📚 A-Level Chemistry: The Effect of Concentration on Reaction Rate | A-Level 化学：浓度对反应速率的影响
In any chemical reaction, the rate measures how quickly reactants are converted into products. For most reactions, changing the concentration of a dissolved or gaseous reactant produces a clear and measurable change in rate. In A-Level Chemistry, concentration is one of the four key factors — together with temperature, pressure and catalysts — that control reaction kinetics, and it is closely linked to collision theory.
在任何化学反应中，反应速率衡量反应物转化为产物的快慢。对大多数反应而言，改变溶解状态或气态反应物的浓度，会引起反应速率明显且可测量的变化。在 A-Level 化学中，浓度与温度、压强和催化剂并列为影响反应动力学的主要因素之一，并且与碰撞理论密切相关。
1. What Is Reaction Rate? | 什么是反应速率？
Reaction rate is usually defined as the change in concentration of a reactant or product per unit time. The units for reaction rate are typically mol dm⁻³ s⁻¹.
反应速率通常定义为反应物或产物浓度在单位时间内的变化量。反应速率的常用单位是 mol dm⁻³ s⁻¹。
For a reactant A, the rate can be written as:
Rate = −Δ[A] / Δt
For a product P, the rate can be written as:
Rate = Δ[P] / Δt
The minus sign for a reactant is used because the concentration of A decreases with time. At A-Level, you are also expected to think about the initial rate: the instantaneous rate at the moment the reaction begins.
对反应物 A 而言，速率可写成：Rate = −Δ[A]/Δt；对产物 P 而言，速率可写成：Rate = Δ[P]/Δt。反应物浓度随时间下降，因此需要负号。在 A-Level 中，你还需要理解“初始速率”的概念：即反应刚开始那一瞬间的瞬时速率。
2. Collision Theory | 碰撞理论
Collision theory states that reacting particles must collide before they can react. However, not every collision leads to a reaction. A collision is successful only when the colliding particles have total kinetic energy at least equal to the activation energy Eₐ, and when they collide with the correct orientation.
碰撞理论指出：反应粒子必须先发生碰撞，才可能发生反应。但并非每次碰撞都会引发反应。只有碰撞粒子的总动能至少达到活化能 Eₐ，并且采取正确取向时，碰撞才是“有效碰撞”。
The rate of reaction depends on two things: how often collisions occur, called the collision frequency, and what fraction of those collisions are successful. Concentration mainly changes the collision frequency.
反应速率取决于两个因素：一是碰撞发生的频率，称为碰撞频率；二是有多少比例的碰撞属于有效碰撞。浓度主要改变的是碰撞频率。
3. Why Higher Concentration Increases Rate | 为什么浓度升高会加快反应速率？
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