LAB: Applying Hess’ Law
Magnesium reacts rapidly with oxygen in a highly exothermic formation reaction:
Mg(s) + ½ O2(g) → MgO(s)
H = ?
The enthalpy change of this reaction can be measured using a bomb calorimeter, but not with a Styrofoam
cup calorimeter. The enthalpy change for the formation of magnesium oxide can be determined by
applying Hess’ law to the following three chemical equations.
MgO(s) + 2 HCl(aq) → MgCl2(aq) + H2O(l)
H =
?
Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
H =
?
H2(g) + ½ O2(g) → H2O(l)
H = -285.8 kJ
In this lab, you will use these equations and the collected observations to calculate an experimental value
for the molar enthalpy of formation of magnesium oxide. At that point, you will be able to compare it to
the theoretical molar enthalpy of formation (∆fH = -601.6 kJ/mol, as found in the data booklet) by
completing a percent error analysis.
Procedure:
1. Measure out approximately 1 g of magnesium oxide into an empty Styrofoam calorimeter.
2. Measure out about 50 mL of excess hydrochloric acid into a graduated cylinder.
3. Measure the initial temperature of the acid solution.
4. Mix the two reactants together in the Styrofoam calorimeter.
5. Measure and record the final temperature of the reaction mixture.
6. Repeat steps 1 – 5, using a coiled strip of magnesium ribbon (no longer than 15 cm) instead of the
magnesium oxide powder.
Assignment:
1. Complete an Observations section, recording all relevant data. (See sample data table shown below.)
2. Using the calorimetry evidence from the first reaction, calculate rH (in kJ/mol) for MgO(s).
3. Using the calorimetry evidence from the second reaction, calculate rH (in kJ/mol) for Mg(s).
4. Using Hess’s law and the three thermochemical equations listed above, calculate the H (in kJ) for the
formation of MgO(s) based on the balanced reaction at the top of the page. Then, convert this value
into a fH (in kJ/mol) for MgO(s).
5. Calculate the percent error associated with your lab result, by comparing your answer in #4 to the
theoretical fH (in kJ/mol) for MgO(s) listed in the data booklet.
Sample of Observations table:
type of solid reactant
mass of solid reactant
(g)
volume of HCl(aq)
(mL)
initial temperature of solution
(oC)
final temperature of solution
(oC)
MgO(s)
Mg(s)