📚 Energy Changes in Reactions | 化学反应中的能量变化
Every chemical reaction involves a transfer of energy between the reacting substances and their surroundings. Some reactions release heat, making the container feel warm, while others absorb heat, causing a drop in temperature. Understanding these energy changes is fundamental in IGCSE Edexcel Science, as it connects the concepts of bond breaking and bond making with measurable temperature differences. In this article, we will explore exothermic and endothermic reactions, energy level diagrams, activation energy, and the role of catalysts, as well as practical ways to calculate energy changes using bond energies.
每一个化学反应都伴随着反应物与周围环境之间的能量转移。有些反应释放热量，使容器变热；有些反应吸收热量，导致温度下降。理解这些能量变化是 IGCSE Edexcel 科学课程的基础，因为它将化学键断裂和形成的概念与可测量的温度差异联系起来。在本文中，我们将深入探讨放热反应和吸热反应、能级图、活化能以及催化剂的作用，并介绍利用键能计算能量变化的实用方法。
1. What Are Energy Changes in Reactions? | 什么是反应中的能量变化？
During a chemical reaction, the atoms in the reactants rearrange to form products. This rearrangement involves breaking existing bonds and forming new bonds. Breaking bonds requires an input of energy, while forming bonds releases energy. The overall energy change of a reaction, often represented by the symbol ΔH, is the net result of these two processes. If more energy is released during bond formation than is absorbed during bond breaking, the reaction is exothermic and ΔH is negative. Conversely, if more energy is absorbed than released, the reaction is endothermic and ΔH is positive.
在化学反应过程中，反应物中的原子重新排列生成产物。这一重组过程涉及旧键的断裂和新键的形成。断裂化学键需要吸收能量，而形成化学键则释放能量。反应的总能量变化通常用符号 ΔH 表示，它是这两个过程的净结果。如果成键过程中释放的能量大于断键过程中吸收的能量，反应就是放热的，ΔH 为负值。反之，如果吸收的能量大于释放的能量，反应就是吸热的，ΔH 为正值。
2. Exothermic Reactions | 放热反应
An exothermic reaction is one that transfers thermal energy from the system to the surroundings. As a result, the temperature of the surroundings increases. Common examples include combustion, neutralisation reactions between acids and alkalis, and many oxidation reactions. In an exothermic reaction, the total energy stored in the bonds of the products is lower than the total energy stored in the bonds of the reactants. The excess energy is released, often as heat and sometimes as light or sound. A familiar demonstration is adding magnesium ribbon to hydrochloric acid: the test tube becomes noticeably warm.
放热反应是将热能由系统传递到周围环境中的反应。因此，周围环境的温度会升高。常见的例子包括燃烧反应、酸与碱的中和反应以及许多氧化反应。在放热反应中，产物化学键中储存的总能量低于反应物化学键中储存的总能量。多余的能量被释放出来，通常表现为热量，有时也表现为光或声。一个常见的演示实验是将镁条加入盐酸中，试管会明显变热。
3. Endothermic Reactions | 吸热反应
An endothermic reaction absorbs thermal energy from the surroundings, causing a decrease in the temperature of the surroundings. In these reactions, the energy required to break bonds in the reactants is greater than the energy released when new bonds form in the products. The mixture therefore takes in heat from the environment. Typical examples include the thermal decomposition of calcium carbonate, the reaction between citric acid and sodium hydrogencarbonate, and photosynthesis. When solid ammonium nitrate dissolves in water, the solution becomes cold, demonstrating an endothermic process.
吸热反应从周围环境中吸收热能，导致周围环境温度下降。在这类反应中，断开反应物中的化学键所需的能量大于产物中新键形成时释放的能量，因此混合物会从环境中吸收热量。典型的例子包括碳酸钙的热分解、柠檬酸与碳酸氢钠的反应以及光合作用。当固体硝酸铵溶于水时，溶液变冷，这是一个吸热过程的体现。
4. Energy Level Diagrams | 能级图
Energy level diagrams provide a visual way to represent energy changes during a reaction. The y-axis shows the energy stored in chemical bonds, and the x-axis represents the progress of the reaction. For an exothermic reaction, the energy level of the products is lower than that of the reactants, and the arrow pointing downwards indicates an energy decrease. For an endothermic reaction, the products sit at a higher energy level than the reactants, and the arrow points upwards. These diagrams also allow us to show the activation energy, which is the minimum energy required for a reaction to occur.
能级图能直观地表示反应过程中的能量变化。y 轴表示储存在化学键中的能量，x 轴表示反应的进程。在放热反应中，产物的能级低于反应物的能级，向下的箭头表示能量降低。在吸热反应中，产物位于比反应物更高的能级，箭头指向上方。这些图还能帮助我们展示活化能——即发生反应所需的最低能量。
Exothermic: reactants → products + energy, ΔH negative
放热：反应物 → 产物 + 能量，ΔH 为负值
5. Activation Energy | 活化能
Activation energy (Eₐ) is the minimum energy that colliding particles must possess for a reaction to take place. It is needed to break the initial bonds in the reactants. Even in an exothermic reaction, where the overall energy change is a release, a certain amount of energy must first be supplied to get the reaction started. This is represented on an energy level diagram as the peak between reactants and products. A high activation energy means that the reaction will be slow at room temperature because only a small proportion of particles will have sufficient energy to react.
活化能（Eₐ）是发生有效碰撞的反应粒子所必须具备的最低能量。它是打破反应物中初始化学键所必需的。即使是在整体释放能量的放热反应中，也必须先提供一定的能量来启动反应。在能级图上，这表现为反应物与产物之间的能量峰值。活化能高意味着在室温下反应会很慢，因为只有极小部分粒子具备足够的能量进行反应。
6. Catalysts and Activation Energy | 催化剂与活化能
A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process. It works by providing an alternative reaction pathway that has a lower activation energy. With a lower energy barrier, a greater proportion of particle collisions will have enough energy to result in a reaction. Importantly, a catalyst does not change the energy of the reactants or products; it only lowers the activation energy. Therefore, the overall ΔH of the reaction remains unchanged. Enzymes are biological catalysts that work on the same principle.
催化剂是一种能加快化学反应速率而自身在过程中不被消耗的物质。它的作用原理是提供一条活化能较低的替代反应路径。能量壁垒降低了，就会有更大比例的粒子碰撞具备足够能量发生反应。重要的是，催化剂不会改变反应物或产物的能量，它只降低活化能。因此，反应的总 ΔH 保持不变。酶是生物催化剂，遵循相同的原理。
7. Calculating Energy Changes Using Bond Energies | 利用键能计算能量变化
Bond energy is the amount of energy required to break one mole of a particular covalent bond in a gaseous molecule. It is usually measured in kilojoules per mole (kJ mol⁻¹). The overall energy change for a reaction can be estimated by comparing the total energy required to break all the bonds in the reactants with the total energy released when all the bonds in the products are formed. The formula is:
键能是断裂气态分子中某一特定共价键所需的能量，通常以千焦每摩尔（kJ mol⁻¹）为单位。通过比较断开反应物中所有化学键所需的总能量与形成产物中所有化学键所释放的总能量，可以估算反应的总能量变化。公式如下：
ΔH = Σ (bond energies of bonds broken) − Σ (bond energies of bonds formed)
ΔH = Σ（断键的键能总和）− Σ（成键的键能总和）
If the value calculated is negative, the reaction is exothermic; if positive, the reaction is endothermic. This method is commonly used for reactions involving simple gases such as hydrogen, chlorine, methane and water.
如果计算出的数值为负，说明反应是放热的；如果为正，则是吸热的。这种方法通常用于涉及氢气、氯气、甲烷和水等简单气体的反应。
8. Worked Example: Bond Energy Calculation | 键能计算示例
Consider the reaction between hydrogen and chlorine to form hydrogen chloride:
以氢气和氯气反应生成氯化氢为例：
H₂ + Cl₂ → 2HCl
Bond energies: H–H = 436 kJ mol⁻¹, Cl–Cl = 243 kJ mol⁻¹, H–Cl = 432 kJ mol⁻¹.
键能：H–H = 436 kJ mol⁻¹，Cl–Cl = 243 kJ mol⁻¹，H–Cl = 432 kJ mol⁻¹
Bonds broken: 1 × H–H and 1 × Cl–Cl. Total energy absorbed = 436 + 243 = 679 kJ.
断裂的化学键：1 个 H–H 键和 1 个 Cl–Cl 键。吸收总能量 = 436 + 243 = 679 kJ
Bonds formed: 2 × H–Cl. Total energy released = 2 × 432 = 864 kJ.
形成的化学键：2 个 H–Cl 键。释放总能量 = 2 × 432 = 864 kJ
ΔH = energy absorbed − energy released = 679 − 864 = −185 kJ. The reaction is exothermic.
ΔH = 吸收能量 − 释放能量 = 679 − 864 = −185 kJ。该反应为放热反应。
| Bond Type 键型 | Bond Energy (kJ mol⁻¹) 键能 | Number Broken 断裂数 | Number Formed 形成数 |
|---|---|---|---|
| H–H | 436 | 1 | 0 |
| Cl–Cl | 243 | 1 | 0 |
| H–Cl | 432 | 0 | 2 |
9. Factors Affecting Reaction Rate and Energy Profile | 影响反应速率与能量曲线的因素
While the overall energy change (ΔH) is fixed for a given reaction, the rate at which the reaction occurs can be influenced by several factors. Increasing temperature gives particles more kinetic energy, so a higher proportion of collisions exceed the activation energy. Raising concentration or pressure increases the frequency of collisions. Using a catalyst lowers the activation energy, thereby increasing the reaction rate without altering ΔH. These changes can all be illustrated on the same energy level diagram by showing a lower activation energy hump when a catalyst is used.
虽然对于一个特定反应，总能量变化 ΔH 是固定的，但反应发生的速率会受到多种因素的影响。升高温度会使粒子获得更多动能，从而使更高比例的碰撞超过活化能。提高浓度或压强会增加碰撞频率。使用催化剂则降低活化能，从而在不改变 ΔH 的情况下提高反应速率。这些变化都可以在同一张能级图上表示，使用催化剂时活化能峰的高度较低。
10. Practical Investigation: Measuring Temperature Changes | 实验探究：测量温度变化
A straightforward experiment to investigate energy changes involves measuring the temperature change when an acid reacts with an alkali. The method typically uses a polystyrene cup as a calorimeter to minimise heat loss. Measured volumes of hydrochloric acid and sodium hydroxide solution are mixed, and the maximum temperature rise is recorded. By varying the volume or concentration, students can observe how the amount of energy released relates to the quantities of reactants. The reaction is exothermic, and the temperature rise can be used to calculate the heat energy released using the formula Q = mcΔT, where m is the mass of the solution, c is the specific heat capacity, and ΔT is the temperature change.
一个探究能量变化的简单实验是测量酸与碱反应时的温度变化。该方法通常使用聚苯乙烯杯作为量热计，以尽量减少热量散失。将量取好的一定体积的盐酸和氢氧化钠溶液混合，记录升高的最高温度。通过改变体积或浓度，学生可以观察释放的能量与反应物量之间的关系。该反应是放热的，利用温度升高值可以计算放出的热量，公式为 Q = mcΔT，其中 m 是溶液的质量，c 是比热容，ΔT 是温度变化。
11. Applications of Exothermic and Endothermic Reactions | 放热与吸热反应的应用
Exothermic reactions have many practical uses. Combustion of fuels provides energy for heating, cooking and transport. Hand warmers often use the exothermic oxidation of iron powder when exposed to air. Self-heating cans of drinks exploit the reaction between quicklime (calcium oxide) and water. Endothermic reactions are also useful; for example, instant cold packs for sports injuries typically contain ammonium nitrate and water, which absorb heat upon mixing. Photosynthesis, the most important endothermic reaction on Earth, absorbs sunlight energy to convert carbon dioxide and water into glucose and oxygen.
放热反应具有许多实际用途。燃料的燃烧为取暖、烹饪和交通提供能量。暖手宝通常利用铁粉暴露在空气中发生放热的氧化反应。自热饮料罐则利用了生石灰（氧化钙）与水的反应。吸热反应同样很有用；例如，运动损伤用的速冷冰袋通常含有硝酸铵和水，混合时吸收热量。光合作用是地球上最重要的吸热反应，它吸收太阳光能，将二氧化碳和水转化为葡萄糖和氧气。
12. Summary and Key Points | 总结与要点
Energy changes in chemical reactions can be classified as exothermic (release energy, ΔH negative) or endothermic (absorb energy, ΔH positive). The overall energy change depends on the balance between energy required to break bonds and energy released when forming bonds. Energy level diagrams provide a clear visual representation of these processes and include activation energy and overall energy change. Catalysts lower the activation energy without affecting ΔH. Bond energy calculations allow us to determine whether a reaction is exothermic or endothermic by using average bond enthalpies. Mastering these concepts is essential for success in IGCSE Edexcel Science examinations and for understanding the energetic principles behind everyday chemical processes.
化学反应中的能量变化可分为放热（释放能量，ΔH 为负值）和吸热（吸收能量，ΔH 为正值）两类。总能量变化取决于断键所需能量与成键释放能量之间的平衡。能级图直观地展示了这些过程，包括活化能和总能量变化。催化剂在不改变 ΔH 的情况下降低活化能。利用平均键焓进行键能计算，可以判断一个反应是放热还是吸热。掌握这些概念对于在 IGCSE Edexcel 科学考试中取得成功以及理解日常化学过程背后的能量原理至关重要。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图