Heat of Combustion: Magnesium

Recommended for grades 9–12.

Figure from experiment 19 from Chemistry with Vernier

Introduction

In Experiment 18, you learned about the additivity of reaction heats as you confirmed Hess’s Law. In this experiment, you will use this principle as you determine a heat of reaction that would be difficult to obtain by direct measurement—the heat of combustion of magnesium ribbon. The reaction is represented by the equation

${\text{(4) Mg(s) + 1/2 }}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{MgO(s)}}$

This equation can be obtained by combining equations (1), (2), and (3):

${\text{(1) MgO(s) + 2 HCl(aq)}} \to {\text{MgC}}{{\text{l}}_{\text{2}}}{\text{(aq) + }}{{\text{H}}_{\text{2}}}{\text{O(1)}}$ ${\text{(2) Mg(s) + 2 HCl(aq)}} \to {\text{MgC}}{{\text{l}}_{\text{2}}}{\text{(aq) + }}{{\text{H}}_{\text{2}}}{\text{(g)}}$ ${\text{(3) }}{{\text{H}}_{\text{2}}}{\text{(g) + 1/2 }}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {{\text{H}}_{\text{2}}}{\text{O(1)}}$

The pre-lab portion of this experiment requires you to combine equations (1), (2), and (3) to obtain equation (4) before you do the experiment. Heats of reaction for equations (1) and (2) will be determined in this experiment. As you may already know, ΔH for reaction (3) is –285.8 kJ.

Objectives

In this experiment, you will

Combine three chemical equations to obtain a fourth.
Use prior knowledge about the additivity of reaction heats.
Determine the heat of combustion of magnesium ribbon.

Sensors and Equipment

This experiment requires each of the following Vernier sensors and equipment (unless otherwise noted):

Stainless Steel Temperature Probe

Additional Requirements

You may also need an interface and software for data collection. What do I need for data collection?

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The student-version preview includes:

Step-by-step instructions for computer-based data collection
List of materials and equipment

Note: The experiment preview of the computer edition does not include essential teacher information, safety tips, or sample data. Instructions for Logger Pro and other software (such as LabQuest App or TI handheld software, where available) are on the CD that accompanies the book. We strongly recommend that you purchase the book before performing experiments.

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