SCH4U1
Name: ____________________________
Molar Enthalpy of Combustion of Magnesium Metal
Purpose:
To determine the molar enthalpy of combustion of magnesium metal using Hess’ Law.
Materials:
calorimeter
thermometer
electronic balance
stirring rod
magnesium metal (~ 0.5 g)
100 mL graduated cylinder
scoopula
magnesium oxide (~ 1.0 g)
steel wool
1.0 mol/L HCl (aq) solution
Background: The combustion of magnesium metal releases thermal energy and light energy. The
combustion reaction is represented by the balanced equation:
1
𝑀𝑔(𝑠) + 𝑂2 (𝑔) → 𝑀𝑔𝑂(𝑠)
2
∆𝐻𝑐𝑜𝑚𝑏 = ?
It is very difficult to experimentally determine the molar enthalpy of this reaction directly using the
equipment available. However, we are able to use Hess’ Law and the following three
thermochemical equations:
1
𝑀𝑔(𝑠) + 2𝐻𝐶𝑙(𝑎𝑞) → 𝐻2 (𝑔) + 𝑀𝑔𝐶𝑙2 (𝑎𝑞)
∆𝐻1 = ?
2
𝑀𝑔𝑂(𝑠) + 2𝐻𝐶𝑙(𝑎𝑞) → 𝐻2 𝑂 (𝑙) + 𝑀𝑔𝐶𝑙2 (𝑎𝑞)
1
𝐻2 (𝑔) + 𝑂2 (𝑔) → 𝐻2 𝑂 (𝑙)
2
∆𝐻2 = ?
3
∆𝐻3 = −285.8 𝑘𝐽/𝑚𝑜𝑙
For the first two reactions, you will determine the enthalpy values (ΔH1 and ΔH2) using data
collected in a calorimeter. The enthalpy value for the third reaction is provided.
Method:
CAUTION: HYDROCHLORIC ACID IS CORROSIVE TO THE SKIN AND MAY CAUSE BLINDNESS IF IT GETS IN
YOUR EYES. WEAR EYE PROTECTION AT ALL TIMES.
Part 1: Determination of the Enthalpy Change of Reaction 1
1) Using the graduated cylinder, measure out 100.0 mL of 1.0 mol/L hydrochloric acid and place it in
the calorimeter. Stir carefully with a thermometer until a constant temperature is reached.
Measure and record this temperature as precisely as possible.
2) Rub a piece of magnesium metal ribbon with steel wool until shiny. Use a balance to obtain about
0.5 g of magnesium metal ribbon. Record the actual mass.
3) Put the magnesium metal ribbon into the hydrochloric acid in the calorimeter. Place the
thermometer into the solution and stir gently but continuously until the magnesium metal is
completely reacted. Record the highest temperature reached.
4) Dispose the contents of the calorimeter into the acid waste. Rinse and dry the calorimeter to
prepare it for Part 2.
Part 2: Determination of the Enthalpy Change of Reaction 2
1) Repeat step 1 from Part 1.
2) Using the balance, obtain about 1.0 g of solid magnesium oxide. Record the actual mass.
3) Repeat step 3 from Part 1 but substitute magnesium oxide for magnesium metal ribbon.
4) Dispose the contents of the calorimeter into the acid waste. Rinse the calorimeter and put it away.
Rinse all other glassware with water and return them to their original locations.
Observations:
Properly prepare a table in your lab book. Be sure to include a table title and units. The following
information will need to be collected in Part 1 and Part 2 of the experiment.
Initial temperature of HCl
Final temperature of solution
Temperature change
Volume of 1.0 mol/L HCl
Mass of magnesium metal ribbon
Mass of magnesium oxide
Analysis:
1. Calculate the molar enthalpy change for Reactions 1 and 2 in terms of kJ/mol Mg and MgO
respectively.
2. Using Hess’ Law, determine the molar enthalpy of combustion of magnesium. Use the additivity
of heats method (ie write out and add the individual step equations).
3. Calculate the percent error. The theoretical value for the molar enthalpy of combustion (ΔHcomb)
of magnesium is -602 kJ/mol.
Error  Experimental Value  Theoretical Value
Percentage Error 
Error
 100
Theoretical Value
Discussion:
1. Several assumptions that are made when using calorimetry can result in systematic
experimental error. Describe how each of the following assumptions would affect your
temperature measurements and ΔH calculation. Be as specific as possible (ie. will the values be
higher, lower or unchanged?).
a) The assumption that the calorimeter is a closed system (no kinetic energy is transferred to or
from the surroundings).
b) The assumption that the heat capacity of any aqueous solutions is 4.18 J/goC.
2. Suppose you used 2.00 g of magnesium metal in reaction 1:
a) How would this have affected the change in temperature?
b) How would this affect the number of kJ produced in the new experiment 1?
c) What effect would this change have on the resulting molar enthalpy value (Hcomb)?
Conclusion
Write a conclusion by restating the purpose and summarizing your results. Discuss how well your
results agree with the published (theoretical) value and if Hess’ Law appears to accurately predict
the enthalpy change for the combustion of magnesium.