A-Level Edexcel Chemistry: Chemical Equilibrium Key Points | A-Level Edexcel 化学：化学平衡考点精讲

📚 A-Level Edexcel Chemistry: Chemical Equilibrium Key Points | A-Level Edexcel 化学：化学平衡考点精讲

Chemical equilibrium is a fundamental concept in A-Level Edexcel Chemistry that describes the state in which the forward and reverse reactions proceed at the same rate, resulting in constant concentrations of reactants and products. This topic underpins industrial processes, acid-base behaviour, and many biological systems. A thorough understanding of dynamic equilibrium, equilibrium constants (Kc and Kp), and Le Chatelier’s principle is essential for success in both AS and A2 papers.

化学平衡是A-Level Edexcel化学的核心概念，它描述的是正反应和逆反应速率相等、反应物和生成物浓度保持恒定的状态。这一主题是工业流程、酸碱行为以及许多生物系统的基础。彻底理解动态平衡、平衡常数（Kc 和 Kp）以及勒夏特列原理，对在AS和A2试卷中取得好成绩至关重要。

1. Reversible Reactions and Dynamic Equilibrium | 可逆反应与动态平衡

A reversible reaction is one that can proceed in both the forward and backward directions under the same conditions. At the start, only reactants are present, so the forward rate is high. As products form, the reverse reaction begins. Eventually, the rates of the forward and reverse reactions become equal; this is the point of dynamic equilibrium. At equilibrium, the reaction has not stopped – both forward and backward reactions continue, but the macroscopic amounts of all species remain unchanged.

可逆反应是在相同条件下既能正向进行又能逆向进行的反应。开始时只有反应物，正向速率很高。随着生成物的出现，逆反应开始发生。最终，正反应和逆反应的速率相等；这就是动态平衡点。达到平衡时，反应并未停止——正逆反应仍在进行，但所有物质的宏观量保持恒定。

Dynamic equilibrium can only be established in a closed system where no matter enters or leaves.
动态平衡只能在封闭系统中建立，即没有物质进入或离开系统。
At equilibrium, the concentrations of reactants and products are constant, but not necessarily equal.
平衡时，反应物和生成物的浓度恒定，但不一定相等。
The equilibrium position describes the relative amounts of reactants and products; it may lie to the left (more reactants) or to the right (more products).
平衡位置描述反应物和生成物的相对含量；可能偏向左边（反应物较多）或右边（生成物较多）。

2. The Equilibrium Constant Kc | 平衡常数 Kc

The equilibrium constant in terms of concentration, Kc, provides a quantitative measure of the equilibrium position for reactions in solution. For the general reaction aA + bB ⇌ cC + dD, the expression is:

基于浓度的平衡常数 Kc 为溶液中的反应提供了平衡位置的定量度量。对于一般反应 aA + bB ⇌ cC + dD，其表达式为：

Kc = [C]ᶜ [D]ᵈ / [A]ᵃ [B]ᵇ

Square brackets denote equilibrium concentrations in mol dm⁻³. The powers correspond to the stoichiometric coefficients in the balanced equation. Kc is temperature-dependent; a change in temperature alters the value of Kc, while changes in concentration or pressure do not. A large Kc (much greater than 1) means the equilibrium favours products, whereas a very small Kc indicates a reactant-favoured equilibrium.

方括号表示以 mol dm⁻³为单位的平衡浓度。指数对应配平方程式中各物质的化学计量数。Kc 与温度有关；温度变化会改变 Kc 的数值，而浓度或压强的变化则不会。较大的 Kc（远大于1）表示平衡倾向于生成物，而非常小的 Kc 则表示平衡偏向反应物。

Note that pure solids and pure liquids are omitted from the Kc expression because their concentrations are essentially constant.

注意，纯固体和纯液体不出现在 Kc 表达式中，因为它们的浓度本质上恒定。

3. The Equilibrium Constant Kp and Partial Pressure | 平衡常数 Kp 与分压

For gaseous equilibria, it is often more convenient to use the equilibrium constant in terms of partial pressure, Kp. The partial pressure of a gas is the pressure it would exert if it alone occupied the total volume. The mole fraction of gas A, xA, is given by xA = moles of A / total moles of all gases in the mixture. Then the partial pressure of A is pA = xA × P, where P is the total pressure.

对于气体平衡，通常更方便的是使用基于分压的平衡常数 Kp。某气体的分压是指它单独占据整个体积时所施加的压力。气体A的摩尔分数 xA 由 xA = A的物质的量 / 混合物中所有气体的总物质的量给出。那么A的分压为 pA = xA × P，其中P为总压。

For the reaction aA(g) + bB(g) ⇌ cC(g) + dD(g), the expression is:

对于反应 aA(g) + bB(g) ⇌ cC(g) + dD(g)，其表达式为：

Kp = (pC)ᶜ (pD)ᵈ / (pA)ᵃ (pB)ᵇ

The units of Kp depend on the difference in the sum of the stoichiometric coefficients of gaseous products and reactants, Δn. For example, if Δn = 0, Kp has no units; if Δn = 1, the unit is pressure (typically atm or Pa). Always show units in calculations, as the Edexcel mark scheme often requires them.

Kp 的单位取决于气体生成物与反应物的化学计量数之和的差值 Δn。例如，若 Δn = 0，Kp 无量纲；若 Δn = 1，单位为压强（通常为 atm 或 Pa）。计算时务必标明单位，因为Edexcel评分标准通常要求呈现单位。

4. Factors Affecting Equilibrium: Le Chatelier’s Principle | 影响平衡的因素：勒夏特列原理

Le Chatelier’s principle states that if a system at dynamic equilibrium is subjected to a change in conditions, the position of equilibrium shifts to oppose that change. This principle helps predict the effect of alterations in concentration, pressure (for gases), and temperature.

勒夏特列原理指出，如果处于动态平衡的体系受到外界条件改变，平衡位置会向削弱这种改变的方向移动。这一原理有助于预测浓度、压强（对气体而言）和温度变化的影响。

Concentration change: Increasing the concentration of a reactant shifts equilibrium to the right (towards products); removing a product also shifts equilibrium to the right to replace what was removed.
浓度变化：增大反应物浓度，平衡右移（朝向生成物）；移除生成物同样使平衡右移以补充被移除的物质。
Pressure change: An increase in pressure shifts equilibrium to the side with fewer moles of gas. If the number of gas moles is equal on both sides, pressure has no effect on equilibrium position (though the reaction rate may increase).
压强变化：增大压强，平衡向气体分子总数较少的一侧移动。若两侧气体分子总数相等，压强对平衡位置没有影响（但反应速率可能增大）。
Temperature change: An increase in temperature favours the endothermic direction. The equilibrium shifts to absorb some of the added heat. For an exothermic forward reaction, raising the temperature decreases the equilibrium yield of products.
温度变化：升高温度有利于吸热方向。平衡会移动以吸收部分新增的热量。若正反应放热，升高温度会降低产物的平衡产率。

Importantly, only a change in temperature alters the value of the equilibrium constant Kc or Kp. Concentration and pressure changes shift the equilibrium position but leave the constant unchanged at a given temperature.

重要的是，只有温度变化才会改变平衡常数 Kc 或 Kp 的数值。浓度和压强的变化会移动平衡位置，但在给定温度下平衡常数保持不变。

5. Effect of a Catalyst on Equilibrium | 催化剂对平衡的影响

A catalyst provides an alternative reaction pathway with lower activation energy, increasing the rates of both the forward and reverse reactions equally. Therefore, a catalyst does not alter the position of equilibrium or the value of the equilibrium constant. It simply allows equilibrium to be reached more quickly.

催化剂提供了具有较低活化能的替代反应路径，同等程度地提高正反应和逆反应的速率。因此，催化剂不会改变平衡位置或平衡常数的数值。它只是让平衡更快地达到。

This is particularly important in industrial processes like the Haber process, where an iron catalyst is used to speed up the attainment of equilibrium without affecting the maximum yield. However, the catalyst does not increase the actual yield at equilibrium; the yield is determined solely by the thermodynamic conditions (temperature, pressure, concentration) at equilibrium.

这在哈伯法等工业流程中尤为重要，铁催化剂用于加快达到平衡的速度，而不影响最大产率。然而，催化剂并不增加平衡时的实际产率；产率仅由平衡时的热力学条件（温度、压强、浓度）决定。

6. Kc Calculations and ICE Tables | Kc 计算与 ICE 表格

Many examination questions require you to calculate Kc from given initial and equilibrium amounts. The standard method uses an ICE table (Initial, Change, Equilibrium). Set up the table with initial moles (or concentrations), then express the change in terms of x, using the stoichiometric ratios. Finally, deduce equilibrium moles (or concentrations). If the volume is known, convert moles to concentrations before substituting into the Kc expression.

许多考试题目要求根据给定的初始量和平衡量计算 Kc。标准方法是使用 ICE 表格（初始、变化、平衡）。建立表格，填入初始物质的量（或浓度），然后用 x 根据化学计量比表示变化量。最后推导出平衡物质的量（或浓度）。如果已知体积，在代入 Kc 表达式之前将物质的量转换为浓度。

For example, consider the dissociation of N₂O₄(g) ⇌ 2NO₂(g). Initially 1.0 mol of N₂O₄ is placed in a 1.0 dm³ vessel. At equilibrium, 0.20 mol of N₂O₄ remains. Deduce the change: 1.0 – 0.20 = 0.80 mol of N₂O₄ reacted, so 2 × 0.80 = 1.60 mol of NO₂ is formed. Then Kc = [NO₂]² / [N₂O₄] = (1.60)² / 0.20 = 12.8 mol dm⁻³.

例如，考虑 N₂O₄(g) ⇌ 2NO₂(g) 的解离。初始时将 1.0 mol N₂O₄ 放入 1.0 dm³容器中。平衡时剩余 0.20 mol N₂O₄。推导变化量：1.0 – 0.20 = 0.80 mol N₂O₄ 已反应，因此生成 2 × 0.80 = 1.60 mol NO₂。则 Kc = [NO₂]² / [N₂O₄] = (1.60)² / 0.20 = 12.8 mol dm⁻³。

7. Kp Calculations and Units | Kp 计算与单位

Kp calculations follow a similar pattern but require careful handling of partial pressures. First, calculate mole fractions for each gaseous species at equilibrium. Then multiply each mole fraction by the total pressure to obtain partial pressures. Insert these into the Kp expression. Always check and state the units.

Kp 的计算遵循类似模式，但需要仔细处理分压。首先，计算平衡时每种气体的摩尔分数。然后将每个摩尔分数乘以总压得到分压。再将这些值代入 Kp 表达式。务必检查并写明单位。

For instance, in the synthesis of methanol: CO(g) + 2H₂(g) ⇌ CH₃OH(g). At equilibrium, the mole fractions are: CO = 0.10, H₂ = 0.20, CH₃OH = 0.70, and the total pressure is 100 atm. Partial pressures: pCO = 10 atm, pH₂ = 20 atm, pCH₃OH = 70 atm. Kp = (pCH₃OH) / (pCO × (pH₂)²) = 70 / (10 × 20²) = 70 / 4000 = 0.0175. The units: Δn = 1 – (1+2) = –2, so units are atm⁻². Thus Kp = 0.0175 atm⁻².

例如，甲醇合成：CO(g) + 2H₂(g) ⇌ CH₃OH(g)。平衡时摩尔分数为：CO = 0.10，H₂ = 0.20，CH₃OH = 0.70，总压为 100 atm。分压：pCO = 10 atm，pH₂ = 20 atm，pCH₃OH = 70 atm。Kp = (pCH₃OH) / (pCO × (pH₂)²) = 70 / (10 × 20²) = 70 / 4000 = 0.0175。单位：Δn = 1 – (1+2) = –2，所以单位为 atm⁻²。因此 Kp = 0.0175 atm⁻²。

8. Homogeneous vs. Heterogeneous Equilibria | 均相平衡与非均相平衡

A homogeneous equilibrium is one in which all reactants and products are in the same phase (all gases or all aqueous). In such systems, all species appear in the equilibrium constant expression (assuming they are not pure solids or liquids). Most Edexcel Kc and Kp problems involve homogeneous gas-phase or solution equilibria.

均相平衡是指所有反应物和生成物都处于同一相态（全部为气体或全部为溶液）。在这种体系中，所有物质都出现在平衡常数表达式中（假设它们不是纯固体或液体）。大多数Edexcel的 Kc 和 Kp 题目涉及均相气相或溶液平衡。

A heterogeneous equilibrium involves substances in more than one phase, such as the decomposition of calcium carbonate: CaCO₃(s) ⇌ CaO(s) + CO₂(g). Here, the solids are omitted from the Kc or Kp expression, so Kp = pCO₂, and Kc = [CO₂]. This simplification often catches students out; remember that the concentration of a pure solid is a constant and is incorporated into the equilibrium constant.

非均相平衡涉及多于一种相态的物质，例如碳酸钙的分解：CaCO₃(s) ⇌ CaO(s) + CO₂(g)。在此，固体不出现在 Kc 或 Kp 表达式中，因此 Kp = pCO₂，Kc = [CO₂]。这一简化常使学生困惑；记住纯固体的浓度是常数，并被并入平衡常数中。

9. Temperature Dependence of the Equilibrium Constant | 平衡常数对温度的依赖性

The equilibrium constant is sensitive to temperature changes. According to the van’t Hoff equation (qualitative understanding is required for Edexcel), for an exothermic forward reaction, Kc decreases as temperature increases. This is because the system shifts to the endothermic (reverse) direction to absorb the added heat, reducing product concentration. Conversely, for an endothermic forward reaction, Kc increases with rising temperature.

平衡常数对温度变化很敏感。根据范特霍夫方程（Edexcel要求定性理解），对于正向放热反应，Kc 随温度升高而减小。这是因为体系向吸热（逆向）移动以吸收新增热量，从而降低生成物浓度。相反，对于正向吸热反应，Kc 随温度升高而增大。

Be prepared to interpret graphs showing Kc or Kp as a function of temperature. A downward slope indicates an exothermic forward reaction; an upward slope indicates an endothermic forward reaction. This concept is often tested alongside Le Chatelier’s principle to explain industrial conditions.

准备好解释显示 Kc 或 Kp 随温度变化的图形。向下的斜率表示正向放热反应；向上的斜率表示正向吸热反应。这一概念常与勒夏特列原理结合考查，用于解释工业条件。

10. Industrial Applications: The Haber and Contact Processes | 工业应用：哈伯法与接触法

Edexcel frequently tests the application of equilibrium principles to industrial processes. In the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic (ΔH = –92 kJ mol⁻¹). According to Le Chatelier, high pressure shifts equilibrium to the right (fewer moles of gas: 4 → 2), while low temperature favours product because the forward reaction is exothermic. However, a compromise temperature of about 400–450°C is used because a very low temperature would make the rate too slow. An iron catalyst speeds up the reaction without affecting equilibrium yield.

Edexcel经常考查平衡原理在工业流程中的应用。在哈伯法中，N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，正反应放热（ΔH = –92 kJ mol⁻¹）。根据勒夏特列原理，高压使平衡右移（气体分子数 4 → 2），低温有利于生成物，因为正反应放热。然而，实际采用约 400–450°C 的折中温度，因为过低的温度会使速率太慢。铁催化剂加速反应但不影响平衡产率。

The Contact process for sulfuric acid involves the oxidation of SO₂: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = –197 kJ mol⁻¹. Again, high pressure would favour fewer moles, but only a slightly raised pressure (1–2 atm) is used because the equilibrium already lies far to the right at moderate temperatures. A vanadium(V) oxide catalyst is used, and temperatures around 450°C are chosen to balance rate and yield.

硫酸生产的接触法涉及 SO₂ 的氧化：2SO₂(g) + O₂(g) ⇌ 2SO₃(g)，ΔH = –197 kJ mol⁻¹。同样，高压有利于分子数较少的一侧，但实际只采用略高于常压（1–2 atm），因为在中等温度下平衡已大幅向右。使用五氧化二钒催化剂，温度选择在约 450°C，以平衡速率和产率。

11. Common Pitfalls and Exam Tips | 常见错误与应试技巧

One common mistake is forgetting to convert moles to concentrations when calculating Kc. Always divide moles by the volume (dm³) to obtain mol dm⁻³ unless the vessel is exactly 1 dm³. Another frequent error is omitting units for Kc and Kp – even if the unit is just concentration raised to a power, you must state it explicitly.

一个常见错误是在计算 Kc 时忘记将物质的量转换为浓度。除非容器容积恰好为 1 dm³，否则务必用物质的量除以体积（dm³）得到 mol dm⁻³。另一个常犯错误是遗漏 Kc 和 Kp 的单位——即使单位只是浓度的幂次，也必须明确写出。

Always use an ICE table to organise data; it reduces arithmetic mistakes.
务必使用 ICE 表格来组织数据；这能减少计算错误。
Remember that solids and pure liquids do not appear in K expressions.
记住固体和纯液体不出现在平衡常数表达式中。
When explaining equilibrium shifts, refer to the “number of moles of gas”, not “number of moles” alone, for pressure effects.
解释压强引起的平衡移动时，要说明“气体分子总数”，而非单纯的“物质的量”。
Distinguish between a shift in equilibrium position and a change in the equilibrium constant, especially when temperature is altered.
尤其在温度改变时，要区分平衡位置的移动和平衡常数的变化。
Practice drawing and interpreting graphs of concentration/pressure versus time, showing how equilibrium is approached and re-established after a disturbance.
练习绘制和解释浓度/压强随时间变化的图形，展示如何趋向平衡以及受干扰后重新建立平衡。

12. Summary of Key Relationships | 关键关系总结

Change	Effect on equilibrium position	Effect on Kc/Kp
Increase reactant concentration	Shifts right	None
Increase pressure (Δn < 0)	Shifts to side with fewer gas moles	None
Increase temperature (exothermic forward)	Shifts left	Decreases
Increase temperature (endothermic forward)	Shifts right	Increases
Add catalyst	No shift	None

中文对照：

变化	对平衡位置的影响	对 Kc/Kp 的影响
增加反应物浓度	右移	无
增大压强 (Δn < 0)	移向气体分子数少的一侧	无
升高温度（正反应放热）	左移	减小
升高温度（正反应吸热）	右移	增大
加入催化剂	不移动	无

Mastering these core concepts and frequently practising numerical and graphical problems will give you confidence in tackling chemical equilibrium questions on the Edexcel A-Level Chemistry examination.

掌握这些核心概念并经常练习数值和图表问题，将使你在应对Edexcel A-Level化学考试中有关化学平衡的题目时充满信心。

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A-Level Edexcel Chemistry: Chemical Equilibrium Key Points | A-Level Edexcel 化学：化学平衡 考点精讲