Chemical Equilibrium Crash Course | A-Level CIE Chemical Equilibrium Exam Focus

📚 Chemical Equilibrium Crash Course | A-Level CIE 化学平衡考点精讲

Chemical equilibrium is one of the most fundamental and heavily examined topics in CIE A-Level Chemistry. A deep understanding of dynamic equilibrium, Le Chatelier’s principle and equilibrium constants (Kc and Kp) is essential for scoring top marks. This guide breaks down every key concept with paired English and Chinese explanations.

化学平衡是 CIE A-Level 化学中最基础、最常考的主题之一。深刻理解动态平衡、勒夏特列原理以及平衡常数（Kc 和 Kp）对夺取高分至关重要。本文用中英对照的方式，逐一击破所有核心考点。

1. Dynamic Equilibrium | 动态平衡

A reversible reaction reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the backward reaction, and the concentrations of reactants and products remain constant. The reaction has not stopped; both forward and backward reactions continue at the same rate.

当可逆反应的正反应速率与逆反应速率相等，且反应物和产物的浓度保持恒定时，体系达到动态平衡。反应并未停止，正逆反应仍在以相同的速率进行。

Dynamic equilibrium can only be established in a closed system where no matter can enter or leave. At the macroscopic level, observable properties such as colour, pressure and concentration do not change.

动态平衡只能在封闭系统中建立，物质不能进出。在宏观层面，颜色、压强、浓度等可观察性质不再改变。

2. Characteristics of Equilibrium | 化学平衡的特征

Equilibrium is dynamic, not static. It requires a closed system and can be approached from either direction. At equilibrium, the Gibbs free energy ΔG is zero, and the composition of the mixture remains unchanged.

平衡是动态的，而非静止的。它需要封闭体系，且可以从正反应或逆反应任一方向达到。平衡时吉布斯自由能 ΔG 为零，混合物的组成保持不变。

The position of equilibrium indicates the relative amounts of reactants and products. If the equilibrium lies to the right, products dominate; if to the left, reactants dominate. This position is influenced by concentration, pressure and temperature.

平衡位置反映反应物与产物的相对数量。平衡偏右时产物占优，偏左时反应物占优。平衡位置受浓度、压强和温度影响。

3. Le Chatelier’s Principle | 勒夏特列原理

Le Chatelier’s principle states that if a system at dynamic equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will shift to oppose the change and restore a new equilibrium.

勒夏特列原理指出，如果处于动态平衡的体系受到浓度、压强或温度的改变，平衡位置将向削弱该改变的方向移动，并建立新的平衡。

This principle is qualitative and helps predict the direction of shift. It does not explain the rate at which equilibrium is re-established, nor does it apply to the addition of a catalyst.

该原理是定性的，用于预测平衡移动的方向。它并不解释重新达到平衡的速率，也不适用于催化剂的加入。

4. Effect of Concentration Changes | 浓度影响

Increasing the concentration of a reactant shifts the equilibrium to the right, favouring the forward reaction to consume the added reactant. Increasing the concentration of a product shifts the equilibrium to the left.

增大反应物的浓度将使平衡向右移动，有利于正反应消耗加入的反应物。增大产物的浓度则使平衡向左移动。

Conversely, decreasing the concentration of a substance causes the equilibrium to shift in the direction that produces that substance. In industry, a product is often removed to drive the equilibrium towards product formation.

相反，减小某种物质的浓度会使平衡向生成该物质的方向移动。工业上常通过不断移除产物，推动平衡向生成产物方向移动。

5. Effect of Pressure Changes | 压强影响

Pressure changes only affect equilibria involving gases. According to Le Chatelier’s principle, increasing the total pressure shifts the equilibrium towards the side with fewer moles of gas molecules to reduce the pressure.

压强改变只影响有气体参与的平衡体系。根据勒夏特列原理，增大总压会使平衡向气体分子总摩尔数较少的一侧移动，以降低压强。

If the number of gas moles is the same on both sides of the equation, a change in pressure has no effect on the position of equilibrium. For example, H₂(g) + I₂(g) ⇌ 2HI(g) has equal moles of gas on each side.

如果反应方程式两边气体分子摩尔数相等，则压强的改变不影响平衡位置。例如 H₂(g) + I₂(g) ⇌ 2HI(g) 两边气体摩尔数相同。

6. Effect of Temperature Changes | 温度影响

Temperature is the only factor that changes the value of the equilibrium constant (Kc or Kp). For an exothermic reaction (ΔH negative), increasing temperature shifts equilibrium to the left; for an endothermic reaction (ΔH positive), it shifts to the right.

温度是唯一能改变平衡常数（Kc 或 Kp）数值的因素。对于放热反应（ΔH 为负），升温使平衡左移；对于吸热反应（ΔH 为正），升温使平衡右移。

Cooling the system will favour the exothermic direction to release heat. In the Haber process, N₂ + 3H₂ ⇌ 2NH₃ (exothermic), a lower temperature increases the equilibrium yield of ammonia but slows the rate, so a compromise temperature is used.

降温则有利于放热方向以释放热量。在哈伯法 N₂ + 3H₂ ⇌ 2NH₃（放热）中，低温可提高氨的平衡产率，但会降低反应速率，因此采用折中温度。

7. Effect of a Catalyst | 催化剂的影响

A catalyst provides an alternative pathway with lower activation energy, increasing the rate of both forward and backward reactions equally. It allows equilibrium to be reached faster but has no effect on the position of equilibrium or the equilibrium constant.

催化剂通过提供活化能较低的替代路径，同等程度地加快正、逆反应速率。它使平衡更快达到，但对平衡位置和平衡常数均无影响。

This is a common exam trap: students often claim that a catalyst increases the yield of a product. In reality, it only speeds up the attainment of equilibrium, which is crucial for industrial cost efficiency.

这是考试中常见陷阱：学生常误以为催化剂能提高产物产率。事实上它只加速达到平衡，这对工业上的成本效益至关重要。

8. Equilibrium Constant Kc | 平衡常数 Kc

For a general reaction aA + bB ⇌ cC + dD, the equilibrium constant in terms of concentration is:

对于一般反应 aA + bB ⇌ cC + dD，以浓度表示的平衡常数为：

K_c = [C]^c[D]^d / [A]^a[B]^b

Square brackets denote equilibrium concentrations in mol dm⁻³. The value of Kc is constant only if temperature is constant. Solids and pure liquids are omitted from the Kc expression because their concentrations are effectively constant.

方括号表示平衡浓度，单位为 mol dm⁻³。Kc 的数值仅在温度不变时为常数。固体和纯液体的浓度视为常数，不写入 Kc 表达式中。

A large Kc (much greater than 1) means the equilibrium position lies far to the right and the reaction nearly goes to completion. A very small Kc (much less than 1) indicates the reaction barely proceeds forward.

Kc 很大（远大于 1）表示平衡位置严重偏右，反应几乎进行完全。Kc 非常小（远小于 1）则表示正反应几乎不发生。

9. Kc Calculations and Units | Kc 的计算与单位

Calculating Kc requires equilibrium concentrations, not initial concentrations. A typical problem provides initial amounts, volume and the equilibrium amount of one species; use an ICE table (Initial, Change, Equilibrium) to find all equilibrium concentrations.

计算 Kc 需要平衡浓度，而非起始浓度。典型题目会给出初始量、体积和某一组分的平衡量；使用 ICE 表格（初始、变化、平衡）求出所有平衡浓度。

The units of Kc depend on the stoichiometry of the reaction. For example, in the reaction N₂ + 3H₂ ⇌ 2NH₃, Kc has units of mol⁻² dm⁶. Always calculate the units by substituting the units of concentration into the Kc expression.

Kc 的单位取决于反应的化学计量关系。例如在 N₂ + 3H₂ ⇌ 2NH₃ 中，Kc 的单位为 mol⁻² dm⁶。务必通过将浓度单位代入 Kc 表达式来推导单位。

Examiners often award marks for the correct units. Remember that if the number of moles on both sides of the equation is equal, Kc has no units (dimensionless).

阅卷者常给正确的单位打分。记住，若方程式两边摩尔数相等，Kc 无单位（量纲为 1）。

10. Equilibrium Constant Kp | 平衡常数 Kp

For gas-phase reactions, the equilibrium constant can be expressed in terms of partial pressures. The partial pressure of a gas is the pressure it would exert if it occupied the container alone. The total pressure is the sum of all partial pressures.

对于气相反应，平衡常数可用分压表示。某气体的分压是其单独占据容器时所施加的压强。总压等于所有分压之和。

The expression for Kp mirrors that of Kc, using partial pressures instead of concentrations:

Kp 的表达式与 Kc 类似，用分压代替浓度：

K_p = (P_C^c P_D^d) / (P_A^a P_B^b)

Partial pressure = mole fraction × total pressure. Mole fraction of A = moles of A / total moles of gas. Kp, like Kc, is only affected by temperature.

分压 = 摩尔分数 × 总压。A 的摩尔分数 = A 的物质的量 / 气体总物质的量。Kp 和 Kc 一样，只受温度影响。

11. Kp Calculations and Units | Kp 的计算与单位

Kp problems require finding the mole fraction of each gas at equilibrium and then multiplying by the total pressure to obtain partial pressures. The units of Kp are derived from the unit of pressure, typically atm or Pa, raised to the appropriate power.

Kp 的计算需要先求平衡时各气体的摩尔分数，再乘以总压得到分压。Kp 的单位由压强单位（通常是 atm 或 Pa）的相应幂次决定。

For instance, in the reaction 2SO₂ + O₂ ⇌ 2SO₃, the total number of gas moles changes, so Kp has units of atm⁻¹ if pressure is in atmospheres. As with Kc, if Δn = 0, Kp is dimensionless.

例如，在反应 2SO₂ + O₂ ⇌ 2SO₃ 中，气体总摩尔数发生变化，如果压强用大气压，Kp 的单位为 atm⁻¹。与 Kc 相同，若 Δn = 0，Kp 无量纲。

Always relate Kp to the partial pressures at equilibrium. A common mistake is to use initial pressures; only equilibrium values should appear in the expression.

始终坚持将 Kp 与平衡时的分压联系起来。常见错误是使用初始分压；表达式中只能代入平衡时的数值。

12. Common Pitfalls and Exam Tips | 常见失分点与应试技巧

Students often confuse rate and equilibrium: a fast reaction does not mean a high yield at equilibrium. Always distinguish between thermodynamic (equilibrium position) and kinetic (rate) considerations.

学生常混淆速率与平衡：快速反应并不意味着平衡产率高。务必区分热力学（平衡位置）与动力学（反应速率）的概念。

When using Le Chatelier’s principle, specify the direction of shift (left or right) and explain why. Do not state that a catalyst increases yield or changes Kc/Kp. Adding an inert gas at constant volume does not affect equilibrium because partial pressures of reacting gases stay the same.

运用勒夏特列原理时，要明确指出移动方向（左或右）并解释理由。不要宣称催化剂能提高产率或改变 Kc/Kp。恒容条件下加入惰性气体不影响平衡，因为反应气体的分压未变。

For Kc and Kp calculations, always set up an ICE table clearly and check the stoichiometry. Give units unless Δn = 0. Temperature changes are the only perturbations that alter the value of the equilibrium constant.

对于 Kc 和 Kp 计算，一定要清晰列出 ICE 表格并检查化学计量关系。除非 Δn = 0，否则需要给出单位。温度改变是唯一能改变平衡常数数值的外界扰动。

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