📚 Energy Changes in Chemical Reactions | 化学反应中的能量变化
Chemical reactions often involve energy changes. In KS3 science, we focus on how heat energy moves between reacting chemicals and their surroundings. Understanding energy changes helps explain why some reactions feel hot and others feel cold.
化学反应通常伴随着能量变化。在 KS3 科学中，我们重点关注热能在反应物与周围环境之间的传递。理解能量变化有助于解释为什么有些反应感觉发热，而另一些反应感觉变冷。
1. What Are Energy Changes? | 什么是能量变化
Energy change is the transfer of energy between the reacting substances and the surroundings. The energy is usually in the form of heat, but light and electrical energy can also be produced or absorbed. We record temperature changes to tell whether heat energy is released or taken in.
能量变化是指反应物与周围环境之间的能量传递。能量通常以热能形式存在，但也可以产生或吸收光能和电能。我们记录温度变化，以判断热能是释放还是吸收。
2. Exothermic Reactions | 放热反应
An exothermic reaction transfers energy from the chemicals to the surroundings. This makes the surroundings warmer. Common signs include a rise in temperature on a thermometer, heat given off, and sometimes light or sound.
放热反应将能量从化学物质传递到周围环境，使周围温度升高。常见迹象包括温度计读数上升、释放热量，有时还会产生光或声音。
3. Endothermic Reactions | 吸热反应
An endothermic reaction takes energy from the surroundings into the chemicals. This makes the surroundings cooler. The temperature of the reaction mixture drops, and you may feel the container become cold.
吸热反应从周围环境吸收能量进入化学物质，使周围温度降低。反应混合物的温度下降，容器摸起来可能变冷。
4. Examples of Exothermic Reactions | 放热反应示例
Many everyday reactions are exothermic. Table 1 shows some common examples.
许多日常反应是放热的。表 1 列出了一些常见例子。
| Reaction 反应 | Notes 说明 |
|---|---|
| Combustion of fuels 燃料燃烧 | Burning methane, petrol or wood releases heat and light. 甲烷、汽油或木材燃烧释放热量和光。 |
| Neutralisation 中和反应 | Acid + alkali reactions give out heat. 酸与碱的反应放出热量。 |
| Respiration 呼吸作用 | Cells release energy from glucose. 细胞从葡萄糖中释放能量。 |
| Magnesium + oxygen 镁与氧气 | Bright white flame and heat. 明亮的白色火焰和热量。 |
In each case, heat energy moves to the surroundings, so the temperature of the surrounding area rises.
在每种情况下，热能传递到周围环境，因此周围区域的温度升高。
5. Examples of Endothermic Reactions | 吸热反应示例
Endothermic reactions are less common in everyday life but very important. Examples include thermal decomposition of calcium carbonate, the reaction between citric acid and sodium hydrogencarbonate, and photosynthesis in plants.
吸热反应在日常生活中较少见，但非常重要。例子包括碳酸钙的热分解、柠檬酸与碳酸氢钠的反应，以及植物的光合作用。
When citric acid solution and sodium hydrogencarbonate powder are mixed, the mixture fizzes and the temperature drops. This principle is used in some fizzy sweets and cold packs.
当柠檬酸溶液与碳酸氢钠粉末混合时，混合物会冒泡并且温度下降。这一原理被用于一些起泡糖和冷敷包中。
6. Detecting Energy Changes | 检测能量变化
To identify an energy change, measure the temperature of a known volume of liquid before and after adding a solid or another solution. Use a polystyrene cup as an insulator to reduce heat loss to the air. Record the maximum or minimum temperature reached.
要检测能量变化，可在加入固体或另一种溶液前后测量已知体积液体的温度。使用聚苯乙烯杯作为隔热层以减少向空气的热量损失。记录达到的最高或最低温度。
A rise in temperature indicates an exothermic reaction; a fall indicates an endothermic reaction. Always stir gently and read the thermometer at eye level for accurate results.
温度升高表示放热反应；温度降低表示吸热反应。轻轻搅拌并在与眼睛平齐的位置读取温度计，以获得准确结果。
7. Energy and Chemical Bonds | 能量与化学键
Chemical reactions involve breaking old bonds and making new bonds. Breaking bonds requires energy, so it is endothermic. Making bonds releases energy, so it is exothermic. The overall energy change tells us which process dominates.
化学反应涉及断裂旧键和形成新键。断裂化学键需要能量，因而是吸热的；形成化学键释放能量，因而是放热的。总能量变化告诉我们哪个过程占主导。
In an exothermic reaction, the energy released when new bonds form is greater than the energy needed to break old bonds. In an endothermic reaction, the energy needed to break old bonds is greater than the energy released when new bonds form.
在放热反应中，形成新键释放的能量大于断裂旧键所需的能量。在吸热反应中，断裂旧键所需的能量大于形成新键释放的能量。
8. Energy Level Diagrams | 能级图
An energy level diagram shows the relative energy of reactants and products. For an exothermic reaction, the products sit at a lower energy level than the reactants, so the difference in energy is released. For an endothermic reaction, the products sit at a higher energy level, so energy is absorbed.
能级图显示反应物和产物的相对能量。对于放热反应，产物处于比反应物更低的能级，因此能量差被释放。对于吸热反应，产物处于更高的能级，因此能量被吸收。
thermic: products lower → energy released; endothermic: products higher → energy absorbed.
你可能会看到放热用向下箭头表示，吸热用向上箭头表示：放热时产物能量更低 → 能量释放；吸热时产物能量更高 → 能量吸收。
9. Everyday Applications | 日常应用
Hand warmers contain iron powder that reacts with oxygen in the air. This exothermic reaction releases heat slowly and keeps hands warm. Cold packs often contain ammonium nitrate and water; when mixed, the endothermic process absorbs heat and cools injuries.
暖手宝含有铁粉，铁粉与空气中的氧气反应。这一放热反应缓慢释放热量，使手保持温暖。冷敷包通常含有硝酸铵和水；混合时，吸热过程吸收热量，为受伤部位降温。
Self-heating drinks cans use an exothermic reaction between quicklime and water to warm the drink. These applications show how controlling energy changes is useful in daily life.
自热饮料罐利用生石灰与水之间的放热反应来加热饮料。这些应用表明，控制能量变化在日常生活中非常有用。
10. Key Summary and Exam Tips | 总结与考试提示
heat exits, temperature rises; endothermic = heat enters, temperature falls. Use the words 'surroundings', 'released' and 'absorbed' carefully. Always give a named example for each type and link temperature change to energy transfer.
记住这些定义：放热 = 热量向外传递，温度升高；吸热 = 热量向内传递，温度下降。仔细使用 '周围环境'、'释放' 和 '吸收' 等词语。为每种类型各举一个具体例子，并将温度变化与能量传递联系起来。
In questions, read whether the thermometer reading rises or falls, then name the reaction type and explain the direction of energy transfer. Practice drawing simple energy level diagrams with labelled arrows.
在题目中，注意温度计读数是上升还是下降，然后指出反应类型并解释能量传递方向。练习绘制带有箭头标注的简单能级图。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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