📚 Energy Changes in Chemical Reactions | 化学反应中的能量变化
Energy is neither created nor destroyed; it can only be converted from one form to another. In chemistry, energy changes help us predict whether a reaction will release heat or absorb heat from its surroundings.
能量既不会凭空产生，也不会凭空消失，它只能从一种形式转化为另一种形式。在化学中，能量变化帮助我们预测反应是向环境释放热量，还是从环境中吸收热量。
1. Chemical Bonds and Energy | 化学键与能量
Chemical reactions involve the breaking of bonds in the reactants and the formation of new bonds in the products. Each chemical bond stores a certain amount of energy.
化学反应涉及断裂反应物中的化学键，并在产物中形成新的化学键。每个化学键都储存着一定量的能量。
Breaking bonds is an endothermic process because it requires an input of energy. Forming bonds is an exothermic process because it releases energy.
断键是一个吸热过程，因为它需要输入能量。成键则是一个放热过程，因为它会释放能量。
Whether a reaction is exothermic or endothermic depends on the difference between the energy required to break bonds and the energy released when new bonds form.
一个反应是放热还是吸热，取决于断键所需能量与原子重新成键所释放能量之间的差值。
2. Exothermic Reactions | 放热反应
In an exothermic reaction, the total energy released in forming new bonds is greater than the total energy needed to break old bonds. As a result, energy is transferred to the surroundings, usually as heat.
在放热反应中，形成新键所释放的总能量大于破坏旧键所需的总能量。因此，能量以热的形式转移给周围环境。
| Exothermic examples | Observations |
| Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O | Heat and light are produced. |
| Neutralisation of acid and alkali | Temperature of mixture rises. |
放热反应例子：甲烷燃烧 CH₄ + 2O₂ → CO₂ + 2H₂O，产生热和光；酸碱中和，混合液温度升高。
Feeling the test tube get hot when a reaction occurs is a simple sign of an exothermic reaction.
当反应发生时，触摸试管感觉变热是放热反应的一个简单标志。
3. Endothermic Reactions | 吸热反应
In an endothermic reaction, more energy is required to break bonds than is released when new bonds form. This causes the surroundings to become cooler because the reaction takes in energy.
在吸热反应中，断键所需的能量大于新键形成时释放的能量。这会导致周围环境变冷，因为反应吸收了能量。
+ 6H₂O → C₆H₁₂O₆ + 6O₂, and thermal decomposition of calcium carbonate: CaCO₃ → CaO + CO₂.
例子包括光合作用：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂，以及碳酸钙的热分解：CaCO₃ → CaO + CO₂。
In the laboratory, barium hydroxide and ammonium chloride can produce a cold flask when mixed.
在实验室中，氢氧化钡与氯化铵混合时能使烧瓶变冷。
4. Reaction Profiles | 反应能线图
Reaction profiles show the energy levels of reactants and products during a reaction. In an exothermic profile, the products have a lower energy level than the reactants.
反应能线图显示反应过程中的反应物和产物的能量水平。在放热反应的能线图中，产物的能量水平低于反应物。
In an endothermic profile, the products have a higher energy level than the reactants.
在吸热反应能线图中，产物的能量水平高于反应物。
The difference between the energy of the reactants and the energy of the products is the overall energy change (ΔH).
反应物能量与产物能量之差就是总能量变化（ΔH）。
ΔH = Energy of products − Energy of reactants
ΔH = 产物能量 − 反应物能量
5. Activation Energy | 活化能
Activation energy is the minimum amount of energy that colliding particles must have for a reaction to happen. Even exothermic reactions need an initial input of energy to start.
活化能是粒子发生碰撞时发生反应所需的最低能量。即使是放热反应也需要初始能量输入才能启动。
This energy is used to break the existing bonds in the reactants. On a reaction profile, activation energy is shown as the height of the "energy hill" from the reactants to the transition state.
这部分能量用于断裂反应物中的现有化学键。在反应能线图上，活化能表现为从反应物到过渡态的"能量峰"高度。
A catalyst lowers the activation energy by providing an alternative pathway, making the reaction go faster without being used up.
催化剂通过提供另一条路径来降低活化能，使反应更快而不被消耗。
6. Bond Energy Calculations | 键能计算
To calculate the energy change of a reaction, we need to know the average bond energies of the bonds broken and formed. The bond energy is the energy required to break one mole of a covalent bond in the gaseous state.
要计算反应的能量变化，我们需要知道断裂和形成的化学键的平均键能。键能是指在气态下断裂一摩尔共价键所需的能量。
The formula is:
公式如下：
Energy change = Total energy of bonds broken − Total energy of bonds formed
能量变化 = 断键所需总能量 − 成键释放总能量
+ Cl₂ → 2HCl. The bond energies are H-H: 436 kJ/mol, Cl-Cl: 242 kJ/mol, H-Cl: 431 kJ/mol.
例如，对于反应 H₂ + Cl₂ → 2HCl，键能分别为 H-H：436 kJ/mol，Cl-Cl：242 kJ/mol，H-Cl：431 kJ/mol。
rgy change = 678 − 862 = −184 kJ. The negative sign shows that the reaction is exothermic.
断键所需能量 = 436 + 242 = 678 kJ。成键释放能量 = 2 × 431 = 862 kJ。能量变化 = 678 − 862 = −184 kJ。负号表示该反应是放热反应。
7. Observing Energy Changes: Experiments | 观察能量变化：实验
A simple experiment involves adding a solid to a liquid and measuring the temperature change. For example, dissolve ammonium nitrate in water; the temperature drops, showing an endothermic process.
一个简单的实验是将固体加入液体并测量温度变化。例如，将硝酸铵溶于水，温度下降，表明是吸热过程。
In contrast, dissolving sodium hydroxide in water causes the temperature to rise, which shows an exothermic process.
相反，将氢氧化钠溶于水会使温度升高，这显示出放热过程。
When calculating energy changes from temperature data, we use the equation:
当根据温度数据计算能量变化时，我们使用公式：
q = mcΔT
q = mcΔT
where q is heat energy (J), m is mass of solution (g), c is specific heat capacity (J/g°C), and ΔT is the temperature change (°C).
其中 q 是热能（J），m 是溶液质量（g），c 是比热容（J/g°C），ΔT 是温度变化（°C）。
This experimental value is often lower than the theoretical value because heat is lost to the surroundings.
由于热量会散失到环境中，实验值通常低于理论值。
8. Applications in Daily Life and Industry | 在日常生活中和工业中的应用
Exothermic reactions are used in hand warmers, self-heating cans, and combustion engines. Quicklime (CaO) reacts with water to release a lot of heat.
放热反应用于暖手宝、自热罐和内燃机。生石灰（CaO）与水反应会释放大量热量。
Endothermic reactions are used in instant cold packs. For example, ammonium nitrate absorbs heat when dissolved in water.
吸热反应用于一次性冰袋。例如，硝酸铵溶于水时吸收热量。
In industry, controlling energy changes is vital for safety and efficiency. Catalysts are often used to lower activation energy and reduce energy costs.
在工业中，控制能量变化对于安全和效率至关重要。通常使用催化剂来降低活化能并降低能源成本。
9. Summary | 总结
Chemical reactions always involve energy changes. Understanding whether a reaction is exothermic or endothermic helps us predict temperature changes and is applied in many fields.
化学反应总是伴随着能量变化。理解反应是放热还是吸热有助于我们预测温度变化，并在许多领域中应用。
Key ideas: bond breaking absorbs energy, bond forming releases energy; activation energy must be overcome; catalysts speed up reactions by lowering activation energy; bond energy calculations allow us to estimate energy changes.
关键概念：断键吸收能量，成键释放能量；必须越过活化能；催化剂通过降低活化能来加速反应；键能计算使我们可以估算能量变化。
Remember to use the correct sign: negative ΔH means exothermic, positive ΔH means endothermic.
记住使用正确的符号：ΔH 为负表示放热，ΔH 为正表示吸热。
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