Data Collection:
Time: 60 sec
Volume of Water: 100 cm3
Initial Temperature: 23°C
Final Temperature: 52° C
∆T = 52°C – 23°C
ΔT = 29°C
Data Processing:
- Finding out how many moles of propane were combusted.
-
P→ 101.3 kPa which is equal to R.T.P
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V→ Volume of gas per second X time
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R→ 8.314
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T→ 23°C
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N→ Unknown Variable
n = PV .
RT
n = (101.3kPa)(5.35cm3 x 60 seconds)
(8.314)(23 + 273)
n = (101.3kPa)(0.32dm3)
(8.314)(296k)
n = 0.013 moles
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The amount of heat released. (Specific Heat Capacity)
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Volume of water→ 100 cm3
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Mass of water→ 100g
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Density of water→ 1g/ cm3
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C= 4.18 Jg-1k-1
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∆T = 29.0 °C
Q = (100g) (4.18 Jg-1k-1) (29.0 C°)
Q= 11890
Q= 12.0 kJ
- Finding the enthalpy of combustion. (Per mole)
∆HC = Q/mol
∆HC = 12.0kJmol
0.013mol
= 923.0 kJ
-
Yet, this combustion is exothermic (releases heat)
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Making it a negative answer.
-
∆HC = - 9.23 x 102 kJ/mol
Uncertainties: (using the Data Booklet)
Uncertainties:
Thermometer: ± 0.5°C
Beaker: ± 0.5 ml
Stopwatch: ±0.04 sec
Graduated Cylinder: ± 2.5 cm3
Uncertainty for final temperature:
(0.5°C/23°C) x 100
± 2.17%
Uncertainty for n = PV
RT
= 2.19 + 2.17
= ±4. 36%
Uncertainty for volume of water:
(2.5cm3/100cm3) x 100
2.50%
Uncertainty for Change of temperature:
(0.5°C/29°C) x 100
= 1.72 %
Uncertainty for Q=mc∆T:
= 2.50 + 1.72
= ±4.22 %
Uncertainty for ∆HC = Q/mol:
= 4.36 + 4.22
-
∆HC = - 9.23 x 102 kJ/mol ± 8.58%
Percentage Error
- Using the data booklet we could find the error.
- Percentage error of the literature value of the Enthalpy change of combustion.
-2220kJ – (-923kJ) x 100
- 2220 kJ
58.42% error
Conclusion:
The purpose of this experiment is to be able to determine the enthalpy change of combustion of propane. After a series of steps, the experiment was successful, however, the experiment could be done in a more efficient and accurate way. After all the calculations, this experiment proved to have a 58.42% error when compared to the percentage error of the literature value. Therefore, we can conclude that this experiment wasn’t the ideal way of finding out the enthalpy change of propane combustion.
First and foremost, one of the biggest errors that occurred in this lab that most of the groups encountered the approximation of the amount of gas flowing per second. As this experiment has to be done in a series of trials, we have to close and open the tap many times. However, the mistake occurs when many students don’t know how to keep the gas flow consistent throughout all three trials. In addition, another big error that was exhibited during the experiment is the wastage of heat to the air. This tremendously affects the uncertainty because when the beaker heated with the Bunsen burner, most of the heat was able to escape, causing a shortage of energy. Since the combustion of propane is exothermic, meaning that it releases heat, contradicts the experiment because when the water is heated it looses most of its energy. Subsequently, another error would be that the pressure could have been different, however in this case the pressure was the R.T.P (23°C). Moreover, other errors this we encountered has to deal with the percent uncertainties these materials produce that affect the end result. As an individual uncertainty it won’t affect the experiment, however, with all the uncertainties combined it makes a drastic change.
To encapsulate, this experiment was more of a demonstration to explain how an experiment can have errors, which can drastically change the results. In this case, there are several ways of improving.
- Find the exact pressure measurement using the barometer.
- Find the exact density using pycnometers.
- Use materials that have lower uncertainties.