📚 Edexcel A-Level Combined Science 089: Energetics and Equilibrium | Edexcel A-Level 综合科学 089：能量学与平衡
This revision guide covers the key Edexcel Combined Science 089 ideas in energetics and equilibrium: enthalpy changes, Hess’s law, calorimetry, entropy, Gibbs free energy, Le Chatelier’s principle, and equilibrium constants Kc and Kp.
本复习指南涵盖 Edexcel 综合科学 089 中能量学与平衡的关键概念：焓变、盖斯定律、量热法、熵、吉布斯自由能、勒夏特列原理以及平衡常数 Kc 与 Kp。
1. Enthalpy and Energy Changes | 焓变与能量变化
Enthalpy, H, is the heat content of a system at constant pressure. An exothermic reaction releases heat to the surroundings and has a negative ΔH, while an endothermic reaction absorbs heat and has a positive ΔH.
焓 H 是恒压下系统的热含量。放热反应向环境释放热量，ΔH 为负；吸热反应吸收热量，ΔH 为正。
Always quote standard conditions: 298 K, 100 kPa and solution concentration 1 mol dm⁻³. Standard enthalpy changes are written with the symbol ᵒ, for example ΔHᵒ.
务必引用标准条件：298 K、100 kPa、溶液浓度 1 mol dm⁻³。标准焓变用符号 ᵒ 表示，例如 ΔHᵒ。
ΔH = H(products) − H(reactants)
A negative ΔH means the products are lower in enthalpy than the reactants, so excess energy is transferred to the surroundings as heat.
ΔH 为负意味着产物的焓低于反应物，多余的能量以热量形式传递给环境。
2. Standard Enthalpy Definitions | 标准焓变定义
You must learn precise definitions for Edexcel exams. The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions.
你必须为 Edexcel 考试记忆精确定义。标准生成焓是在标准条件下由元素生成 1 mol 化合物时的焓变。
The standard enthalpy of combustion is the enthalpy change when one mole of a substance burns completely in excess oxygen. The standard enthalpy of neutralisation is the enthalpy change when one mole of water is formed from an acid and a base.
标准燃烧焓是 1 mol 物质在过量氧气中完全燃烧时的焓变。标准中和焓是酸与碱反应生成 1 mol 水时的焓变。
| Definition | 定义 | Symbol | 符号 |
| Standard enthalpy of formation | 标准生成焓 | ΔHfᵒ |
| Standard enthalpy of combustion | 标准燃烧焓 | ΔHcᵒ |
| Standard enthalpy of neutralisation | 标准中和焓 | ΔHneutᵒ |
| Standard enthalpy of reaction | 标准反应焓 | ΔHrᵒ |
For formation and combustion, the definition specifies one mole of the substance named. This is a common source of sign and scale errors in calculations.
对于生成焓和燃烧焓，定义均针对 1 mol 指定物质。这是计算中常见的正负号和比例错误来源。
3. Hess’s Law and Enthalpy Cycles | 盖斯定律与焓循环
Hess’s law states that the total enthalpy change for a reaction is independent of the route taken. This allows you to calculate unknown ΔH values from known enthalpy changes.
盖斯定律指出，反应的总焓变与路径无关。这使你能够利用已知焓变计算未知 ΔH。
ΔH = ΣΔHfᵒ(products) − ΣΔHfᵒ(reactants)
When using formation data, construct a cycle with elements at the bottom. Alternatively, combustion data gives the reverse equation.
使用生成焓数据时，可以将元素放在循环底部。使用燃烧焓数据时，则得到相反方向的表达式。
ΔH = ΣΔHcᵒ(reactants) − ΣΔHcᵒ(products)
Always check that you multiply each enthalpy value by the correct stoichiometric coefficient from the balanced equation.
务必根据配平方程式中的化学计量数，将每个焓变乘以正确系数。
4. Bond Enthalpies | 键焓
Mean bond enthalpy is the average energy needed to break one mole of a covalent bond in the gas phase. Bond breaking is endothermic; bond making is exothermic.
平均键焓是气相中断裂 1 mol 共价键所需的平均能量。断键吸热，成键放热。
ΔH = ΣE(bonds broken) − ΣE(bonds formed)
Bond enthalpy calculations are less accurate than Hess’s law because average bond energies are used. They also require all species to be in the gas phase.
键焓计算不如盖斯定律精确，因为使用的是平均键能，而且要求所有物质都处于气相。
5. Calorimetry and Experimental Measurement | 量热法与实验测量
The heat transferred, q, is calculated from q = mcΔT, where m is mass of water or solution, c is specific heat capacity and ΔT is temperature change.
传递的热量 q 由 q = mcΔT 计算，其中 m 是水或溶液的质量，c 是比热容，ΔT 是温度变化。
q = mcΔT
The enthalpy change per mole is then found using ΔH = −q / n, where n is the moles of the limiting reactant. The negative sign shows heat lost by the reaction.
摩尔焓变由 ΔH = −q / n 求得，其中 n 是限制反应物的物质的量。负号表示反应放出热量。
ΔH = −q / n
In simple solution calorimetry, assume the density of the solution is 1 g cm⁻³ and the specific heat capacity is 4.18 J g⁻¹ K⁻¹. Heat loss to the surroundings is the main source of error.
在简单溶液量热法中，假设溶液密度为 1 g cm⁻³，比热容为 4.18 J g⁻¹ K⁻¹。热量散失到环境是主要误差来源。
6. Entropy and Gibbs Free Energy | 熵与吉布斯自由能
Entropy, S, measures the dispersal of energy. Gases have higher entropy than liquids, and liquids have higher entropy than solids. A reaction is feasible when ΔG is negative.
熵 S 衡量能量的分散程度。气体的熵高于液体，液体的熵高于固体。当 ΔG 为负时，反应可自发进行。
ΔG = ΔH − TΔS
The table below summarises how the signs of ΔH and ΔS affect feasibility. Temperature must be in kelvin for this equation.
下表总结了 ΔH 和 ΔS 的正负号如何影响反应的可自发进行性。该方程中的温度必须使用开尔文。
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