📚 Energy Changes in Chemical Reactions | 化学反应中的能量变化
Chemical reactions always involve energy being taken in or given out. Understanding these energy changes helps explain why some reactions feel hot, others feel cold, and how fuels release useful energy.
化学反应总是伴随着能量的吸收或释放。理解这些能量变化有助于解释为什么有些反应会发热、有些会变冷，以及燃料如何释放有用的能量。
1. What Are Energy Changes? | 什么是能量变化？
In every chemical reaction, existing bonds in reactants are broken and new bonds in products are formed. Breaking bonds requires energy, while forming bonds releases energy.
在每一个化学反应中，反应物中已有的化学键会断裂，生成物中新的化学键会形成。断键需要吸收能量，而成键会释放能量。
The overall energy change depends on the balance between these two processes. If more energy is released in bond formation than is taken in for bond breaking, the reaction gives out heat to the surroundings.
总能量变化取决于这两个过程之间的平衡。如果成键释放的能量大于断键吸收的能量，反应就会向周围环境放出热量。
- System: the chemical reaction itself
- Surroundings: everything around the reaction, such as the solution, air, or container
- 体系：化学反应本身
- 环境：反应周围的一切，例如溶液、空气或容器
2. Exothermic Reactions | 放热反应
An exothermic reaction transfers thermal energy from the system to the surroundings. As a result, the temperature of the surroundings increases.
放热反应将热能由体系传递到环境中。因此，环境的温度会升高。
Common examples include combustion of fuels, neutralisation of acids and alkalis, and respiration in living cells.
常见的例子包括燃料的燃烧、酸碱中和以及活细胞中的呼吸作用。
In an exothermic reaction, the energy of the products is lower than the energy of the reactants. The difference in energy is released, usually as heat.
在放热反应中，生成物的能量低于反应物的能量。两者之间的能量差被释放出来，通常以热的形式放出。
| Example | Observable sign |
| Burning methane | Flame and heat |
| Magnesium + acid | Temperature rises |
| Adding water to quicklime | Steam and heat |
例子：燃烧甲烷可观察到火焰和热量；镁与酸反应会使温度升高；生石灰加水会放出蒸汽和热量。
3. Endothermic Reactions | 吸热反应
An endothermic reaction takes in thermal energy from the surroundings. The temperature of the surroundings decreases.
吸热反应从环境中吸收热能。环境的温度会降低。
Typical examples include thermal decomposition of carbonates, photosynthesis in plants, and dissolving certain salts such as ammonium nitrate in water.
典型的例子包括碳酸盐的热分解、植物的光合作用以及某些盐类（如硝酸铵）溶于水。
In an endothermic reaction, the products have more energy than the reactants. The extra energy is absorbed from the environment.
在吸热反应中，生成物的能量高于反应物。额外的能量从环境中吸收。
| Example | Observable sign |
| Thermal decomposition of calcium carbonate | Continuous heating needed |
| Photosynthesis | Light energy absorbed |
| Dissolving ammonium nitrate | Beaker feels cold |
例子：碳酸钙热分解需要持续加热；光合作用吸收光能；硝酸铵溶解时烧杯摸起来变冷。
4. Reaction Profile Diagrams | 反应历程图
A reaction profile diagram shows the energy of the reactants and products, and the activation energy of the reaction.
反应历程图展示反应物和生成物的能量，以及反应的活化能。
For an exothermic reaction, the products are drawn at a lower energy level than the reactants. The energy difference is labelled as ΔH, and it is negative.
对于放热反应，生成物的能级低于反应物。能量差标记为 ΔH，且为负值。
For an endothermic reaction, the products are drawn at a higher energy level than the reactants. The energy difference ΔH is positive.
对于吸热反应，生成物的能级高于反应物。能量差 ΔH 为正值。
Exothermic: ΔH = H(products) − H(reactants) < 0
放热反应：ΔH = H(生成物) − H(反应物) < 0
Endothermic: ΔH = H(products) − H(reactants) > 0
吸热反应：ΔH = H(生成物) − H(反应物) > 0
You should be able to sketch these diagrams and label the activation energy, reactants, products, and ΔH clearly.
你应当能够画出这些曲线图，并清晰地标注活化能、反应物、生成物和 ΔH。
5. Bond Breaking and Bond Making | 键的断裂与形成
Bond breaking is always endothermic because energy must be supplied to pull atoms apart.
断键总是吸热的，因为必须提供能量才能把原子拉开。
Bond making is always exothermic because energy is released when new bonds form and the atoms become more stable.
成键总是放热的，因为形成新键时原子变得更稳定，会释放能量。
The overall thermal change can be calculated by comparing the total energy absorbed in breaking bonds with the total energy released in forming bonds.
总的热变化可以通过比较断键吸收的总能量与成键释放的总能量来计算。
ΔH = ΣE(bonds broken) − ΣE(bonds formed)
ΔH = ΣE(断裂的键) − ΣE(形成的键)
If breaking bonds absorbs more energy than forming bonds releases, the reaction is endothermic. If forming bonds releases more energy, the reaction is exothermic.
如果断键吸收的能量大于成键释放的能量，反应为吸热反应。如果成键释放的能量更多，反应则为放热反应。
6. Calculating Enthalpy Change from Bond Energies | 从键能计算焓变
Bond energy is the energy needed to break one mole of a particular covalent bond in the gaseous state.
键能是指断裂气态中1摩尔某种共价键所需的能量。
To calculate ΔH, use the formula:
要计算 ΔH，可使用以下公式：
ΔH = ΣE(bonds broken) − ΣE(bonds formed)
ΔH = ΣE(断裂的键) − ΣE(形成的键)
Worked example: calculate the energy change for the reaction H₂ + Cl₂ → 2HCl, given E(H−H) = 436 kJ/mol, E(Cl−Cl) = 242 kJ/mol, and E(H−Cl) = 431 kJ/mol.
计算示例：计算 H₂ + Cl₂ → 2HCl 的能量变化，已知 E(H−H) = 436 kJ/mol、E(Cl−Cl) = 242 kJ/mol、E(H−Cl) = 431 kJ/mol。
Energy in = 436 + 242 = 678 kJ
吸收能量 = 436 + 242 = 678 kJ
Energy out = 2 × 431 = 862 kJ
释放能量 = 2 × 431 = 862 kJ
ΔH = 678 − 862 = −184 kJ/mol
ΔH = 678 − 862 = −184 kJ/mol
The negative sign shows that the reaction is exothermic. Always state the sign and unit in your answer.
负号表示该反应是放热反应。答题时一定要写明符号和单位。
7. Activation Energy | 活化能
Activation energy is the minimum energy required for a reaction to start. It is needed to break the existing bonds in the reactants before new bonds can form.
活化能是反应开始所需的最低能量。在形成新键之前，必须先提供能量来断裂反应物中已有的化学键。
On a reaction profile diagram, activation energy is the energy difference between the reactants and the highest point on the curve, called the transition state.
在反应历程图中，活化能是反应物与曲线最高点（即过渡态）之间的能量差。
Reactions with high activation energy often require heating or a catalyst to proceed at a reasonable rate.
活化能高的反应通常需要加热或使用催化剂，才能以合理的速率进行。
Activation energy = energy of transition state − energy of reactants
活化能 = 过渡态能量 − 反应物能量
8. Catalysts and Energy Changes | 催化剂与能量变化
A catalyst speeds up a chemical reaction by providing an alternative reaction pathway with lower activation energy.
催化剂通过提供一条活化能较低的替代反应路径来加快化学反应。
A catalyst does not change the energy of the reactants or the products. Therefore, it does not change the overall ΔH of the reaction.
催化剂不会改变反应物或生成物的能量。因此，它不会改变反应的总 ΔH。
In a reaction profile diagram, a catalysed reaction has a lower curve for the activation energy barrier, but the starting and ending energy levels stay the same.
在反应历程图中，有催化剂的反应其活化能垒更低，但起点和终点的能级保持不变。
This is important in industry because lower activation energy means less fuel is needed to run the process, reducing costs.
这在工业上很重要，因为较低的活化能意味着运行过程需要的燃料更少，从而降低成本。
9. Practical Examples and Applications | 实际例子与应用
Exothermic reactions are used in hand warmers, self-heating cans, and the combustion of fuels for transport and electricity generation.
放热反应用于暖手宝、自热罐以及用于交通和发电的燃料燃烧。
Endothermic reactions are used in cold packs for sports injuries. When ammonium nitrate dissolves in water, it absorbs heat and cools the surroundings quickly.
吸热反应用于运动损伤的冷敷包。硝酸铵溶于水时会吸收热量，使周围迅速降温。
Photosynthesis is an essential endothermic process that stores light energy as chemical energy in glucose.
光合作用是一个重要的吸热过程，它将光能储存为葡萄糖中的化学能。
Respiration is an exothermic process that releases energy from glucose for cells to use in movement, growth, and repair.
呼吸作用是一个放热过程，它从葡萄糖中释放能量，供细胞用于运动、生长和修复。
10. Exam Tips for Cambridge Science | 剑桥科学考试技巧
Always link the sign of ΔH to the direction of heat transfer: negative ΔH means heat is released to the surroundings, positive ΔH means heat is absorbed from the surroundings.
始终将 ΔH 的符号与热传递方向联系起来：ΔH 为负表示热量释放到环境中，ΔH 为正表示从环境中吸收热量。
When calculating bond energies, draw the displayed formula of each molecule first. This helps you count every bond accurately.
在计算键能时，首先画出每种分子的结构式。这有助于你准确数出每一个键。
Make sure your reaction profile diagrams show a curve from reactants to products, with the activation energy hump clearly labelled and ΔH marked as a vertical energy difference.
确保你的反应历程图画出从反应物到生成物的曲线，并清晰标注活化能的峰值以及用竖直能量差表示 ΔH。
- State whether the reaction is exothermic or endothermic
- Give the correct sign and unit for ΔH
- Mention bond breaking as endothermic and bond forming as exothermic
- Explain the effect of a catalyst on activation energy
- 说明反应是放热还是吸热
- 给出 ΔH 正确的符号和单位
- 提到断键吸热、成键放热
- 解释催化剂对活化能的影响
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