Disclaimer:
The exact reagents, reactions, processes and equipment used in practical activities may vary between schools, colleges, and exam board exemplar methods. The essential techniques, skills, and learning objectives remain the same. Always follow the instructions, risk assessments, and safety guidance provided by your teacher or centre.

PRACTICAL 2: Measurement of an Enthalpy Change

Aim:
To measure the enthalpy change (ΔH) for the dissolution of anhydrous and hydrated copper(II) sulfate using calorimetry, and apply Hess’s Law to determine the enthalpy of hydration.

Overview:
The experiment involves recording temperature changes when dissolving anhydrous and hydrated copper(II) sulfate in water, using calorimetric data to calculate ΔH values, and applying Hess’s Law to obtain the enthalpy of hydration.

Essential Ideas:

Indirect determination of enthalpy change via Hess’s Law.
Measurement of temperature changes during dissolving processes.
Use of calorimetry and specific heat capacity calculations.

Hess’s Cycle

This required practical is based around the following Hess Cycle for dissolving anhydrous and hydrated copper (II) sulfate.

ΔH₃ = ΔH₁ – ΔH₂

where:

ΔH₃ is the hydration of copper(II) sulfate
ΔH₁ is the dissolving of anhydrous CuSO₄
ΔH₂ is the dissolving of hydrated CuSO₄

Apparatus and Chemicals

Apparatus:

Anhydrous copper(II) sulfate powder
Hydrated copper(II) sulfate crystals
Two polystyrene cups with lids
250 cm³ or 400 cm³ beaker (for holding cups)
Stand and clamp
Thermometer (0°C to 50°C, glass or digital)
Two 25 cm³ measuring cylinders
Two weighing bottles
Stopwatch
Graph paper
Stirrer
Distilled or deionised water
Digital balance (2 or 3 decimal places)

Experiment 1 – Determination of ΔH₁ (dissolving anhydrous CuSO₄)

AQA A-Level Chemistry calorimetry setup and temperature versus time method for determining enthalpy change

Weigh 3.90–4.10 g of anhydrous copper(II) sulfate into a dry weighing bottle. Record the exact mass.
Construct a results table for temperature readings at 1-minute intervals (up to 15 minutes).
Measure 25 cm³ of deionised water into a polystyrene cup. Record the starting temperature.
Begin timing (t = 0), then record the temperature every minute.
At minute 4, quickly add the anhydrous CuSO₄ and stir continuously.
Do not record temperature at the moment of addition.
Resume recording temperatures from minute 5 onward.
Plot temperature vs. time. Draw two lines of best fit (pre- and post-addition) and extrapolate to t = 4 min to determine ΔT.

Experiment 2 – Determination of ΔH₂ (dissolving hydrated CuSO₄)

Weigh 6.20–6.30 g of hydrated copper(II) sulfate into a dry weighing bottle. Record the exact mass.
Prepare a table for 1-minute temperature recordings (as in Experiment 1).
Measure 24 cm³ of deionised water into a polystyrene cup (adjusted to match water content in hydrated crystals).
Repeat the same procedure as Experiment 1 using hydrated copper(II) sulfate.
Plot and analyse the temperature change graph to determine ΔT.

Calculating Enthalpy Change (ΔH₁ and ΔH₂)

Use:
q = mcΔT

Where:
q = heat energy (J)
m = mass of water (25 g or 24 g)
c = specific heat capacity of water = 4.18 J g⁻¹ K⁻¹
ΔT = temperature change in K

Convert q to kJ (÷ 1000)
Then calculate:
ΔH = q / moles of solute (CuSO₄)

Finally:
ΔH₃ = ΔH₁ – ΔH₂
This is the enthalpy change for:
CuSO₄(s) + aq → CuSO₄·5H₂O(s)

Safety Notes

Avoid skin contact with copper(II) sulfate (both forms are irritants).
Use a dry container to weigh the solids.
Dispose of copper solutions as directed by teacher or technician.

Sources of Error

Heat loss to surroundings: Minimise by using a lid and insulating the cup.
Incomplete dissolution: Stir thoroughly.
Inaccurate temperature reading: Use digital thermometer for precision.
Delay in temperature recording: Ensure consistent timing.