In class by now you should have started discussing the concepts of thermodynamics. In this lab we will try to bring a purpose to some of those discussions. Specifically, why in the world do you need to know 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:

As you can see from the video, the reaction is quite exothermic and trying to measure the enthalpy of this reaction would be difficult and possibly even dangerous. But we can still make this determination of the DH_Rxn by use of both calorimetry and Hess's Law.

We will construct and use a constant pressure calorimeter and by measuring the heat of reaction of two related reactions which are not so exothermic. The DH_Rxn of magnesium with hydrochloric acid and magnesium oxide with hydrochloric acid is much easier to determine and when used in conjunction with Hess's Law will allow us to calculate the DH_Rxn for our target reaction.

Hess's Law Reactions:

Our reaction of interest: