Mastering End-of-Chapter Questions in A-Level Chemistry: Equilibrium Worked Example | 攻克 A-Level 化学章末题：平衡计算精讲实例

📚 Mastering End-of-Chapter Questions in A-Level Chemistry: Equilibrium Worked Example | 攻克 A-Level 化学章末题：平衡计算精讲实例

End-of-chapter questions in Cambridge A-Level Chemistry are designed to test not just recall but the ability to apply concepts across multiple topics. This article takes a typical equilibrium question from the Haber process and works through it step by step, showing how to handle Kp calculations, Le Chatelier’s principle, and catalyst effects under exam conditions.

剑桥 A-Level 化学的章末题不仅考查记忆，更考查跨主题综合运用概念的能力。本文以一道哈伯法平衡的典型题为例，逐步演示在考试情境下如何处理 Kp 计算、勒夏特列原理以及催化剂的影响。

1. Why End-of-Chapter Questions Matter | 为什么章末题重要

End-of-chapter questions consolidate several learning objectives in one scenario. They often combine calculations, explanations, and predictions, which mirrors the style of Cambridge Paper 4 and Paper 5 questions.

章末题将多个学习目标整合到一个情境中。它们通常融合计算、解释和预测，与剑桥 Paper 4 和 Paper 5 的题型风格一致。

Practising these questions helps you identify weak areas before the final examination and improves your ability to manage multi-step problems under time pressure.

练习这些题目有助于你在最终考试前发现薄弱环节，并提高在时间压力下处理多步骤问题的能力。

2. Anatomy of a Typical End-of-Chapter Question | 典型章末题的结构

A well-written end-of-chapter question usually begins with a chemical context, provides numerical data, and then asks for a calculation, an explanation, and an evaluation. In our worked example, the context is the synthesis of ammonia, and the parts require Kp calculation, temperature and pressure effects, and the role of a catalyst.

一道设计良好的章末题通常先给出化学背景和数值数据，然后要求计算、解释和评价。在本例题中，背景是氨的合成，各个小问分别要求 Kp 计算、温度和压力影响以及催化剂作用。

Recognising this pattern helps you allocate time efficiently: secure calculation marks first, then move to explanation and evaluation.

识别这种模式有助于你高效分配时间：先确保计算分，再处理解释和评价部分。

3. Worked Example: The Haber Process Equilibrium | 例题：哈伯法平衡

Consider this question: ‘A mixture of 1.00 mol N₂(g) and 3.00 mol H₂(g) is placed in a 2.00 dm³ vessel at 400 °C. At equilibrium, the partial pressure of NH₃ is 2.00 atm and the total pressure is 10.0 atm. The reaction is N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ mol⁻¹. (a) Write the Kp expression and calculate its value. (b) State and explain the effect of increasing temperature on the yield of NH₃. (c) State and explain the effect of increasing pressure on the yield of NH₃. (d) Iron is used as a catalyst. Explain how a catalyst affects the rate and the equilibrium position.’

考虑这道题：“将 1.00 mol N₂(g) 和 3.00 mol H₂(g) 置于 2.00 dm³ 容器中，温度 400 °C。达到平衡时，NH₃ 的分压为 2.00 atm，总压为 10.0 atm。反应为 N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ mol⁻¹。(a) 写出 Kp 表达式并计算其值。(b) 说明并解释升高温度对 NH₃ 产率的影响。(c) 说明并解释增大压力对 NH₃ 产率的影响。(d) 铁用作催化剂。解释催化剂如何影响速率和平衡位置。”

4. Part (a): Writing the Kp Expression | 第(a)问：书写 Kp 表达式

For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the equilibrium constant in terms of partial pressures is Kp = (p(NH₃))² / (p(N₂) × (p(H₂))³). The powers come from the stoichiometric coefficients: 2 for NH₃, 1 for N₂, and 3 for H₂.

对于平衡 N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，用分压表示的平衡常数 Kp = (p(NH₃))² / (p(N₂) × (p(H₂))³)。指数来自化学计量数：NH₃

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