📚 AQA International AS Chemistry Unit 2: Energetics, Kinetics and Equilibria | AQA 国际 AS 化学第 2 单元：能量学、动力学与平衡
Unit 2 of AQA International AS Chemistry brings together three quantitative physical topics: energetics, kinetics and chemical equilibria. These ideas explain why reactions release or absorb heat, how quickly reactions proceed, and how far reversible reactions go under given conditions.
AQA 国际 AS 化学第 2 单元整合了三个定量物理化学主题：能量学、动力学和化学平衡。这些概念解释了反应为何放热或吸热、反应进行的快慢以及可逆反应在给定条件下进行的程度。
1. Enthalpy Changes and Reaction Profiles | 焓变与反应能量图
Enthalpy change, ΔH, is the heat energy transferred in a reaction at constant pressure. It is usually quoted in kJ mol⁻¹ and refers to the molar quantities shown in the balanced equation.
焓变 ΔH 是在恒压条件下反应中传递的热能。它通常以 kJ mol⁻¹ 表示，并对应配平方程中所示的摩尔数量。
In an exothermic reaction, ΔH is negative because heat energy is released to the surroundings. In an endothermic reaction, ΔH is positive because heat energy is absorbed from the surroundings.
在放热反应中，ΔH 为负值，因为热能释放到周围环境；在吸热反应中，ΔH 为正值，因为热能从周围环境中吸收。
A reaction profile shows the energy of reactants, products and the activation energy barrier. The activation energy, Ea, is the minimum energy colliding particles need for a successful reaction.
反应能量图显示反应物、产物的能量以及活化能垒。活化能 Ea 是碰撞粒子发生有效反应所需的最低能量。
Standard enthalpy changes are measured under standard conditions: 100 kPa pressure, a stated temperature, usually 298 K, and solutions at concentration 1 mol dm⁻³. Common standard enthalpy changes include ΔHf for formation, ΔHc for combustion and ΔHneut for neutralisation.
标准焓变是在标准条件下测量的：压强 100 kPa、规定温度通常为 298 K、溶液浓度为 1 mol dm⁻³。常见的标准焓变包括生成焓 ΔHf、燃烧焓 ΔHc 和中和焓 ΔHneut。
2. Calorimetry: Measuring Enthalpy Changes | 量热法：测量焓变
g q = mcΔT, where m is mass, c is specific heat capacity and ΔT is the temperature change.
简单的量热实验测量已知质量的水或溶液的温度变化。传递的热量用 q = mcΔT 计算，其中 m 为质量，c 为比热容，ΔT 为温度变化。
q = mcΔT
For dilute aqueous solutions, we often assume the density is 1.00 g cm⁻³ and the specific heat capacity is 4.18 J g⁻¹ K⁻¹. These assumptions allow the mass of the solution to be read directly from its volume.
对于稀水溶液，我们通常假设其密度为 1.00 g cm⁻³，比热容为 4.18 J g⁻¹ K⁻¹。这些假设使我们能够直接从溶液体积得出其质量。
To find ΔH per mole, divide the heat energy by the number of moles of the limiting reactant. A common error is using the mass of the solid rather than the total mass of the solution.
要计算每摩尔焓变，需要用热量除以限制反应物的摩尔数。常见的错误是使用固体的质量而不是溶液的总质量。
Calorimetry experiments always lose some heat to the surroundings, so measured values are usually smaller than theoretical values. Using a lid, insulation and a polystyrene cup reduces heat loss.
量热实验总是会向周围环境散失部分热量，因此测量值通常小于理论值。使用盖子、保温材料和聚苯乙烯杯可以减少热量散失。
3. Hess's Law and Energy Cycles | 赫斯定律与能量循环
Hess's law states that the total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. This allows unknown enthalpy changes to be calculated indirectly.
赫斯定律指出，只要始态和终态相同，反应的总焓变与所采取的途径无关。这使得未知的焓变可以通过间接方法计算。
An energy cycle can be drawn using standard enthalpies of formation. The enthalpy change of a reaction equals the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants.
可以利用标准生成焓绘制能量循环。反应的焓变等于产物的生成焓之和减去反应物的生成焓之和。
ΔH = ΣΔHf(products) − ΣΔHf(reactants)
Alternatively, standard enthalpies of combustion can be used in a cycle. In that case, the enthalpy change of reaction equals the sum of the enthalpies of combustion of the reactants minus the sum of the enthalpies of combustion of the products.
另外，也可以在能量循环中使用标准燃烧焓。此时，反应焓变等于反应物的燃烧焓之和减去产物的燃烧焓之和。
When drawing cycles, make sure the arrows point in the direction of the defined change. If an arrow is reversed in the calculation, change the sign of the enthalpy value.
绘制能量循环时，要确保箭头指向所定义变化的方向。如果计算时箭头方向相反，则要改变该焓值的正负号。
4. Mean Bond Enthalpies | 平均键焓
Mean bond enthalpy is the energy required to break one mole of a given covalent bond in the gaseous state, averaged over a range of different compounds. It is always positive because bond breaking is endothermic.
平均键焓是在气态下断裂 1 摩尔某共价键所需的能量，是不同化合物中的平均值。它始终为正值，因为断键过程吸热。
In a chemical reaction, bonds in the reactants are broken and new bonds in the products are formed. Breaking bonds absorbs energy, while making bonds releases energy.
在化学反应中，反应物中的键断裂，产物中的新键生成。断键吸收能量，成键释放能量。
The enthalpy change of a reaction can be estimated from mean bond enthalpies using the relationship: ΔH equals the total energy absorbed in breaking bonds minus the total energy released in forming bonds.
反应的焓变可以用平均键焓估算：ΔH 等于断键吸收的总能量减去成键释放的总能量。
ΔH ≈ ΣE(bonds broken) − ΣE(bonds formed)
Mean bond enthalpy values only give approximate enthalpy changes. This is because they are averaged over many molecules, and they apply strictly to gaseous species rather than liquids or solids.
平均键焓只能给出近似的焓变值。这是因为它们是许多分子的平均值，而且严格适用于气态物质，而非液态或固态。
5. Collision Theory and the Maxwell-Boltzmann Distribution | 碰撞理论与麦克斯韦-玻尔兹曼分布
Collision theory states that for a reaction to occur, particles must collide with the correct orientation and with energy greater than or equal to the activation energy. Only successful collisions lead to products.
碰撞理论指出，反应发生需要粒子以正确取向碰撞，并且碰撞能量大于或等于活化能。只有有效碰撞才会生成产物。
The Maxwell-Boltzmann distribution shows the spread of kinetic energies of gas particles at a constant temperature. The curve starts at the origin, rises to a maximum and then tails off towards high energies.
麦克斯韦-玻尔兹曼分布表示在恒定温度下气体粒子动能的分布。曲线从原点开始，上升到最高点，然后向高能量方向拖尾。
The area under the curve beyond the activation energy represents the number of particles with enough energy to react. This area is small at moderate temperatures because most particles have energies below Ea.
曲线中超过活化能的区域面积代表具有足够能量参与反应的粒子数。在中等温度下，这个区域较小，因为大多数粒子的能量低于 Ea。
Increasing the temperature changes the shape of the Maxwell-Boltzmann distribution. The peak shifts to the right and becomes lower, and a much larger proportion of particles now have energy greater than Ea.
升高温度会改变麦克斯韦-玻尔兹曼分布的形状。峰值向右移动并降低，能量大于 Ea 的粒子比例大幅增加。
6. Factors Affecting the Rate of Reaction | 影响反应速率的因素
Increasing the concentration of a solution, or the pressure of a gas, increases the rate of reaction. This is because more particles are present in the same volume, so successful collisions occur more frequently.
增大溶液的浓度或气体的压强会加快反应速率。这是因为相同体积内有更多粒子，因此有效碰撞更频繁。
Increasing the surface area of a solid reactant increases the rate. A powdered solid exposes more particles to the other reactant, increasing the frequency of collisions.
增大固体反应物的表面积会加快反应速率。粉末状固体使更多粒子暴露于另一反应物，从而增加碰撞频率。
Raising the temperature increases reaction rate for two reasons. Particles collide more often, and more importantly, a much higher proportion of collisions now have energy equal to or greater than Ea.
升高温度从两个方面加快反应速率。粒子碰撞更频繁，更重要的是，能量等于或大于 Ea 的碰撞比例大大提高。
These effects can be explained using collision theory: a factor increases rate only if it increases the frequency of successful collisions. Temperature is usually the most powerful factor because it increases both collision frequency and collision energy.
这些影响可以用碰撞理论解释：只有增加有效碰撞频率的因素才能加快反应速率。温度通常是最强的因素，因为它同时增加了碰撞频率和碰撞能量。
7. Catalysts and Activation Energy | 催化剂与活化能
A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process. It provides an alternative reaction pathway with a lower activation energy.
催化剂是能加快化学反应速率而本身在反应过程中不被消耗的物质。它提供了一条活化能较低的另一条反应途径。
On a Maxwell-Boltzmann distribution, a catalyst does not change the curve itself. Instead, it moves the activation energy line to the left, so a much larger area of the curve lies beyond Ea.
在麦克斯韦-玻尔兹曼分布图中，催化剂不会改变曲线本身，而是将活化能线向左移动，因此曲线中超过 Ea 的区域面积大大增加。
Homogeneous catalysts are in the same phase as the reactants, while heterogeneous catalysts are in a different phase. Many industrial catalysts are heterogeneous solids, such as iron in the Haber process.
均相催化剂与反应物处于同一相，而非均相催化剂则处于不同的相。许多工业催化剂是非均相固体，例如哈伯法中的铁。
Catalysts are economically and environmentally important. They allow lower temperatures and pressures to be used, saving energy and reducing costs without changing the position of equilibrium or the value of Kc.
催化剂在经济和环境方面都很重要。它们允许使用较低的温和压强，节约能源、降低成本，同时不会改变平衡位置或 Kc 的值。
8. Reversible Reactions and Dynamic Equilibrium | 可逆反应与动态平衡
A reversible reaction can occur in both the forward and backward directions. In a closed system, the forward and backward reactions eventually take place at the same rate.
可逆反应可以同时向正反应方向和逆反应方向进行。在封闭体系中，正反应和逆反应最终会以相同的速率进行。
Dynamic equilibrium is reached when the rates of the forward and backward reactions are equal and the concentrations of reactants and products remain constant, although both reactions continue at the molecular level.
当正反应和逆反应速率相等、反应物和产物浓度保持恒定时，就达到了动态平衡。尽管两种反应仍在分子水平上持续进行，但宏观浓度不再改变。
Equilibrium can only be established in a closed system where no matter is allowed to escape. If products or reactants are removed, the equilibrium position shifts to oppose the change.
只有在没有物质逸出的封闭体系中才能建立平衡。如果移除产物或反应物，平衡位置会移动以抵消这种变化。
A homogeneous equilibrium involves reactants and products in the same phase, usually all gases or all aqueous ions. This is the main focus for Kc calculations in Unit 2.
均相平衡涉及处于同一相的反应物和产物，通常全部为气体或全部为水溶液离子。这是第 2 单元 Kc 计算的主要对象。
9. Le Chatelier's Principle | 勒夏特列原理
Le Chatelier's principle states that if a system at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium shifts to oppose that change.
勒夏特列原理指出，如果改变平衡体系的浓度、压强或温度，平衡位置会移动以减弱这种改变。
Increasing the concentration of a reactant shifts the equilibrium position towards the products. Removing a product has the same effect, while removing a reactant shifts the position back towards the reactants.
增大反应物的浓度会使平衡位置向产物方向移动。移除产物也有同样的效果，而移除反应物则会使平衡位置向反应物方向移动。
For gaseous equilibria, increasing pressure shifts the position towards the side with fewer gas molecules. Decreasing pressure favours the side with more gas molecules.
对于气体平衡体系，增大压强会使平衡位置向气体分子较少的一侧移动；减小压强则有利于气体分子较多的一侧。
Increasing temperature favours the endothermic direction and decreasing temperature favours the exothermic direction. For example, in the Haber process N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic, so lower temperature increases the yield of ammonia.
升高温度有利于吸热方向，降低温度有利于放热方向。例如，在哈伯法中 N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，正反应为放热反应，因此较低温度会提高氨的产率。
Changes in concentration, pressure or temperature affect the equilibrium position but not the value of the equilibrium constant. Only a change in temperature changes the value of Kc.
浓度、压强或温度的变化会影响平衡位置，但不会影响平衡常数的值。只有温度变化才会改变 Kc 的值。
10. The Equilibrium Constant Kc | 平衡常数 Kc
For the general homogeneous equilibrium aA + bB ⇌ cC + dD, the equilibrium constant expression is Kc equals the product of the equilibrium concentrations of the products divided by the product of the equilibrium concentrations of the reactants, each raised to the power of its stoichiometric coefficient.
对于一般均相平衡 aA + bB ⇌ cC + dD，平衡常数表达式为：Kc 等于各产物平衡浓度的幂乘积除以各反应物平衡浓度的幂乘积，每个浓度以化学计量数为指数。
Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ
The units of Kc depend on the powers of concentration in the expression. They can be worked out by substituting mol dm⁻³ into the expression and cancelling common terms.
Kc 的单位取决于表达式中浓度的幂。将 mol dm⁻³ 代入表达式并约去相同项即可求出单位。
To calculate Kc, first determine the equilibrium concentrations of all species. If initial amounts and one equilibrium amount are given, use an ICE table to find the changes and the remaining equilibrium concentrations.
要计算 Kc，首先要确定所有物种的平衡浓度。如果已知初始量和某一个平衡量，可以使用 ICE 表求出变化量以及其余的平衡浓度。
A change in concentration or pressure does not alter Kc. However, increasing the temperature increases Kc for an endothermic reaction and decreases Kc for an exothermic reaction.
浓度或压强变化不会改变 Kc。然而，升高温度会使吸热反应的 Kc 增大，使放热反应的 Kc 减小。
Catalysts have no effect on the value of Kc. They speed up both the forward and backward reactions equally, allowing equilibrium to be reached more quickly.
催化剂对 Kc 的值没有影响。它同等程度地加快正反应和逆反应，使体系更快达到平衡。
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