Chemistry
Sensors:
Loggers:
Temperature
Any EASYSENSE
Logging time: EasyLog
Teacher’s notes
13 Heat of combustion: Magnesium and oxygen
Read
If no thermo chemical data is available for a particular reaction or the reaction is considered to be too
dangerous to make in one step, the enthalpy change can still be predicted by calculation. Hess found
that whatever the route taken to make a product the enthalpy change was always the same, provided
the changes are determined under the same conditions.
In this investigation the heat of combustion of magnesium ribbon is to be found by producing
magnesium oxide indirectly.
The reaction of magnesium and oxygen is:
(4) Mg(s) + ½O2
MgO(s)
By using other reactions with Mg the oxide MgO can be produced. The ΔH of each of these other
reactions, when added will give the ΔH of reaction (4)
(1) MgO(s) + 2HCl(aq)
MgCl2(aq) + H2O(l)
(2) Mg(s) + 2HCl(aq
MgCl2(aq) + H2(g)
(3) H2(g) + ½O2
H2O(l)
The ΔH of reaction (3) is given in data books as -285.8 kJ. The ΔH of reactions 1 and 2 will be derived
by experiment.
Apparatus
1.
An EASYSENSE logger.
2. A Smart Q Temperature sensor.
3. 100 cm3 measuring cylinder.
4. 250 cm3 beaker.
5. Expanded polystyrene cup.
6. Magnesium oxide.
7. Magnesium ribbon.
8. 1 mol dm-3 Hydrochloric acid.
9. Balance accurate to 0.1 g.
10. Retort stand, boss and clamp.
L3 Chemistry
T13 - 1(V2)
Hazards
Magnesium ribbon
Highly
flammable
This needs to be carefully monitored. Follow local guidelines about
storage, quantities, etc. Do not encourage burning of the magnesium. The
light from burning magnesium is intense and can cause eye damage. Dry
sand should be available for use as a fire extinguisher. Do not use or
attempt to use water on burning magnesium.
The hydrochloric acid + magnesium reaction may produce an acid vapour. A lid for the apparatus is
recommended.
Set up of the software and logger
Use EasyLog and select Overlay to collect all experiments on the same graph.
Notes
It is best to have the magnesium pre-cut to sections that weigh about 0.5 g. They should be kept in a
tin with a tight fitting lid. The pieces can then be weighed by the students to find the exact mass.
The accepted value for the ΔH is – 602 kJ/mol. Results from the investigation will vary but should
easily be of the same magnitude. Results can be very close to the published figure, errors of less than
1% from published are possible.
Analysis and results
The students are lead through the calculation in the work sheet. They will need to calculate:
1.
The temperature change for each reaction.
2. The energy trapped in the calorimeter through Specific heat x mass x temperature change.
This is defined on the students sheet as Q = Cp x m x ∆T. e specific heat is that of the water
in the experiment and is assumed to be 4.18. This assumption is of course on source of error.
3. The value calculated for Q will be positive value, placing a negative in front of the value will
convert it to the ∆H value.
4. Moles of MgO and Mg are calculated from the mass used divided by the sum of the formula
masses. Mg is taken to be 24.3 and MgO is 40.29. Values from different data tables may be
different according to number of decimals used and the reference atom used. This is another
source of “error” in the final value.
L3 Chemistry
T13 - 2(V2)
Worked example
Reaction 1
Reaction 2
Volume of hydrochloric acid
(cm3)
100
100
Final temperature (T2)
29.5
39.7
Starting temperature (T1)
22.1
22.3
Change in temperature ΔT =
(T2-T1)
7.4
17.4
Mass of solid (g)
1.0
0.5
Energy Q = Cp x m x Δt
ΔH
Moles
1.18 x 7.4 x 101 = 3124 J
4.18 x 17.4 x 100.5 = 7309 J
3.124 kJ
7.309 kJ
-3.124 kJ
-7.309 kJ
1.0 / 40.3 = 0.0248
0.5 / 24.3 = 0.0206
Moles MgO
Moles Mg
-3.124 / 0.0248 =
ΔH mol-1
-125.97 kJ mol-1
(value rounded up)
-7.309 / 0.0206 =
-354.80 kJ mol-1
(1) MgCl2(aq) + H2O(l)
MgO(s) + 2HCl(aq)
+125.97
(2) Mg(s) + 2HCl(aq
MgCl2(aq) + H2(g)
-354.80
(3) H2(g) + ½O2
H2O(l)
-286.0
(1) + (2) + (3) = (4)
+125.97 + (-354.80) + (286.0) =
(4) Mg(s) + ½O2 → MgO(s)
-515 kJ mol-1
Note reaction (1) has been reversed to provide the energy
data for the creation of MgO from MgCl2 and H20
Published value is -603 kJ
mol-1
Percent error
L3 Chemistry
T13 - 3(V2)
14.6%