PRACTICAL 4.2: Enthalpy Changes in Combustion Reactions
4.1.1
–
–
4.1.2
conduct practical investigations to measure temperature changes in examples of endothermic and
exothermic reactions, including:
4.1.1a combustion
4.1.1b dissociation of ionic substances in aqueous solution
investigate enthalpy changes in reactions using calorimetry and 𝑞 = 𝑚𝑐𝛥𝑇 (heat capacity formula) to
calculate, analyse and compare experimental results with reliable secondary-sourced data, and to
explain any differences
Aim: To calculate the molar enthalpy of combustion for a series of fuels and compare these
values to theoretical values.
Equipment/Materials:
Spirit burners containing different alcohols
250 mL aluminium beaker
digital thermometer
Risk Analysis:
Hazard
100mL measuring cylinder
digital balance
Risk
Minimisation
Bunsen burner flame
Burns to body
Don't wear loose clothing / tie hair up
Alcohols
Damage to eyes
Protective glasses
Procedure:
1. Set up the apparatus as shown.
2. Accurately measure 120 mL of water into the aluminium
beaker.
3. Measure and record the initial temperature of the water.
4. Accurately weigh the spirit burner and cap.
5. Remove the cap and light the spirit burner.
6. Allow the flame to heat the water in the beaker until a temperature change of approx. 40 °C is achieved.
NOTE: you may need to adjust the height of the beaker so that the bottom of the beaker is just above the top of the flame.
7. Extinguish the flame with the cap and continue to monitor the temperature of the water until a maximum
has been reached. Record this value.
8. Reweigh the burner and lid and record the mass.
9. Repeat Steps 1-8 for a second trial (and 3rd trial if time permits).
Results:
Fuel: Methanol (CH4OH)
Fuel: Ethanol (C2H5OH)
Fuel: Propan-1-ol (C3H7OH)
Trial
1
Trial
2
Trial
3
mH2O
mi(fuel
)
mf(fuel
)
98.28
264.6
5
263.6
5
96.65
263.6
5
263.0
6
98.47
263.0
6
262.1
5
Ti
16.7
19.0
18.6
Tf
37.9
32.6
40.5
mH2O
mi(fuel
)
mf(fuel
)
Trial
1
100.6
8
247.6
5
246.8
9
Trial
2
100.5
239.7
9
239.2
5
Trial
3
100.3
6
238.7
5
238.1
8
Ti
19.6
17.2
18.1
Tf
48.2
37.1
39.0
mH2O
mi(fuel
)
mf(fuel
)
Trial
1
101.1
3
247.3
8
246.8
0
Trial
2
101.4
2
246.8
0
246.2
8
Trial
3
99.64
246.0
0
245.4
5
Ti
17.1
18.7
17.0
Tf
38.4
40.6
37.7
Note: mi = initial mass in g; mf = final mass in g
Ti = initial temperature in °C
Discussion:
For propan-1-ol:
1. Write a chemical equation to show its complete combustion.
2 C3H7OH + 9 O2 → 6 CO2 + 8 H2O
2. Use q = mcΔT to calculate the heat of reaction for Trial 1.
q = 101.13 x 4.18 x 21.3 = 9004J or 9kJ
3. Divide the “q” value by the mass of propan 1 used to establish the energy content (J/g) for this substance.
9004 / 0.58 = 15524J/g
4. Determine the enthalpy of combustion (ΔHcombustion) in (kJ/mol) by multiplying your value from Q.3 by the
molar mass of this substance, then converting units as required.
15524J/g x 60.095 g/mol = 932914J/mol = 932.9kJ/mol released therefore enthalpy = 932.9kJ/mol
5. Repeat the calculations for the other two trials.
Trial 2 q =101.42 x 4.18 x 21.9 = 9284J 9284/0.52 = 17853J/g 17853 x 60.095 = 1072876J/mol =
1072.9kJ/mol = -1072.9kJ/mol
Trial 3 q = 99.64 x 4.18 x 20.7 = 8621J 8621/0.55 = 15675J/g 15675 x 60.095 = 941989J/mol =
942.0kJ/mol = -942.0kJ/mol
6. Determine the average enthalpy of combustion from the three trials conducted.
(-932.9 + -1072.9 + -942.0) / 3 = -982.6kJ/mol
7. Find the theoretical value for the enthalpy of combustion of propan-1-ol (a.k.a. n-propanol). Compare your
experimental values to theoretical data.
Experimental -982kJ/mol vs theoretical -2008kJ/mol
8. Calculate the efficiency of this calorimetric process:
% Efficiency = 100 x experimental value
theoretical value
48.9% efficient
9. Account for differences by identifying major error sources and justify the impact of these errors on your
calculated values.
The system in which the experiment was conducted was very much open to the world around it
allowing massive amounts of energy to escape into the surrounding environment, large amounts of
energy would have also been lost to the glassware in which the water was being held. These sources of
error should drastically decrease the value for enthalpy (which it did)
10. Comment on:
a. The reliability of this investigation.
Fairly reliable as consistent results were obtained across multiple different alcohols using the
same procedure
b. The validity of the data obtained (consider what assumption(s) must be made in your calculations).
The data obtained however was not valid whatsoever as the experimental value was wildly
different to the theoretical
Conclusion:
Consistent molar enthalpies for a variety of alcohols/fuels can be obtained using a simplistic setup,
however this simplistic setup has many limitations and does not give accurate/valid results due to the
massive amounts of energy which leave the system. (This escape of energy is what lead to the large
discrepancy between the experimental and theoretical values)
(Note: Data from three different fuels is provided to enable you to practice these calculations, although only the
data for propan-1-ol is used in this virtual investigation.)