IGCSE AQA Chemistry: Thermochemistry Key Points | IGCSE AQA 化学：热化学 考点精讲

📚 IGCSE AQA Chemistry: Thermochemistry Key Points | IGCSE AQA 化学：热化学 考点精讲

Thermochemistry deals with the heat changes that accompany chemical reactions. Understanding energy transfer is essential for explaining why reactions occur and for practical applications like fuels, hand warmers, and cold packs. This revision guide covers the key concepts required for IGCSE AQA Chemistry, including exothermic and endothermic reactions, energy profile diagrams, bond energy calculations, and calorimetry experiments.

热化学研究伴随化学反应的热量变化。理解能量转移对于解释反应发生的原因以及燃料、暖手宝和冷敷袋等实际应用至关重要。本复习指南涵盖了 IGCSE AQA 化学所需的关键概念，包括放热与吸热反应、能量变化图、键能计算和量热实验。

1. Exothermic and Endothermic Reactions | 放热反应与吸热反应

Exothermic reactions transfer energy to the surroundings, usually causing an increase in temperature. Common examples include combustion (burning fuels), neutralisation (acid + alkali), and respiration. The energy released comes from the chemical bonds in the reactants being rearranged into more stable products.

放热反应将能量传递给周围环境，通常导致温度升高。常见例子包括燃烧（燃料燃烧）、中和（酸与碱的反应）和呼吸作用。释放的能量来源于反应物中的化学键重新组合形成更稳定的生成物。

In an endothermic reaction, energy is absorbed from the surroundings, leading to a temperature decrease. Thermal decomposition (e.g., breaking down calcium carbonate) and photosynthesis are typical endothermic processes. The reaction system cools down because it takes in heat.

在吸热反应中，能量从周围环境被吸收，导致温度下降。热分解（如碳酸钙的分解）和光合作用是典型的吸热过程。反应体系因吸收热量而变冷。

The overall energy change is described by the enthalpy change, ΔH. Exothermic reactions have a negative ΔH (energy given out), while endothermic reactions have a positive ΔH (energy taken in).

总能量变化用焓变 ΔH 来描述。放热反应的 ΔH 为负值（能量放出），吸热反应的 ΔH 为正值（能量吸收）。

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2. Energy Profile Diagrams | 能量变化图

An energy profile diagram shows how the energy of the reacting particles changes during a reaction. The vertical axis is energy, and the horizontal axis is the reaction progress. The reactants and products are shown as flat lines, and the difference in their energy levels equals ΔH.

能量变化图展示反应过程中粒子能量的变化。纵坐标为能量，横坐标为反应进程。反应物和生成物用水平线表示，它们的能量差等于 ΔH。

For an exothermic reaction, the products are lower in energy than the reactants, so ΔH is negative. The energy level goes down overall. For an endothermic reaction, the products are higher in energy, so ΔH is positive, and the energy level goes up overall.

对于放热反应，生成物的能量低于反应物，因此 ΔH 为负，总能量下降。对于吸热反应，生成物能量更高，ΔH 为正，总能量上升。

The highest point on the curve is the activation energy, Ea. It is the minimum energy that particles must collide with for a reaction to occur. A catalyst provides an alternative pathway with a lower activation energy, but it does not change the enthalpy change ΔH.

曲线最高点代表活化能 Ea，即粒子发生反应所需的最低碰撞能量。催化剂提供了一条活化能较低的反应路径，但不改变焓变 ΔH。

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3. Bond Energies and Enthalpy Change Calculation | 键能与焓变计算

Every chemical reaction involves breaking bonds in reactants and forming new bonds in products. Bond breaking requires energy (endothermic process), while bond making releases energy (exothermic process). The overall enthalpy change can be calculated using average bond energies with the equation:

每个化学反应都涉及反应物中旧键的断裂和生成物中新键的形成。断键需要吸收能量（吸热过程），成键则释放能量（放热过程）。利用平均键能可以计算总焓变，公式为：

ΔH = Σ (bond energies of bonds broken) − Σ (bond energies of bonds formed)

The sum of the bond energies for all bonds broken is found first. Then the sum for all bonds formed is subtracted. A negative result means the reaction is exothermic (more energy released making bonds than absorbed breaking bonds). A positive result means the reaction is endothermic.

首先求出所有断裂键的键能之和，再减去所有形成键的键能之和。若结果为负，说明反应放热（成键释放的能量多于断键吸收的能量）；若结果为正，则反应吸热。

Worked example: Hydrogen reacts with chlorine: H₂ + Cl₂ → 2HCl. Given: H−H = 436 kJ/mol, Cl−Cl = 243 kJ/mol, H−Cl = 432 kJ/mol. Bonds broken: 1 × 436 + 1 × 243 = 679 kJ. Bonds formed: 2 × 432 = 864 kJ. ΔH = 679 − 864 = −185 kJ/mol. The negative sign indicates an exothermic reaction.

例题：氢气与氯气反应：H₂ + Cl₂ → 2HCl。已知 H−H = 436 kJ/mol，Cl−Cl = 243 kJ/mol，H−Cl = 432 kJ/mol。断裂键能总和 = 1×436 + 1×243 = 679 kJ。形成键能总和 = 2×432 = 864 kJ。ΔH = 679 − 864 = −185 kJ/mol。负号表明该反应放热。

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4. Calorimetry Experiment for Neutralisation | 中和反应的量热实验

The enthalpy change of neutralisation can be measured simply using a polystyrene cup as a calorimeter. Equal volumes of a strong acid (e.g., hydrochloric acid) and a strong alkali (e.g., sodium hydroxide) are mixed, and the temperature rise is recorded with a thermometer. The polystyrene cup provides insulation to reduce heat loss.

中和焓变可通过使用聚苯乙烯杯作为量热计简单测定。将等体积的强酸（如盐酸）与强碱（如氢氧化钠）混合，用温度计记录温度上升值。聚苯乙烯杯起隔热作用以减少热损失。

The solution mass is taken as the total volume of the mixture (since the density can be assumed to be 1 g/cm³, close to water). The heat energy transferred, Q, is calculated using:

Q = m c ΔT

where m is the total mass of solution, c is the specific heat capacity of water (4.18 J g⁻¹ °C⁻¹), and ΔT is the temperature change. The molar enthalpy change is then determined by dividing Q by the number of moles of water formed (or the limiting reactant) and adding a negative sign if the reaction is exothermic.

其中 m 为溶液总质量，c 为水的比热容 (4.18 J g⁻¹ °C⁻¹)，ΔT 为温度变化。然后通过 Q 除以生成水的物质的量（或限制反应物的物质的量），并视反应放热加上负号，即得摩尔焓变。

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5. Worked Example: Neutralisation Calculation | 计算实例：中和热

A typical experiment: 50.0 cm³ of 1.0 mol/dm³ HCl is mixed with 50.0 cm³ of 1.0 mol/dm³ NaOH in a polystyrene cup. The initial temperature of both solutions is 21.0°C, and the final temperature after mixing is 27.8°C. Calculate the enthalpy change of neutralisation.

一个典型实验：将 50.0 cm³ 1.0 mol/dm³ HCl 与 50.0 cm³ 1.0 mol/dm³ NaOH 在聚苯乙烯杯中混合。两种溶液初始温度均为 21.0°C，混合后的最高温度为 27.8°C。计算中和焓变。

Total volume = 100 cm³, so mass m ≈ 100 g (assuming 1 g/cm³). Temperature change ΔT = 27.8 − 21.0 = 6.8°C. Q = 100 × 4.18 × 6.8 = 2842.4 J ≈ 2.84 kJ (to 3 s.f.). Moles of HCl used = 0.050 dm³ × 1.0 mol/dm³ = 0.050 mol. The neutralisation produces 0.050 mol of water. Therefore, ΔH = −2.84 kJ / 0.050 mol = −56.8 kJ/mol. The negative sign is essential because heat was released.

总体积 100 cm³，故溶液质量 m ≈ 100 g（假设密度 1 g/cm³）。温度变化 ΔT = 27.8 − 21.0 = 6.8°C。Q = 100 × 4.18 × 6.8 = 2842.4 J ≈ 2.84 kJ（保留三位有效数字）。HCl 物质的量 =

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