Heat of Formation of MgO
AP Chemistry Lab #6
Name_______________________________ Date__________________ Period____
Magnesium reacts with oxygen in air to form magnesium oxide. In this reaction
a great deal of heat and light are produced—the temperature of the flame can reach as
high as 2400°C. The amount of heat energy produced in this reaction cannot be
measured directly in the high school lab. It is possible, however, to determine the
amount of heat produced by an indirect method, using Hess’s Law.
In this lab you will use Hess’s Law to determine the standard enthalpy change for
the combustion of magnesium. You will determine the standard enthalpy change for
two other reactions which can be combined algebraically with a third reaction (given
below) to obtain the standard enthalpy of combustion for magnesium. Given that the
calorimeters being used are made only of coffee cups and cardboard, you will first
determine the heat capacity of the calorimeter itself (rather than assuming that it is not
absorbing any heat energy at all).
Procedure
Part A: Determination of the Heat Capacity of the Calorimeter
1) At your table, heat about 125 mL of water to about 70°C. You may share this
water with the other group at your table. As long as you have more than 100 mL
of water, the exact amount is not important.
2) Construct a calorimeter of two nested Styrofoam cups with a cover which has a
hole in it for the temperature probe.
3) Measure 50.0 mL of water from the tap and pour it into the calorimeter.
4) Gently stir the water with your thermometer. After several minutes record the
temperature of the water.
5) Measure 50.0 mL of the hot water into a graduated cylinder; record its
temperature.
6) Immediately pour the hot water into the room temperature water in the
calorimeter. Cover the calorimeter and insert the temperature probe.
7) Stir the water for about ten seconds and then record the highest temperature
that you see. Use caution with the probe so that you do not accidently poke a
hole in the cup!
8) Pour the water down the drain, turn off the hot plate, pour out any excess hot
water.
Part B: Reaction of Magnesium Oxide with Hydrochloric Acid
1) Mass a clean, dry calorimeter to the nearest 0.01 g.
2) Using a graduated cylinder, obtain 15.0 mL of 1.0 M HCl and add it to the
calorimeter; measure and record the combined mass of the calorimeter and
hydrochloric acid.
3) Take a small weighing boat and add about 0.20 g of magnesium oxide. Measure
the exact mass of the magnesium oxide to the nearest 0.01 g.
4) Using a digital thermometer, measure the initial temperature of the hydrochloric
acid solution to the nearest 0.1°C. Hold the coffee cups or they will fall over with
the temperature probe in them!
5) Carefully add the magnesium oxide to the acid in the calorimeter. Immediately
put the cover on and stir with the temperature probe for about ten seconds.
Then record the highest temperature that you observe to the nearest 0.1°C.
6) Pour the reaction mixture down the drain with excess water, then rinse and dry
the calorimeter.
7) Repeat steps 1-6 with a second sample of magnesium oxide.
Part C: Reaction of Magnesium with Hydrochloric Acid
8) Obtain a 7.0-cm strip of magnesium ribbon and cut it into two pieces of unequal
length, roughly 3- and 4-cm each.
9) Measure the exact length of each piece of magnesium ribbon to the nearest 0.1
cm
10) Mass a clean, dry calorimeter to the nearest 0.01 g.
11) Using a graduated cylinder, obtain 15.0 mL of 1.0 M HCl and add it to the
calorimeter; measure and record the combined mass of the calorimeter and
hydrochloric acid.
12) Using a digital thermometer, measure the initial temperature of the hydrochloric
acid solution to the nearest 0.1°C. Hold the coffee cups or they will fall over with
the temperature probe in them!
13) Add the first (shorter) piece of magnesium ribbon to the acid and then
immediately put the cover on and stir with the temperature probe for about ten
seconds. Then record the highest temperature that you observe to the nearest
0.1°C.
14) Rinse the contents of the calorimeter down the drain with excess water.
15) Dry the calorimeter and mass it again to the nearest 0.01 g.
16) Repeat steps 11-13 with the second (larger) piece of magnesium ribbon.
17) Rinse the contents of the calorimeter down the drain with excess water. Dry the
calorimeter and return it to where you found it.
Analysis
1) Calculate the average temperature of the hot and cold water by adding their
initial temperatures and dividing by two.
2) Calculate the heat absorbed by the calorimeter. You can assume that qcal = -qwater.
For qwater the temperature change will be the difference between the average
temperature that you calculated in step 1 and the final temperature that you
recorded in part A (Tfinal – Taverage). The density of water is 1.00 g/mL.
3) Calculate the heat capacity of the calorimeter by dividing your answer to #2 by
the change in temperature for the cold water (since it was in the calorimeter to
begin with).
4) Write the balanced equation for the reaction of magnesium oxide with
hydrochloric acid.
5) Determine the heat gained by the solution in the calorimeter in each trial for part
B. Use 4.18 J/g°C as the specific heat for the solution and 1.00 g/mL for the
density because it is mostly water. The mass here is the total mass of the
reactants. Remember that qrxn = − qsolution + qcalorimeter and you can calculate qcalorimeter
(
)
by multiplying the heat capacity for the calorimeter (from question 3) by the
change in temperature for this reaction.
6) Use the mass of magnesium oxide to determine the ΔH (kJ/mole) for each trial in
part B. Then find the average enthalpy change for this reaction (part B).
7) Write the balanced equation for the reaction of magnesium with hydrochloric
acid.
8) Use the conversion factor provided in class to determine the exact mass of each
piece of magnesium used in part C.
9) Repeat step 5 for each trial in part C.
10) Use the mass of magnesium to determine the ΔH (kJ/mole) for each trial in part
C. Then find the average enthalpy change for this reaction (part C).
11) Use Hess’ Law to determine the standard enthalpy of formation of magnesium
oxide. You want to solve for the equation:
Mg(s) + ½ O2(g) ! MgO(s)
The following equation is necessary for figuring this out:
H 2 + ½ O2 ! H2O
ΔH = -286.8 kJ
You’ll need to rearrange at least one equation in order to do this.
Conclusion
Write an appropriate conclusion. You should look up the standard enthalpy of
formation for magnesium oxide in your textbook. Calculate the percent error in
your experimental determination and give at least one specific possible source of
error.