A-Level Chemistry: Chemical Equilibrium — Complete Guide | A-Level 化学：化学平衡完全指南

Introduction | 引言

Chemical equilibrium is one of the most fundamental concepts in A-Level Chemistry. It bridges the gap between reaction kinetics and thermodynamics, explaining why many reactions never go to completion. Mastering equilibrium is essential not only for exam success but also for understanding real-world processes like the Haber process, the Contact process, and biochemical pathways.

化学平衡是 A-Level 化学中最基础的概念之一。它连接了反应动力学和热力学，解释了为什么许多反应永远不会进行到底。掌握平衡不仅对考试成功至关重要，而且对理解现实世界的过程（如哈伯法、接触法和生化途径）也至关重要。

1. What is Dynamic Equilibrium? | 什么是动态平衡？

A reversible reaction reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the backward reaction. At this point, the concentrations of reactants and products remain constant — but notice the word “dynamic”: reactions are still occurring in both directions. The system appears static from the outside, but at the molecular level, it is constantly in motion.

当正反应速率等于逆反应速率时，可逆反应达到动态平衡。此时，反应物和产物的浓度保持不变——但注意”动态”这个词：两个方向的反应仍在进行。从外部看系统似乎是静态的，但在分子水平上，它始终在运动。

Key Characteristics | 关键特征

• Closed system required: No matter can enter or leave. If you open the system, equilibrium is disturbed.
• Macroscopic constancy: Observable properties (colour, pressure, concentration) do not change.
• Microscopic activity: Forward and reverse reactions continue at equal rates.
• Can be approached from either direction: Whether you start with pure reactants or pure products, the same equilibrium mixture is eventually reached.

• 需要封闭系统：物质不能进入或离开。如果打开系统，平衡就会被破坏。
• 宏观恒定：可观察的性质（颜色、压力、浓度）不发生变化。
• 微观活动：正反应和逆反应以相等的速率继续进行。
• 可从任一方向达到：无论从纯反应物还是纯产物开始，最终都会达到相同的平衡混合物。

2. The Equilibrium Constant (Kc) | 平衡常数 (Kc)

For a general reaction at equilibrium:

对于一般的平衡反应：

aA + bB ⇌ cC + dD

The equilibrium constant expression is:

平衡常数表达式为：

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

Where square brackets denote equilibrium concentrations in mol dm⁻³. The stoichiometric coefficients become the powers in the expression.

其中方括号表示平衡浓度，单位为 mol dm⁻³。化学计量系数成为表达式中的幂次。

What Kc Tells Us | Kc 告诉我们什么

• Kc >> 1: The equilibrium lies far to the right — products dominate. The reaction essentially goes to completion.
• Kc ≈ 1: Significant amounts of both reactants and products are present at equilibrium.
• Kc << 1: The equilibrium lies far to the left — reactants dominate. Very little product forms.

• Kc >> 1：平衡远远偏向右侧——产物占主导地位。反应基本上进行到底。
• Kc ≈ 1：平衡时存在大量的反应物和产物。
• Kc << 1：平衡远远偏向左侧——反应物占主导地位。生成的产物非常少。

Important Notes on Kc | 关于 Kc 的重要说明

• Kc depends only on temperature. Changing concentration or pressure does NOT change Kc — it changes the position of equilibrium, but the ratio at the new equilibrium remains the same Kc value at that temperature.
• Solids and pure liquids are omitted from the Kc expression because their concentrations are effectively constant.
• Kc has units that depend on the specific reaction. Always calculate and state the units in your exam answer.

• Kc 仅取决于温度。改变浓度或压力不会改变 Kc — 它会改变平衡位置，但在新平衡时的比值仍然是该温度下的 Kc 值。
• 固体和纯液体从 Kc 表达式中省略，因为它们的浓度实际上是常数。
• Kc 有单位，取决于具体反应。在考试答案中始终计算并注明单位。

3. Le Chatelier’s Principle | 勒夏特列原理

Le Chatelier’s Principle states: if a system at dynamic 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：如果处于动态平衡的系统受到变化（浓度、压力或温度），平衡位置会移动以抵消该变化。

3.1 Effect of Concentration | 浓度的影响

Consider the equilibrium: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) (blood-red complex)

考虑平衡：Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)（血红色络合物）

• Adding Fe³⁺ or SCN⁻: Equilibrium shifts right → more FeSCN²⁺ forms → solution becomes darker red.
• Removing FeSCN²⁺: Equilibrium shifts right to replace what was removed.
• Adding FeSCN²⁺: Equilibrium shifts left → some FeSCN²⁺ dissociates.

• 加入 Fe³⁺ 或 SCN⁻：平衡向右移动 → 生成更多 FeSCN²⁺ → 溶液变成更深的红色。
• 移除 FeSCN²⁺：平衡向右移动以补充被移除的物质。
• 加入 FeSCN²⁺：平衡向左移动 → 部分 FeSCN²⁺ 解离。

3.2 Effect of Pressure | 压力的影响

Pressure changes only affect equilibria involving gases where the number of moles changes.

压力变化仅影响涉及气体且摩尔数发生变化的平衡。

Change | 变化	Shift Direction | 移动方向
Increase pressure | 增加压力	Towards fewer moles of gas | 向气体摩尔数较少的方向
Decrease pressure | 降低压力	Towards more moles of gas | 向气体摩尔数较多的方向
Equal moles on both sides | 两侧摩尔数相等	No effect | 无影响

Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

4 moles of gas on the left, 2 moles on the right. Increasing pressure shifts equilibrium to the right — more ammonia is produced. This is why the Haber process uses high pressure!

例子：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

左侧 4 mol 气体，右侧 2 mol。增加压力使平衡向右移动 — 产生更多氨。这就是为什么哈伯法使用高压！

3.3 Effect of Temperature | 温度的影响

This is the only factor that changes Kc. The direction depends on whether the reaction is exothermic or endothermic.

这是唯一改变 Kc 的因素。方向取决于反应是放热还是吸热。

• Exothermic (ΔH negative): Increasing temperature shifts equilibrium LEFT (Kc decreases). Decreasing temperature shifts equilibrium RIGHT (Kc increases).
• Endothermic (ΔH positive): Increasing temperature shifts equilibrium RIGHT (Kc increases). Decreasing temperature shifts equilibrium LEFT (Kc decreases).

• 放热反应（ΔH 为负）：升高温度使平衡向左移动（Kc 减小）。降低温度使平衡向右移动（Kc 增大）。
• 吸热反应（ΔH 为正）：升高温度使平衡向右移动（Kc 增大）。降低温度使平衡向左移动（Kc 减小）。

3.4 Effect of a Catalyst | 催化剂的影响

A catalyst does NOT change the position of equilibrium and does NOT change Kc. It simply speeds up both forward and reverse reactions equally, allowing equilibrium to be reached faster. In industry, catalysts are crucial because they allow equilibrium to be reached at lower temperatures, saving energy costs.

催化剂不会改变平衡位置，也不会改变 Kc。它只是同等程度地加速正反应和逆反应，使平衡更快达到。在工业中，催化剂至关重要，因为它们允许在较低温度下达到平衡，节省能源成本。

4. The Haber Process — A Perfect Case Study | 哈伯法 — 完美的案例研究

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

The Haber process is the industrial synthesis of ammonia from nitrogen and hydrogen. It perfectly illustrates the compromise between equilibrium yield and reaction rate:

哈伯法是从氮气和氢气工业合成氨的方法。它完美地说明了平衡产率和反应速率之间的折中：

Condition | 条件	Equilibrium Yield Effect | 平衡产率影响	Rate Effect | 速率影响	Industrial Choice | 工业选择
High pressure | 高压	Increases yield ✓ | 增加产率 ✓	Increases rate ✓ | 提高速率 ✓	200 atm (compromise: cost vs yield)
High temperature | 高温	Decreases yield ✗ (exothermic) | 降低产率 ✗ (放热)	Increases rate ✓ | 提高速率 ✓	400-450°C (compromise)
Iron catalyst | 铁催化剂	No effect | 无影响	Increases rate ✓ | 提高速率 ✓	Used | 使用

5. Common Exam Calculations | 常见考试计算

Calculating Kc from Equilibrium Amounts | 从平衡量计算 Kc

Worked Example: 0.50 mol of ethanoic acid and 0.50 mol of ethanol are mixed in a 2.0 dm³ flask. At equilibrium, 0.30 mol of ethyl ethanoate is formed. Calculate Kc.

例题：在 2.0 dm³ 烧瓶中混合 0.50 mol 乙酸和 0.50 mol 乙醇。平衡时生成 0.30 mol 乙酸乙酯。计算 Kc。

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

	CH₃COOH	C₂H₅OH	CH₃COOC₂H₅	H₂O
Initial (mol)	0.50	0.50	0	0
Change (mol)	-0.30	-0.30	+0.30	+0.30
Equilibrium (mol)	0.20	0.20	0.30	0.30
Concentration (mol dm⁻³)	0.10	0.10	0.15	0.15

Kc = (0.15 × 0.15) / (0.10 × 0.10) = 0.0225 / 0.0100 = 2.25

Units: (mol dm⁻³ × mol dm⁻³) / (mol dm⁻³ × mol dm⁻³) = no units

6. Exam Tips & Common Mistakes | 考试技巧与常见错误

1. Always state Kc units. Many students lose marks by omitting units from their final answer.
2. Kc depends ONLY on temperature. If a question asks what changes Kc, the answer is temperature alone — not pressure, not concentration.
3. Use Le Chatelier’s Principle precisely: Say “the equilibrium shifts to oppose the change” rather than vague statements like “it moves right”.
4. In Kc calculations, show your ICE table (Initial, Change, Equilibrium). Examiners award marks for the method even if the final answer is wrong.
5. For gas-phase equilibria, you can use partial pressures instead of concentrations, giving Kp. The same principles apply.
6. Catalysts do NOT affect yield or Kc. This is a classic trick question. A catalyst only speeds up the rate at which equilibrium is reached.

1. 始终注明 Kc 的单位。许多学生因遗漏最终答案的单位而失分。
2. Kc 仅取决于温度。如果问题问什么会改变 Kc，答案仅是温度——不是压力，不是浓度。
3. 精确使用勒夏特列原理：说”平衡移动以抵消变化”，而不是模糊地说”它向右移动”。
4. 在 Kc 计算中，展示你的 ICE 表格（初始、变化、平衡）。即使最终答案错误，考官也会给方法分。
5. 对于气相平衡，可以用分压代替浓度，得到 Kp。原理相同。
6. 催化剂不影响产率或 Kc。这是经典的陷阱题。催化剂只加速达到平衡的速率。

Summary | 总结

Chemical equilibrium is about understanding the balance between forward and reverse reactions. The key tools — Le Chatelier’s Principle, the equilibrium constant Kc, and ICE tables — allow you to predict and quantify how a system responds to changes. From the industrial Haber process to biological buffer systems, equilibrium chemistry is everywhere. Master these fundamentals, practice the calculations, and you will excel in your A-Level Chemistry exams.

化学平衡是理解正反应和逆反应之间平衡的关键。核心工具——勒夏特列原理、平衡常数 Kc 和 ICE 表格——让你能够预测和量化系统如何响应变化。从工业哈伯法到生物缓冲系统，平衡化学无处不在。掌握这些基础知识，练习计算，你将在 A-Level 化学考试中脱颖而出。