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# Hess's Law (17 Favorites)

LESSON PLAN in Exothermic & Endothermic, Hess's Law, Enthalpy. Last updated March 25, 2020.

### Summary

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

High school

### AP Chemistry Curriculum Framework

This lesson supports the following unit, topic, and learning objectives:

• Unit 6: Thermodynamics
• Topic 6.9: Hess’s Law
• ENE-3.C: Represent a chemical or physical process as a sequence of steps.
• ENE-3.D: Explain the relationship between the enthalpy of a chemical or physical process and the sum of the enthalpies of the individual steps.

### Objectives

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

### Time

Teacher Preparation: 10 minutes

Lesson: 30 minutes

• PowerPoint

### Safety

• 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