A-Level Chemistry Unit 5 (Jan 20) Core Principles | A-Level 化学第五单元(2020年1月)核心原理

📚 A-Level Chemistry Unit 5 (Jan 20) Core Principles | A-Level 化学第五单元(2020年1月)核心原理

The AQA A-Level Chemistry Unit 5 (CHEM5) examination paper from January 2020 provides a rigorous assessment of advanced topics, including thermodynamics, equilibrium, acid–base chemistry, redox processes, and transition metal behaviour. This article distils the core principles tested in that paper, offering a bilingual revision guide that mirrors the depth and style of questions candidates faced.

2020年1月的AQA A-Level化学第五单元(CHEM5)试卷对热力学、平衡、酸碱、氧化还原和过渡金属等高级课题进行了严格考查。本文提炼该试卷中考察的核心原理，提供一份双语复习指南，再现考生遇到的题目深度与风格。

1. Thermodynamics: Born-Haber Cycles and Lattice Enthalpy | 热力学：玻恩-哈伯循环与晶格焓

A Born‑Haber cycle applies Hess’s Law to relate the enthalpy of formation of an ionic compound to its atomisation, ionisation, electron affinity, and lattice enthalpy. Students must be able to construct the cycle and determine an unknown term, usually lattice enthalpy.

玻恩-哈伯循环运用赫斯定律将离子化合物的生成焓与其原子化、电离、电子亲和及晶格焓关联起来。学生须能构建循环并求解未知项，通常是晶格焓。

In the January 2020 paper, a typical task involved a cycle for MgCl₂, requiring candidates to identify the standard enthalpy change for each arrow and to calculate the lattice dissociation enthalpy (ΔH_L°), which is endothermic. The cycle uses ΔH_at° for Mg(s) → Mg(g) and ½Cl₂(g) → Cl(g), the first and second ionisation energies of magnesium, the electron affinity of chlorine, and the overall ΔH_f°.

在2020年1月试卷中，一道典型题给出MgCl₂的循环，要求考生识别每一步的标准焓变并计算晶格解离焓(ΔH_L°，吸热)。该循环用到Mg(s) → Mg(g)和½Cl₂(g) → Cl(g)的原子化焓(ΔH_at°)、镁的第一和第二电离能、氯的电子亲和能以及总的标准生成焓ΔH_f°。

Accurate sign convention is essential: lattice formation enthalpy is exothermic (negative), whereas lattice dissociation enthalpy is positive. The general summation is ΔH_f° = sum of all other enthalpy changes, which allows any missing value

Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com

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