📚 7.5.6 Energy Changes in Chemical Reactions | 7.5.6 化学反应中的能量变化
Energy changes are a fundamental part of chemical reactions. In IGCSE Science, understanding whether a reaction releases or absorbs heat helps us explain everything from hand warmers to photosynthesis. This article covers exothermic and endothermic processes, reaction profile diagrams, energy calculations using bond energies, and practical methods for measuring energy changes.
能量变化是化学反应的基本组成部分。在IGCSE科学中，理解一个反应是释放热量还是吸收热量，有助于我们解释从暖手宝到光合作用的各种现象。本文涵盖放热过程和吸热过程、反应历程图、利用键能进行能量计算以及测量能量变化的实验方法。
1. What Are Energy Changes in Reactions? | 什么是反应中的能量变化？
Almost every chemical reaction involves a transfer of energy between the reacting system and its surroundings. This transfer is usually in the form of thermal energy, which causes temperature changes that we can measure. By convention, the system is the chemicals themselves, and the surroundings are everything else, such as the air, the reaction container, and any solution present.
几乎每个化学反应都涉及反应体系与周围环境之间的能量转移。这种转移通常以热能形式发生，引起我们可以测量的温度变化。按照惯例，体系指化学物质本身，环境则是指其他一切，如空气、反应容器以及存在的任何溶液。
The energy change of a reaction is represented by the symbol ΔH (delta H). When the sign of ΔH is negative, the reaction releases energy to the surroundings (exothermic); when positive, energy is taken in from the surroundings (endothermic). This concept is crucial for the IGCSE Edexcel specification, linking energy changes to bond energies and practical investigations.
反应的能量变化用符号ΔH（德尔塔H）表示。当ΔH为负值时，反应向环境释放能量（放热）；为正值时，反应从环境吸收能量（吸热）。这一概念对于IGCSE Edexcel考试至关重要，它将能量变化与键能以及实验探究联系起来。
2. Exothermic Reactions – Definition and Examples | 放热反应——定义与示例
An exothermic reaction transfers energy from the reacting system to the surroundings. As a result, the temperature of the surroundings increases. In terms of ΔH, exothermic reactions have a negative ΔH value because the products have less energy than the reactants.
放热反应将能量从反应体系转移到周围环境中。因此，环境的温度升高。就ΔH而言，放热反应的ΔH为负值，因为生成物比反应物具有更少的能量。
Common examples of exothermic reactions include combustion (burning of fuels), neutralisation (acid + alkali), respiration in living cells, and the reaction of metals with acids. The reaction between magnesium and hydrochloric acid is often used in the school laboratory to demonstrate a rapid temperature increase.
放热反应的常见实例包括燃烧（燃料的燃烧）、中和反应（酸+碱）、活细胞中的呼吸作用以及金属与酸的反应。镁与盐酸的反应常在学校实验室中用来演示温度的快速升高。
- Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + energy
- Neutralisation: HCl + NaOH → NaCl + H₂O + energy
- Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
- 燃烧：CH₄ + 2O₂ → CO₂ + 2H₂O + 能量
- 中和：HCl + NaOH → NaCl + H₂O + 能量
- 呼吸作用：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量
3. Endothermic Reactions – Definition and Examples | 吸热反应——定义与示例
An endothermic reaction takes in energy from the surroundings, causing the temperature of the surroundings to drop. The ΔH value for such a reaction is positive, meaning the products have more stored energy than the reactants.
吸热反应从环境中吸收能量，导致环境的温度下降。此类反应的ΔH值为正，意味着生成物比反应物储存了更多的能量。
Typical endothermic reactions include thermal decomposition, photosynthesis, and the reaction of citric acid with sodium hydrogencarbonate. In the laboratory, the reaction of barium hydroxide with ammonium chloride can make a beaker so cold that it freezes to a wet block of wood.
典型的吸热反应包括热分解、光合作用以及柠檬酸与碳酸氢钠的反应。在实验室中，氢氧化钡与氯化铵的反应可以使烧杯变得非常冰冷，甚至冻结在一块湿木头上。
- Thermal decomposition: CaCO₃ → CaO + CO₂ (requires heating)
- Photosynthesis: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
- Citric acid + sodium hydrogencarbonate: temperature decrease observed
- 热分解：CaCO₃ → CaO + CO₂（需要加热）
- 光合作用：6CO₂ + 6H₂O + 光能 → C₆H₁₂O₆ + 6O₂
- 柠檬酸 + 碳酸氢钠：观察到温度下降
4. Reaction Profiles – Energy Level Diagrams | 反应历程——能级图
A reaction profile is a graph showing how the energy of a reaction mixture changes as the reaction progresses from reactants to products. The vertical axis represents energy, and the horizontal axis represents the progress of the reaction. These diagrams help us visualise ΔH and activation energy.
反应历程图是展示反应混合物的能量如何随着反应从反应物到生成物进行而变化的图表。纵轴代表能量，横轴代表反应进程。这些图帮助我们直观地看到ΔH和活化能。
For an exothermic reaction, the energy level of the products is lower than that of the reactants, so the arrow showing ΔH points downwards. For an endothermic reaction, the products have a higher energy, so the ΔH arrow points upwards. Both types of diagram include a hump representing the activation energy barrier.
对于放热反应，生成物的能级低于反应物，因此表示ΔH的箭头指向下方。对于吸热反应，生成物的能量更高，因此ΔH箭头指向上方。两种图都包含一个代表活化能障碍的峰。
| Feature | Exothermic Diagram | Endothermic Diagram |
|---|---|---|
| Energy of products vs reactants | Lower | Higher |
| ΔH sign | Negative | Positive |
| Activation energy hump | Present | Present |
| 特征 | 放热图 | 吸热图 |
|---|---|---|
| 生成物能量 vs 反应物能量 | 更低 | 更高 |
| ΔH符号 | 负 | 正 |
| 活化能峰 | 存在 | 存在 |
5. Activation Energy | 活化能
Activation energy (Eₐ) is the minimum amount of energy required for a reaction to occur. It is the energy needed to break the bonds in the reactants so that new bonds can start to form. In a reaction profile, it appears as the difference between the energy of the reactants and the highest point on the curve (the transition state).
活化能（Eₐ）是反应发生所需的最小能量。它是打断反应物中键所需的能量，这样新键才能开始形成。在反应历程图中，它表现为反应物能量与曲线最高点（过渡态）之间的差值。
Even exothermic reactions need an initial energy input to get started. For example, a spark is needed to ignite a mixture of methane and air, even though combustion releases a large amount of energy. Catalysts work by providing an alternative pathway with a lower activation energy, which speeds up the reaction without being used up.
即使是放热反应也需要初始的能量输入才能启动。例如，需要火花才能点燃甲烷和空气的混合物，尽管燃烧会释放大量能量。催化剂的作用是提供一条活化能较低的替代途径，从而加快反应速率而不被消耗。
6. Bond Breaking and Bond Making | 键的断裂与形成
Chemical reactions involve the breaking of bonds in reactants and the forming of new bonds in products. Bond breaking is an endothermic process (energy is absorbed), whereas bond making is exothermic (energy is released). The overall energy change depends on the balance between these two processes.
化学反应涉及反应物中键的断裂和生成物中新键的形成。键的断裂是吸热过程（吸收能量），而键的形成是放热过程（释放能量）。总体的能量变化取决于这两个过程之间的平衡。
If the energy released from forming new bonds is greater than the energy absorbed in breaking old bonds, the reaction is exothermic. If the energy absorbed to break bonds is greater than the energy released when new bonds form, the reaction is endothermic. This is the basis for calculating enthalpy changes using bond energies.
如果形成新键释放的能量大于断裂旧键吸收的能量，反应就是放热的。如果断裂旧键吸收的能量大于形成新键释放的能量，反应就是吸热的。这是利用键能计算焓变的基础。
7. Calculating Enthalpy Change Using Bond Energies | 利用键能计算焓变
Bond energy (also called bond enthalpy) is the energy needed to break one mole of a particular covalent bond in gaseous molecules, averaged over a range of compounds. It is given in kilojoules per mole, kJ/mol. We can use these values to estimate the overall enthalpy change of a reaction.
键能（也称为键焓）是指断裂气态分子中一摩尔特定共价键所需的能量，基于一系列化合物的平均值。其单位是千焦每摩尔，kJ/mol。我们可以利用这些数值来估算反应的总体焓变。
The calculation follows a straightforward formula:
ΔH = Σ (bond energies of bonds broken) – Σ (bond energies of bonds formed)
计算遵循一个简单的公式：
ΔH = Σ（断裂的键的键能总和） – Σ（生成的键的键能总和）
Example for the combustion of hydrogen: 2H₂ + O₂ → 2H₂O. Bonds broken: 2 H–H (436 kJ/mol each) + 1 O=O (498 kJ/mol) = 872 + 498 = 1370 kJ. Bonds formed: 4 O–H (464 kJ/mol each) = 1856 kJ. ΔH = 1370 – 1856 = −486 kJ per 2 moles of water, indicating an exothermic reaction.
以氢气燃烧为例：2H₂ + O₂ → 2H₂O。断裂的键：2个H–H（每个436 kJ/mol）+ 1个O=O（498 kJ/mol）= 872 + 498 = 1370 kJ。生成的键：4个O–H（每个464 kJ/mol）= 1856 kJ。ΔH = 1370 – 1856 = −486 kJ（每2摩尔水），表明这是一个放热反应。
Students must remember to multiply the bond energy by the number of each type of bond in the balanced equation and keep track of units. Edexcel IGCSE questions frequently provide bond energy tables and ask for the calculated ΔH.
学生必须记住将键能乘以配平方程式中每种键的数量，并注意单位。Edexcel IGCSE考试经常提供键能数据表并要求计算ΔH。
8. Energy Changes and Reversible Reactions | 能量变化与可逆反应
In a reversible reaction, the forward and reverse processes have opposite energy changes. If the forward reaction is exothermic, the backward reaction will be endothermic by exactly the same amount of energy, and vice versa. This means the magnitude of ΔH is the same, but the sign is reversed.
在可逆反应中，正反应和逆反应具有相反的能量变化。如果正反应是放热的，那么逆反应将是吸热的，且能量变化的绝对值完全相同，反之亦然。这意味着|ΔH|相同，但符号相反。
A classic example is the hydration and dehydration of copper(II) sulfate: CuSO₄·5H₂O (blue) ⇌ CuSO₄ (white) + 5H₂O. The forward dehydration is endothermic (requires heat), which is why we must gently heat the blue crystals to make them turn white. The reverse hydration is exothermic – adding water to anhydrous copper sulfate releases heat and restores the blue colour.
一个经典的例子是硫酸铜（II）的水合与脱水：CuSO₄·5H₂O（蓝色）⇌ CuSO₄（白色）+ 5H₂O。正向脱水是吸热的（需要加热），这就是为什么我们要微微加热蓝色晶体使其变白。逆向水合是放热的——向无水硫酸铜中加水会释放热量并恢复蓝色。
Another important reversible reaction is the Haber process: N₂ + 3H₂ ⇌ 2NH₃. The forward reaction (production of ammonia) is exothermic. According to Le Chatelier’s principle, lower temperatures favour the exothermic direction, but in practice an optimum temperature of about 450 °C is used to balance rate and yield. Understanding energy changes helps explain such industrial conditions.
另一个重要的可逆反应是哈伯法：N₂ + 3H₂ ⇌ 2NH₃。正反应（生成氨）是放热的。根据勒夏特列原理，较低温度有利于放热方向，但实际生产中采用约450 °C的优化温度以平衡速率和产率。理解能量变化有助于解释这样的工业条件。
9. Practical: Measuring Temperature Changes | 实验：测量温度变化
A core practical in the Edexcel IGCSE Science specification involves measuring the temperature change when a reaction takes place. The simplest method uses a polystyrene cup (good insulator) fitted with a lid and a thermometer. The temperature is recorded at regular intervals before, during, and after mixing reactants.
Edexcel IGCSE科学课程中的一个核心实验涉及测量反应发生时的温度变化。最简单的方法是使用一个配有盖子和温度计的聚苯乙烯杯（良好的绝热体）。在混合反应物之前、期间和之后以固定时间间隔记录温度。
For an exothermic reaction, such as mixing hydrochloric acid and sodium hydroxide solution, the temperature rises and then gradually falls as the cup loses heat to the surroundings. The maximum temperature reached can be used to calculate the energy released, typically using the formula:
q = m × c × ΔT
对于放热反应，例如混合盐酸和氢氧化钠溶液，温度会上升，然后随着杯子向环境散热而逐渐下降。达到的最高温度可用来计算释放的能量，通常使用以下公式：
q = m × c × ΔT
where m is the mass (or volume assuming 1 g/cm³) of the solution, c is the specific heat capacity (4.18 J/g°C for water), and ΔT is the temperature rise. The result can then be divided by the number of moles of limiting reactant to find ΔH in kJ/mol.
其中m是溶液的质量（或假定密度为1 g/cm³的体积），c是比热容（水的比热容为4.18 J/g°C），ΔT是温度升高值。然后将结果除以限制反应物的摩尔数，即可求得ΔH，单位为kJ/mol。
For endothermic reactions, students need to monitor the temperature drop. Sources of error include heat loss to the air, inaccurate thermometer readings, and incomplete reaction. The experiment can be improved by using a lid, stirring continuously, and repeating to obtain an average.
对于吸热反应，学生需要监测温度下降。误差来源包括向空气散热、温度计读数不准确以及反应不完全。可以通过使用盖子、持续搅拌和重复实验取平均值来改进实验。
10. Everyday Applications of Energy Changes | 能量变化的日常应用
Exothermic and endothermic processes are not confined to the lab; they are part of daily life. Exothermic reactions are harnessed in self-heating cans, hand warmers, and combustion engines. Hand warmers typically contain iron powder, which oxidises slowly in air, releasing a steady stream of heat.
放热和吸热过程不仅限于实验室；它们是日常生活的一部分。放热反应被应用于自热罐头、暖手宝和内燃机中。暖手宝通常含有铁粉，铁粉在空气中缓慢氧化，持续释放热量。
Endothermic processes find use in instant cold packs used for sports injuries. These packs often contain ammonium nitrate and water separated by a barrier. When the barrier is broken, the dissolving of ammonium nitrate absorbs heat, producing a rapid cooling effect. This same principle applies to the chilling sensation from the evaporation of volatile liquids like ethanol on the skin.
吸热过程则用于运动损伤的速冷冰袋中。这些冰袋通常含有用隔层分开的硝酸铵和水。当隔层破裂时，硝酸铵的溶解吸收热量，产生快速冷却效果。同样的原理也适用于像乙醇这样的挥发性液体在皮肤上蒸发时的冷感。
Photosynthesis, the most important endothermic reaction on Earth, absorbs light energy to convert carbon dioxide and water into glucose and oxygen. Without this energy-absorbing process, life as we know it would not exist.
光合作用是地球上最重要的吸热反应，它吸收光能，将二氧化碳和水转化为葡萄糖和氧气。没有这个吸收能量的过程，我们所知的生命就无法存在。
11. Exam Tips and Common Mistakes | 应试技巧与常见错误
IGCSE Edexcel exam questions on energy changes often target the correct use of terminology and the ability to interpret reaction profiles and calculate ΔH from bond energies. One common mistake is confusing the direction of energy transfer: always remember that exothermic means heat exits the system, endothermic means heat enters the system.
IGCSE Edexcel考试中有关能量变化的问题通常会针对术语的正确使用、解释反应历程图以及根据键能计算ΔH的能力。一个常见的错误是混淆能量转移的方向：永远要记住，exothermic（放热）意味着热量离开体系，endothermic（吸热）意味着热量进入体系。
When drawing reaction profiles, students must label the axes correctly (energy and progress of reaction), show the activation energy and ΔH clearly, and indicate whether the reaction is exothermic or endothermic based on the relative energy levels of products and reactants. Many candidates lose marks by omitting labels or drawing the activation energy hump incorrectly.
在绘制反应历程图时，学生必须正确标注坐标轴（能量和反应进程），清晰地标出活化能和ΔH，并根据反应物和生成物的相对能级标明反应是放热还是吸热。许多考生因遗漏标签或错误地绘制活化能峰而丢分。
For bond energy calculations, always write out the balanced equation, then list all bonds broken and formed with their respective bond energies. Double-check that you have multiplied by the correct stoichiometric coefficients. If the calculated ΔH is negative, state that the reaction is exothermic; if positive, endothermic. Units (kJ/mol) are essential.
在进行键能计算时，一定要先写出配平的方程式，然后列出所有断裂和形成的键及其相应的键能。仔细检查是否乘上了正确的化学计量数。如果计算出的ΔH为负，说明反应是放热的；如果为正，则是吸热的。单位（kJ/mol）必不可少。
For practical-based questions, be ready to suggest improvements such as using a lid, better insulation, or a digital thermometer, and to explain how temperature changes relate to exothermic or endothermic classifications.
对于基于实验的问题，要准备好提出改进建议，例如使用盖子、更好的保温措施或数字温度计，并解释温度变化如何与放热或吸热分类相关联。
12. Summary and Key Points | 总结与要点
Energy changes in chemical reactions are summarised by the sign of ΔH: negative for exothermic (heat given out), positive for endothermic (heat taken in). Reaction profile diagrams show the energy pathway and the activation energy barrier. Bond breaking is endothermic, bond making is exothermic; the overall ΔH can be estimated using average bond energies. Practical investigations, such as temperature measurements in a polystyrene cup, allow us to quantify these energy changes and link classroom chemistry to real-world applications.
化学反应中的能量变化通过ΔH的符号来概括：负值代表放热（释放热量），正值代表吸热（吸收热量）。反应历程图显示了能量路径和活化能障碍。键的断裂是吸热的，键的形成是放热的；总体的ΔH可以利用平均键能进行估算。实验探究，例如在聚苯乙烯杯中测量温度变化，使我们能够量化这些能量变化，并将课堂化学与现实世界的应用联系起来。
Published by TutorHao | IGCSE 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