The Rate of Reaction | 反应速率

📚 The Rate of Reaction | 反应速率

Understanding reaction rates is central to chemistry and helps us explain why some reactions are fast while others are slow. In this article, we will explore what reaction rate means, how to measure it, the factors that affect it, and how the collision theory explains these observations.

理解反应速率是化学的核心，它帮助我们解释为什么有些反应很快而有些却很慢。在这篇文章中，我们将探讨反应速率的含义、如何测量、影响反应速率的因素，以及碰撞理论如何解释这些现象。

1. What is Rate of Reaction? | 什么是反应速率？

The rate of a reaction is the change in concentration (or amount) of a reactant or product per unit time. It tells us how quickly reactants are used up and products are formed.

反应速率是指单位时间内反应物或产物浓度（或物质的量）的变化量。它告诉我们反应物被消耗和多快、产物生成多快。

For example, when magnesium reacts with hydrochloric acid, the mass of magnesium decreases and hydrogen gas is released. The rate can be found by measuring the volume of gas produced per second.

例如，镁与盐酸反应时，镁的质量减少并释放氢气。速率可以通过测量每秒产生气体的体积来确定。

Rate = change in amount of reactant or product ÷ time taken

速率 = 反应物或产物的变化量 ÷ 时间

The unit of rate is often mol dm⁻³ s⁻¹ or cm³/s, depending on how the amount is measured.

速率的单位通常是 mol dm⁻³ s⁻¹ 或 cm³/s，取决定量测量的方式。

2. Measuring Reaction Rate | 测量反应速率

There are several experimental methods to measure how fast a reaction proceeds. The most common methods include collecting a gas, measuring mass loss, and observing colour changes or turbidity.

测量反应进行快慢有几种实验方法。最常用的方法包括收集气体、测量质量损失，以及观察颜色变化或浑浊度。

Gas collection – count bubbles or use a gas syringe to measure the volume of gas over time.
Mass loss – place the reaction on a balance and record the decrease in mass as gas escapes.
Colour change – use a colorimeter or time how long it takes for a solution to become opaque.

气体收集 – 数气泡或用气体注射器测量气体体积随时间的变化。
质量损失 – 把反应容器放在天平上，记录气体逸出时质量减少。
颜色变化 – 使用比色计，或计时溶液变得不再透明所需的时间。

For reactions producing a solid precipitate, such as sodium thiosulfate with hydrochloric acid, we can measure the time taken for a cross drawn under the flask to disappear.

对于产生固体沉淀的反应，例如硫代硫酸钠与盐酸反应，我们可以测量烧杯下面画上十字被完全遮挡所需的时间。

3. Collision Theory | 碰撞理论

For particles to react, they must collide with each other with sufficient energy and the correct orientation. This idea is called the collision theory.

粒子要发生反应，它们必须以足够的能量和正确的方向相互碰撞。这个观点称为碰撞理论。

Only a small fraction of collisions lead to a reaction. These are called successful collisions or effective collisions. The minimum energy needed for a successful collision is the activation energy.

只有一小部分碰撞能引发反应，这些碰撞被称为有效碰撞。发生有效碰撞所需的最低能量称为活化能。

A collision must have energy ≥ activation energy and the correct geometry.

碰撞必须满足能量 ≥ 活化能，并且方向正确。

Increasing the frequency of successful collisions increases the reaction rate.

增加有效碰撞的频率会增大反应速率。

4. Factor: Concentration and Pressure | 因素：浓度与压强

When the concentration of a dissolved reactant is increased, there are more particles in the same volume. This makes collisions more frequent, so the reaction rate increases.

当溶解的反应物浓度增大时，相同体积内的粒子更多。这使得碰撞更频繁，因此反应速率增大。

For gases, increasing the pressure has a similar effect because it reduces the volume and pushes particles closer together.

对于气体，增大压强有类似效果，因为它减小体积并使粒子更加靠近。

Doubling the concentration roughly doubles the frequency of collisions, although the exact effect depends on the order of the reaction. In IGCSE, you usually need to state that rate increases.

浓度加倍，碰撞频率大约加倍，但确切效果取决于反应级数。在 IGCSE 中，通常只需说明速率增大。

5. Factor: Surface Area | 因素：表面积

When a solid reactant is broken into smaller pieces, its total surface area increases. More reactant particles are exposed and available to collide.

当固体反应物被粉碎成更小的颗粒时，其总表面积增大。更多反应物粒子暴露在外并可用于碰撞。

For example, powdered calcium carbonate reacts with hydrochloric acid much faster than marble chips of the same mass.

例如，粉末状碳酸钙与盐酸反应比同质量的大理石颗粒快得多。

With a larger surface area, the frequency of collisions between solid particles and aqueous or gaseous particles increases, so the rate increases.

表面积更大，固体颗粒与液体或气体颗粒之间的碰撞频率增加，因此速率增大。

6. Factor: Temperature | 因素：温度

Raising the temperature increases the average kinetic energy of particles. Particles move faster and collide more often.

升高温度会增加粒子的平均动能。粒子运动更快，碰撞更频繁。

More importantly, a greater proportion of particles now have energy equal to or greater than the activation energy. This dramatically increases the number of successful collisions.

更重要的是，现在有更大比例的粒子具有等于或大于活化能的能量。这会显著增加有效碰撞的次数。

A typical rule of thumb is that for every 10 °C rise in temperature, the reaction rate roughly doubles. This is because the number of energetic particles increases sharply.

一个常见经验法则是，温度每升高 10 °C，反应速率大约加倍。这是因为高能量粒子的数量急剧增加。

7. Factor: Catalysts | 因素：催化剂

A catalyst is a substance that speeds up a chemical reaction without being used up. It works by providing an alternative reaction pathway with a lower activation energy.

催化剂是一种能加快化学反应而不被消耗的物质。它通过提供一条活化能更低的替代反应路径来起作用。

With a lower activation energy, a much larger fraction of collisions becomes successful. Catalysts do not change the amount of product formed, only how quickly it is produced.

由于活化能降低，很大比例的碰撞变成有效碰撞。催化剂不改变产物的量，只改变生成产物的速率。

Enzymes are biological catalysts. In industry, catalysts like iron in the Haber process help save energy and reduce costs.

酶是生物催化剂。在工业中，如哈伯法中的铁催化剂，可以节约能源并降低成本。

8. Reaction Rate Graphs | 反应速率图

When we record volume of gas or mass loss against time, we obtain a curve that becomes flatter as the reaction proceeds. The slope at any point gives the rate at that moment.

当我们记录气体体积或质量损失随时间变化时，会得到一条曲线，随着反应进行曲线逐渐变平。任意点的斜率给出该时刻的速率。

Initially, the slope is steep because reactant concentration is high. As reactants are consumed, the slope decreases until the gradient becomes zero, meaning the reaction has stopped.

初始时，斜率较大，因为反应物浓度高。随着反应物消耗，斜率减小，直到梯度变为零，说明反应停止。

Steep slope = fast reaction
Flat slope = slow or no reaction
The final amount of product is the same if the same mass of limiting reactant is used

斜率陡 = 反应快
斜率平 = 反应慢或不再反应
若使用相同质量的限制反应物，最终产物的量相同

More reactant particles (higher concentration, surface area, or temperature) give a steeper initial slope and a shorter time to completion.

更多反应物粒子（更高的浓度、表面积或温度）会产生更陡的初始斜率和更短的完成时间。

9. Activation Energy and Energy Profiles | 活化能与能量-反应进程图

An energy profile diagram shows the energy changes during a reaction. The activation energy is the energy barrier between reactants and the activated complex.

能量-反应进程图显示反应过程中的能量变化。活化能是反应物与活化络合物之间的能量屏障。

For an exothermic reaction, the products have less energy than the reactants. For an endothermic reaction, the products have more energy than the reactants.

对于放热反应，产物的能量低于反应物。对于吸热反应，产物的能量高于反应物。

When a catalyst is added, the profile shows a second, lower hump representing the alternative pathway. The overall energy change ΔH stays the same.

当加入催化剂时，图中显示第二个更低的峰，代表替代路径。总能量变化 ΔH 保持不变。

10. Real-World Applications | 实际应用

Controlling reaction rates is important in everyday life and industry. Food is kept in a fridge to slow down the spoilage reactions caused by microorganisms and enzymes.

控制反应速率在日常生活和工业中都很重要。食物保存在冰箱中，是为了减缓由微生物和酶引起的变质反应。

Flour dust in a factory can explode because the very large surface area of fine particles leads to a very fast combustion reaction.

工厂中的面粉粉尘可能发生爆炸，因为细颗粒具有非常大的表面积，导致燃烧反应非常迅速。

Catalytic converters in cars use platinum to speed up the conversion of toxic gases like carbon monoxide into less harmful products.

汽车中的催化转化器使用铂来加速一氧化碳等有毒气体转化为危害较小的产物。

By understanding and controlling rate factors, chemists can design safer and more efficient industrial processes.

通过理解和控制速率因素，化学家可以设计更安全、更高效的工业过程。

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The Rate of Reaction | 反应速率