📚 Exothermic and Endothermic Reactions | 放热与吸热反应
Energy changes are at the heart of all chemical reactions. Whether a reaction gives out heat to the surroundings or takes in heat from them defines one of its most fundamental properties. In IGCSE Edexcel Science, understanding exothermic and endothermic processes is essential for explaining everything from combustion to photosynthesis, and for predicting how changes in temperature, pressure and concentration can shift equilibrium. This article unpacks the key concepts, practical examples and bond‑energy calculations you need to master.
能量变化是所有化学反应的核``心。一个反应是向环境释放热量，还是从环境吸收热量，决定了它最基本的性质之一。在IGCSE爱德思科学课程中，理解放热与吸热过程对于解释燃烧、光合作用等各种现象，以及预测温度、压强和浓度变化如何使平衡发生移动至关重要。本文详细解析你需要掌握的核心概念、实际例子以及键能计算。
1. Introduction to Energy Changes | 能量变化简介
Whenever a chemical reaction takes place, existing bonds in the reactants are broken and new bonds are formed to make the products. Bond breaking requires energy, while bond making releases energy. The overall energy change for a reaction depends on the balance between these two processes. If more energy is released when new bonds form than is needed to break the old bonds, the reaction is exothermic. If the opposite is true, the reaction is endothermic.
每当发生化学反应时，反应物中已有的化学键会断裂，随后形成新键生成产物。断键需要能量，而成键则释放能量。反应的总能量变化取决于这两个过程的能量平衡。如果成键释放的能量多于断键所需的能量，反应为放热反应；反之，则为吸热反应。
Temperature change is the most common observable indicator. In an exothermic reaction, the temperature of the surroundings rises because thermal energy is transferred from the reacting system to the outside. In an endothermic reaction, the surroundings get colder as the system absorbs heat.
温度变化是最常见的可观察指标。在放热反应中，环境温度上升，因为反应体系将热能传递给外界；吸热反应中，环境变冷，因为体系从外界吸收热量。
2. What Are Exothermic Reactions? | 什么是放热反应？
Exothermic reactions are chemical reactions that release energy, usually in the form of heat, causing the temperature of the surroundings to increase. The energy released can also take the form of light or sound, such as in combustion reactions. On an energy level diagram, the products of an exothermic reaction sit at a lower energy level than the reactants; the difference in height represents the energy released, often labelled as ΔH (enthalpy change) with a negative sign.
放热反应是指释放能量（通常以熱的形式）并导致环境温度升高的化学反应。释放的能量也可以光或声的形式出现，例如燃烧反应。在能级图中，放热反应的生成物能量水平比反应物低；高度差代表释放的能量，常用ΔH（焓变）的负值表示。
robic respiration) and the reaction of acids with alkalis (neutralisation). In each case, the temperature rises noticeably.
常见例子包括燃料的燃烧，如甲烷：CH₄ + 2O₂ → CO₂ + 2H₂O，细胞中的呼吸作用（有氧呼吸），以及酸与碱的中和反应。这些反应中，温度会明显升高。
3. What Are Endothermic Reactions? | 什么是吸热反应？
Endothermic reactions absorb energy from the surroundings, resulting in a temperature drop. Energy must be supplied for the reaction to proceed; often this input is in the form of heat, but light or electricity can also drive the process. On an energy level diagram, the products end up higher than the reactants, giving a positive ΔH value.
吸热反应从周围环境吸收能量，导致温度下降。反应必须获得能量才能进行；输入的能量通常是熱能，但光能或电能也可以驱动反应。在能级图中，生成物的能量水平高于反应物，因此ΔH为正值。
ertain salts such as ammonium nitrate in water. In each case, the container feels cold to the touch.
典型的吸热过程包括光合作用（6CO₂ + 6H₂O + 光能 → C₆H₁₂O₆ + 6O₂）、碳酸钙的热分解（CaCO₃ → CaO + CO₂），以及某些盐类（如硝酸铵）溶于水的过程。在这些情况中，容器摸上去是凉的。
4. Energy Level Diagrams | 能级图
An energy level diagram (or reaction profile) shows the relative energies of reactants and products and the overall energy change. For an exothermic reaction, the reactant line is drawn higher than the product line; the arrow pointing downwards represents the energy released to the surroundings. For an endothermic reaction, the reactant line is lower, and the arrow points upward, showing energy absorbed.
能级图（或称反应曲线图）展示了反应物与生成物的相对能量以及总能量变化。对于放热反应，反应物的能级线高于生成物；向下的箭头表示释放到环境中的能量。对于吸热反应，反应物的能级线较低，箭头指向向上，表示吸收的能量。
These diagrams also incorporate the activation energy peak, which will be discussed later. Being able to sketch and label these diagrams correctly – including the labels 'reactants', 'products', 'ΔH' and 'activation energy' – is a fundamental skill tested in the exam.
这些图中还包含活化能峰，将在后面讨论。能够正确绘制并标注这些图——包括“反应物”、“生成物”、“ΔH”和“活化能”——是考试中需要掌握的基本技能。
5. Bond Breaking and Bond Making: The Heart of Energy Change | 断键与成键：能量变化的核心
Every reaction involves breaking reactant bonds (energy input) and forming product bonds (energy output). The overall energy change ΔH can be expressed as:
ΔH = total energy to break bonds – total energy released when making bonds
每个反应都包含反应物断键（能量输入）和生成物成键（能量输出）。总能量变化ΔH可以表示为：
ΔH = 断键所需总能量 - 成键释放的总能量
If bond making releases more energy than bond breaking absorbs, ΔH is negative (exothermic). If bond breaking absorbs more energy, ΔH is positive (endothermic). Therefore, the strengths of the covalent bonds involved directly determine whether a reaction feels hot or cold.
如果成键释放的能量多于断键吸收的能量，ΔH为负值（放热）；如果断键吸收的能量更多，ΔH为正值（吸热）。因此，所涉及共价键的强弱直接决定了反应是放热还是吸热。
6. Activation Energy | 活化能
The activation energy (Eₐ) is the minimum amount of energy colliding particles need in order to start a reaction. Even exothermic reactions, which overall give out heat, require an initial input of energy to break the first bonds. This energy is represented by the peak on an energy level diagram.
活化能（Eₐ）是碰撞粒子引发反应所需的最少能量。即使是总体上释放热量的放热反应，也需要初始的能量输入来断裂第一批化学键。这一能量用能级图中的峰表示。
A reaction with a high activation energy proceeds slowly unless a catalyst is used to provide an alternative pathway with a lower Eₐ. It is important to note that a catalyst lowers the activation energy but does not change the enthalpy change ΔH, so the reactant and product energy levels remain the same.
活化能高的反应进行缓慢，除非使用催化剂提供一条活化能较低的反应路径。需要注意，催化剂降低活化能但不改变焓变ΔH，因此反应物和生成物的能级保持不变。
7. Simple Practical Investigations | 简单的实验探究
One common class practical measures the temperature change when different metals react with a dilute acid or when an acid neutralises an alkali. For example, adding magnesium ribbon to hydrochloric acid produces hydrogen gas and a sharp temperature rise (exothermic). By recording the initial and maximum (or minimum) temperature, you can calculate the temperature change and compare the magnitude of energy released.
一个常见的课堂实验是测量不同金属与稀酸反应，或者酸与碱中和时的温度变化。例如，将镁条加入盐酸中会产生氢气并引起温度急剧上升（放热）。通过记录初始温度和最高（或最低）温度，可以计算温度变化并比较释放或吸收能量的多少。
Another experiment involves dissolving ammonium chloride in water, which causes a temperature drop, clearly demonstrating an endothermic process. Students often plot temperature against time to visualise the energy transfer before and after mixing.
另一个实验是将氯化铵溶于水中，会导致温度下降，清楚地展示吸热过程。学生通常绘制温度-时间图，以直观显示混合前后的能量传递。
8. Everyday Examples and Applications | 日常实例与应用
Exothermic reactions have countless real‑world uses. Combustion of petrol and diesel powers vehicles; the exothermic reaction in hand warmers (often the crystallisation of supersaturated sodium acetate or the oxidation of iron) provides portable heat; self‑heating food cans rely on the reaction between quicklime (CaO) and water.
放热反应在现实生活中有无数用途。汽油和柴油的燃烧为车辆提供动力；暖手器中的放热反应（通常是过饱和醋酸钠的结晶或铁的氧化）提供便携式热源；自热食品罐则依靠生石灰（CaO）与水的反应。
Endothermic processes are equally important. Photosynthesis is the ultimate endothermic reaction, capturing light energy to build glucose. Sports injury cold packs use the endothermic dissolution of ammonium nitrate or urea to provide instant cold.
吸热过程同样重要。光合作用是最根本的吸热反应，捕获光能生成葡萄糖。运动损伤冷敷包利用硝酸铵或尿素溶解时的吸热过程来提供即时冷却。
9. Energy Changes and Reversible Reactions | 能量变化与可逆反应
In a reversible reaction, the forward and backward processes have opposite energy profiles. For example, the hydration of copper(II) sulfate (white to blue) is exothermic, while the dehydration (blue to white) is endothermic:
CuSO₄·5H₂O (blue) ⇌ CuSO₄ (white) + 5H₂O
在可逆反应中，正向与逆向过程具有相反的能量变化。例如，硫酸铜的水合（白色变蓝色）是放热的，而脱水（蓝色变白色）是吸热的：
CuSO₄·5H₂O （蓝色） ⇌ CuSO₄ （白色） + 5H₂O
This relationship is useful to illustrate that if the forward reaction is exothermic, the reverse must be endothermic by the same magnitude. The direction of energy transfer can be controlled by temperature: heating tips the equilibrium in the endothermic direction, while cooling favours the exothermic direction, in accordance with Le Chatelier's principle.
这一关系有助于说明，若正向反应放热，则逆向反应必定吸热且数值相等。能量传递的方向可通过温度控制：根据勒夏特列原理，加热使平衡向吸热方向移动，冷却则有利于放热方向。
10. The Haber Process: A Case Study | 哈伯制氨法：案例分析
The Haber process for producing ammonia is an excellent example to link energy changes with industrial equilibrium. The reaction is:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = –92 kJ mol⁻¹
哈伯制氨法是联系能量变化与工业平衡的绝佳例子。其反应为：
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = –92 kJ mol⁻¹
Since ΔH is negative, the forward reaction is exothermic. Lower temperatures would shift the equilibrium towards more ammonia, but the rate would be too slow. A catalyst (iron) is used to lower activation energy, and a compromise temperature of about 450 °C is chosen, together with high pressure, to achieve a viable yield quickly. Understanding the energy profile of each step helps explain why these conditions are selected.
由于ΔH为负值，正向反应是放热的。较低的温度会使平衡向生成更多氨的方向移动，但速率太慢。工业上使用催化剂（铁）来降低活化能，并选择约450°C的折中温度，配合高压，以迅速获得可行的产率。了解每一步的能量曲线有助于解释为何选择这些条件。
11. Calculating Energy Changes Using Bond Energies | 用键能计算能量变化
IGCSE Edexcel Science often requires you to calculate the overall enthalpy change from given average bond energies (in kJ mol⁻¹). The method follows three steps:
- Write a balanced equation and identify all the bonds broken in the reactants and all the bonds formed in the products.
- Multiply each type of bond by its bond energy and add them up separately.
- Apply the formula: ΔH = Σ (bond energies of bonds broken) – Σ (bond energies of bonds formed).
IGCSE 爱德思科学考试常要求利用给出的平均键能（单位 kJ mol⁻¹）计算总焓变。方法分为三步：
- 写出配平的方程式，识别反应物中断开的所有键和生成物中形成的新键。
- 将每种键的数目乘以其键能，并分别加总。
- 套用公式：ΔH = Σ（断键的键能之和） – Σ（成键的键能之和）。
For example, in the reaction H₂ + Cl₂ → 2HCl, bonds broken are one H–H (436 kJ) and one Cl–Cl (243 kJ); bonds formed are two H–Cl (2 × 431 = 862 kJ). So ΔH = (436+243) – 862 = – exothermic. Practising several such calculations builds confidence for the written paper.
例如，在反应 H₂ + Cl₂ → 2HCl 中，断开的键为一个 H–H（436 kJ）和一个 Cl–Cl（243 kJ）；形成的键为两个 H–Cl（2 × 431 = 862 kJ）。因此 ΔH = (436+243) – 862 = –183 kJ。负值表示反应放热。多做几道此类计算题有助于提升在笔试卷中的信心。
12. Key Points and Exam Tips | 要点与备考提示
Always remember that exothermic reactions cause a temperature rise and have a negative ΔH, while endothermic reactions cause a temperature drop and have a positive ΔH. Use the energy level diagram to visualise whether the products are higher or lower than the reactants.
务必牢记：放热反应导致温度升高，ΔH为负；吸热反应导致温度降低，ΔH为正。利用能级图来形象地判断生成物是高于还是低于反应物。
When interpreting practical data, state clearly whether the reaction is exothermic or endothermic based on the temperature change, and link your conclusion to energy released or absorbed. In bond‑energy calculations, double‑check that you have counted every bond in the displayed formulae, especially in molecules with double or triple bonds.
在解释实验数据时，要根据温度变化清晰指明反应是放热还是吸热，并将结论与能量释放或吸收联系起来。在键能计算中，要仔细核对已数清结构式中的每一个键，特别是含有双键或三键的分子。
| Exothermic | Endothermic |
| Temperature of surroundings increases | Temperature of surroundings decreases |
| ΔH negative | ΔH positive |
| Products have less energy than reactants | Products have more energy than reactants |
| Examples: combustion, neutralisation | Examples: photosynthesis, thermal decomposition |
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