Edexcel A-Level Chemistry: Chemical Equilibria and Kc | 爱德思A-Level化学：化学平衡与平衡常数Kc

📚 Edexcel A-Level Chemistry: Chemical Equilibria and Kc | 爱德思A-Level化学：化学平衡与平衡常数Kc

Chemical equilibria are central to understanding how reversible reactions behave at A-Level. For Edexcel A-Level Chemistry, you must be able to explain dynamic equilibrium, write and calculate Kc expressions, and predict how changes in conditions shift the position of equilibrium using Le Chatelier’s principle.

化学平衡是A-Level阶段理解可逆反应行为的核心。爱德思A-Level化学要求你能够解释动态平衡、书写并计算Kc表达式，并运用勒夏特列原理预测条件变化如何移动平衡位置。

1. Dynamic Equilibrium | 动态平衡

In a reversible reaction, reactants form products while products simultaneously re-form reactants. Dynamic equilibrium is reached in a closed system when the rate of the forward reaction equals the rate of the backward reaction. At this point, the concentrations of all species remain constant, but the reactions have not stopped.

在可逆反应中，反应物生成产物的同时，产物也在重新生成反应物。当正反应速率等于逆反应速率时，封闭体系中达到动态平衡。此时所有物质的浓度保持不变，但反应并没有停止。

It is essential to remember that equilibrium can only be established in a closed system. If any product or reactant escapes, the system is open and true equilibrium cannot be reached.

必须记住，平衡只能在封闭体系中建立。如果任何产物或反应物逸出，体系就属于开放体系，无法达到真正的平衡。

2. Reversible Reactions and Closed Systems | 可逆反应与封闭体系

A reversible reaction is shown with the symbol ⇌. For example, the reaction between nitrogen and hydrogen to form ammonia is reversible:

可逆反应用符号 ⇌ 表示。例如，氮气与氢气生成氨的反应是可逆的：

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Equilibrium can only be established in a closed system where no matter is lost or gained. In an open system, products or reactants may escape, preventing the rates from becoming equal and constant.

平衡只能在物质不损失或增加的封闭体系中建立。在开放体系中，产物或反应物可能逸出，导致正逆反应速率无法相等并保持恒定。

3. The Equilibrium Constant Kc | 平衡常数Kc

For a general homogeneous reaction aA + bB ⇌ cC + dD, the equilibrium constant Kc is defined as:

对于一般的均相反应 aA + bB ⇌ cC + dD，平衡常数Kc定义为：

Kc = [C]ᶜ [D]ᵈ / [A]ᵃ [B]ᵇ

Square brackets represent concentrations in mol dm⁻³. The powers a, b, c and d are the stoichiometric coefficients from the balanced equation. Kc is valid for a given temperature; changing temperature changes Kc.

方括号表示浓度，单位为 mol dm⁻³。幂指数 a、b、c、d 是配平方程式中的化学计量系数。Kc 只在特定温度下成立；改变温度会改变 Kc。

Kc has no units if the total number of moles of gaseous or aqueous products equals the total number of moles of gaseous or aqueous reactants. Otherwise, units must be calculated from the concentration terms.

如果气态或水溶液中生成物的总物质的量等于反应物的总物质的量，则 Kc 没有单位。否则，单位必须由浓度项计算得出。

4. Writing Kc Expressions | 书写Kc表达式

Only gases and aqueous species appear in Kc expressions. Pure solids and pure liquids are omitted because their concentrations are effectively constant. For example, in the esterification reaction CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l), water is not a solvent, so all species are included.

只有气体和水溶液中的物种才出现在 Kc 表达式中。纯固体和纯液体因其浓度基本恒定而被省略。例如，在酯化反应 CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l) 中，水不是溶剂，因此所有物种都要写入表达式。

However, if water is the solvent and is present in large excess, its concentration is effectively constant, so it is omitted from the Kc expression.

但是，如果水是溶剂且大量过量，其浓度基本恒定，因此要在 Kc 表达式中省略。

5. Calculating Kc from Experimental Data | 由实验数据计算Kc

To calculate Kc, you need equilibrium concentrations of all reactants and products. An ICE table (Initial moles, Change in moles, Equilibrium moles) is a useful tool. Consider this example:

要计算 Kc，你需要所有反应物和产物的平衡浓度。ICE 表（初始物质的量、变化量、平衡物质的量）是一种很有用的工具。请看下面的例子：

0.50 mol of N₂ and 0.50 mol of H₂ are placed in a 1.0 dm³ vessel. At equilibrium, 0.20 mol of NH₃ is present. Calculate Kc.

将 0.50 mol N₂ 和 0.50 mol H₂ 放入 1.0 dm³ 容器中。达到平衡时，NH₃ 的物质的量为 0.20 mol。计算 Kc。

Species	N₂	3H₂	2NH₃
Initial / mol	0.50	0.50	0
Change / mol	-0.10	-0.30	+0.20
Equilibrium / mol	0.40	0.20	0.20

Since the volume is 1.0 dm³, the equilibrium concentrations are 0.40 mol dm⁻³ for N₂, 0.20 mol dm⁻³ for H₂ and 0.20 mol dm⁻³ for NH₃. Therefore:

由于体积为 1.0 dm³，平衡浓度分别为 N₂ 0.40 mol dm⁻³、H₂ 0.20 mol dm⁻³、NH₃ 0.20 mol dm⁻³。因此：

Kc = [NH₃]² / ([N₂][H₂]³) = (0.20)² / (0.40 × (0.20)³) = 12.5 dm⁶ mol⁻²

Always calculate and state the units of Kc unless the expression is dimensionless. In this case, the units are dm⁶ mol⁻².

除非 Kc 表达式无量纲，否则务必计算并写明 Kc 的单位。本例题中，单位为 dm⁶ mol⁻²。

6. 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 temperature, pressure or concentration, the position of equilibrium shifts to oppose the change. This principle helps predict the direction of shift.

勒夏特列原理指出，如果处于平衡状态的体系受到温度、压强或浓度的改变，平衡位置会向减弱这种改变的方向移动。这一原理有助于预测平衡移动的方向。

For example, if the concentration of a reactant is increased, the equilibrium shifts to the right to consume some of the added reactant. If a product is removed, the equilibrium also shifts to the right to produce more product.

例如，如果增加某一反应物的浓度，平衡会向右移动，以消耗部分新增反应物。如果移除某一产物，平衡也会向右移动，以生成更多产物。

7. Effect of Temperature | 温度的影响

The effect of temperature depends on whether the forward reaction is exothermic or endothermic. For an exothermic forward reaction, increasing temperature shifts equilibrium to the left, decreasing Kc. For an endothermic forward reaction, increasing temperature shifts equilibrium to the right, increasing Kc.

温度的影响取决于正反应是放热还是吸热。若正反应放热，升高温度会使平衡向左移动，Kc 减小。若正反应吸热，升高温度会使平衡向右移动，Kc 增大。

Temperature is the only factor that changes the value of Kc. Concentration and pressure changes move the position of equilibrium but do not change Kc.

温度是唯一能改变 Kc 数值的因素。浓度和压强的变化只会移动平衡位置，不会改变 Kc。

8. Effect of Pressure and Concentration | 压强和浓度的影响

Pressure changes only affect equilibria involving gases with different numbers of moles on each side. Increasing pressure favours the side with fewer gas molecules. For example, in N₂ + 3H₂ ⇌ 2NH₃, the forward reaction reduces gas moles from 4 to 2, so high pressure favours ammonia formation.

压强变化只影响两侧气体分子数不同的气体平衡。增大压强有利于气体分子数较少的一侧。例如，在 N₂ + 3H₂ ⇌ 2NH₃ 中，正反应使气体物质的量从 4 减少到 2，因此高压有利于生成氨。

If the number of gas moles is the same on both sides, changing pressure has no effect on the position of equilibrium.

如果两侧气体物质的量相同，改变压强不会影响平衡位置。

9. Catalysts and Equilibrium | 催化剂与平衡

A catalyst speeds up both forward and backward reactions equally, so it does not change the position of equilibrium or Kc. It only allows equilibrium to be reached more quickly.

催化剂同等程度地加快正反应和逆反应速率，因此不会改变平衡位置或 Kc。它只能使体系更快达到平衡。

Catalysts are important in industrial processes because they reduce the time needed to reach equilibrium without reducing yield.

催化剂在工业过程中非常重要，因为它们缩短了达到平衡所需的时间，却不会降低产率。

10. Industrial Application: The Haber Process | 工业应用：哈伯法

The Haber process combines N₂ and H₂ to produce NH₃:

哈伯法将 N₂ 和 H₂ 转化为 NH₃：

N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = -92 kJ mol⁻¹

Typical conditions are 400–450°C, 200 atm, and an iron catalyst. The high pressure favours product yield, but the high temperature is a compromise: lower temperature favours yield but makes the reaction too slow, so a moderate temperature and catalyst are used.

典型条件是 400–450°C、200 atm 以及铁催化剂。高压有利于提高产率，但高温是一种折中方案：较低温度有利于提高产率，但反应过慢，因此使用适中的温度和催化剂。

This is a classic Edexcel example linking equilibrium principles with industrial practice, and exam questions often ask you to justify the chosen conditions.

这是爱德思考试中一个将平衡原理与工业实践联系起来的经典例子，试题经常要求你对所选条件进行解释。

11. Exam Tips and Common Mistakes | 考试技巧与常见错误

Always write Kc with products over reactants and balance the equation first.
Include units for Kc unless the expression is dimensionless; cancel units carefully.
Do not include solids or pure liquids in Kc expressions.
Remember that changing temperature changes Kc, but changing concentration or pressure does not change Kc; it only shifts the position of equilibrium.
Quote Le Chatelier’s principle precisely in answers rather than giving vague statements.
Use ICE tables for calculations and check that equilibrium moles are all positive.

始终先将产物写在 Kc 分子上、反应物写在分母上，并先配平方程式。

除非 Kc 表达式无量纲，否则要写明单位，并仔细约去单位。

不要在 Kc 表达式中写入固体或纯液体。

记住改变温度会改变 Kc，而改变浓度或压强不会改变 Kc，只会移动平衡位置。

回答简答题时要准确表述勒夏特列原理，避免模糊表述。

计算时使用 ICE 表，并检查平衡物质的量是否全部为正。

12. Summary | 小结

Chemical equilibrium is a dynamic balance of forward and backward reaction rates. Kc quantifies the equilibrium position at a given temperature. Le Chatelier’s principle helps predict shifts in equilibrium caused by changes in temperature, pressure and concentration. Edexcel exam questions often combine calculation and explanation, so practise ICE tables and written justifications.

化学平衡是正逆反应速率的动态平衡。Kc 定量描述了特定温度下的平衡位置。勒夏特列原理有助于预测由温度、压强和浓度变化引起的平衡移动。爱德思试题常将计算与解释结合起来，因此要练习 ICE 表和文字论证。

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Edexcel A-Level Chemistry: Chemical Equilibria and Kc | 爱德思A-Level化学：化学平衡与平衡常数Kc