📚 AQA A Level Practical Handbook Chemistry 4.2: Measuring an Enthalpy Change | AQA A水平化学实验手册4.2：测量焓变
Section 4.2 of the AQA A Level Chemistry practical handbook focuses on Required Practical 2, which asks students to measure an enthalpy change using a simple calorimeter.
AQA A Level 化学实验手册第 4.2 节对应必修实验 2，要求学生使用简易量热计测量焓变。
This practical builds core skills in temperature measurement, heat energy calculation and error analysis, and it is frequently assessed in written papers.
该实验培养温度测量、热能计算和误差分析等核心技能，也是笔试中的常见考查内容。
1. What Section 4.2 Covers | 4.2节涵盖内容
Section 4.2 covers the measurement of an enthalpy change in solution using a simple expanded polystyrene cup calorimeter.
第 4.2 节涵盖利用简易聚苯乙烯泡沫杯量热计测量溶液中的焓变。
The most common reactions studied are neutralisation of a strong acid with a strong base, or the enthalpy change of solution of an ionic solid.
最常研究的反应包括强酸与强碱的中和反应，或离子固体的溶解焓变。
You need to record temperature changes accurately, calculate the heat energy transferred, and convert this into an enthalpy change per mole of limiting reactant.
你需要准确记录温度变化，计算传递的热能，并将其转换为每摩尔限量反应物的焓变。
2. Core Principle and Enthalpy Definitions | 核心原理与焓变定义
Enthalpy change, ΔH, is the heat energy transferred in a reaction at constant pressure. If the temperature rises, the reaction is exothermic and ΔH is negative.
焓变 ΔH 是恒压条件下反应中传递的热能。如果温度升高，反应放热，ΔH 为负值。
For an endothermic process, the temperature falls and ΔH is positive.
对于吸热过程，温度降低，ΔH 为正值。
is the mass of solution, c is the specific heat capacity and ΔT is the temperature change.
反应混合物吸收或放出的热能由 Q = mcΔT 计算，其中 m 为溶液质量，c 为比热容，ΔT 为温度变化。
For dilute aqueous solutions, c is taken as 4.18 J g⁻¹ K⁻¹ and the density as 1.00 g cm⁻³.
对于稀水溶液，比热容取 4.18 J g⁻¹ K⁻¹，密度取 1.00 g cm⁻³。
Q = mcΔT
ΔH = −Q / n
In this equation, n is the moles of the limiting reactant that reacted.
式中，n 是发生反应的限量反应物的物质的量。
3. Apparatus and Chemicals | 仪器与试剂
The apparatus needed for a neutralisation enthalpy measurement is simple but must be chosen to minimise heat loss.
中和焓变测量所需的仪器虽简单，但必须尽量减小热量损失。
- Expanded polystyrene cup and lid | 聚苯乙烯泡沫杯及盖子
- 250 cm³ beaker | 250 cm³ 烧杯
- Thermometer with 0.1 °C or 0.2 °C resolution | 分辨率为 0.1 °C 或 0.2 °C 的温度计
- Two 25.0 cm³ measuring cylinders or burettes | 两个 25.0 cm³ 量筒或滴定管
- Stirring rod or magnetic stirrer | 玻璃搅拌棒或磁力搅拌器
- 1.0 mol dm⁻³ hydrochloric acid | 1.0 mol dm⁻³ 盐酸
- 1.0 mol dm⁻³ sodium hydroxide | 1.0 mol dm⁻³ 氢氧化钠
- Safety goggles and lab coat | 护目镜和实验服
4. Step-by-Step Method | 分步操作步骤
Step 1: Rinse the cup, measuring cylinders and thermometer with distilled water and dry them.
步骤1：用蒸馏水冲洗泡沫杯、量筒和温度计并擦干。
Step 2: Measure 25.0 cm³ of 1.0 mol dm⁻³ HCl and transfer it to the polystyrene cup.
步骤2：量取 25.0 cm³ 的 1.0 mol dm⁻³ 盐酸并转移至聚苯乙烯杯中。
Step 3: Place the lid on the cup, insert the thermometer, and record the initial temperature of the acid.
步骤3：盖上杯盖，插入温度计，记录酸的初始温度。
Step 4: Measure 25.0 cm³ of 1.0 mol dm⁻³ NaOH and record its initial temperature separately.
步骤4：量取 25.0 cm³ 的 1.0 mol dm⁻³ 氢氧化钠溶液，并单独记录其初始温度。
Step 5: Add the NaOH to the acid in the cup, replace the lid immediately, and stir gently.
步骤5：将氢氧化钠溶液加入杯中盐酸中，立即盖好盖子，轻轻搅拌。
Step 6: Record the temperature every 30 seconds until a maximum temperature is observed.
步骤6：每 30 秒记录一次温度，直到观察到最高温度。
Step 7: Continue recording for a further 5 minutes to enable a cooling correction graph to be drawn.
步骤7：继续记录 5 分钟，以便绘制冷却校正曲线。
Step 8: Plot temperature against time and extrapolate the cooling line back to the time of mixing to estimate the true maximum temperature change.
步骤8：绘制温度-时间图，并将冷却线外推回混合时刻，以估算真实的最高温度变化。
5. Sample Data and Results Table | 示例数据与结果表
The table below shows a typical data set for the neutralisation of 25.0 cm³ of 1.0 mol dm⁻³ HCl with 25.0 cm³ of 1.0 mol dm⁻³ NaOH.
下表是用 25.0 cm³ 1.0 mol dm⁻³ HCl 与 25.0 cm³ 1.0 mol dm⁻³ NaOH 中和的一组典型数据。
| Quantity | 量 | Example value | 示例值 |
|---|---|
| Volume of HCl | 盐酸体积 | 25.0 cm³ |
| Volume of NaOH | 氢氧化钠体积 | 25.0 cm³ |
| Initial temperature of acid | 酸的初始温度 | 21.0 °C |
| Initial temperature of base | 碱的初始温度 | 21.0 °C |
| Average initial temperature | 平均初始温度 | 21.0 °C |
| Final corrected temperature | 校正后最终温度 | 27.5 °C |
| Temperature change ΔT | 温度变化 ΔT | 6.5 °C |
6. Calculation Walkthrough | 计算详解
ΔT. The total volume of solution is 25.0 cm³ + 25.0 cm³ = 50.0 cm³, so its mass is 50.0 g.
首先使用 Q = mcΔT 计算热能。溶液总体积为 25.0 cm³ + 25.0 cm³ = 50.0 cm³，因此其质量为 50.0 g。
Q = 50.0 g × 4.18 J g⁻¹ K⁻¹ × 6.5 K = 1358.5 J = 1.3585 kJ
Next calculate the moles of the limiting reactant. Both HCl and NaOH have concentration 1.0 mol dm⁻³ and volume 0.0250 dm³.
接着计算限量反应物的物质的量。HCl 和 NaOH 浓度均为 1.0 mol dm⁻³，体积均为 0.0250 dm³。
