Introduction

It is likely that you have started discussing the concepts of thermodynamics in class.  In this lab we will illustrate some of those concepts. Specifically, why in the world do you need to know about processes like calorimetry and Hess's Law?

The purpose of this lab is to determine the enthalpy of reaction for the burning of magnesium in oxygen:

The reaction is quite exothermic. Therefore trying to measure the enthalpy of this reaction would be difficult and possibly even dangerous. But we can still determine the DHRxn by using calorimetry to collect experimental data and then using Hess's Law to manipulate that data to yield the answer we need.
We will construct and use a constant pressure calorimeter to measure the DHRxn of two reactions related by their reagents to our target reaction.  These values of DHRxn are much easier to measure as they are much less exothermic.  The DHRxn of magnesium with hydrochloric acid and of magnesium oxide with hydrochloric acid will be determined and then used in conjunction with Hess's Law to calculate the DHRxn for our target reaction.

Hess's Law Reactions: