Icon KEEP YOUR EYES ON THE JOULES - A Hess's Law Experiment

This thermochemistry experiment tracks the movement of heat during an experiment incorporating measurement, data collection, data analysis, and drawing conclusions.

This printable education experiment contains a detailed teachers' guide, student information and question sheets, and answer sheets for printing and distributing in the classroom.

Age Level

This experiment is designed for ages 15-18. It can be adapted for physical science or introductory chemistry classes, but it is most appropriate for advanced classes.


  1. Applying the Law of Conservation of Energy to Hess's Law.
  2. Enthalpy of a reaction (∆H) and thermochemical equations.
  3. Calculating the energy change for a reaction when the energy change cannot be measured directly.
  4. Energy diagrams for reactions.
  5. State functions.


  1. To show how the enthalpy of a reaction changes when the reaction's is modified.
  2. To apply the Law of Conservation of energy to a series of related reactions.
  3. To use an energy diagram to explain Hess's Law.
  4. To interpret Hess's Law.

Activity Overview

The heat of formation of magnesium oxide is not easily measured in the laboratory. In order to obtain the enthalpy for forming magnesium oxide, two related reactions will be conducted in the lab. Summing the equations leads to the net overall equation, 2Mg + O2 -> 2MgO. Hess's Law states that if a series of equations can be summed to give the net equation for a reaction, then the energy change associated with each equation can also be summed to give the net energy change for the overall reaction.

This lesson is designed to be easily and quickly integrated into existing curriculum. It comes complete with a two-page student hand-out, standards correlations, vocabulary lists, plus extensions and assessment tools with answer keys. This experiment is appropriate for remedial, review, reinforcement or extension purposes.