Rates of Reaction | 化学反应速率

📚 Rates of Reaction | 化学反应速率

In GCSE Science, the rate of reaction tells us how quickly reactants are turned into products. This idea is central to Edexcel IGCSE Chemistry and is also tested in the Science Double Award course because it connects experimental observations with particle theory.

在 IGCSE 科学中，反应速率描述的是反应物转化为产物的快慢。这个概念是 Edexcel IGCSE 化学的核心，也出现在 Science Double Award 考试中，因为它把实验现象与粒子理论联系在一起。

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

Rate is a measure of how much something changes in a given time. For a chemical reaction, it is the change in the amount of a reactant or product per unit time.

速率是单位时间内某个量变化多少的度量。对于化学反应，它指的是反应物或产物的量在单位时间内的变化。

Rate = change in quantity ÷ time taken
速率 = 量的变化 ÷ 所用时间

For example, if 50 cm³ of hydrogen gas is produced in 20 seconds, the average rate is 50 ÷ 20 = 2.5 cm³/s.

例如，如果 20 秒内生成了 50 cm³ 氢气，那么平均速率就是 50 ÷ 20 = 2.5 cm³/s。

2. Measuring Rates | 测定反应速率

Rates can be measured by following how fast a product appears or how fast a reactant disappears. Common methods include gas collection, loss of mass and the disappearing cross experiment.

反应速率可以通过观察产物生成的快慢或反应物消失的快慢来测定。常用方法有气体收集、质量损失和“十字消失”实验。

Gas collection
气体收集

A gas syringe measures the volume of gas produced at regular time intervals. The steeper the graph slope, the faster the reaction.

用气筒定时测量生成气体的体积。图像斜率越陡，说明反应越快。

Loss of mass
质量损失

When the reaction releases a gas, the flask loses mass. This is measured on a balance placed below the flask.

当反应放出气体时，锥形瓶质量会减少。此时可将锥形瓶放在天平上测量质量变化。

Disappearing cross
十字消失

In reactions that form a precipitate, a cross drawn on paper becomes unclear as the mixture turns cloudy. The time taken for it to disappear is proportional to the rate.

在生成沉淀的反应中，纸上的十字会随着混合液变浑浊而看不清。十字消失所需的时间与速率成反比，时间越短则速率越快。

3. Collision Theory | 碰撞理论

For particles to react, they must collide with enough energy and with the correct orientation. This is known as the collision theory.

粒子要发生反应，必须具有足够的能量并以正确的方向碰撞。这就是碰撞理论。

A successful collision occurs when the total kinetic energy of the particles is greater than or equal to the activation energy, Eₐ.

当粒子的总动能大于或等于活化能 Eₐ 时，碰撞才是有效碰撞。

The activation energy is the minimum energy needed for colliding particles to react. A higher rate means more successful collisions per second.

活化能是碰撞粒子发生反应所需的最低能量。反应速率更高意味着每秒内有效碰撞次数更多。

4. Surface Area | 表面积

If a solid reactant is broken into smaller pieces, its total surface area increases. This means more particles are exposed to collisions with the other reactant.

如果将固体反应物弄成更小的碎片，它的总表面积会增大。这意味着更多粒子暴露出来，与其他反应物碰撞的机会增加。

For example, powdered calcium carbonate reacts with hydrochloric acid faster than one large marble chip of the same mass.

例如，粉末状碳酸钙与盐酸的反应速率比同质量的大理石颗粒更快。

Why? Smaller pieces have a larger surface-area-to-volume ratio, so the frequency of collisions between particles increases.

原因是：小块固体具有更大的表面积与体积之比，因此粒子之间的碰撞频率增加。

5. Concentration and Pressure | 浓度和压力

Concentration measures how many particles of a solute are in a given volume of solution. Higher concentration means more particles in the same volume, so collisions are more frequent.

浓度表示单位体积溶液中溶质粒子的多少。浓度越高，相同体积内粒子越多，碰撞就越频繁。

Pressure applies to gases. Increasing the pressure of a gas pushes the same number of particles into a smaller volume, increasing the collision frequency.

压力适用于气体。增大气体压力会使相同数目的粒子被压缩到更小的体积中，从而增加碰撞频率。

In both cases, the rate increases because there are more successful collisions per second, assuming the temperature stays the same.

在这两种情况下，由于每秒内有效碰撞次数增多，反应速率会加快（前提是温度保持不变）。

6. Temperature | 温度

Temperature is related to the average kinetic energy of particles. When temperature increases, particles move faster and collide more often.

温度与粒子的平均动能有关。温度升高时，粒子运动更快，碰撞更频繁。

More importantly, a larger fraction of particles now have energy equal to or greater than the activation energy. This makes a much bigger effect than simply increasing collision frequency.

更重要的是，现在有更大比例的粒子拥有等于或大于活化能的能量。这比单纯增加碰撞频率的影响要大得多。

As a rule of thumb, for many reactions, a 10 °C rise in temperature roughly doubles the rate of reaction.

粗略地说，对于许多反应，温度每升高 10 °C，反应速率大约会加倍。

7. Catalysts | 催化剂

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

催化剂是一种能够加快反应速率但自身不被消耗的物质。它通过提供一条具有更低活化能的替代反应路径来起作用。

With a lower activation energy, more colliding particles have enough energy to react. So the rate increases even at the same temperature.

由于活化能降低，更多碰撞粒子有足够的能量发生反应。因此即使在相同温度下，速率也会增大。

Catalysts are specific and can be recovered at the end of the reaction. Enzymes are biological catalysts.

催化剂具有专一性，并且可以在反应结束时回收。酶是生物催化剂。

8. Practical: Marble Chips and Acid | 实验：大理石与酸

This classic experiment studies the reaction between hydrochloric acid and calcium carbonate:

下面是一个经典实验：研究盐酸与碳酸钙的反应：

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Carbon dioxide is released, so the mass of the flask decreases. By measuring mass loss every 10 seconds, the average rate can be calculated.

反应会放出二氧化碳，因此锥形瓶的质量减少。通过每 10 秒测量一次质量损失，可以计算出平均反应速率。

To compare rates for different sizes of marble chips, keep the same temperature, acid concentration and total mass. Only vary the particle size.

在比较不同大小大理石颗粒的反应速率时，应保持温度、酸浓度和固体总质量相同，只改变颗粒大小。

9. Graph Interpretation | 曲线图解读

Reaction graphs usually show the amount of product or reactant against time. The line starts steep and becomes less steep as the reaction slows down.

反应曲线通常展示产物量或反应物量随时间的变化。曲线开始时较陡，然后逐渐变缓，说明反应在减慢。

When the line becomes horizontal, the reaction has stopped. This happens because at least one reactant has been used up.

当曲线变为水平时，反应已经停止。这是因为至少有一种反应物已被消耗完。

Part of graph 曲线部分	Meaning 含义
Steep slope 斜率较陡	Fast rate; many successful collisions per second 速率快；每秒有效碰撞次数多
Flat line 水平线	Reaction finished; no more product formed 反应结束；不再生成产物

10. Exam Tips | 考试要点

Always state the factor being changed and the factor kept constant when comparing experiments.

在比较实验时，务必说清哪个因素发生了变化，哪个因素保持不变。

Use the language of particles: “more frequent collisions” or “a higher proportion of particles have energy greater than the activation energy” rather than vague phrases like “particles move more”.

使用粒子层面的语言：“碰撞更频繁”或“有更高比例的粒子能量超过活化能”，不要使用“粒子动得更多”这种含糊的说法。

For a higher mark, link the shape of a graph to the depletion of reactants or the build-up of products.

要拿高分，需要把曲线形状与反应物消耗或产物累积联系起来。

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Rates of Reaction | 化学反应速率