Rates of Reaction | 化学反应速率

📚 Rates of Reaction | 化学反应速率

Chemical reactions occur at different speeds. Some, like a firework exploding, are almost instantaneous; others, such as the rusting of iron, take years. In this article, we explore what is meant by the rate of a reaction, how it can be measured, and the four main factors that affect it.

化学反应发生的速度各不相同。有些反应，如烟花爆炸，几乎瞬间完成；而另一些反应，如铁的生锈，则需要数年时间。在本文中，我们探讨什么是反应速率、如何测量它，以及影响它的四个主要因素。

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

The rate of a reaction tells us how quickly reactants are used up or products are formed. It is a measure of the change in concentration (or mass, volume, or colour) over a given time.

反应速率告诉我们反应物被消耗或产物生成的快慢。它是在一定时间内浓度（或质量、体积、颜色）的变化量。

We can write this as:

rate = change in amount ÷ time taken

Common units include g/s, cm³/s, or mol/(dm³·s). For a fast reaction, the amount changes quickly; for a slow reaction, it changes slowly.

常用单位有 g/s、cm³/s 或 mol/(dm³·s)。对于快速反应，物质的量变化很快；对于慢速反应，变化很慢。

2. Measuring the Rate | 测量反应速率

Rates are usually found by measuring a property that changes during the reaction. In the reaction between hydrochloric acid and marble chips (calcium carbonate), carbon dioxide gas is produced.

通常通过测量反应过程中变化的某种性质来求得速率。在盐酸与大理石碎片（碳酸钙）的反应中，会产生二氧化碳气体。

Three common methods are:

Measuring gas volume – using a gas syringe or measuring cylinder over time.
Measuring mass loss – placing the flask on a balance and recording the decrease as gas escapes.
Measuring turbidity / colour change – timing how long it takes for a cross to disappear when a precipitate forms.

测量气体体积 – 使用气体注射器或量筒随时间记录。
测量质量损失 – 将烧瓶放在天平上，记录因气体逸出而减少的质量。
测量浑浊度/颜色变化 – 当沉淀形成时，计时十字标记消失所需的时间。

For example, in the marble-chip reaction, the volume of CO₂ can be recorded every 10 seconds. The steeper the slope on a concentration–time graph, the faster the reaction.

例如，在大理石碎片反应中，可以每10秒记录一次 CO₂ 的体积。在浓度–时间图上，斜率越陡，反应越快。

3. Collision Theory | 碰撞理论

For a reaction to happen, reactant particles must collide with each other. However, most collisions do not lead to a reaction. Only collisions with enough energy – greater than or equal to the activation energy – and the correct orientation result in a successful reaction.

反应发生的前提是反应物粒子必须相互碰撞。然而，大多数碰撞并不会导致反应。只有那些能量足够——达到或超过活化能——且方向正确的碰撞才是有效碰撞。

Activation energy (Eₐ) is the minimum energy needed for a reaction to occur. At a given temperature, only a fraction of particles have this energy.

活化能（Eₐ）是反应发生所需的最低能量。在给定温度下，只有一部分粒子具有这样的能量。

Increasing the frequency of successful collisions increases the rate of reaction. This idea is called collision theory.

增加有效碰撞的频率会提高反应速率。这一思想被称为碰撞理论。

4. Temperature | 温度

Raising the temperature increases the average kinetic energy of the particles. They move faster, so they collide more often. More importantly, a greater proportion of particles have energy equal to or greater than the activation energy.

升高温度会增大粒子的平均动能。粒子运动更快，因此碰撞更频繁。更重要的是，能量达到或超过活化能的粒子比例会增大。

As a rough guide, for many reactions, increasing the temperature by 10 °C roughly doubles the rate of reaction.

粗略来说，对于许多反应，温度每升高 10 °C，反应速率大约会翻倍。

This explains why food decays faster in summer than in winter, and why chemical reactions in laboratories are often heated to speed them up.

这解释了为什么食物在夏天比冬天腐败得更快，也解释了为什么实验室中的化学反应常常需要加热以加快速度。

5. Concentration and Pressure | 浓度与压强

In a solution, increasing the concentration of a reactant means there are more particles in the same volume. This increases the frequency of collisions per unit time, so the rate of reaction increases.

在溶液中，增大反应物浓度意味着相同体积内粒子数增多。单位时间内碰撞频率增加，因此反应速率增大。

For gases, increasing the pressure has the same effect: the same number of particles is squeezed into a smaller volume, so collisions are more frequent.

对于气体，增大压强也有同样的效果：相同数量的粒子被压缩到更小的体积中，碰撞更频繁。

For example:

Condition	Particle density	Collision frequency	Rate
Low concentration	Low	Low	Slow
High concentration	High	High	Fast

条件	粒子密度	碰撞频率	速率
低浓度	低	低	慢
高浓度	高	高	快

6. Surface Area | 表面积

If a solid reactant is divided into smaller pieces, its total surface area increases. This exposes more particles to the other reactant, so the frequency of collisions at the surface is greater.

如果固体反应物被分割成更小的碎块，其总表面积增大。这使得更多粒子暴露在另一种反应物中，因此表面上的碰撞频率更高。

For example, a pile of powdered calcium carbonate reacts with acid much faster than one large marble chip of the same mass. That is why antacid tablets are often crushed before being swallowed.

例如，一定质量的粉末状碳酸钙与酸反应的速度远快于一大块相同质量的大理石碎片。这也是为什么抗酸药片常被碾碎后再服用。

Rate increases as surface area increases, provided all other factors are kept constant.

在其余条件不变时，表面积增大，反应速率增大。

7. Catalysts | 催化剂

A catalyst is a substance that speeds up a reaction but is chemically unchanged at the end. It works by providing an alternative reaction pathway with a lower activation energy.

催化剂是一种能加快反应速率但在反应结束时化学性质不变的物质。它通过提供一条活化能更低的替代反应路径来起作用。

Because the activation energy is lowered, more particles have enough energy to react, so the rate increases. Catalysts are specific: a catalyst for one reaction may not work for another.

由于活化能降低，更多粒子具有足够的能量去反应，因此速率增大。催化剂具有专一性：对某一反应有效的催化剂可能对另一反应无效。

Common examples include:

Iron in the Haber process (ammonia synthesis).
Platinum in catalytic converters.
Enzymes as biological catalysts in living cells.

哈伯法（合成氨）中的铁。
催化转换器中的铂。
活细胞中作为生物催化剂的酶。

Catalysts are important industrially because they reduce energy costs and increase reaction rates without being used up.

催化剂在工业上非常重要，因为它们能降低能量成本并提高反应速率，而自身不会被消耗。

8. Graphs and Rate Calculations | 图像与速率计算

A graph of product volume against time shows how the reaction progresses. At the start, the slope is steep because the concentration of reactants is high. As reactants are used up, the slope becomes less steep. When one reactant runs out, the reaction stops and the graph becomes horizontal.

产物体积随时间变化的图像展示了反应的进程。开始时斜率较陡，因为反应物浓度较高。随着反应物消耗，斜率变缓。当某一反应物耗尽时，反应停止，图像变为水平。

To find the average rate from a graph:

average rate = total change ÷ total time taken

To find the rate at a particular instant (instantaneous rate), draw a tangent to the curve at that point and calculate its gradient.

要由图求平均速率：

平均速率 = 总变化量 ÷ 总时间

要求某一时刻的瞬时速率，可在曲线该点作切线，并计算切线的斜率。

9. Reversible Reactions and Equilibrium | 可逆反应与平衡

Some reactions are reversible: they can go in both the forward and backward directions. When a reversible reaction is in a closed system, it can reach a state of dynamic equilibrium, where the rates of the forward and reverse reactions are equal.

有些反应是可逆的：它们既能正向进行，也能逆向进行。当可逆反应处于封闭体系中时，可以达到动态平衡状态，此时正逆反应的速率相等。

Changing the conditions (temperature, pressure, or concentration) can shift the position of equilibrium. The Haber process is an important industrial example that uses this principle to maximise the yield of ammonia.

改变条件（温度、压强或浓度）可以移动平衡的位置。哈伯法是一个重要的工业实例，它利用这一原理来最大化氨的产率。

The rate of reaction tells us how fast equilibrium is reached, while the position of equilibrium tells us how much product is formed at equilibrium. These are two different ideas – both are affected by conditions, but not always in the same way.

反应速率告诉我们到达平衡有多快，而平衡位置告诉我们平衡时生成了多少产物。这是两个不同的概念——它们都受条件影响，但影响方式不一定相同。

10. Summary of Key Points | 关键要点总结

To summarise the main factors affecting reaction rate:

总结影响反应速率的主要因素：

Factor	How it increases rate
Temperature ↑	More energetic and more frequent collisions
Concentration / Pressure ↑	More particles in same volume → more frequent collisions
Surface area ↑	More exposed particles → more frequent collisions
Catalyst	Lowers activation energy → higher proportion of successful collisions

因素	如何提高速率
温度升高	碰撞更有能量且更频繁
浓度/压强增大	相同体积内粒子更多 → 碰撞更频繁
表面积增大	暴露的粒子更多 → 碰撞更频繁
催化剂	降低活化能 → 有效碰撞比例更高

Always remember: the rate of reaction depends on the frequency of successful collisions and the proportion of particles with energy ≥ activation energy.

始终记住：反应速率取决于有效碰撞的频率以及能量 ≥ 活化能的粒子比例。

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Rates of Reaction | 化学反应速率