The Determination of the Molar Heat of Reaction for the Combustion of Magnesium
Objective:
Determine the value for the molar heat of reaction for the combustion of magnesium:
Mg(s) + (1/2)O2(g) ---> MgO(s)
(1)
Brief Overview
In many cases a given chemical reaction can be expressed as the algebraic sum of several other chemical reactions. Such
is the case with the above reaction. In every case thus far tested, the heat of reaction (expressed in kJ/mol) for the given
reaction is also the algebraic sum of the several other chemical reactions. This generalization is called Hess' Law, in
honor of its discoverer: Germain Henri Hess.
In this lab, we will determine the heat of reaction for three reactions that will sum up
to the above reaction. The are:
Mg(s) + 2HCl(ag) ---> MgCl2(ag) + H2(g)
(A)
MgO(s) + 2HCl(ag) ---> MgCl2(ag) + H2O(l)
(B)
H2(g) + (1/2)O2(g) ---> H2O(l)
(C)
Safety
1) Make sure to wear goggles and aprons during the entire course of the lab, which includes all clean-up time.
2) This lab involves handling glassware, which is breakable. Handle all glass with proper care, making sure to not
drop or knock over any pieces.
3) This lab involves handling hydrochloric acid. Take care to not spill any. Inform your teacher immediately about any
spills. Avoid breathing vapors from the acid, especially during the time it is reacting.
Materials
thermometer
Two Styrofoam cups
100 mL graduated cylinder
weighing boat
400-mL beaker
1.00-molar hydrochloric acid magnesium oxide powder magnesium ribbon
Procedure
1) Make sure the Styrofoam cup is clean and dry. Put the cup inside the 400-mL beaker and never separate them,
except for cleaning, during the lab.
Reaction A: Magnesium with Hydrochloric Acid
2) Cut approx. 32 cm of magnesium ribbon. Weigh the Mg ribbon on the electronic balance and record the value in
your data table. Cut the ribbon into 3 or 4 pieces. Be careful not to lose any pieces.
3) Using the 100 mL graduated cylinder, obtain 95-100 mL of the hydrochloric acid. Measure and record the mass
to the hundredth place.
4) Record the initial temperature of the acid solution with the thermometer. Record this value in your data table.
5) While stirring gently with the thermometer probe, add all the Mg ribbon pieces to the acid and put on the lid
immediately. Be careful not to breathe any vapors as the reaction proceeds. Make sure 100% of the Mg reacts.
Be careful that a piece does not stick to the side of the cup.
6) Keep stirring, making sure the entire cup contents are mixed well. Do not stir vigorously. Use highest
temperature for calculations.
7) Discard the solution down the drain with the water running. Rinse the Styrofoam cup and the thermometer.
Reaction B: Magnesium Oxide with Hydrochloric Acid
1) Obtain ~ 1 g of MgO on weigh paper. Record the exact mass in your data table.
2) Now, repeat steps 3-7 above with the MgO in place of the Mg. Please dry the cup and thermometer before using
them again.
Reaction C: Hydrogen gas with Oxygen gas
1) For safety reasons, this value will not be experimentally determined in class. It has been professionally
determined and verified and the value is listed in standard reference sources. Your assignment is to find the
value, in kilojoules per mole for the formation of liquid water.