AS Chemistry Unit 2: Calculations from the June 2019 Mark Scheme | AS化学第二单元：2019年6月评分方案计算题型解析

📚 AS Chemistry Unit 2: Calculations from the June 2019 Mark Scheme | AS化学第二单元：2019年6月评分方案计算题型解析

Calculation questions in Edexcel International AS Chemistry Unit 2 test your ability to apply quantitative reasoning to energetics, kinetics and organic chemistry. Analysing the June 2019 mark scheme reveals the specific steps and key points where marks are awarded, enabling you to perfect your exam technique. This article breaks down the core calculation types, provides worked examples, and shows how to use the mark scheme to avoid common pitfalls.

在Edexcel国际AS化学第二单元中，计算题考察你在能量学、动力学和有机化学中应用定量推理的能力。分析2019年6月的评分方案，可以揭示评分的关键步骤和得分点，帮助你完善考试技巧。本文将剖析核心计算题型，提供示例，并展示如何利用评分方案避免常见错误。

1. Using q = mcΔT | 利用q = mcΔT计算热量

The most fundamental calculation is determining the heat energy absorbed or released by a solution or water using q = mcΔT. In the June 2019 Unit 2 exam, students were required to calculate q when a mass of water was heated by a combustion reaction. The mark scheme awarded one mark for correct substitution of mass (m), specific heat capacity (c = 4.18 J g⁻¹ K⁻¹) and temperature change (ΔT = T_final – T_initial), and a second mark for the correct numerical answer with appropriate units (J or kJ). Remember that if the mass is given in grams, c should be 4.18 J g⁻¹ K⁻¹, and ΔT can be in °C as the interval is identical to Kelvin.

最基本的计算是利用 q = mcΔT 求溶液或水吸收释放的热量。在2019年6月Unit 2考试中，学生需要计算水在燃烧反应中被加热时的q值。评分方案：正确代入质量(m)、比热容(c=4.18 J g⁻¹ K⁻¹)和温度变化(ΔT=最终温度-初始温度)得一分，数值结果及正确单位(J或kJ)再得一分。注意如果质量单位是克，c取4.18 J g⁻¹ K⁻¹，温度差ΔT可用摄氏度，因为温差间隔与开尔文相同。

q = mcΔT

Many candidates lose a mark by forgetting to convert the mass of water into grams, or by misreading the thermometer. The mark scheme in June 2019 also insisted on using exactly 4.18, not 4.2, for the specific heat capacity unless instructed otherwise.

许多考生因为忘记将水的质量换算成克，或读错温度计而失分。2019年6月的评分方案还明确要求使用精确值4.18，而不是4.2，除非题目另有说明。

2. Determining ΔH per Mole | 计算每摩尔焓变

Once q is known, the molar enthalpy change ΔH is found by dividing by the number of moles of the limiting reactant or fuel burned: ΔH = –q / n. The negative sign indicates an exothermic reaction (temperature rise). The June 2019 mark scheme required candidates to calculate moles from mass and molar mass, and then use ΔH = –q/n. Marks were also given for converting q from joules to kilojoules before dividing, or for giving ΔH in kJ mol⁻¹.

一旦求得q，每摩尔焓变ΔH由q除以限制试剂或燃料的摩尔数而得：ΔH = –q / n。负号表示放热反应（温度升高）。2019年6月评分方案要求考生通过质量和摩尔质量求出摩尔数，然后使用ΔH = –q/n。将q从焦耳转换为千焦再除以n，或者结果以kJ mol⁻¹表示，都能得分。

ΔH = –q / n

For example, if 0.92 g of ethanol (Mᵣ = 46.0) is burned and q = 17.6 kJ, then n = 0.92/46.0 = 0.0200 mol, giving ΔH = –17.6/0.0200 = –880 kJ mol⁻¹. The mark scheme would award one mark for moles, one for the division, and one for the negative sign and units.

例如，燃烧0.92 g乙醇（Mᵣ = 46.0），测得q = 17.6 kJ，则n = 0.92/46.0 = 0.0200 mol，ΔH = –17.6/0.0200 = –880 kJ mol⁻¹。评分方案会给摩尔计算一分，除法一分，负号和单位再一分。

3. Hess’s Law Cycles | 赫斯定律循环

In the June 2019 paper, a Hess’s Law cycle was used to determine the enthalpy change of reaction from enthalpies of formation or combustion. The mark scheme expected candidates to correctly draw and label the cycle, or to apply the formula ΔH°_reaction = ΣΔH°_f(products) – ΣΔH°_f(reactants). One mark was typically for the cycle, another for the correct expression, and a third for the numerical answer with sign and units.

在2019年6月试卷中，利用赫斯定律循环从生成焓或燃烧焓求反应焓变。评分方案要求正确绘制并标注循环图，或者应用公式ΔH°反应 = ΣΔH°_f(产物) – ΣΔH°_f(反应物)。通常绘制循环得一分，正确表达式再一分，数值加符号单位再一分。

ΔH°_reaction = ΣΔH°_f(products) – ΣΔH°_f(reactants)

When using combustion data, the relationship becomes ΔH°_reaction = ΣΔH°_c(reactants) – ΣΔH°_c(products). The mark scheme rewards the careful handling of signs and stoichiometric coefficients. In the 2019 exam, many students lost a mark by incorrectly multiplying the enthalpy of combustion for a reactant without considering the coefficient in the balanced equation.

若使用燃烧焓数据，则关系变为 ΔH°反应 = ΣΔH°_c(反应物) – ΣΔH°_c(生成物)。评分方案重视符号和化学计量数的正确使用。在2019年考试中，许多学生因为未按配平方程的系数乘以反应物的燃烧焓而失分。

4. Bond Enthalpy Calculations | 键焓计算

The mark scheme for a typical bond enthalpy question requires listing all bonds broken and formed, calculating the total energy absorbed (bonds broken) and energy released (bonds formed), then applying ΔH = Σ(energy of bonds broken) – Σ(energy of bonds formed). In June 2019, a mark was given for each correct summation, and a final mark for the correct ΔH value. Be careful with bond multiplicities: e.g. in C₂H₄ there is a C=C double bond, and four C–H bonds.

典型键焓题的评分方案要求列出所有断裂和生成的键，计算总吸能（键断裂）和总放能（键形成），然后应用ΔH = Σ(键断裂能) – Σ(键形成能)。2019年6月考试中，每个正确的加和得一分，最终ΔH正确再得一分。注意键的数目：如C₂H₄中含一个C=C双键和四个C–H键。

• Step 1: Draw displayed formulae to identify every bond.
• 步骤1：画出结构式以识别所有键。
• Step 2: Sum the bond energies for all reactants (bonds broken).
• 步骤2：加和所有反应物的键能（断裂的键）。
• Step 3: Sum the bond energies for all products (bonds formed).
• 步骤3：加和所有生成物的键能（生成的键）。
• Step 4: ΔH ≈ Σ(BEs broken) – Σ(BEs formed).
• 步骤4：ΔH ≈ Σ(键断裂能) – Σ(键形成能)。

The term “approximate” is used because mean bond enthalpies are averaged over many compounds. The mark scheme sometimes awards a mark for stating that the calculated value is not exact for a specific molecule.

这里用“近似”一词，因为平均键焓是对许多化合物取平均。评分方案有时会给分，如果考生说明计算值对特定分子并不精确。

5. Standard Enthalpy of Combustion from Experimental Data | 实验数据求标准燃烧焓

These questions extend the q = mcΔT calculation by accounting for the mass of fuel burned. The mark scheme highlights the need to use the temperature rise of water, mass of water, and the mass of fuel to find ΔH_c. One common pitfall is using the mass of the spirit burner instead of the mass of fuel actually combusted. The June 2019 mark scheme allocated marks for: calculating q, calculating moles of fuel burned, dividing q by moles, and multiplying by –1 with correct sign.

Published by TutorHao | AS Chemistry Revision Series | aleveler.com

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