📚 Mastering AS Chemistry Unit 2 Calculation Questions (June 2019 Insert) | 攻克 AS 化学单元 2 计算题型（2019 年六月数据表）
The June 2019 AS Chemistry Unit 2 exam comes with an Insert packed with essential data — bond enthalpies, standard enthalpies, molar masses — and you will be expected to use these values to solve a range of calculation problems. This article breaks down every major calculation type that appears in this paper, showing you how to read the Insert and apply the numbers with confidence.
2019 年六月 AS 化学单元 2 考试附带的数据表汇集了关键数据——键焓、标准焓变、摩尔质量——试卷要求你运用这些数值解答多种计算题。本文将逐一拆解该试卷中出现的所有重要计算题型，教你如何读懂数据表并自信地运用这些数字。
1. Familiarising Yourself with the Insert | 熟悉数据表内容
Before tackling any question, scan the Insert to locate the specific data you’ll need. The June 2019 Insert for Unit 2 typically includes a table of average bond enthalpies, some standard enthalpy changes of formation (ΔHf⁰) and combustion (ΔHc⁰), along with an updated periodic table and relative atomic masses. Having a clear mental map of where each piece of information lies saves crucial seconds in the exam.
在解答任何题目之前，先快速浏览数据表，定位所需数据。2019 年六月单元 2 数据表通常包含一张平均键焓表、若干标准生成焓（ΔHf⁰）和标准燃烧焓（ΔHc⁰）数据，以及一张最新版本的元素周期表与相对原子质量。在脑海中清楚记住每类信息的位置，可以为考试争得宝贵时间。
The Insert often gives bond enthalpies for common diatomic and polyatomic bonds, such as C–H (413 kJ mol⁻¹), C=O (799 kJ mol⁻¹), O–H (463 kJ mol⁻¹), and H–H (436 kJ mol⁻¹). You will use these when the question explicitly asks for an estimate based on mean bond enthalpies.
数据表通常给出常见双原子与多原子键的键焓，例如 C–H (413 kJ mol⁻¹)、C=O (799 kJ mol⁻¹)、O–H (463 kJ mol⁻¹)、H–H (436 kJ mol⁻¹) 等。当题目明确要求用平均键焓进行估算时，你就需要使用这些数值。
- Check the units: all energy values are in kJ mol⁻¹, so make sure your answers end up with the same unit.
- 检查单位：所有能量数值均以 kJ mol⁻¹ 给出，务必确保最终答案单位一致。
- Note that the Insert may provide molar masses for compounds like butane or ethanol; these are needed for mole calculations in calorimetry or titrations.
- 留意数据表可能会给出丁烷、乙醇等化合物的摩尔质量，这在量热或滴定中的摩尔计算里必不可少。
2. Using Mean Bond Enthalpies to Estimate ΔH | 用平均键焓估算反应焓变
When the reaction can be represented by all reactants and products in the gaseous state, you can estimate the enthalpy change using the bond-breaking minus bond-making formula. The Insert supplies the bond enthalpy values, so the key is to draw out all the bonds present in each species.
当反应物和产物均以气态存在时，可以用“断键吸热减去成键放热”的公式估算焓变。数据表提供了键焓值，因此关键是把每种分子里所有的键都画出来。
For example, for the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Bonds broken: 4 × C–H (4 × 413) and 2 × O=O (2 × 498). Bonds made: 2 × C=O (2 × 799) and 4 × O–H (4 × 463). The calculation becomes:
比如甲烷的燃烧：CH₄ + 2O₂ → CO₂ + 2H₂O。断键：4 个 C–H（4 × 413）与 2 个 O=O（2 × 498）。成键：2 个 C=O（2 × 799）与 4 个 O–H（4 × 463）。计算过程如下：
ΔH ≈ [4(413) + 2(498)] – [2(799) + 4(463)]
ΔH ≈ [1652 + 996] – [1598 + 1852] = 2648 – 3450 = –802 kJ mol⁻¹
Remember, mean bond enthalpies give only an approximate value because the actual bond energies depend on the molecular environment. In the exam, a question will often ask you to compare this estimate with the standard enthalpy of combustion found in the Insert.
要记住，平均键焓只能得到近似值，因为实际键能会受分子环境影响。考试题常常要求你将这一估算值与数据表中给出的标准燃烧焓进行比较。
- Always write the balanced equation first; check that all reactants and products are gases if the question doesn’t specify otherwise.
- 务必先写出配平的反应方程式；若题目未特别说明，则应确保所有反应物和产物均为气态。
- Watch out for molecules with multiple bonds, like O=O and C≡N, and use the correct bond enthalpy from the Insert.
- 注意含有多重键的分子，比如 O=O 和 C≡N，务必选用数据表中正确的键焓。
3. Constructing Hess’s Law Cycles | 构建盖斯定律循环
Many questions in Unit 2 require you to calculate an unknown enthalpy change by applying Hess’s Law, often using standard enthalpies of formation or combustion that are provided in the Insert. The two most common routes are via formation or via combustion.
单元 2 中许多题目要求你运用盖斯定律计算未知焓变，通常会用到数据表所提供的标准生成焓或燃烧焓。最常见的两种路径分别是生成路径与燃烧路径。
Route 1: Using ΔHf⁰. The enthalpy change of a reaction equals the total enthalpy of formation of the products minus the total enthalpy of formation of the reactants, each multiplied by their stoichiometric coefficients. This is the direct ‘products minus reactants’ method only if you use formation data.
路径一：利用 ΔHf⁰。反应的焓变等于产物的总生成焓减去反应物的总生成焓，各项均需乘以化学计量数。只要使用的全是生成焓数据，就可以直接用“产物减反应物”的方法。
ΔH = Σ ΔHf⁰ (products) – Σ ΔHf⁰ (reactants)
Route 2: Using ΔHc⁰. When you are given combustion data, you must build a cycle where both reactants and products are combusted completely to the same combustion products (CO₂, H₂O, etc.). Then the enthalpy change of the reaction equals the total combustion enthalpy of the reactants minus the total combustion enthalpy of the products.
路径二：利用 ΔHc⁰。当题目给出燃烧焓数据时，必须构建一个循环，将反应物和产物都完全燃烧成相同的燃烧产物（如 CO₂、H₂O 等）。此时，反应的焓变等于反应物的总燃烧焓减去产物的总燃烧焓。
ΔH = Σ ΔHc⁰ (reactants) – Σ ΔHc⁰ (products)
The Insert may list ΔHf⁰ for species like CO₂ (–394 kJ mol⁻¹), H₂O(l) (–286 kJ mol⁻¹), and C₂H₅OH(l) (–278 kJ mol⁻¹). Drawing a cycle with arrows up (for formation) or down (for combustion) can help you avoid sign errors.
数据表中可能会列出 CO₂ (–394 kJ mol⁻¹)、H₂O(l) (–286 kJ mol⁻¹) 和 C₂H₅OH(l) (–278 kJ mol⁻¹) 等物质的 ΔHf⁰。绘制循环图，用向上箭头表示生成、向下箭头表示燃烧，有助于避免符号错误。
4. Processing Calorimetry Data | 量热实验数据处理
A classic calculation in Unit 2 involves a simple calorimetry experiment — for instance, measuring the temperature rise when a known mass of fuel is burned, or when two solutions are mixed. The heat energy transferred, q, is calculated using q = mcΔT, and then scaled to molar quantities.
单元 2 中的经典计算题涉及简单的量热实验——例如，测量一定质量燃料燃烧或两种溶液混合时的温度升高。传递的热量 q 用 q = mcΔT 计算，然后再换算为摩尔量。
q = m × c × ΔT
where m is the mass (or volume) of water/solution (in g or cm³, assuming density = 1 g cm⁻³), c is the specific heat capacity (4.18 J g⁻¹ K⁻¹ for water), and ΔT is the temperature change in °C or K. Don’t forget to convert q from joules to kilojoules if your final answer is in kJ mol⁻¹.
其中 m 是水或溶液的质量（或体积，g 或 cm³，假设密度为 1 g cm⁻³），c 是比热容（水的比热容为 4.18 J g⁻¹ K⁻¹），ΔT 是温度变化（°C 或 K）。若最终答案要表示为 kJ mol⁻¹，千万别忘了把 q 从焦耳转换为千焦。
After finding q, calculate the amount in moles of the fuel burned or the limiting reactant, using n = m/M (molar mass from the Insert). Then the molar enthalpy change is:
求得了 q 后，再通过 n = m/M（摩尔质量可从数据表获得）计算燃料或限制反应物的物质的量，接着按下式计算摩尔焓变：
ΔH = –q / n
The negative sign is added because the heat released in exothermic reactions is shown as a negative ΔH. If temperature decreases, q is negative and ΔH becomes positive (endothermic).
公式中加上负号是因为放热反应释放的热量用负的 ΔH 表示。如果温度下降，q 为负，则 ΔH 为正（吸热）。
5. Calculating Standard Enthalpy of Combustion | 计算标准燃烧焓
Combustion problems often combine the calorimetry equation with the data from the Insert. You may be asked to determine the ΔHc⁰ of an alcohol or a hydrocarbon using a spirit burner and a water calorimeter. The Insert gives molar masses, so you can find the moles of fuel.
燃烧问题常将量热方程与数据表信息结合起来。题目可能会要求你使用酒精灯和水量热计测定醇类或烃类的 ΔHc⁰。数据表提供了摩尔质量，你可以据此求出燃料的物质的量。
A typical procedure: measure the mass of the spirit burner before and after burning; record the temperature rise of a known mass of water; calculate q = mcΔT; find the mass of fuel burned; calculate n (fuel); then ΔHc⁰ = –q / n. Be ready to comment on experimental limitations, such as heat loss to the surroundings and incomplete combustion, and to suggest improvements like using a draught shield or a bomb calorimeter.
典型步骤：测量酒精灯燃烧前后的质量；记录一定质量水的温度升高值；计算 q = mcΔT；计算燃烧掉的燃料质量；求出燃料的物质的量 n；再计算 ΔHc⁰ = –q / n。要做好准备评论实验的局限性，例如向环境散热和不完全燃烧，并给出改进措施，例如使用挡风罩或弹式量热计。
The Insert value for ΔHc⁰ will be more negative than the experimental value because the experiment usually underestimates the energy released.
数据表中给出的 ΔHc⁰ 会比实验值更负，因为实验通常会低估释放的能量。
6. Understanding the Equilibrium Constant Kc | 理解平衡常数 Kc
Equilibrium calculations are a major feature of Unit 2. The Insert provides no direct equilibrium data, but you are expected to write the expression for Kc and calculate its value from concentrations or from initial amounts together with the equilibrium amount of one species.
平衡计算是单元 2 的重头戏。虽然数据表不直接提供平衡数据，但要求你写出 Kc 表达式，并根据浓度或根据初始量以及某一物种的平衡量来计算 Kc 值。
For a general reaction: aA + bB ⇌ cC + dD, the equilibrium constant is:
对于一般反应：aA + bB ⇌ cC + dD，平衡常数表达式为：
Kc = [C]ᶜ [D]ᵈ / [A]ᵃ [B]ᵇ
The square brackets denote concentrations in mol dm⁻³. Remember, solids and pure liquids do not appear in the Kc expression; only gases and aqueous species are included.
方括号表示以 mol dm⁻³ 为单位的浓度。记住，固体和纯液体不写入 Kc 表达式，只有气体和溶液中的物质才出现。
If the Insert provides molar masses, you may need to convert grams to moles and then to concentrations by dividing by the total volume. Be meticulous with units — Kc has units that depend on the difference in the number of moles of gaseous products and reactants (Δn).
如果数据表提供了摩尔质量，你可能需要先将质量换算为物质的量，再除以总体积得到浓度。务必仔细处理单位——Kc 的单位取决于气态产物与反应物计量数之差（Δn），且不同反应的单位形式不同。
7. Using ICE Tables to Find Kc | 利用 ICE 表求算 Kc
Questions often provide the initial amounts of all reactants and the equilibrium amount of one substance, expecting you to complete an ICE table (Initial, Change, Equilibrium) and then deduce the equilibrium amounts of all others. Let’s set up a typical example.
题目常常给出所有反应物的初始量以及某一种物质的平衡量，要求你填写 ICE 表（初始、变化、平衡），进而推出所有其他物质的平衡量。下面搭建一个典型例子。
Consider the equilibrium: H₂ + I₂ ⇌ 2HI. Initially, a container holds 0.50 mol of H₂ and 0.50 mol of I₂. At equilibrium, 0.20 mol of H₂ remains. The ICE table looks like this:
考虑平衡：H₂ + I₂ ⇌ 2HI。初始时，容器内有 0.50 mol H₂ 和 0.50 mol I₂。平衡时，剩余 0.20 mol H₂。ICE 表如下：
| Species | Initial (mol) | Change (mol) | Equilibrium (mol) |
|---|---|---|---|
| H₂ | 0.50 | –0.30 | 0.20 |
| I₂ | 0.50 | –0.30 | 0.20 |
| HI | 0.00 | +0.60 | 0.60 |
If the volume is V dm³, the concentrations are 0.20/V, 0.20/V, and 0.60/V. Then Kc = [HI]² / ([H₂][I₂]) = (0.60/V)² / ((0.20/V)(0.20/V)) = 9.00 (no units because Δn = 0).
若体积为 V dm³，各物质的浓度分别为 0.20/V、0.20/V 和 0.60/V。则 Kc = [HI]² / ([H₂][I₂]) = (0.60/V)² / ((0.20/V)(0.20/V)) = 9.00（无单位，因为 Δn = 0）。
Be careful to subtract the change for reactants and add for products; the ‘Change’ row must reflect the stoichiometry. Practice several examples, including those where the initial amounts are given in moles and the volume is stated.
仔细扣减反应物的变化量、增加产物的变化量；“变化”行必须反映化学计量关系。多做几个练习，包括那些给出物质的量（mol）和明确体积的题目。
8. Determining Rate Orders from Experimental Data | 从实验数据确定反应级数
Kinetics problems in Unit 2 require you to deduce the orders of reaction with respect to individual reactants using the method of initial rates. The Insert normally gives no kinetic data, but the question supplies a table of initial concentrations and initial rates.
单元 2 的动力学问题要求你利用初始速率法推断各反应物的反应级数。数据表通常不给动力学数据，但题目会提供一张包含初始浓度和初始速率的表格。
For a rate equation: rate = k [A]ᵐ [B]ⁿ, compare two experiments where the concentration of only one reactant changes. For instance, if doubling [A] doubles the rate while [B] stays constant, then m = 1. If doubling [A] quadruples the rate, m = 2. If changing [A] has no effect, m = 0.
对于速率方程 rate = k [A]ᵐ [B]ⁿ，应比较仅有一个反应物浓度改变的两个实验。例如，固定 [B] 不变，若 [A] 加倍使速率也加倍，则 m = 1；若 [A] 加倍使速率变成 4 倍，则 m = 2；若 [A] 改变而速率不变，则 m = 0。
Once the orders are found, substitute the values from any experiment to calculate the rate constant k. Pay attention to the units of k, which depend on the overall order. In Unit 2, you are typically expected to state the units: for first order, k has units s⁻¹; for second order, dm³ mol⁻¹ s⁻¹, etc.
一旦确定了级数，代入任意一组实验数据即可求出速率常数 k。注意 k 的单位取决于总反应级数。单元 2 通常要求同学们能给出单位：一级反应 k 的单位为 s⁻¹；二级反应 k 的单位为 dm³ mol⁻¹ s⁻¹，以此类推。
- Always pair experiments where only one concentration varies. If more than one varies, you may need to use one of them as a reference after calculating one order first.
- 务必选取仅有一个浓度改变的实验对进行对比。如果多个浓度同时改变，可能需要先求出一个级数，再以此为基准进行后续计算。
- Write the full rate equation only after confirming the orders, and include units for k.
- 务必在确认级数后再写出完整的速率方程，并标出 k 的单位。
9. Redox Titration Calculations | 氧化还原滴定计算
Redox titrations appear in Unit 2 and often involve the use of potassium manganate(VII) or sodium thiosulfate. The Insert may provide molar masses for some reagents, and the periodic table helps you calculate others. Knowing the stoichiometric ratio is the core of the calculation.
氧化还原滴定是单元 2 的常客，常涉及高锰酸钾或硫代硫酸钠。数据表可能给出某些试剂的摩尔质量，周期表则帮助你计算其他物质的摩尔质量。掌握化学计量比是计算的核心。
A typical question: ‘25.0 cm³ of a solution of iron(II) sulfate was acidified and titrated with 0.0200 mol dm⁻³ KMnO₄. The average titre was 23.50 cm³. Calculate the concentration of Fe²⁺.’ The balanced half-equations give the overall ratio: 5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O, so 1 mol MnO₄⁻ reacts with 5 mol Fe²⁺.
典型题目：“取 25.0 cm³ 硫酸亚铁溶液，酸化后用 0.0200 mol dm⁻³ KMnO₄ 滴定，平均滴定体积为 23.50 cm³。计算 Fe²⁺ 的浓度。” 配平后的半反应方程式给出总计量关系：5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O，因此 1 mol MnO₄⁻ 与 5 mol Fe²⁺ 反应。
Step 1: calculate moles of MnO₄⁻ used = (23.50 / 1000) × 0.0200. Step 2: moles of Fe²⁺ in 25.0 cm³ = 5 × moles of MnO₄⁻. Step 3: concentration of Fe²⁺ = moles / (25.0/1000). The key is to remember the 5:1 ratio and to convert volumes to dm³.
第一步：计算所用 MnO₄⁻ 的物质的量 = (23.50 / 1000) × 0.0200。第二步：25.0 cm³ 溶液中 Fe²⁺ 的物质的量 = 5 × MnO₄⁻ 的物质的量。第三步：Fe²⁺ 浓度 = 物质的量 / (25.0/1000)。关键在于牢记 5:1 的比例，并将体积换算为 dm³。
Other redox titrations involve iodine-sodium thiosulfate: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. Here the ratio is 1:2. The Insert supplies relevant relative atomic masses if you need to calculate an unknown molar mass from titration data.
其他氧化还原滴定涉及碘与硫代硫酸钠：I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻，计量比为 1:2。如果需要从滴定数据推算未知物的摩尔质量，数据表可提供相关的相对原子质量。
10. Combining Moles, Masses and Enthalpy | 摩尔、质量与焓变的综合计算
Some of the trickiest questions in the June 2019 paper involve multi-step calculations that link moles, mass, concentration, and enthalpy. For instance, you might have to calculate the mass of a fuel needed to raise the temperature of a given volume of water by a certain amount, using the ΔHc⁰ from the Insert and the calorimetry equation.
2019 年六月试卷中某些最棘手的题目涉及多步计算，将摩尔、质量、浓度和焓变串联在一起。例如，你可能需要利用数据表中的 ΔHc⁰ 和量热方程，计算将一定体积的水加热指定温度所需的燃料质量。
Approach: first use q = mcΔT to find the energy needed (in J). Convert to kJ. From the Insert, ΔHc⁰ is negative; use its magnitude (without sign) to find moles of fuel required: n = energy required / |ΔHc⁰|. Finally, multiply n by M (from the Insert) to obtain the mass. Always check that units cancel correctly — energy should be in kJ if ΔHc⁰ is in kJ mol⁻¹.
解题思路：先用 q = mcΔT 求出所需能量（J），换算为 kJ。数据表中 ΔHc⁰ 为负值；取绝对值（不计符号）求得所需燃料的物质的量：n = 所需能量 / |ΔHc⁰|。最后，将 n 乘以数据表上的摩尔质量 M，即得质量。务必检查单位能否正确抵消——若 ΔHc⁰ 以 kJ mol⁻¹ 给出，能量就应为 kJ。
Similarly, you might need to link a titration result to an enthalpy change, by first finding the concentration of a reactant, then the number of moles that reacted, and finally the heat change per mole. Breaking the problem into clear stages — moles, energy, scaling — ensures you don’t get lost.
类似地，也可能需要将滴定结果与焓变联系起来：先求出某反应物的浓度，再算出实际反应的物质的量，最后得到每摩尔的热量变化。把问题拆解为摩尔、能量、比例换算等清晰的阶段，就能保证思路清晰。
11. Avoiding Common Pit
Published by TutorHao | AS Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导