📚 IB Chemistry: Core Concepts of Chemical Equilibrium | IB化学：化学平衡核心概念
Chemical equilibrium is one of the most fundamental topics in IB Chemistry. It explains why many reactions do not simply go to completion, but instead reach a state where reactants and products coexist at constant concentrations. Mastering this concept is essential for understanding acid–base chemistry, solubility, and many industrial processes.
化学平衡是IB化学中最基础的主题之一。它解释了为什么许多反应不会简单地进行到底，而是达到一种反应物和产物浓度保持恒定的状态。掌握这一概念对于理解酸碱化学、溶解度以及许多工业过程至关重要。
1. Dynamic Equilibrium | 动态平衡
Equilibrium occurs in a closed system when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant, but both reactions are still happening continuously. This is called dynamic equilibrium.
平衡发生在封闭体系中，当正反应速率等于逆反应速率时。此时反应物和产物的浓度保持不变，但两个反应仍在持续进行。这被称为动态平衡。
Only occurs in a closed system – no matter can enter or leave.
仅在封闭体系中发生——物质不能进出。
Macroscopic properties (colour, pressure, concentration) are constant.
宏观性质（颜色、压力、浓度）保持不变。
Microscopic processes continue – molecules keep reacting.
微观过程仍在继续——分子不断反应。
Forward rate = Reverse rate ≠ 0
正反应速率 = 逆反应速率 ≠ 0
2. Equilibrium Constant Kc | 平衡常数Kc
For a general reaction aA + bB ⇌ cC + dD, the equilibrium constant is defined as the ratio of the product concentrations raised to their stoichiometric coefficients to the reactant concentrations raised to their coefficients.
对于一般反应 aA + bB ⇌ cC + dD，平衡常数定义为产物浓度以其化学计量系数为幂的乘积与反应物浓度以其化学计量系数为幂的乘积之比。
Kc = [C]c[D]d / ([A]a[B]b)
Concentrations are in mol dm⁻³.
浓度单位是 mol dm⁻³。
Kc is dimensionless in terms of activities, but in IB we usually quote it with units.
从活度角度看 Kc 无单位，但在IB中我们通常给它加上单位。
Only temperature can change the value of Kc.
只有温度能改变 Kc 的数值。
3. Kp and Gaseous Equilibria | Kp与气相平衡
For reactions involving gases, partial pressures can be used instead of concentrations. The equilibrium constant is then called Kp. For aA + bB ⇌ cC + dD, where all species are gases:
对于涉及气体的反应，可以用分压代替浓度。此时的平衡常数称为Kp。对于aA + bB ⇌ cC + dD，其中所有物质均为气体：
Kp = (PC)c(PD)d / [(PA)a(PB)b]
Partial pressure = mole fraction × total pressure.
分压 = 摩尔分数 × 总压。
Kp is only used for gases; pure solids and pure liquids do not appear in the expression.
Kp 仅用于气体；纯固体和纯液体不写入表达式中。
Units of Kp depend on the sum of stoichiometric coefficients.
Kp 的单位取决于化学计量系数之和。
4. Reaction Quotient Q | 反应商Q
The reaction quotient Q has the same mathematical form as Kc or Kp, but it uses the concentrations or pressures at any moment during the reaction, not necessarily at equilibrium.
反应商Q与Kc或Kp具有相同的数学形式，但它使用的是反应过程中任意时刻的浓度或分压，不一定是平衡时的值。
If Q < K, the reaction will proceed in the forward direction to reach equilibrium.
如果 Q < K，反应将向正方向进行以达到平衡。
If Q > K, the reaction will proceed in the reverse direction.
如果 Q > K，反应将向逆方向进行。
If Q = K, the system is already at equilibrium.
如果 Q = K，体系已经处于平衡状态。
Q vs K: the direction of change is determined by their comparison.
Q与K的比较决定了反应移动的方向。
5. Le Chatelier's Principle | 勒夏特列原理
Le Chatelier's Principle states that if a system at equilibrium is disturbed, the equilibrium will shift in the direction that counteracts the disturbance. This is a powerful qualitative tool for predicting the effect of changes in concentration, pressure, temperature, or volume.
勒夏特列原理指出，如果平衡体系受到干扰，平衡将向抵消该干扰的方向移动。这是一个强大的定性工具，用于预测浓度、压力、温度或体积变化的影响。
System responds to minimise the applied stress.
体系通过对抗外界应力的方式作出响应。
6. Effect of Concentration and Pressure | 浓度和压力的影响
Adding a reactant or removing a product shifts the equilibrium to the right (towards products). Adding a product or removing a reactant shifts it to the left. Changing pressure only affects equilibria where the number of gas molecules differs between reactants and products.
增加反应物或移去产物会使平衡向右（产物方向）移动。增加产物或移去反应物会使平衡向左移动。改变压力只影响反应物与产物气体分子总数不同的平衡。
Increasing pressure favours the side with fewer gas moles.
增大压力有利于气体物质的量较少的一侧。
Decreasing pressure favours the side with more gas moles.
减小压力有利于气体物质的量较多的一侧。
Adding an inert gas at constant volume does not change the equilibrium position.
在恒容条件下加入惰性气体不会改变平衡位置。
7. Effect of Temperature | 温度的影响
Temperature changes alter the value of the equilibrium constant itself. For an exothermic forward reaction (ΔH < 0), increasing temperature decreases Kc, shifting equilibrium towards reactants. For an endothermic forward reaction (ΔH > 0), increasing temperature increases Kc, shifting equilibrium towards products.
温度变化会改变平衡常数本身的值。对于正向放热反应（ΔH < 0），升高温度会使Kc减小，平衡向反应物方向移动。对于正向吸热反应（ΔH > 0），升高温度会使Kc增大，平衡向产物方向移动。
Temperature is the only factor that changes Kc/Kp.
温度是唯一改变Kc/Kp的因素。
8. Catalysts and Equilibrium | 催化剂与平衡
Catalysts lower the activation energy for both the forward and reverse reactions equally. They therefore do NOT change the position of equilibrium or the value of Kc. A catalyst only increases the rate at which equilibrium is reached.
催化剂同等程度地降低正反应和逆反应的活化能。因此，它不会改变平衡位置或Kc的值。催化剂只是加快了达到平衡的速率。
Both forward and reverse rates increase equally.
正反应和逆反应的速率同等增加。
Composition at equilibrium remains unchanged.
平衡时的组成保持不变。
Catalysts are important in industrial processes like the Haber process.
催化剂在哈伯法等工业过程中非常重要。
9. Kc and Temperature: van't Hoff Equation | Kc与温度：范特霍夫方程
The relationship between the equilibrium constant and temperature is described by the van't Hoff equation. In its integrated form, it shows that plotting ln K against 1/T gives a straight line.
平衡常数与温度之间的关系由范特霍夫方程描述。其积分形式表明，以 ln K 对 1/T 作图得到一条直线。
ln(K₂/K₁) = -ΔH°/R × (1/T₂ - 1/T₁)
For an exothermic reaction, K decreases as T increases.
对于放热反应，K随T升高而减小。
For an endothermic reaction, K increases as T increases.
对于吸热反应，K随T升高而增大。
The slope of ln K vs 1/T equals -ΔH°/R.
ln K 对 1/T 作图的斜率等于 -ΔH°/R。
10. Equilibrium in Solubility: Ksp | 溶解度平衡：Ksp
For a sparingly soluble salt such as AgCl, an equilibrium exists between the solid and its ions in solution. The solubility product Ksp is the equilibrium constant for this dissolution process.
对于像AgCl这样的微溶盐，固体与其溶液中的离子之间存在平衡。溶度积Ksp就是这一溶解过程的平衡常数。
AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq)
Ksp = [Ag⁺][Cl⁻]
Pure solids do not appear in the Ksp expression.
纯固体不写入Ksp表达式中。
Comparing Q with Ksp predicts precipitation.
比较Q与Ksp可以预测沉淀是否生成。
Common ion effect reduces solubility by shifting equilibrium to the left.
同离子效应通过使平衡向左移动来降低溶解度。
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