How can enthalpy changes be measured?

In this session, students carry out an experiment to observe what happens when water is added to anhydrous copper(II) sulfate, and when hydrated copper(II) sulfate is dissolved in water. This is used to introduce them to Hess’s Law and its application. Find guidance and more ideas in How to teach enthalpy cycles post-16.

This activity can follow on from the combustion of fuels experiment used for exploring where energy comes from. 

Learning objectives

• Recognise that some enthalpy changes can’t be measured directly.
• Know that Hess’s Law shows that whichever route is taken to a product, the overall enthalpy change is the same.

The teacher notes contain a full teaching sequence, inlcuding prompts for questions and commentary. 

What is Hess’s law?

Hess’s law says that the enthalpy change for a reaction will be the same regardless of the route taken.

Imagine a chemical journey from A → B, the enthalpy change will be the same even if you were to get there going through C, D and/or E!

Practical activities

The equipment list, preparation, safety information and disposal instructions for the practical activities are included in the technician notes. The student sheet provides step-by-step instructions for each activity. Read our standard health and safety guidance and carry out a risk assessment before running any live practical.

Learners must:

• Wear safety glasses throughout.
• Avoid skin contact with the solids or solutions.
• Not exceed the amounts stated.

Activity: what happens when water is added to anhydrous copper(II) sulfate?

Equipment

• Safety glasses
• Test tube
• Test tube rack
• Spatula
• Pipette
• Small beaker containing water
• Anhydrous copper(II) sulfate powder (DANGER: corrosive, irritant, environmental hazard)

Method

Wear eye protection.

1. Use a spatula to put a small amount (thumbnail covering) of anhydrous copper(II) sulfate into a test tube.
2. Add water drop by drop.
3. Carefully touch the outside of the test tube.

Key points

• It is difficult to measure the enthalpy change of hydration accurately in a direct way because the hydration process can’t be controlled directly.
• Instead, anhydrous and hydrated copper(II) sulfate can be dissolved in water.
• These enthalpy changes can be measured easily.
• Each ‘route’ produces a solution of hydrated copper(II) sulfate.
• Hydrated copper(II) sulfate has some water in the structure already. By dissolving the two salts and determining the enthalpy changes, we can work out the enthalpy change of hydration.

Experiment: an enthalpy change you cannot measure directly

Equipment

• Anhydrous copper(II) sulfate powder (DANGER: corrosive, irritant, environmental hazard)
• Hydrated copper(II) sulfate crystals (DANGER: corrosive, irritant, environmental hazard)
• Spatula
• Weighing boat (or similar)
• Insulated cup
• Beaker
• Thermometer reading 0–110°C in 0.1°C increments
• Water
• 50 cm³ pipette or measuring cylinder
• Access to a mass balance
• Eye protection

Method

Wear eye protection.

• Calculate the mass of 0.025 moles of anhydrous copper(II) sulfate.
• Use a spatula to measure out this mass into a weighing boat.
• Stand the cup in the beaker. Measure out 50 cm³ of water into the cup.
• Record the temperature of the water in the cup.
• Add the copper(II) sulfate to the water and stir with the thermometer.
• Record the highest temperature reached.
• Repeat these steps with hydrated copper(II) sulfate.

How do you work out enthalpy change?

1. Calculate the energy released. 

Energy (Joules) = 50 x 4.2 x temperature rise

2. Divide the energy figure by the number of moles.

3. Write equations with state symbols for the enthalpy changes you have measured. Write their enthalpy changes alongside them.

Questions to develop thinking

• What is the equation for the enthalpy change we want to measure?
• What are the enthalpy changes for the two reactions we have measured?
• How can we make a triangle between these reactions?
• Why do both routes to the copper(II) sulfate solution involve the same amount of energy?
• Why is it useful to be able to measure enthalpy changes?

The assessment for learning approach

Asking lots of questions provides a structured approach to revealing what your learners understand of the topic. The experimental work grounds the questions in reality, giving the opportunity to consolidate learning from the preliminary test, but questions are important to gauge understanding.

Written feedback will confirm correct ideas and will indicate whether learners need any additional support.

Answers for writing a summary

When evaluating learners’ summaries, pay particular attention to:

• Understanding why the enthalpy change cannot be measured directly.
• Understanding the idea of the two routes to the same substance.
• The application of Hess’s law to find the enthalpy change of hydration.

Points to look for:

• The enthalpy change is the ‘enthalpy change of hydration’.
• It can’t be measured directly as we cannot physically measure how much energy is released when five moles of water react with one mole of copper(II) sulfate.
• Both routes end up with a solution of copper(II) sulfate. The difference between the two routes is the enthalpy change of hydration.
• A typical value for the enthalpy change is of the order of –5 kJ mol^-1.