Experiment
Heat of Combustion: Magnesium 18
In Experiment 16, you learned about the additivity of reaction heats as you confirmed Hess’s
Law. In this experiment, you will use this principle as you determine a heat of reaction that
would be difficult to obtain by direct measurement—the heat of combustion of magnesium
ribbon. The reaction is represented by the equation
(4) Mg(s) + 1/2 O2(g) 
 MgO(s)
This equation can be obtained by combining equations (1), (2), and (3):
(1) MgO(s) + 2 HCl(aq) 
 MgCl2(aq) + H2O(l)
(2) Mg (s) + 2 HCl(aq) 
 MgCl2(aq) + H2 (g)
(3) H2(g) + 1/2 O2(g)

 H2O(l)
The pre-lab portion of this experiment requires you to combine equations (1), (2), and (3) to
obtain equation (4) before you do the experiment. Heats of reaction for equations (1) and (2) will
be determined in this experiment. As you may already know, H for reaction (3) is -285.8 kJ.
Figure 1
MATERIALS
Power Macintosh or Windows PC
Vernier computer interface
Logger Pro
Temperature Probe
ring stand
utility clamp
100-mL graduated cylinder
250-mL beaker
Styrofoam cup
1.00 M HCl
magnesium oxide, MgO
magnesium ribbon, Mg
stirring rod
balance
weighing paper
PRE-LAB EXERCISE
In the space provided below, combine equations (1), (2), and (3) to obtain equation (4).
Chemistry with Computers
18 - 1
Experiment 18
Reversed
(1) MgCl2(aq) + H2O(l) 
 MgO(s) + 2 HCl(aq)
H1 = opposite sign
(2) Mg (s) + 2 HCl(aq) 
 MgCl2(aq) + H2 (g) H2 =
(3) H2(g) + 1/2 O2(g)
H3 = -285.8 kJ

 H2O(l)
+ _______________________________________________
(4) Mg(s) + 1/2 O2(g) 
 MgO(s)
H4 = H1 + H2 + H3
PROCEDURE
1. Obtain and wear safety glasses and an apron.
2. Prepare the computer for data collection by opening the Experiment 19 folder from
Chemistry with Computers. Then open the experiment file that matches the probe you are
using. The vertical axis has temperature scaled from 0 to 50°C. The horizontal axis has time
scaled from 0 to 500 seconds.
Reaction 1
3. Place a Styrofoam cup into a 250-mL beaker as shown in Figure 1. Measure out 100.0 mL of
1.00 M HCl into the Styrofoam cup. CAUTION: Handle the HCl solution with care. It can
cause painful burns if it comes in contact with the skin.
4. Use a utility clamp and a slit stopper to suspend a Temperature Probe from a ring stand as
shown in Figure 1. Lower the Temperature Probe into the solution in the Styrofoam cup.
5. Weigh out about 1.00 g of magnesium oxide, MgO, on a piece of weighing paper. Record the
exact mass used in your data table. CAUTION: Avoid inhaling magnesium oxide dust.
6. Click Collect to begin data collection and obtain the initial temperature, t1. After three or four
readings at the same temperature (t1) have been obtained, add the white magnesium oxide
powder to the solution. Use a stirring rod to stir the cup contents until a maximum
temperature has been reached and the temperature starts to drop. Click Stop to end data
collection.
7. Examine the initial readings in the Table window to determine the initial temperature, t1. To
determine the final temperature, t2, click the Statistics button, . The maximum temperature
is listed in the statistics box on the graph. Record t1 and t2 in your data table.
8. Discard the solution as directed by your teacher.
Reaction 2
9. Repeat Steps 3-8 using about 0.50 g of magnesium ribbon rather than magnesium oxide
powder. The magnesium ribbon has been pre-cut to the proper length by your teacher. Be
sure to record the measured mass of the magnesium. CAUTION: Do not breathe the vapors
produced in the reaction!
18 - 2
Chemistry with Computers
Heat of Combustion: Magnesium
PROCESSING THE DATA
1. In the spaces provided, calculate the change in temperature, t, for Reactions 1 and 2.
2. Calculate the heat released by each reaction, q, using the formula
q = Cp•m•t
Cp = 4.18 J/g°C, and m = 100.0 g of HCl solution. Convert joules to kJ in your final answer.
3. Determine H. (H = -q)
4. Determine the moles of MgO and Mg used.
5. Use your Step 3 and Step 4 results to calculate H/mol for MgO and Mg.
6. Determine H/mol Mg for Reaction 4. (Use your Step 5 results, your pre-lab work, and
H = -285.8 kJ for Reaction 3).
7. Determine the percent error for the answer you obtained in Step 6. The accepted value for
this reaction can be found in a table of standard heats of formation.
Chemistry with Computers
18 - 3
Experiment 18
DATA AND CALCULATIONS
Reaction 1 (MgO)
Reaction 2 (Mg)
1. Volume of 1.00 M HCl
101.58 g
102.06 g
2. Final temperature, t2
33.07 °C
51.17 °C
3. Initial temperature, t1
25.43 °C
25.43 °C
33.07°C – 25.43 °C = 7.64 °C
51.17 °C – 25.43 °C = 25.74°C
1.07 g MgO
0.54 g Mg
4. Change in temperature, t
5. Mass of solid
q = m • c • T
q = m • c • T
q = 101.58 • 4.184 • 7.64
q = 102.06• 4.184 • 25.74
6. Heat, q
q = m • c • T
where c = 4.184 J/g•C
q = 3,247 J x
1 kJ
1000J
q = 10,990 J x
1 kJ
1000J
3.25 kJ
10.99 kJ
H = -q
H = -q
H = - 3.25 kJ
H = -10.99 kJ
7. H
H = -q
8. Moles
From Periodic Table
Mg = 24.31 g/mol
O = 16.00 g/mol
MgO = 24.31+16.0=40.31g/mol
moles = mass/molar mass
moles = mass/molar mass
1.07 g MgO x 1 mol MgO .
40.31 g MgO
0.54 g Mg x 1 mol Mg
24.31 g Mg
0.0265 mol MgO
0.022 mol Mg
- 3.25
kJ
0.0265 mol
-10.99 kJ
0.022 mol
=-499.54
- 500 kJ/mol
9. H/mol
- 123 kJ/mol
10. Determine H/mol Mg for reaction (4)*.
(1) ___ MgCl2(aq)
+ H2O(l) → MgO(s) + 2 HCl(aq)____
H1 = +123 kJ/mol
(2) ____ Mg (s) + 2
HCl(aq) → MgCl2(aq) + H2 (g)_____ H2 = - 500 kJ/mol
(3) ____ H2(g) + 1/2 O2(g) → H2O(l)________________ H3 = - 285.8 kJ/mol
(4)* ____ Mg(s)
+ 1/2 O2(g) → MgO(s)_______________ H4 = H1 + H2 +H3
H4 = +123 + (-500) + (-285.8)
H4 = -662.8 = -660 kJ/mol
11. Percent Error
% error 
18 - 4
Accepted value - experiment al value
Accepted value
x100
% error 
 602 kJ/mol - (-660 kJ/mol)
 602kJ / mol
x100 
60
x100  10%
602
Chemistry with Computers