Measurement of an Enthalpy Change (Oxford AQA International A Level Chemistry)

Required Practical 2: Measuring Enthalpy Change

Objective

To determine the enthalpy change for the conversion of anhydrous copper(II) sulfate into hydrated copper(II) sulfate, ΔH₃

This enthalpy change cannot be measured directly but can be calculated from:

• The enthalpy change for the conversion of anhydrous copper sulfate to copper sulfate solution, ΔH₁

• The enthalpy change for the conversion of hydrate copper sulfate to copper sulfate solution, ΔH₂

Apparatus

• Hydrated copper(II) sulfate crystals

• Anhydrous copper(II) sulfate powder

• Polystyrene cups with lids

• 250 cm³ for holding polystyrene cup

• Stand and clamp

• Thermometer

• 25 cm³ measuring cylinders x 2

• Weighing bottles x 2

• Stopwatch

• Graph paper

• Stirrer

• Distilled water

• Digital balance

Method

Experiment 1: Determining ΔH₁

1. Weigh out between 3.90 g and 4.10 g of anhydrous copper(II) sulfate in a weighing bottle and record the mass

2. Measure 25 cm³ of deionised water into a polystyrene cup using a measuring cylinder

3. Record its temperature at the beginning (t=0)

4. Start the timer and record the temperature every minute for a further three minutes

5. At the fourth minute, add the powdered anhydrous copper(II) sulfate rapidly to the polystyrene cup and continue to stir, but do not record the temperature.

6. At the fifth minute and for every minute up to 15 minutes, stir and record the temperature of the solution in the polystyrene cup.

Experiment 2: Determining ΔH₂

1. Weigh out between 6.20 g and 6.30 g of hydrated copper(II) sulfate in a weighing bottle and record the mass

2. Measure 24 cm³ of deionised water into a polystyrene cup using a measuring cylinder

3. Record its temperature at the beginning (t=0)

4. Start the timer and record the temperature every minute for a further three minutes

5. At the fourth minute, add the powdered hydrated copper(II) sulfate rapidly to the polystyrene cup and continue to stir, but do not record the temperature.

6. At the fifth minute and for every minute up to 15 minutes, stir and record the temperature of the solution in the polystyrene cup.

How to set up your equipment

Practical Tip

Be careful not to get the anhydrous copper(II) on your skin as it can cause irritation.

Results

Record your results in a table like this:

The temperature is not recorded at minute 4 as this is when you have added the hydrated / anhydrous copper(II) sulfate.

Evaluation

For each set of results:

1. Plot a graph of temperature (on the y-axis) against time (on the x-axis)

2. Draw two separate best fit lines; one, which joins the points before the addition of hydrated / anhydrous copper(II) sulfate, and one which joins the points after the addition

3. Extrapolate both lines to the fourth minute

4. Use your graph to determine the temperature change at the fourth minute

5. Use q = m x c x ΔT and ΔH = q/ n to determine ΔH₁ and ΔH₂

6. Determine the enthalpy change for ΔH₃ using:

   ΔH₃ = ΔH₁ – ΔH₂