AS Chemistry Unit 4 Insert (Jun19) – Experimental Techniques | AS 化学 第四单元插入页 (Jun19) – 实验操作

📚 AS Chemistry Unit 4 Insert (Jun19) – Experimental Techniques | AS 化学 第四单元插入页 (Jun19) – 实验操作

The June 2019 AS Chemistry Unit 4 insert provided a detailed calorimetry experiment for determining the enthalpy change of neutralisation between hydrochloric acid and sodium hydroxide. This article breaks down the experimental steps, data handling, and common pitfalls shown in that insert, helping students master the practical technique and the associated calculations.

2019年6月的AS化学第四单元插入页给出了一个详细的中和反应量热实验，用于测定盐酸与氢氧化钠反应的焓变。本文拆解该插入页展示的实验步骤、数据处理和常见陷阱，帮助学生掌握这项实用技术以及相关计算。

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1. Introduction to the Insert | 插入页介绍

The insert, labelled ‘Unit 4: Energy and Kinetics’, presented a simple coffee-cup calorimetry experiment. It included a diagram of the apparatus, a stepwise procedure, and a results table for mixing 50.0 cm³ of 2.00 mol dm⁻³ HCl with 50.0 cm³ of 2.00 mol dm⁻³ NaOH, recording the maximum temperature reached.

这份标注为”第四单元：能量与动力学”的插入页展示了一个简单的咖啡杯量热实验。它包含装置图、分步操作流程，以及一份将50.0 cm³ 2.00 mol dm⁻³盐酸与50.0 cm³ 2.00 mol dm⁻³氢氧化钠混合并记录最高温度的结果表格。

The purpose was to calculate the standard enthalpy change of neutralisation, ΔHₙₑᵤᵗ°, and to assess the accuracy of the method. Students were expected to extract data, apply the formula q = m × c × ΔT, and relate the heat evolved to the amount of water formed.

实验目的是计算标准中和焓变 ΔHₙₑᵤᵗ°，并评估方法的准确度。要求学生提取数据，应用公式 q = m × c × ΔT，并将释放的热量与生成水的物质的量联系起来。

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2. Experimental Setup | 实验装置

The insert showed a polystyrene cup resting in a glass beaker for support, with a lid made of card or foil. A thermometer was placed through a hole in the lid, reaching the liquid but not touching the bottom. A stirring rod or magnetic stirrer was used to ensure even heat distribution.

插入页显示了一个放在烧杯中支撑的聚苯乙烯杯，杯上有纸板或箔片制成的盖子。温度计从盖子的小孔插入，浸入液体但不触碰杯底。使用搅拌棒或磁力搅拌器确保热量分布均匀。

Polystyrene cups are chosen for their low heat capacity and good insulating properties, minimising heat loss to the surroundings. The lid further reduces evaporative cooling and prevents splashing during mixing.

选用聚苯乙烯杯是因为其热容低且隔热性好，最大限度地减少向环境散失热量。盖子进一步减少蒸发冷却并防止混合时液体溅出。

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3. Safety Precautions | 安全注意事项

Even though the reagents are dilute, both hydrochloric acid and sodium hydroxide are corrosive and can cause eye damage and skin burns. The insert reminded students to wear chemical-splash goggles, lab coat, and nitrile gloves throughout the experiment.

尽管试剂是稀溶液，盐酸和氢氧化钠仍具有腐蚀性，可造成眼部损伤和皮肤灼伤。插入页提醒学生在整个实验过程中佩戴防化学飞溅护目镜、实验服和丁腈手套。

Any spills should be wiped immediately with a damp cloth, and hands must be washed after handling. The stirrer should be clean and dry before use to avoid contamination that could affect the reaction.

任何溅出液应立即用湿布擦拭，实验后必须洗手。搅拌棒在使用前应清洁干燥，避免污染影响反应。

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4. Procedure Steps | 操作步骤

The insert instructed to measure 50.0 cm³ of 2.00 mol dm⁻³ NaOH using a burette or a volumetric pipette and transfer it into the polystyrene cup. The initial temperature of the NaOH solution was recorded every 30 seconds for 3 minutes to establish a baseline.

插入页要求用滴定管或移液管量取50.0 cm³ 2.00 mol dm⁻³ NaOH溶液，转移至聚苯乙烯杯。每30秒记录一次NaOH溶液的初始温度，持续3分钟以建立基线。

Then 50.0 cm³ of 2.00 mol dm⁻³ HCl at the same room temperature was added rapidly but carefully. The mixture was stirred continuously, and the temperature was monitored every 15 seconds for another 5 minutes. The maximum temperature reached was noted.

然后迅速但小心地加入相同室温的50.0 cm³ 2.00 mol dm⁻³ HCl。持续搅拌混合物，并在接下来的5分钟内每15秒监测一次温度。记录达到的最高温度。

On the insert’s graph grid, students would plot temperature against time, extrapolate the cooling curve back to the time of mixing, and read the corrected maximum temperature rise ΔT.

在插入页的坐标格纸上，学生需绘制温度-时间图，将冷却曲线外推至混合瞬间，读取校正后的最大温升 ΔT。

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5. Data Recording | 数据记录

The results table in the insert listed time intervals and corresponding temperatures. For the NaOH solution, the readings showed a steady 20.5 °C. After HCl addition, the temperature rose rapidly to a peak of 29.2 °C at 1.5 minutes, then slowly decreased.

插入页的结果表格列出了时间间隔和对应的温度。NaOH溶液读数稳定在20.5 °C。加入HCl后，温度迅速上升，在1.5分钟时达到29.2 °C的峰值，随后缓慢下降。

Using the extrapolation method, the theoretical instantaneous temperature rise was determined to be from 20.5 °C to 30.0 °C, giving a corrected ΔT of 9.5 °C. This correction compensates for the heat lost to the environment during the reaction.

采用外推法，确定理论瞬间温度从20.5 °C升至30.0 °C，校正后的 ΔT 为9.5 °C。这一校正补偿了反应过程中散失到环境中的热量。

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6. Calculation of Heat Energy (q) | 热量计算 (q)

The heat energy transferred to the solution is calculated using the equation:

传递给溶液的热量用以下公式计算：

q = m × c × ΔT

where m is the total mass of the solution, c is the specific heat capacity of the solution, and ΔT is the corrected temperature change. The insert assumed c = 4.18 J g⁻¹ K⁻¹, the value for water.

其中 m 是溶液的总质量，c 是溶液的比热容，ΔT 是校正后的温度变化。插入页假定 c = 4.18 J g⁻¹ K⁻¹，即水的数值。

Since the density of the dilute solution is roughly 1.00 g cm⁻³, the total mass of the mixed solution (100.0 cm³) is 100.0 g. Therefore:

由于稀溶液的密度约为1.00 g cm⁻³，混合溶液（100.0 cm³）的总质量为100.0 g。因此：

q = 100.0 g × 4.18 J g⁻¹ K⁻¹ × 9.5 K = 3971 J

This value represents the heat released by the reaction, so q = -3971 J (or -3.971 kJ) for an exothermic process.

该值代表反应释放的热量，因此对于放热过程，q = -3971 J（或 -3.971 kJ）。

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7. Determining Moles of Reaction | 确定反应的摩尔数

The neutralisation reaction is: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l). The limiting reactant determines the amount of water formed. The insert provided volumes and concentrations: 50.0 cm³ × 2.00 mol dm⁻³ gives 0.100 mol of NaOH and 0.100 mol of HCl.

中和反应为：NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)。限制试剂决定生成水的物质的量。插入页给出了体积和浓度：50.0 cm³ × 2.00 mol dm⁻³ 得到0.100 mol NaOH和0.100 mol HCl。

Since they react in a 1:1 mole ratio, both are completely consumed, producing exactly 0.100 mol of water. This is the ‘n’ used in the enthalpy change formula.

由于以1:1的摩尔比反应，两者完全消耗，恰好生成0.100 mol水。这就是焓变公式中使用的 ‘n’。

If either reactant were in excess, the number of moles of the one not in excess would be used. The insert data ensures an exact stoichiometric mixture to simplify the calculation.

如果任一反应物过量，则使用不足量的反应物的物质的量。插入页的数据确保精确的化学计量混合，以简化计算。

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8. Calculating Enthalpy Change (ΔH) | 计算焓变 (ΔH)

The enthalpy change per mole of water formed is given by:

每摩尔生成水的焓变由下式给出：

ΔH = -q / n

Substituting the values from the insert:

代入插入页的数值：

ΔH = -(-3.971 kJ) / 0.100 mol = -39.71 kJ mol⁻¹

However, typical experimental values are around -57 kJ mol⁻¹. The lower magnitude obtained here indicates significant heat loss, but the insert’s purpose is to teach the calculation process rather than achieve a text-book value.

然而，典型的实验值约为 -57 kJ mol⁻¹。此处得到的数值偏低，表明热量损失较大，但插入页的目的是教授计算过程，而非取得教科书数值。

Students should present the final answer to an appropriate number of significant figures (e.g., -39.7 kJ mol⁻¹) and include the negative sign to show the reaction is exothermic.

学生应以合适的有效数字位数呈现最终答案（如 -39.7 kJ mol⁻¹），并包含负号以表明反应放热。

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9. Evaluating Errors | 误差评估

The insert often asks students to identify sources of error. Heat loss to the polystyrene cup, the lid, and the thermometer is unavoidable but can be reduced. The extrapolation in the temperature-time graph helps correct for this, but it is not perfect.

插入页常要求学生找出误差来源。向聚苯乙烯杯、盖子和温度计的热量散失不可避免，但可以降低。温度-时间图的外推有助于校正，但并不完美。

Other errors include the assumption that the specific heat capacity of the solution equals that of pure water, and that the density is exactly 1.00 g cm⁻³. These simplifications introduce a small systematic error.

其他误差包括假设溶液的比热容等于纯水的比热容，以及密度恰好为1.00 g cm⁻³。这些简化引入了小的系统误差。

Parallax errors when reading the thermometer and volumetric glassware, and delays in adding the acid, can affect ΔT. Incomplete mixing or heat from the stirrer motor may also contribute.

读取温度计和容量玻璃仪器时的视差，以及加酸延迟，都会影响 ΔT。混合不充分或搅拌器马达产生的热量也可能造成影响。

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10. Suggested Improvements | 改进建议

To increase accuracy, a temperature sensor and data logger could replace the mercury thermometer, giving continuous digital readings and reducing reading errors. A vacuum-flask calorimeter or a double polystyrene cup can further minimise heat loss.

为提高准确性，可用温度传感器和数据记录仪代替水银温度计，进行连续数字读数并减少读数误差。真空烧瓶量热计或双层聚苯乙烯杯可进一步减少热量损失。

The insert suggested pre-warming both solutions to the same initial temperature and rinsing the transfer vessel with a small amount of the solution to avoid dilution errors. Conducting the experiment in a draft-free environment also helps.

插入页建议将两种溶液预热至相同的初始温度，并用少量溶液润洗转移容器以避免稀释误差。在无气流通风的环境中进行实验也有帮助。

Using a lid made of insulating foam and ensuring the thermometer is calibrated before use are simple, cost-effective improvements that boost reliability.

使用隔热泡沫制成的盖子，并确保温度计在使用前经过校准，都是简单经济的改进措施，可提高可靠性。

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11. Interpreting the Insert’s Data Table | 解读插入页数据表

The insert’s table typically had three columns: Time (s), Temperature (°C) for NaOH alone, and Temperature of mixture after HCl addition. At time zero (mixing), there was a blank cell which students had to fill by extrapolation.

插入页表格通常有三栏：时间（秒）、单独NaOH的温度（°C）、加入HCl后混合物的温度。在时间零点（混合瞬间）有一个空白单元格，学生必须通过外推填写。

By plotting the cooling portion of the graph (after the peak) and drawing a straight line of best fit back to the mixing time, the true ΔT could be determined. The insert tested this graphical skill explicitly.

通过绘制图形冷却部分（峰值后）并绘制最佳拟合直线回推到混合时间，即可确定真实的 ΔT。插入页明确考查了这一图形技能。

Missing the extrapolation step and using the raw peak temperature would give a smaller ΔT, leading to a less exothermic enthalpy value. This was a common error highlighted in examiners’ reports.

忽略外推步骤而直接使用原始峰值温度，会得到较小的 ΔT，导致焓变值偏低。这是考官报告中强调的常见错误。

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12. Summary | 总结

The June 2019 AS Chemistry Unit 4 insert offered a classic calorimetry scenario that integrates practical technique, data analysis, and thermodynamic calculations. Mastering the steps from accurate reagent measurement to graph plotting and final ΔH calculation is essential for high marks.

2019年6月的AS化学第四单元插入页提供了一个经典的量热场景，融合了实用技术、数据分析和热力学计算。掌握从精确试剂测量到图形绘制以及最终 ΔH 计算的各个环节，对于取得高分至关重要。

By carefully studying this insert, students reinforce their understanding of exothermic enthalpy changes, the importance of heat-loss correction, and the evaluation of experimental procedures – skills directly transferable to other thermochemistry investigations.

通过仔细研究这份插入页，学生可以加深对放热焓变、热量损失校正重要性以及实验方案评估的理解——这些技能可直接迁移到其他热化学研究中。

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