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Hess's Law (8 Favorites)

LESSON PLAN in Hess's Law. Last updated April 26, 2019.


In this lesson, students learn about the concepts of Hess's Law and how to solve problems involving them.

Grade Level

High school

AP Chemistry Curriculum Framework

  • Big Idea 5: The laws of thermodynamics describe the essential role of energy and explain and predict the direction of changes in matter.
    • 5.4 The student is able to use conservation of energy to relate the magnitudes of the energy changes occurring in two or more interacting systems, including identification of the systems, the type (heat versus work), or the direction of energy flow.
    • 5.8 The student is able to draw qualitative and quantitative connections between the reaction enthalpy and the energies involved in the breaking and formation of chemical bonds.


By the end of this lesson, students should be able to

  • Understand the concepts behind Hess’s Law.
  • Be able to solve problems using Hess’s Law.

Chemistry Topics

This lesson supports students’ understanding of

  • Hess’s Law
  • Enthalpy


Teacher Preparation: 10 minutes

Lesson: 30 minutes


  • PowerPoint


  • No specific safety precautions need to be observed for this activity.

Teacher Notes

  • Students get really involved in this lesson because they view each reaction set as if it were a puzzle.
  • Included in the downloads section is an accompanying worksheet (and answer key) with six problems for students to practice what they’ve learned from the lesson.
  • *Note that the associated PowerPoint Presentation is animated. It is designed so that the examples are easy to follow, in a step-by-step format for the students to easily understand. If you use the PDF version, the final answers for each example will only be shown.

For the Student

Directions: Using Hess’s law, indicate on the line what change is made to the equation and rewrite each equation to find the H for each given reaction. (An answer key is provided in the downloads section.)

  1. Calculate the DH for the reaction: 2 Al(s) +Fe2O3(s) → 2 Fe(s)+ Al2O3(s)

Given the following information:

______2 Al(s)+3/2 O2(g) → Al2O3(s ) ΔH = -1670 kJ/mol

______2 Fe(s)+3/2 O2(g)→Fe2O3(s) ΔH = -824 kJ/mol

  1. Determine the DH for the reaction: NO(g)+½ O2(g) → NO2(g)

Given the following information:

______½ N2(g)+½ O2(g) → NO(g) ΔH = +90.0 kJ/mol

______½ N(g)+O2(g) → NO2(g) ΔH = +34.0 kJ/mol

  1. Calculate the DH for the reaction: Cu(s) + ½ O2(g) → CuO(s)

Given the following information:

______ CuO(s) + Cu(s) → Cu2O(s) ΔH = -11 kJ/mol

______ 2 CuO(s) → Cu2O(s) + ½ O2(g) ΔH = +115 kJ/mol

  1. Calculate the DH for the reaction: PbCl2(s) + Cl2(g) → PbCl4(l)

Given the following information:

______ Pb(s) + 2 Cl2(g) → PbCl4(l) ΔH = -393 kJ/mol

______ Pb(s) + Cl2(g) → PbCl2(s) ΔH = -359 kJ/mol

  1. Calculate the ΔH for the reaction: 2 H2O2(l) → 2 H2O(l) + O2(g)

Given the following information:

______ H2(g) + O2(g) → H2O2(l) ΔH = -188 kJ/mol

______ H2(g) + ½ O2(g) → H2O(l) ΔH = -286 kJ/mol

  1. Calculate the ΔH for the reaction: N2(g) + O2(g) → 2 NO (g)

Given the following information:

______ 4 NH3(g) + 3 O2(g) → 2 N2(g) + 6 H2O(l) ΔH = -1530 kJ/mol

______ 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(l) ΔH = -1170 kJ/mol