Applying Hess' Law:
Rohan Sharma
Purpose:
The purpose of this investigation is to use Hess' law to determine a standard molar enthalpy of
combustion.
Problem:
What is the standard molar enthalpy of combustion for magnesium?
Prediction:
The standard molar enthalpy of combustion for magnesium is -601.6 kJ/mol as per the table of
standard molar enthalpies of formation. (The standard molar enthalpy of combustion of
magnesium is the same as the standard molar enthalpy of formation of magnesium oxide because
both processes have the same chemical equation.)
∆c Hm° = ∆f Hm° = -601.6 kJ/mol
Mg
MgO
Mg (S) + ½ O2 (G)
MgO (S)
∆c H° = ∆f H° = -601.6 kJ
Pre-Lab Calculation:
Materials:
Lab Apron
Polystyrene calorimeter with lid
Eye Protection
Thermometer
magnesium ribbon
50 mL or 100 mL graduated cylinder
(MAXIMUM 15 cm strip)
laboratory scoop or plastic spoon
magnesium oxide powder
steel wool
(MAXIMUM 1.00 g sample)
weighing boat or paper
1.00 mol/L hydrochloric acid
centigram balance
(USE 50 mL each time)
ruler
Procedure:
1. Measure 50 mL of 1.0 mol/L hydrochloric acid in a clean graduated cylinder.
2. Pour the hydrochloric acid into the calorimeter and record the initial temperature to the
nearest tenth of a degree Celsius.
3. Get a magnesium ribbon measuring it to be less than 15 cm and use a centigram balance to
find the mass of the ribbon.
4. Stick the thermometer through the lid (using the hole provided).
5. Gently rub the magnesium ribbon with a steel wool and place it into the lab calorimeter.
6. Place the lid immediately and begin to stir the reactants (magnesium ribbon and
hydrochloric acid) within the calorimeter.
7. Make sure the ribbon is completely dissolved before recording the final temperature.
8. Record the highest temperature seen to the nearest tenth of a degree Celsius.
9. The first half of the lab is finished.
10. Empty the solution and clean the thermometer, the lab calorimeter, and graduated
cylinder.
11. Measure 50 mL of 1.0 mol/L hydrochloric acid in a clean graduated cylinder.
12. Using a weigh boat or paper, measure 1.00 g of magnesium oxide powder. Magnesium
oxide is an extremely fine dust. Do not inhale the powder because it is an irritant.
13. Pour the hydrochloric acid into the calorimeter and record the initial temperature to the
nearest tenth of a degree Celsius.
** Make sure you stick the thermometer back through the lid (using the hole provided).
14. Pour the magnesium oxide powder into the lab calorimeter.
15. Place the lid immediately and begin to stir the reactants (magnesium oxide and
hydrochloric acid) within the calorimeter.
16. Record the highest temperature seen to the nearest tenth of a degree Celsius.
17. Dispose of all waste properly and clean up your lab station.
Observations seen during experiment:
What we're measuring:
1
Initial Temperature
Final Temperature
Volume of HCl (aq)
Mass of Mg (S)
Length of Mg (S) ribbon
Mass of MgO (S)
Analysis (to be completed after the experiment):
2
Standard Molar Enthalpies of Formation at 298.15 K