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.