Required Practical 2 - Measuring Enthalpy Change (AQA A-Level Chemistry): Revision Notes

4.1.3 Required Practical 2 - Measuring Enthalpy Change

Aim

To measure the enthalpy change ($ΔH$) when anhydrous copper(II) sulphate is dissolved in water, or to determine the enthalpy of combustion for a fuel. This involves monitoring the temperature change of a reaction in a calorimeter.

Equipment

Chemicals

• Anhydrous copper(II) sulphate, $CuSO₄$ (3.90 - 4.10 g)
• Deionised (or distilled) water (25 cm³)
• Alternative: fuel for combustion, e.g., ethanol

Apparatus

• Polystyrene cup (as a calorimeter)
• Thermometer or electronic temperature sensor
• Balance (2 decimal places) for weighing chemicals
• Stirrer
• Measuring cylinder (for water)
• Stopwatch or timer
• Graph paper (for data analysis)
• Weighing bottle with stopper
• Beaker (for additional insulation)
• Clamp stand (to hold the thermometer)

Risk Assessment

• Copper(II) sulfate can be harmful if ingested or if it comes into contact with skin:
  • Wear safety goggles, gloves, and a lab coat.
  • Wash hands immediately after handling chemicals.
• Handle the polystyrene cup with care to avoid spills.
• Take care when using hot solutions or handling flammable fuels:
  • Keep flammable materials away from open flames.
  • Use a fume cupboard if working with volatile substances.

Method

Method 1: Dissolving Anhydrous Copper(II) Sulphate

1. Preparation:

• Weigh between 3.90 and 4.10 g of anhydrous copper(II) sulphate using a balance.
• Record the precise mass.
• Prepare a results table to record temperatures every minute for 15 minutes.

2. Initial Measurements:

• Add 25 cm³ of deionised water to a polystyrene cup.
• Record the initial temperature at t = 0 minutes.
• Continue recording the temperature every minute for the first 3 minutes.

3. Reaction:

• At the fourth minute, quickly add the anhydrous copper(II) sulfate to the water.
• Do not record the temperature immediately.
• At the fifth minute, resume temperature recordings every minute up to the 15th minute, stirring gently.

4. Data Analysis:

• Plot a graph of temperature vs. time.
• Draw two best-fit lines: one for data before the addition of the copper(II) sulphate and one for data after.
• Extrapolate these lines to determine the temperature change at the fourth minute, representing the temperature change immediately after addition.

Method 2: Combustion of Fuel

1. Preparation:

• Set up a flame calorimeter (or use a simple metal container above a burning fuel).
• Weigh the fuel before starting.

2. Combustion:

• Ignite the fuel and monitor the temperature change in the water.
• Record the initial and final temperatures, as well as the mass of the fuel burned.

Results

Record all temperature values with 1 decimal place accuracy.

Example calculations for $ΔH$:

$$q = mc\Delta T$$

Where:

• $q$ = heat energy change (J)
• $m$ = mass of water (g)
• $c$ = specific heat capacity of water (4.18 J g⁻¹ K⁻¹)
• $\Delta T$ = temperature change (°C) Convert $q$ to kJ and use it to find enthalpy change per mole of reactant.

Errors and Improvements

• Common errors:
  • Heat loss to the surroundings.
  • Assuming solutions have the same specific heat capacity and density as water.
  • Ignoring the heat absorbed by the calorimeter.
  • Incomplete reactions or slow dissolving.
  • Room temperature fluctuations.
• Improvements:
  • Use an electronic temperature sensor for higher accuracy.
  • Employ a flame calorimeter for combustion experiments to reduce heat loss:
  • A spiral copper chimney can capture heat better.
  • Use pure oxygen instead of air for complete combustion.