📚 Kinetics and Reaction Mechanisms | 动力学与反应机理
Kinetics is the study of reaction rates and the step-by-step pathways that convert reactants into products. In A-Level Chemistry, you need to link experimental rate data to rate equations, rate constants, activation energy, and proposed reaction mechanisms.
动力学研究反应速率以及将反应物转化为产物的分步路径。在 A-Level 化学中，你需要将实验速率数据与速率方程、速率常数、活化能和提出的反应机理联系起来。
This topic combines quantitative analysis with molecular ideas about collisions, energy barriers, and the slow step. A strong grasp of kinetics allows you to judge whether a proposed mechanism is consistent with experimental evidence.
本主题将定量分析与关于碰撞、能垒和慢步骤的分子观点结合起来。掌握动力学可以让你判断所提出的机理是否与实验证据一致。
1. What Is Reaction Kinetics? | 什么是反应动力学？
Reaction kinetics is the branch of chemistry that studies the rate at which a chemical reaction occurs and the factors that affect it. It does not tell us whether a reaction is energetically feasible—that is thermodynamics—but it tells us how fast products form once the reaction is able to proceed.
反应动力学是化学的一个分支，研究化学反应进行的速率及影响速率的因素。它不回答反应在能量上是否可行（这是热力学问题），而是回答一旦反应能够进行，产物生成有多快。
Typical factors that affect rate include concentration, temperature, pressure for gases, surface area for solids, and the presence of a catalyst. Kinetic studies also reveal the sequence of bond-breaking and bond-forming events at the molecular level, which is the reaction mechanism.
影响速率的典型因素包括浓度、温度、气体压强、固体表面积以及催化剂的存在。动力学研究还能揭示分子水平上断键与成键事件的先后顺序，也就是反应机理。
The rate of a reaction is always a positive quantity, conventionally expressed as the change in product concentration per unit time or the negative change in reactant concentration per unit time.
反应速率始终是一个正值，通常表示为单位时间内产物浓度的变化，或单位时间内反应物浓度的负变化。
2. Measuring Reaction Rates | 测量反应速率
Rate is defined as the change in concentration of a reactant or product per unit time. For a reactant A, an average rate can be written as:
速率定义为单位时间内反应物或产物浓度的变化。对于反应物 A，平均速率可以写为：
rate = −Δ[A] ÷ Δt
The negative sign shows that the reactant concentration decreases. Rates are usually expressed in mol dm⁻³ s⁻¹, but when following a gas volume or mass change, the units may first appear as cm³ s⁻¹ or g s⁻¹ and then be converted.
负号表示反应物浓度在下降。速率通常以 mol dm⁻³ s⁻¹ 表示，但当跟踪气体体积或质量变化时，单位可能首先以 cm³ s⁻¹ 或 g s⁻¹ 出现，然后再进行换算。
Common experimental methods include collecting a gas in a syringe, measuring mass loss as a gas escapes, following colour intensity by colorimetry, recording electrical conductivity, and titrating samples at fixed time intervals after quenching.
常见的实验方法包括用注射器收集气体、测量气体逸出导致的质量损失、通过比色法追踪颜色强度、记录电导率，以及在固定时间间隔对淬灭后的样品进行滴定。
For any method, the instantaneous rate at a chosen time is found from the gradient of a concentration–time or volume–time graph. Choosing an appropriate method depends on whether a gas is produced, whether ions are formed or removed, and how quickly the reaction occurs.
无论采用哪种方法，某一时刻的瞬时速率都可以从浓度–时间或体积–时间图的斜率求得。选择合适的方法取决于是否有气体生成、是否形成或消耗离子，以及反应的快慢。
3. Rate Equations and Orders | 速率方程与反应级数
For a general reaction aA + bB → products, the experimentally determined rate equation may have the form:
对于一般反应 aA + bB → 产物，实验测定的速率方程可能具有以下形式：
rate = k[A]ᵐ[B]ⁿ
Here m is the order with respect to A, n is the order with respect to B, and m + n is the overall order. These orders are not necessarily equal to the stoichiometric coefficients a and b; they must be found experimentally.
其中 m 是对 A 的反应级数，n 是对 B 的反应级数，m + n 是总反应级数。这些级数不一定等于化学计量数 a 和 b，必须通过实验确定。
Common orders are zero, first, and second. Zero order means doubling the concentration does not change the rate; first order means rate doubles; second order means rate quadruples. A reactant can also have a fractional order, but at A-Level most examples are integer orders.
常见级数为零级、一级和二级。零级表示浓度加倍时速率不变；一级表示速率加倍；二级表示速率变为原来的四倍。反应物也可能有分数级数，但在 A-Level 中大多数例子是整数级数。
The rate equation is an empirical summary. It does not, by itself, prove which bonds are broken in the rate-determining step, but it gives strong clues about which species are involved before or during that step.
速率方程是一个经验总结。它本身不能证明决速步骤中断裂了哪些键，但它能提供有力线索，说明哪些物质在决速步骤之前或期间参与反应。
4. The Rate Constant and Its Units | 速率常数及其单位
The rate constant k links rate to concentrations at a given temperature. Its units depend on the overall order, so calculating units is a common exam skill.
速率常数 k 在给定温度下将速率与浓度联系起来。其单位取决于总反应级数，因此计算单位是常见考点。
For rate = k[A]ᵐ[B]ⁿ, the units of k are:
对于 rate = k[A]ᵐ[B]ⁿ，k 的单位为：
k units = mol dm⁻³ s⁻¹ ÷ (mol dm⁻³)^(m+n)
| Overall order | Units of k |
|---|---|
| 0 | mol dm⁻³ s⁻¹ |
| 1 | s⁻¹ |
| 2 | dm³ mol⁻¹ s⁻¹ |
| 3 | dm⁶ mol⁻² s⁻¹ |
k increases with temperature and also changes when a catalyst lowers the activation energy, but it is independent of concentration. A large k means a fast reaction under the stated conditions.
k 随温度升高而增大；当催化剂降低活化能时 k 也会增大，但 k 与浓度无关。k 值大表示在所述条件下反应较快。
5. Determining Orders Using Initial Rates | 用初始速率法测定反应级数
The initial rates method keeps all reactant concentrations constant except one, then measures the initial rate. Comparing experiments reveals the order with respect to the changing reactant.
初始速率法保持除一种反应物外所有反应物浓度不变，然后测量初始速率。比较实验即可确定相对于该变化反应物的级数。
For example, if doubling [A] while keeping [B] constant doubles the rate, the reaction is first order in A. If doubling [A] quadruples the rate, it is second order in A. If the rate does not change, it is zero order in A.
例如，如果在保持 [B] 不变时将 [A] 加倍，速率加倍，则对 A 为一级；如果 [A] 加倍后速率变为四倍，则对 A 为二级；如果速率不变，则对 A 为零级。
A common calculation uses the ratio method:
常见的计算采用比值法：
rate₂ ÷ rate₁ = ( [A]₂ ÷ [A]₁ )ᵐ × ( [B]₂ ÷ [B]₁ )ⁿ
This avoids needing the value of k when comparing experiments under the same conditions. Once the orders are known, k can be calculated by substituting any single complete experiment into the rate equation.
这样在相同条件下比较实验时无需先求 k 值。一旦知道级数，只需将任意一组完整实验数据代入速率方程即可计算 k。
Always quote the comparison explicitly: ‘doubling [A] doubled the rate, so the reaction is first order with respect to A.’ This method is reliable only if the initial rate is measured before significant concentrations have changed.
一定要明确写出比较过程：“将 [A] 加倍，速率加倍，因此对 A 为一级。”只有初始速率是在浓度发生显著变化之前测量时，这种方法才可靠。
6. Concentration–Time Graphs and Half-Life | 浓度–时间图与半衰期
Plotting concentration against time gives a curve whose gradient at any point is the instantaneous rate. The shape of the curve can indicate the order with respect to a single reactant.
以浓度为纵轴、时间为横轴作图，曲线在某一点的斜率即为瞬时速率。曲线形状可以提示相对于单一反应物的反应级数。
For a zero-order reaction, concentration falls linearly with time. For first order, the half-life is constant, so each successive half-life takes the same time. For second order, the half-life increases as concentration falls.
零级反应的浓度随时间线性下降；一级反应的半衰期恒定，每个连续半衰期所需时间相同；二级反应的半衰期随浓度降低而增大。
The half-life equation for a first-order reaction is:
一级反应的半衰期方程为：
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