Exothermic and Endothermic Reactions | 放热与吸热反应

📚 Exothermic and Endothermic Reactions | 放热与吸热反应

When a chemical reaction takes place, energy is usually transferred to or from the surroundings. In Edexcel IGCSE Science, you need to describe and explain these energy changes in terms of temperature change, bond breaking and bond forming, and interpret energy level diagrams.

当化学反应发生时，能量通常会在反应体系与环境之间转移。在 Edexcel IGCSE 科学中，你需要从温度变化、化学键断裂与形成以及能量图解读等角度来描述和解释这些能量变化。

1. Why Energy Changes Matter | 为什么能量变化很重要

Every chemical reaction involves a change in energy. Some reactions release energy to the surroundings, making the solution or the container feel hotter. Other reactions take in energy from the surroundings, which makes the temperature drop. Recognising which type of reaction is happening helps chemists control industrial processes, design products such as cold packs and heat packs, and predict whether a reaction will be easy to start.

每一个化学反应都伴随能量变化。有些反应会向环境释放能量，使溶液或容器变热；另一些反应则会从环境吸收能量，导致温度下降。判断反应属于哪种类型，能够帮助化学家控制工业过程、设计如冷敷包和热敷包等产品，并预测一个反应是否容易发生。

In the IGCSE course, energy changes are not just about “hot or cold”. You must be able to use the terms exothermic and endothermic correctly, and link them to bond energy calculations and energy profile diagrams.

在 IGCSE 课程中，能量变化不仅仅是”热或冷”的问题。你必须能够正确使用放热和吸热这两个术语，并将它们与键能计算和能量图联系起来。

2. Exothermic Reactions | 放热反应

An exothermic reaction is one that transfers energy from the system to the surroundings, usually in the form of heat. As a result, the temperature of the reaction mixture or surroundings increases. In an exothermic reaction, the enthalpy change ΔH is negative because the products have less chemical energy than the reactants.

放热反应是将能量从反应体系传递给环境的反应，通常以热能的形式释放。因此，反应混合物或环境的温度会升高。在放热反应中，焓变 ΔH 为负值，因为生成物所含的化学能比反应物少。

Common examples of exothermic reactions at IGCSE level include:

在 IGCSE 阶段常见的放热反应包括：

Combustion of fuels such as methane and ethanol.

燃料的燃烧，如甲烷和乙醇的燃烧。
Neutralisation reactions between acids and alkalis.

酸与碱之间的中和反应。
The reaction between a reactive metal such as magnesium and dilute hydrochloric acid.

活泼金属（如镁）与稀盐酸的反应。
Oxidation reactions, including respiration.

氧化反应，包括呼吸作用。

During combustion, the chemical energy stored in the fuel is released as heat and light. During neutralisation, the formation of water from H⁺ and OH⁻ ions releases a significant amount of energy. You should be prepared to describe a simple experiment where the temperature rise of the solution is measured with a thermometer or a data logger.

在燃烧过程中，燃料中储存的化学能以热和光的形式被释放。在中和反应中，H⁺ 与 OH⁻ 离子结合生成水时会释放大量能量。你应能描述一个简单实验，即用温度计或数据采集器测量溶液温度的升高。

3. Endothermic Reactions | 吸热反应

An endothermic reaction is one that takes in energy from the surroundings, usually as heat. The temperature of the reaction mixture falls because energy is being absorbed from the surrounding water or air. In an endothermic reaction, ΔH is positive because the products have more chemical energy than the reactants.

吸热反应是从环境吸收能量的反应，通常以热能形式吸收。由于能量不断从周围的水或空气中被吸收，反应混合物的温度会下降。在吸热反应中，ΔH 为正值，因为生成物所含的化学能比反应物多。

Important examples include:

重要的例子包括：

Thermal decomposition of calcium carbonate (limestone) into calcium oxide and carbon dioxide.

碳酸钙（石灰石）受热分解为氧化钙和二氧化碳。
Photosynthesis, where plants absorb light energy to convert carbon dioxide and water into glucose and oxygen.

光合作用，植物吸收光能，将二氧化碳和水转化为葡萄糖和氧气。
Dissolving ammonium nitrate or potassium chloride in water, which makes the solution become colder.

将硝酸铵或氯化钾溶于水，会使溶液温度下降。
Thermal decomposition of copper(II) carbonate.

碳酸铜(II) 的受热分解。

A typical exam question asks you to compare exothermic and endothermic reactions. In your answer, mention both the temperature change and the sign of ΔH. You may also need to say whether the reaction is more likely to be spontaneous or require continuous heating.

典型的考试题会要求你比较放热反应与吸热反应。回答时，要同时提到温度变化和 ΔH 的符号。你还可能需要判断反应更容易自发进行，还是需要持续加热。

4. Activation Energy and Energy Profile Diagrams | 活化能与能量图

Every reaction needs a minimum amount of energy before reactant particles can successfully collide and react. This energy is called the activation energy, symbolised as Eₐ. It is the energy required to break the existing bonds before new bonds can form.

每个反应都需要一个最小能量值，反应物微粒才能成功碰撞并发生反应。这个能量称为活化能，用 Eₐ 表示。它是断裂原有化学键所需的能量，之后才能形成新的化学键。

Energy profile diagrams show the energy of the reactants, the energy of the products, and the activation energy hump. In an exothermic reaction, the products are at a lower energy level than the reactants, so the difference in height represents the energy released. In an endothermic reaction, the products are higher than the reactants, and the overall energy change is absorbed.

能量图展示反应物能量、生成物能量以及活化能的”驼峰”。在放热反应中，生成物的能量水平低于反应物，两者的高度差代表释放的能量。在吸热反应中，生成物能量高于反应物，总能量变化为吸收的能量。

ΔH = energy of products − energy of reactants

You should be able to label an energy profile diagram with the activation energy and the overall enthalpy change. A common mistake is to confuse the activation energy with ΔH. Activation energy is the height from the reactants to the top of the curve, while ΔH is the vertical distance between reactants and products.

你应该能够在能量图上标注活化能和总焓变。一个常见错误是混淆活化能与 ΔH。活化能是反应物到曲线最高点的高度，而 ΔH 是反应物与生成物之间的垂直距离。

5. Bond Breaking and Bond Making | 键的断裂与形成

Chemical reactions involve the breaking of bonds in the reactants and the formation of new bonds in the products. Breaking covalent bonds requires energy, so it is an endothermic process. Forming new covalent bonds releases energy, so it is an exothermic process.

化学反应涉及反应物中化学键的断裂和在生成物中新的化学键的形成。断裂共价键需要吸收能量，因此是一个吸热过程；形成新的共价键会释放能量，因此是一个放热过程。

Whether a reaction is exothermic or endothermic overall depends on the balance between these two terms. If more energy is released when new bonds form than is absorbed when old bonds break, the reaction is exothermic. If the opposite is true, the reaction is endothermic.

一个反应总体上是放热还是吸热，取决于这两个过程的能量平衡。如果形成新键释放的能量大于断裂旧键吸收的能量，则反应为放热；反之则为吸热。

ΔH = total energy absorbed to break bonds − total energy released to form bonds

For example, in the reaction H₂ + Cl₂ → 2HCl, you would need to look up the bond energies for H–H, Cl–Cl and H–Cl. Using bond energies is a common calculation question in the Edexcel IGCSE exam, and you must remember to subtract, not add, the two totals.

例如，在反应 H₂ + Cl₂ → 2HCl 中，你需要查表得到 H–H、Cl–Cl 和 H–Cl 的键能。使用键能计算焓变是 Edexcel IGCSE 考试中常见的计算题，你必须记得用两个总值相减，而不是相加。

6. Measuring Energy Changes | 测量能量变化

In the laboratory, you can measure the temperature change of a reaction using a polystyrene cup instead of a glass beaker. Polystyrene is a good insulator, so it reduces heat loss to the surroundings. A lid can be added to reduce heat loss through evaporation and convection.

在实验室中，你可以用聚苯乙烯泡沫杯代替玻璃烧杯来测量反应温度变化。聚苯乙烯泡沫是良好的隔热材料，可以减少热量散失到环境中。还可以加上盖子，以减少蒸发和对流造成的热量损失。

For a neutralisation reaction, a known volume of dilute hydrochloric acid is placed in the cup, and the initial temperature is recorded. A known mass of sodium hydroxide solution is then added quickly while stirring with a thermometer. The highest temperature reached is recorded, and the temperature rise is calculated.

以中和反应为例，将一定体积的稀盐酸放入杯中，记录初始温度。然后迅速加入已知质量的氢氧化钠溶液，同时用温度计搅拌。记录达到的最高温度，并计算温度的升高值。

q = m × c × ΔT

In the above equation, q is the heat energy in joules, m is the mass of the solution in grams, c is the specific heat capacity of water (usually 4.18 J g⁻¹ °C⁻¹), and ΔT is the temperature change in degrees Celsius. You are usually told whether to use 4.18 or a similar value.

在上面的公式中，q 是热能，单位是焦耳；m 是溶液质量，单位是克；c 是水的比热容，通常取 4.18 J g⁻¹ °C⁻¹；ΔT 是温度变化，单位是摄氏度。题目一般会直接给出这个值或类似值。

Sources of error include heat loss to the surroundings, heat absorbed by the thermometer and the cup, and incomplete reaction. To improve accuracy, you can use insulation, a lid, and more precise measuring instruments.

误差来源包括热量散失到环境、温度计与杯子本身吸收热量、以及反应不充分。为了提高准确度，可以使用隔热材料、盖子以及更精密的测量仪器。

7. Everyday Applications | 日常应用

Exothermic reactions are used in hand warmers. Some hand warmers contain iron powder, which reacts with oxygen in the air in an oxidation reaction. The heat released warms the hands. Other types of hand warmers rely on the crystallisation of a supersaturated solution, which is also an exothermic process.

放热反应可用于暖手宝。某些暖手宝中含有铁粉，铁粉与空气中的氧气发生氧化反应，释放的热量使手部温暖。另一种暖手宝利用过饱和溶液的结晶过程，这也是一个放热过程。

Endothermic reactions are used in instant cold packs. A plastic pouch containing water sits inside a larger bag of ammonium nitrate. When the inner pouch is broken, the salt dissolves in water and the temperature drops quickly, which helps reduce swelling in sports injuries.

吸热反应用于一次性冷敷包。一个装有水的塑料小袋被放在装有硝酸铵的大袋中。当内袋被打破，盐溶于水，温度迅速下降，可以帮助缓解运动损伤后的肿胀。

Understanding energy changes also helps explain why some reactions need heating to start. Although thermal decomposition is endothermic, a Bunsen burner provides the continuous energy needed to break the bonds. Once the reaction is underway, it will not release enough heat to keep going by itself.

理解能量变化也有助于解释为什么有些反应需要加热才能开始。虽然受热分解是吸热的，但本生灯提供持续的能量以断裂化学键。一旦反应开始，它自身无法释放足够的热量来维持反应进行。

8. Exam Tips and Common Misconceptions | 考试技巧与常见误区

One of the most common mistakes in the IGCSE exam is describing endothermic reactions as “needing heat” and exothermic reactions as “giving out heat”, but failing to link this to the change in temperature of the surroundings. Always say that the temperature of the surroundings increases or decreases.

IGCSE 考试中最常见的错误之一是只把吸热反应描述为”需要热量”、放热反应描述为”放出热量”，却没有将这与环境温度的变化联系起来。你应当明确指出环境温度是升高还是降低。

Another misconception is thinking that activation energy and ΔH mean the same thing. Activation energy is the energy input needed to start the reaction, while ΔH is the net energy change from reactants to products. A reaction can be highly exothermic yet still have a large activation energy, such as the combustion of charcoal.

另一个误区是认为活化能与 ΔH 含义相同。活化能是引发反应所需的能量输入，而 ΔH 是反应物到生成物的净能量变化。一个反应可以是强放热的，但仍具有很大的活化能，例如木炭的燃烧。

When answering explain questions, use key phrases such as “energy released when bonds are made is greater than energy absorbed when bonds are broken”. This sentence alone can earn you most of the marks on a bond-energy question.

回答解释题时，请使用关键短语，例如”形成化学键释放的能量大于断裂化学键吸收的能量”。仅这一句话就能帮你获得键能题的大部分分数。

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Exothermic and Endothermic Reactions | 放热与吸热反应