Which Order of Reaction? | 化学反应级数怎么判断？

📚 Which Order of Reaction? | 化学反应级数怎么判断？

When chemists study reaction kinetics, one of the first questions they ask is: how does the rate depend on the concentration of each reactant? The answer is summarised by the order of reaction. Understanding how to determine reaction orders from experimental data is a core skill in Cambridge A-Level Chemistry, and it is frequently examined through rate equations, graphs, and mechanisms.

化学家研究反应动力学时，首先会问：反应速率如何随各反应物浓度变化？答案可以用反应级数来概括。掌握如何从实验数据判断反应级数是剑桥 A-Level 化学的核心技能，也是速率方程、图像和机理题中的常见考点。

1. What Is Order of Reaction? | 什么是反应级数？

The order of reaction with respect to a particular reactant is the power to which its concentration is raised in the experimentally determined rate equation. It shows how sensitive the rate is to changes in that reactant’s concentration.

某一反应物的反应级数，是实验测得的速率方程中该反应物浓度的幂指数。它反映了反应速率对该反应物浓度变化的敏感程度。

The overall order of reaction is the sum of the individual orders for all reactants in the rate equation. For example, if rate = k[A][B]², the order with respect to A is 1, with respect to B is 2, and the overall order is 3.

反应的总级数是速率方程中所有反应物各自级数之和。例如，若 rate = k[A][B]²，则对 A 为 1 级，对 B 为 2 级，总级数为 3。

Reaction order cannot be deduced from the stoichiometric coefficients of the balanced equation. It must be found experimentally.

反应级数不能从配平方程式的化学计量数直接推出，必须通过实验测定。

2. The Rate Equation and Rate Constant | 速率方程与速率常数

The rate equation links the reaction rate to the concentrations of the reactants and the rate constant, k. For a reaction A + B → products, a general form is:

速率方程将反应速率与反应物浓度以及速率常数 k 联系起来。对于反应 A + B → 产物，一般形式为：

rate = k[A]ᵐ[B]ⁿ

Here, m and n are the orders with respect to A and B. The rate constant k is temperature dependent but is not affected by concentration changes.

式中 m 和 n 分别是对 A 和 B 的反应级数。速率常数 k 只随温度变化，不受浓度变化影响。

The values of m and n are usually small integers or sometimes zero, or simple fractions such as ½ in very rare school-level examples.

m 和 n 通常是小的整数或零，在学校层面的极少例子中也可能出现 ½ 这样的简单分数。

3. Units of the Rate Constant | 速率常数的单位

The units of k vary with the overall order of reaction. Since rate is usually measured in mol dm⁻³ s⁻¹ and concentration in mol dm⁻³, the units of k can be worked out from the rate equation.

k 的单位随反应总级数而变化。由于速率常用 mol dm⁻³ s⁻¹ 表示，浓度用 mol dm⁻³ 表示，因此可以根据速率方程推导 k 的单位。

For zero order, k has units mol dm⁻³ s⁻¹. For first order, k has units s⁻¹. For second order, k has units mol⁻¹ dm³ s⁻¹. For third order, k has units mol⁻² dm⁶ s⁻¹.

零级反应中 k 的单位为 mol dm⁻³ s⁻¹；一级反应为 s⁻¹；二级反应为 mol⁻¹ dm³ s⁻¹；三级反应为 mol⁻² dm⁶ s⁻¹。

Checking the units of k is a quick way to determine the overall order in an exam question.

检查 k 的单位是考试中快速判断总级数的一种方法。