AQA AS Chemistry Unit 2 January 2021 Mark Scheme: Key Concepts and Exam Techniques | AQA AS 化学第二单元 2021 年 1 月评分方案：核心考点与答题技巧

📚 AQA AS Chemistry Unit 2 January 2021 Mark Scheme: Key Concepts and Exam Techniques | AQA AS 化学第二单元 2021 年 1 月评分方案：核心考点与答题技巧

The January 2021 AQA AS Chemistry Unit 2 mark scheme provides a clear record of how examiners award marks for physical, inorganic and organic chemistry. It shows that answers must be precise, use correct scientific language and link observations to underlying theory.

2021 年 1 月 AQA AS 化学第二单元评分方案清楚展示了考官在物理化学、无机化学和有机化学中如何给分。它表明答案必须准确、使用正确的科学语言，并将观察结果与基础理论联系起来。

1. Energetics and Enthalpy Changes | 能量学与焓变

The mark scheme frequently tested Hess’s law cycles using standard enthalpy of formation data. Candidates were expected to show the cycle or use the expression ΔH = ΣΔHf(products) − ΣΔHf(reactants), and then substitute values with correct signs.

评分方案经常考查使用标准生成焓数据的赫斯定律循环。考生需要展示循环或使用公式 ΔH = ΣΔHf(生成物) − ΣΔHf(反应物)，并正确代入带正负号的数值。

ΔH = ΣΔHf(products) − ΣΔHf(reactants)

A very common error was reversing the sign of the final answer or forgetting to multiply an enthalpy value by the number of moles in the balanced equation. The mark scheme awarded marks only when the arithmetic and the sign were both correct.

一个非常常见的错误是最后的答案正负号写反，或忘记将焓值乘以配平方程中的摩尔数。只有当算术和正负号都正确时，评分方案才给分。

When defining standard enthalpy of combustion, the marking points included ‘one mole of substance’, ‘completely burned in oxygen’ and ‘under standard conditions’. Missing any one of these three elements usually lost the mark.

在定义标准燃烧焓时，给分点包括 “1 摩尔物质”、”在氧气中完全燃烧” 和 “在标准条件下”。缺少这三项中的任何一项通常会失分。

2. Kinetics and the Maxwell-Boltzmann Distribution | 动力学与麦克斯韦-玻尔兹曼分布

The mark scheme required candidates to describe how increasing temperature changes the Maxwell-Boltzmann distribution curve. The key points were that the curve shifts to the right and flattens, with a greater proportion of particles having energy greater than or equal to the activation energy, Eₐ.

评分方案要求考生描述升高温度如何改变麦克斯韦-玻尔兹曼分布曲线。关键点是曲线向右移动并变得平缓，更大比例的粒子具有大于或等于活化能 Eₐ 的能量。

A common misconception was that a catalyst lowers the activation energy by shifting the curve. The mark scheme expected the explanation that a catalyst provides an alternative reaction pathway with a lower Eₐ, so more particles have sufficient energy to react at a given temperature.

一个常见误解是催化剂通过改变曲线来降低活化能。评分方案要求解释为催化剂提供了具有较低 Eₐ 的替代反应路径，因此在给定温度下更多粒子具有足够能量发生反应。

When interpreting the curve, candidates often confused the area under the curve with the total number of particles. The mark scheme accepted that the total area remains constant as temperature changes, but the distribution broadens towards higher energies.

在解读曲线时，考生常将曲线下面积与粒子总数混淆。评分方案认可温度改变时总面积保持不变，但分布向更高能量方向拓宽。

3. Chemical Equilibria and Le Chatelier’s Principle | 化学平衡与勒夏特列原理

Equilibrium questions in the January 2021 mark scheme assessed the effect of changing pressure, temperature and concentration on the position of equilibrium. Answers had to state the direction of shift and justify it by referring to gas moles or the exothermic/endothermic nature of the forward reaction.

2021 年 1 月评分方案中的平衡题考查了改变压强、温度和浓度对平衡位置的影响。答案必须说明移动方向，并通过气体摩尔数或正反应的放热/吸热性质来证明。

For example, if the forward reaction produces fewer gas moles, increasing pressure shifts the equilibrium to the right. Credit was not given for simply saying ‘the equilibrium shifts to oppose the change’ without linking this to the specific reaction.

例如，如果正反应产生的气体摩尔数更少，增大压强会使平衡向右移动。只写 “平衡移动以抵抗这种变化” 而不与具体反应联系，不给分。

Calculating Kc required using equilibrium concentrations, not initial amounts. The mark scheme penalised using moles directly in the Kc expression without dividing by the volume of the container first.

计算 Kc 需要使用平衡浓度，而不是初始物质的量。评分方案对不先除以容器体积而直接在 Kc 表达式中使用摩尔数的做法扣分。

Kc = [products] / [reactants] (balanced coefficients as powers)

4. Redox Reactions and Oxidation States | 氧化还原反应与氧化态

The mark scheme required candidates to determine oxidation states from chemical formulae and use them to identify which species is oxidised and which is reduced. For example, in Group 7 reactions, chlorine changes from 0 to −1 when it gains electrons.

评分方案要求考生从化学式确定氧化态，并利用它们判断哪种物质被氧化、哪种被还原。例如在第 7 族反应中，氯得到电子后氧化态从 0 变为 −1。

Disproportionation was a key concept. The mark scheme accepted an answer only if the same element was shown to both increase and decrease in oxidation state in the same reaction, such as chlorine in the reaction with cold dilute sodium hydroxide.

歧化反应是一个关键概念。只有当同一元素在同一反应中氧化态既升高又降低时，评分方案才给分，例如氯气与冷的稀氢氧化钠溶液反应。

Ionic half-equations had to be balanced in terms of atoms and charge. Candidates frequently lost marks for missing electrons or using the wrong number of water and hydrogen ions when balancing under acidic conditions.

离子半反应式必须在原子数和电荷两方面配平。考生常因遗漏电子或在酸性条件下配平时使用错误数目的水和氢离子而失分。

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