A-Level Chemistry Practical Handbook 4.2: Measuring Enthalpy Change | A-Level化学实验手册4.2：反应焓变测定

📚 A-Level Chemistry Practical Handbook 4.2: Measuring Enthalpy Change | A-Level化学实验手册4.2：反应焓变测定

Enthalpy change (ΔH) is a fundamental concept in thermochemistry, representing the heat energy change at constant pressure. In A-Level Chemistry Practical 4.2, you will determine the enthalpy change for a simple displacement reaction, such as zinc reacting with copper(II) sulfate solution, using a basic calorimeter. This experiment teaches you essential skills in temperature measurement, data recording, and calculation of energy transfers.

焓变（ΔH）是热化学的一个基本概念，表示恒压下的热能量变化。在A-Level化学实验4.2中，你将使用简易量热计测定一个简单置换反应（例如锌与硫酸铜溶液反应）的焓变。本实验将训练你温度测量、数据记录和能量转换计算的基本技能。

1. Concept of Enthalpy Change | 焓变概念

Enthalpy (H) is the heat content of a system. The enthalpy change, ΔH, is the heat absorbed or released during a reaction at constant pressure. Exothermic reactions release heat, resulting in a negative ΔH (e.g., combustion, neutralisation, metal displacement). Endothermic reactions absorb heat, giving a positive ΔH (e.g., dissolving ammonium nitrate). The standard molar enthalpy change, ΔH°, is measured under standard conditions (100 kPa, 298 K, and all solutions at 1 mol dm⁻³).

焓（H）是系统的热含量。焓变ΔH是恒压下反应吸收或释放的热量。放热反应释放热量，ΔH为负值（例如燃烧、中和、金属置换）；吸热反应吸收热量，ΔH为正值（例如溶解硝酸铵）。标准摩尔焓变ΔH°是在标准条件下（100 kPa、298 K，所有溶液浓度1 mol dm⁻³）测量得到的。

2. Principle of Measurement | 测量原理

To measure ΔH for a reaction, we use a simple calorimeter, such as a polystyrene cup with a lid. The heat evolved or absorbed by the reaction (q in joules) is calculated from the temperature change (ΔT) of the solution using the equation:

q = m c ΔT

其中m是溶液质量（通常近似为水的密度1 g cm⁻³，因此体积数值与质量数值相等），c是比热容（水溶液通常取4.18 J g⁻¹ K⁻¹），ΔT是温度变化。

Then, the enthalpy change per mole of limiting reactant is found by ΔH = –q / n, where n is the amount in moles of the limiting reactant. The negative sign indicates that heat lost by the reaction (exothermic) corresponds to a negative enthalpy change for the system.

然后，每摩尔限量反应物的焓变通过ΔH = –q / n计算，其中n是限量反应物的物质的量。负号表示反应放出的热量（放热）对应系统的焓变为负值。

3. Apparatus and Chemicals | 仪器与试剂

For the zinc–copper sulfate displacement reaction, the following apparatus and chemicals are required:

• Polystyrene cup with lid (calorimeter)
• Thermometer (0 – 110 °C, precision ±0.5 °C or better)
• Measuring cylinder (50 cm³)
• Weighing balance (±0.01 g)
• Zinc powder (excess, about 3 g)
• Copper(II) sulfate solution, 1.0 mol dm⁻³ (50 cm³)
• Stirring rod
• Stopclock or timer

实验所需仪器和试剂：

• 带盖聚苯乙烯杯（量热计）
• 温度计（0–110 °C，精度±0.5 °C或更高）
• 量筒（50 cm³）
• 分析天平（±0.01 g）
• 锌粉（过量，约3 g）
• 硫酸铜溶液，1.0 mol dm⁻³（50 cm³）
• 搅拌棒
• 秒表

4. Procedure – Calibration and Initial Temperature | 操作步骤 – 校准与初温

Rinse the polystyrene cup

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