Introduce your students to Hess’s Law as a means of measuring enthalpy changes using this lesson plan with a simple experiment for 16–18 year olds

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.

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

Learning objectives

Students will:

  • 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.

Sequence of activities


  1. Introduce the activity by saying that although we can measure the enthalpy change of combustion for fuels, other enthalpy changes cannot be measured directly.
  2. Ask students for examples, eg the energy released when we eat food.
  3. Mention other reactions where chemists cannot really control what is happening.

Activity: stage 1

  1. Give each student the worksheet ‘How can enthalpy changes be measured?’
  2. Organise the students into pairs.
  3. Introduce the first activity – to test what happens when water is added to anhydrous copper(II) sulfate.
  4. Check that the students observed that heat is released.
  5. Pose questions to develop ideas, such as:
    • Is the enthalpy change for this reaction exothermic or endothermic?
    • What sign should the enthalpy change have?
    • What is the equation for this reaction?
    • What bonds are being broken and formed in this reaction?
    • Why is it difficult to measure this enthalpy change directly?
    • What is this enthalpy change called?
  6. Share the learning objectives at this point.

Key points

Explain the following 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.

Activity: stage 2

Support and supervise as students:

  1. Collect data according to the worksheet.
  2. Calculate the enthalpy changes.
  3. Apply Hess’s Law to find the overall enthalpy change of copper(II) sulfate solution.


In a plenary, use 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?


Take in the accounts of the experiment and give written feedback. 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.


Teacher questioning provides a structured approach to revealing what the students understand of the topic. Student participation in experimental work grounds the questions in reality, giving the opportunity to consolidate learning from the preliminary test, but again, questions are important to gauge understanding.

Written feedback allows the teacher to confirm correct ideas and to indicate how points of weakness can be supported.

Practical notes


For each pair of students:

  • Weighing boat (or similar)
  • Polystyrene cup
  • Thermometer reading 0–110 °C in 0.1 °C increments
  • Pipette or measuring cylinder, 50 cm3
  • Access to a balance weighing to 0.01 g
  • Test tubes, one or two
  • Test tube rack


For each pair of students:

  • Anhydrous copper(II) sulfate powder (TOXIC if ingested), about 5 g, plus a separate small amount of excess to test
  • Hydrated copper(II) sulfate crystals (TOXIC if ingested), about 7 g

Health, safety and technical notes

Principal hazard

  • The harmful nature of the copper salts.


Writing a summary

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.