Hess’s Law
The purpose of this investigation is to use Hess’s law to determine the molar enthalpy of
combustion of magnesium, using Calorimetry.
What is the molar enthalpy of combustion of magnesium?
Prediction
Show how the three known equations and their enthalpies of reaction may be combined, using
Hess’s law, to yield the target equation and its enthalpy of combustion.
Experimental Design
Make a 1.00 M solution of hydrochloric acid and place 100.0 mL of the solution into a
calorimeter. Measure the initial temperature. Polish a piece of magnesium ribbon with steel
wool and measure out approximately 0.50 g and add the solution to the calorimeter. Measure
the temperature change.
Repeat the same steps but replace the magnesium ribbon with 1.00 g of magnesium oxide.
Materials
Steel wool
Balance
Thermometer
Calorimeter
Graduated cylinder
Scoopula
Magnesium ribbon
Magnesium oxide
HCl
Analysis
- Were the changes exothermic or endothermic? Explain
- Calculate the molar enthalpies of magnesium in acid and magnesium oxide in acid.
- Determine the molar enthalpy of combustion of magnesium based of experimental
values.
- Which of the measured values limited the precision of your value? Explain
- Suggest possible sources of error
- Calculate a percentage difference for molar enthalpy of combustion of magnesium
(- 601.6 kJ/mol)
- Is Hess’s law an acceptable method to calculate enthalpies of reaction?
Pre-lab
Pg. 357 #16 (a), (c), (d), (e) and pg. 351 Prediction (a)