Rates of Reaction | 反应速率

📚 Rates of Reaction | 反应速率

The rate of a chemical reaction describes how quickly reactants are converted into products. Understanding this concept is essential for analysing industrial processes, biological enzyme activity, and everyday phenomena like rusting or combustion.

化学反应速率描述了反应物转化为产物的快慢。理解这一概念对于分析工业生产过程、生物酶活性以及铁锈、燃烧等日常现象都至关重要。

1. Defining Rate of Reaction | 定义反应速率

Rate of reaction is the change in concentration of a reactant or product per unit time. It can be measured in grams per second (g/s), cubic centimetres per second (cm³/s), or moles per second (mol/s).

反应速率是指单位时间内反应物或产物浓度的变化。其单位可以是克每秒（g/s）、立方厘米每秒（cm³/s）或摩尔每秒（mol/s）。

The rate can be expressed using the formula:

Rate = Amount of reactant used or product formed ÷ Time

For a given reaction, the rate typically decreases over time as reactants are consumed.

对于某一反应，随着反应物的消耗，速率通常会逐渐下降。

2. Measuring Reaction Rates | 测定反应速率

There are several ways to follow the progress of a reaction. The method chosen depends on the type of reaction and the observable changes involved.

跟踪反应进程的方法有多种，具体选择取决于反应类型以及可观测的变化。

Common techniques include measuring the volume of gas evolved, monitoring loss of mass, measuring turbidity (cloudiness), and using a colour change with a stopwatch.

常用技术包括测定放出气体的体积、监测质量损失、测量浊度（浑浊程度），以及利用颜色变化并用秒表计时。

Gas volume: use a gas syringe or displacement of water.
Mass loss: only when a gas escapes into the atmosphere.
Turbidity: when a precipitate is formed, a cross drawn under the flask disappears.

气体体积：使用气体注射器或排水集气法。
质量损失：仅当气体逸散到空气中时可用。
浊度：当沉淀生成时，烧杯下画出的十字标记变得模糊直至消失。

3. Collision Theory | 碰撞理论

Collision theory states that for a reaction to occur, particles must collide with sufficient energy (greater than or equal to the activation energy) and the correct orientation.

碰撞理论指出，反应发生的条件是颗粒必须具有足够能量（大于或等于活化能）的正确碰撞取向。

Increasing the frequency of successful collisions increases the rate of reaction. Any change that raises the number of effective collisions per second will speed up the reaction.

增加有效碰撞的频率会加快反应速率。任何能提高每秒有效碰撞次数的改变都会加速反应进行。

4. Effect of Concentration | 浓度的影响

When the concentration of a dissolved reactant increases, there are more particles in a given volume. This leads to a higher frequency of collisions and therefore a faster reaction.

当溶解态反应物的浓度增大时，单位体积内的粒子数增多，导致碰撞频率升高，因此反应速率更快。

The relationship is not linear at high concentrations because the effect becomes less pronounced as all available particles are already reacting. However, for simple IGCSE problems, doubling the concentration roughly doubles the rate (if other conditions remain constant).

在高浓度下，这种关系并非完全线性，因为当几乎所有粒子都已参与反应时，效果会变得不明显。然而，对于IGCSE基础题目，若其他条件不变，浓度加倍速率约加倍。

5. Effect of Pressure (for Gases) | 气压的影响（针对气体）

For gaseous reactants, increasing the pressure compresses the gas, so the same number of particles occupies a smaller volume. The particle density increases, leading to more frequent collisions.

对于气态反应物，增大压力会使气体被压缩，相同数量的颗粒占据更小的体积。颗粒密度升高，碰撞更加频繁。

Pressure is essentially the same as concentration in gases. Therefore, increasing pressure increases the rate of reaction, provided the temperature stays constant.

对于气体来说，压力本质上是浓度的另一种体现。因此，在温度恒定的条件下，增大压力会加快反应速率。

6. Effect of Temperature | 温度的影响

Raising the temperature increases the average kinetic energy of particles. This results in more collisions per second, but more importantly, a much larger fraction of collisions have energy equal to or above the activation energy.

升高温度会增加颗粒的平均动能。这意味着每秒碰撞次数更多，但更重要的是，能量达到或超过活化能的碰撞比例大大增加。

As a rough rule, a 10 °C rise in temperature approximately doubles the rate of reaction for many common reactions.

粗略规律是，温度每升高10 °C，许多常见反应的速率约增大一倍。

7. Effect of Surface Area | 表面积的影响

When a solid reactant is broken into smaller pieces, its total surface area increases. More particles are exposed to the other reactant, so collisions at the solid surface become more frequent.

当固体反应物被粉碎成更小的颗粒时，其总表面积增加。更多的颗粒暴露在另一种反应物中，使得固体表面的碰撞更加频繁。

For example, a powdered solid reacts faster than the same mass in a single lump. Finely divided solids also react explosively in some cases.

例如，粉末状固体比相同质量的块状固体反应更快。某些情况下，细小分散的固体甚至会发生爆炸性反应。

8. Catalysts | 催化剂

A catalyst is a substance that speeds up a reaction without being used up in the process. It provides an alternative reaction pathway with a lower activation energy.

催化剂是一种能加快反应速率但自身不被消耗的物质。它为反应提供了具有较低活化能的替代路径。

Catalysts are specific and important in industry. For example, iron is used in the Haber process for making ammonia, and platinum is used in the catalytic converters of car exhausts.

催化剂具有选择性，在工业中非常重要。例如，铁用于哈伯法合成氨，铂用于汽车尾气催化转换器。

Catalysts do not change the position of equilibrium, only the time taken to reach it.

9. Interpreting Rate Graphs | 解读速率图

Reaction progress can be plotted as a graph of product formation (or reactant loss) against time. The gradient of the curve at any point gives the instantaneous rate.

反应进程可以绘制为产物生成量（或反应物消耗量）对时间的曲线。曲线在任意一点的斜率即瞬时速率。

Initially, the gradient is steep because the reactant concentration is high. As the reactant is used up, the curve becomes flatter until it levels off, indicating the reaction has stopped.

初始时，由于反应物浓度高，曲线斜率较大。随着反应物消耗，曲线逐渐平缓，直至趋于水平，说明反应停止。

Curve shape	Meaning
Steep linear start	Constant high rate at the beginning
Decreasing slope	Rate falls as reactants are consumed
Horizontal plateau	Reaction finished; limiting reactant exhausted

10. Practical: Measuring Volume of Gas | 实验：测量气体体积

A standard experiment to find the rate of reaction involves adding hydrochloric acid to marble chips (calcium carbonate) and measuring the volume of carbon dioxide produced over time using a gas syringe.

测定反应速率的经典实验是将盐酸与大理石碎片（碳酸钙）混合，并用气体注射器测定二氧化碳随时间的生成体积。

The experiment can be repeated with different concentrations of acid, different temperatures, or different sizes of marble chips. Each time, the initial rate can be compared from the steepness of the graph.

该实验可以改用不同浓度酸、不同温度或不同粒度的大理石进行。每次根据曲线的初始斜率和生成总量进行比较。

11. Exam Tips and Common Mistakes | 考试技巧与常见错误

When answering questions about rates, always refer to the frequency of effective collisions, not just ‘collisions’. Mention the activation energy when explaining temperature and catalyst effects.

回答速率相关问题时，务必提及“有效碰撞”而不仅仅是“碰撞”。解释温度和催化剂影响时要说明活化能。

Use the correct units for rate (e.g., cm³/s).
Draw tangents to steep curves to calculate instantaneous rate.
Remember that increasing temperature increases both collision frequency and collision energy.
A catalyst provides a lower activation energy pathway, but does not increase the amount of product formed.

正确使用速率单位（如 cm³/s）。
需要计算瞬时速率时，应在曲线上绘制切线。
记住升高温度既增加碰撞频率，也增加碰撞能量。
催化剂降低活化能路径，但不改变最终产物的生成量。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Rates of Reaction | 反应速率