📚 AQA A Level Chemistry Unit 5 Jan 2021: Thermodynamics, Redox and Inorganic Chemistry | AQA A级化学Unit 5 2021年1月真题精讲
The January 2021 AQA A Level Chemistry Unit 5 paper tests three overlapping skill areas: energetic calculations using Born-Haber cycles and Gibbs free energy, electrochemical cells and standard electrode potentials, and the descriptive inorganic chemistry of Period 3 and transition metals. Exam questions in this unit reward precision with sign conventions, unit conversions, and ion-electron half-equations.
2021年1月AQA A Level化学Unit 5试卷主要考查三大交叉板块：运用Born-Haber循环和吉布斯自由能进行能量计算、电化学电池与标准电极电势，以及第三周期元素和过渡金属的描述性无机化学。本单元的考题对符号规则、单位换算和离子-电子半反应式的准确性要求非常高。
This article is not a copy of the live paper but a topic-focused revision walkthrough that reflects the style and demands of the January 2021 Unit 5 questions. It highlights the equations, definitions, common errors, and model-thinking needed to improve your score.
本文并非真题复刻，而是一份紧扣2021年1月Unit 5试题风格的主题复习精讲。文章重点梳理高频方程式、定义、常见错误和答题思路，帮助你稳定提分。
1. Paper Overview and Assessment Style | 试卷结构与考查风格
The January 2021 Unit 5 paper is typically written under timed conditions and contains a mixture of short-answer calculations, structured inorganic questions, and synoptic reaction sequences. Many candidates lose marks not from lack of knowledge but from ignoring the full wording of the question, especially the words "standard conditions", "excess", "dilute", and "alkaline".
2021年1月Unit 5试卷通常在限时条件下完成，题型包括短答案计算、结构化无机题和综合反应推断。很多考生失分并非因为知识欠缺，而是忽略了题干中的完整限定词，尤其是“标准条件”“过量”“稀溶液”“碱性”等关键词。
For calculation questions, you should always show the steps: write the relevant equation, substitute values with units, convert kJ to J where necessary, and express final answers to the correct significant figures. For inorganic description questions, use precise colours, oxidation states, and ligand names rather than vague language.
对于计算题，应始终展示完整步骤：写出相关公式、代入带单位的数值、必要时将kJ转换为J，并按正确的有效数字给出最终答案。对于无机描述题，必须使用准确的颜色、氧化态和配体名称，避免模糊表述。
Typical question stems in this paper ask you to calculate a lattice enthalpy from a Born-Haber cycle, predict whether a redox reaction is feasible using E° values, balance a redox equation in acidic or alkaline solution, or explain an observation in transition-metal aqueous chemistry.
本卷常见的题干要求包括：根据Born-Haber循环计算晶格焓、利用E°值判断氧化还原反应是否可行、在酸性或碱性条件下配平氧化还原方程式，或解释过渡金属水溶液化学中的实验现象。
2. Born-Haber Cycles and Lattice Enthalpy | Born-Haber循环与晶格焓
enthalpy normally reported as a negative value because the process is strongly exothermic.
晶格焓是指在标准条件下，由气态离子生成1摩尔离子化合物时的焓变。例如Na⁺(g) + Cl⁻(g) → NaCl(s)的晶格焓通常为负值，因为该过程强烈放热。
Born-Haber cycles apply Hess's law to ionic solids. A complete cycle for sodium chloride includes atomisation of sodium, atomisation of chlorine, first ionisation energy of sodium, first electron affinity of chlorine, and the lattice enthalpy. The formation enthalpy is the sum of these changes.
Born-Haber循环将赫斯定律应用于离子固体。完整的氯化钠循环包括钠的原子化、氯的原子化、钠的第一电离能、氯的第一电子亲和能以及晶格焓。生成焓等于这些变化的代数和。
ΔHf = ΔHat(Na) + ΔHat(Cl) + IE₁(Na) + EA₁(Cl) + ΔH_lattice
In the January 2021 paper, candidates were often asked to insert the correct species or enthalpy labels into a cycle, or to use the cycle to calculate an unknown value. The two most common mistakes are writing the electron affinity of chlorine as positive instead of negative, and treating atomisation enthalpy as zero for a diatomic gas when it must be half the bond dissociation enthalpy.
在2021年1月试卷中，考生常被要求填入正确的物质或焓变标签，或利用循环计算未知量。最常见的两个错误是：将氯的电子亲和能误写为正号，以及将双原子气体的原子化焓误当作零，而实际上它应等于键解离焓的一半。
Another frequent error is confusing lattice formation enthalpy with lattice dissociation enthalpy. Formation is exothermic and used in the Born-Haber cycle; dissociation is the reverse process and is endothermic. When you solve the cycle, keep the direction of each arrow consistent with the sign.
另一个常见错误是混淆晶格生成焓与晶格解离焓。生成焓为放热过程，用于Born-Haber循环；解离焓是其逆过程，为吸热过程。在计算循环时，必须保证每个箭头方向与正负号一致。
A useful check is that a more exothermic lattice enthalpy gives a more stable compound, usually with smaller ions or higher charges. MgO has a much more negative lattice enthalpy than NaCl because Mg²⁺ and O²⁻ have higher charges than Na⁺ and Cl⁻.
一个有用的判断是：晶格焓越负，化合物越稳定，通常离子半径越小或电荷越高。MgO的晶格焓远低于NaCl，因为Mg²⁺和O²⁻的电荷高于Na⁺和Cl⁻。
3. Enthalpy of Solution and Hydration | 溶解焓与水合焓
The enthalpy of solution is the enthalpy change when one mole of an ionic solid dissolves completely in water to form a very dilute solution. It can be calculated from lattice enthalpy and hydration enthalpies using a simple Hess cycle.
溶解焓是指1摩尔离子固体完全溶于水形成极稀溶液时的焓变。它可通过晶格焓和水合焓构建简单的赫斯循环进行计算。
When an ionic solid dissolves, two energy steps occur: the lattice is broken into gaseous ions, which is endothermic, and the gaseous ions are hydrated by water, which is exothermic. The overall solution enthalpy is the sum of lattice dissociation enthalpy and the total hydration enthalpy of the ions.
离子固体溶解时发生两个能量过程：晶格被破坏生成气态离子，该过程吸热；气态离子与水发生水合作用，该过程放热。总溶解焓等于晶格解离焓与所有离子水合焓之和。
ΔHsol = -ΔH_lattice(formation) + ΔHhyd(Na⁺) + ΔHhyd(Cl⁻)
mcΔT, then divide by moles, and remember that heat lost by the surroundings has a sign that must be interpreted from the temperature change.
在2021年1月试卷中，常考题型包括提供Born-Haber数值求溶解焓，或给出实验温度变化数据计算每摩尔溶解焓。对于量热法问题，应使用q = mcΔT，再除以物质的量，并记住根据温度变化判断热量的正负号。
Hydration enthalpy becomes more exothermic as ion charge density increases. Small, highly charged ions such as Mg²⁺ attract water molecules more strongly than large singly charged ions such as K⁺. However, because lattice enthalpy also increases with charge density, solubility does not always increase in the same order.
离子电荷密度越大，水合焓越负。Mg²⁺等小而高电荷的离子比K⁺等大而单电荷的离子更能吸引水分子。但由于晶格焓也随电荷密度增大而变得更负，溶解度并不一定按相同顺序变化。
Watch out for units when combining hydration and lattice enthalpies: both must be in kJ mol⁻¹, and if a cycle gives lattice dissociation as positive, do not subtract it twice.
在合并水合焓和晶格焓时要注意单位统一：两者都必须使用kJ mol⁻¹；如果循环中的晶格解离焓已经取正，就不要重复减去两次。
4. Entropy, Gibbs Free Energy and Feasibility | 熵、吉布斯自由能与反应可行性
The second law of thermodynamics states that a process is feasible if the total entropy change of the universe is positive. Total entropy change is the sum of the entropy change of the system and the entropy change of the surroundings.
热力学第二定律指出，如果宇宙的总熵变为正，过程就具有可行性。总熵变等于系统的熵变与环境的熵变之和。
ΔStotal = ΔSsystem + ΔS_surroundings
T, where ΔH is the enthalpy change of the system and T is the absolute temperature in kelvin. A strongly exothermic reaction increases the entropy of the surroundings because it releases energy into them.
环境的熵变由ΔS_surroundings = -ΔH/T给出，其中ΔH是系统的焓变，T是以开尔文为单位的绝对温度。强放热反应会向环境释放能量，从而增加环境的熵。
For calculations at constant pressure, it is more convenient to use the Gibbs free energy. The standard equation is:
在恒压条件下，使用吉布斯自由能更方便。标准方程为：
ΔG = ΔH - TΔS
ibility occurs only above a certain temperature, calculated from T = ΔH/ΔS at equilibrium.
当ΔG为零或负值时，反应可行。如果ΔH为负且ΔS为正，反应在任何温度下都可行。如果ΔH为正且ΔS也为正，则只有在高于某一温度时反应才可行，可由平衡条件T = ΔH/ΔS计算。
re using ΔG = ΔH - TΔS. Failure to do this is one of the most frequent calculation errors.
2021年1月试卷常考查kJ mol⁻¹与J K⁻¹ mol⁻¹之间的换算。由于ΔH通常以kJ表示，而熵以J K⁻¹ mol⁻¹表示，在代入ΔG = ΔH - TΔS之前，必须将ΔH乘以1000转换为J。这一换算错误是最常见的计算失误之一。
You may also be asked to explain why a reaction with a positive ΔH can still be spontaneous at high temperature. This requires linking the large positive entropy change, often from gas formation or dissolving, to the TΔS term overtaking the positive ΔH as temperature rises.
你还可能被要求解释为什么ΔH为正的反应在高温下仍能自发进行。这需要联系较大的正熵变，通常来自气体生成或溶解过程，使得TΔS项在温度升高时超过正的ΔH。
5. Electrode Potentials and the Standard Hydrogen Electrode | 电极电势与标准氢电极
Standard electrode potential E° is the electromotive force of a half-cell connected to the standard hydrogen electrode under standard conditions: 298 K, 100 kPa gas pressure, and 1 mol dm⁻³ ion concentrations. The standard hydrogen electrode is assigned a value of 0.00 V.
标准电极电势E°是指在标准条件下，即298 K、气体压力100 kPa、离子浓度1 mol dm⁻³时，将半电池与标准氢电极连接所测得的电动势。标准氢电极被指定为0.00 V。
The standard hydrogen electrode uses hydrogen gas at 100 kPa bubbled over a platinum electrode in contact with a solution of H⁺ ions at 1 mol dm⁻³. Platinum is used because it is inert and provides a surface for the reaction.
标准氢电极使用100 kPa的氢气气泡通过铂电极，并与浓度为1 mol dm⁻³的H⁺溶液接触。使用铂是因为它惰性且能为反应提供表面。
In the January 2021 paper, questions frequently asked for the conditions needed to measure a reliable E° value, or asked you to write a cell diagram for a given half-cell. Remember that a half-cell with a metal electrode is written as M(s) | M²⁺(aq), while a gas electrode is written with Pt as the electrode, for example Pt(s) | H₂(g) | H⁺(aq).
在2021年1月试卷中，常会问到测定可靠E°值所需的条件，或要求写出给定半电池的电池图。记住金属电极半电池写作M(s) | M²⁺(aq)，而气体电极需写出铂电极，例如Pt(s) | H₂(g) | H⁺(aq)。
The more positive the E° value, the greater the tendency of the half-equation to proceed in the reduction direction. Therefore, a half-cell with a more positive E° will oxidise a half-cell with a less positive E° in a spontaneous redox reaction.
E°值越正，半反应向还原方向进行的倾向越大。因此，E°较正的半电池会氧化E°较负的半电池，形成自发氧化还原反应。
When combining half-equations, balance the electrons but do not multiply the E° values. Electrode potential is an intensive property, so it does not depend on the amount of substance and must not be doubled or tripled.
合并半反应时，要配平电子，但不要将E°值相乘。电极电势是强度性质，不随物质的量变化，因此绝不能加倍或乘以任何倍数。
A typical question may present two standard reduction potentials and ask you to calculate the cell EMF and write the overall cell reaction. Use Ecell = Emore positive - Eless positive, then reverse the less positive half-equation and combine.
常见题型会给出两个标准还原电势，要求计算电池电动势并写出总电池反应。可使用Ecell = Emore positive - Eless positive，然后将较负的半反应逆向并合并。
6. Electrochemical Cells and Cell EMF | 电化学电池与电池电动势
Published by TutorHao | Exam Prep Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图