📚 Energy from Chemical Reactions: Mark Scheme Essentials | 化学反应中的能量变化：评分标准要点
In CIE KS3 Science, questions on energy from chemical reactions often test whether you can correctly name an energy change, link observations such as temperature rise or fall to the type of reaction, and use precise mark scheme phrases. This revision guide explains the key ideas and shows you the exact wording examiners expect.
在 CIE KS3 科学考试中，关于化学反应能量变化的题目通常考查你是否能正确说出能量变化的名称、将温度上升或下降等观察现象与反应类型联系起来，并使用评分标准中的准确措辞。本复习指南讲解核心概念，并告诉你考官期望的精确表述。
1. Exothermic and Endothermic Reactions | 放热反应与吸热反应
An exothermic reaction transfers heat energy to the surroundings. The temperature of the surroundings increases. Combustion, neutralisation and respiration are common examples.
放热反应把热量传递到周围环境。周围环境的温度升高。燃烧、中和反应和呼吸作用是常见的例子。
An endothermic reaction absorbs heat energy from the surroundings. The temperature of the surroundings decreases. Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate are typical examples.
吸热反应从周围环境吸收热量。周围环境的温度降低。热分解以及柠檬酸与碳酸氢钠的反应是典型例子。
exothermic: energy released → ΔH < 0; endothermic: energy absorbed → ΔH > 0
放热反应：释放能量，ΔH 为负；吸热反应：吸收能量，ΔH 为正。
2. Mark Scheme Keywords for Exothermic Changes | 放热变化的评分关键词
Examiners look for the idea that heat energy is given out to the surroundings. Simply writing ‘it gets hot’ may not be enough because the mark scheme requires a transfer of energy, not just an observation.
考官希望看到热量被释放到周围环境中的表述。只写“它变热了”可能不够，因为评分标准要求的是能量转移，而不仅仅是观察现象。
- heat energy is released to the surroundings
- temperature of the surroundings increases
- products have less chemical energy than reactants
关键表述包括：热量被释放到周围环境；周围环境温度升高；生成物的化学能比反应物少。
For a named reaction such as neutralisation, you should write: ‘neutralisation is exothermic because it releases heat energy.’
对于中和反应等具体反应，你应该写：“中和反应是放热的，因为它释放热量。”
CH₄ + 2O₂ → CO₂ + 2H₂O (exothermic)
甲烷燃烧生成二氧化碳和水，是放热反应。
3. Mark Scheme Keywords for Endothermic Changes | 吸热变化的评分关键词
For endothermic processes, mark scheme answers must mention heat energy being taken in or absorbed from the surroundings. A fall in temperature alone is an observation, not a complete explanation.
对于吸热过程，评分标准答案必须提到从周围环境吸收热量。仅仅说温度下降是观察现象，不是完整的解释。
- heat energy is absorbed from the surroundings
- temperature of the surroundings decreases
- products have more chemical energy than reactants
关键表述包括：从周围环境吸收热量；周围环境温度降低；生成物的化学能比反应物多。
A common example is the reaction between citric acid and sodium hydrogencarbonate. Write: ‘the reaction is endothermic because it takes in heat energy from the surroundings.’
一个常见例子是柠檬酸与碳酸氢钠的反应。应写：“该反应是吸热的，因为它从周围环境吸收热量。”
CaCO₃ → CaO + CO₂ (endothermic)
碳酸钙热分解生成氧化钙和二氧化碳，是吸热反应。
4. Energy Level Diagrams | 能级图
Energy level diagrams show the relative energy of reactants and products. For an exothermic reaction, the products are at a lower energy level than the reactants, so the energy change arrow points downwards.
能级图显示反应物和生成物的相对能量。对于放热反应，生成物的能级低于反应物，因此能量变化箭头向下。
For an endothermic reaction, the products are at a higher energy level than the reactants, so the energy change arrow points upwards.
对于吸热反应，生成物的能级高于反应物，因此能量变化箭头向上。
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