reaction and does not affect the position of equilibrium.

它在反应结束时化学性质不变，也不影响平衡位置。

Lowering Eₐ increases the fraction of particles that have enough energy to react, so the rate increases without changing the temperature.

降低 Eₐ 会增加具有足够能量反应的粒子比例，因此无需改变温度即可提高速率。

Catalysts are important in industrial processes such as the Haber process and the Contact process.

催化剂在哈伯法合成氨和接触法制硫酸等工业过程中非常重要。

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11. Reaction

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