No. of moles of methanol used = 1.7(12+4+16)
= 0.0531 mol
Heat capacity of the can = (-715 x 0.0531)(-20.6)
= 1.84 kJoC-1
No. of moles of ethanol used = 1.28(12x2 + 6 + 16)
= 0.02783 mol
Enthalpy change of combustion of ethanol = (-21.4x1.84)0.02783
= -1414.9 kJmol-1
No. of moles of propan-1-ol used = 1.27(12x3 + 8 + 16)
= 0.0212 mol
Enthalpy change of combustion of propan-1-ol = (-22.2x1.84)0.0212
= -1926.8 kJmol-1
From experimental result, methanol has the lowest enthalpy change of combustion whereas ethanol is the second. Propan-1-ol has the highest enthalpy change.
Methanol, ethanol and propan-1-ol are all alkanols which belong to the same homologous series. When the number of carbon atoms is increasing, it is difficult for the alcohols to undergo complete combustion as van der Waals’ forces increases. Therefore, the energy required for 1 mole of alcohol to undergo complete combustion increasing along the homologous series.
-Accuracy of Results and Source of Errors
The value obtained from the experiment is greater than the actual value. It may due to some sources of errors in the experiment:
- Heat may be lost to the surroundings
To reduce heat lost to the surroundings, the hole of the can was blocked by tissue paper to avoid heat loss by convection. Moreover, cotton wool or cloth was used to wrap the can to reduce heat loss by conduction.
- Some heat is brought away by air instead of absorbed by the calorimeter
A shield formed by insulating mats was installed for the experimental set-up so that heat will not be brought away by air current easily.
- Incomplete combustion
Due to insufficient supply of oxygen, carbon or carbon monoxide may have been produced rather than carbon dioxide. To avoid incomplete combustion, a flame calorimeter can be used instead of aluminium can calorimeter.
- Uneven distribution of heat
To avoid this problem, we should stir the water at frequent intervals throughout the experiment.
- Reusing aluminium cans
When the experiment is repeated using the same aluminium can, carbon formed from incomplete combustion may stick on the bottom of the can. It will add weight to the can thus the heat capacity of can is different.
We should clean the can before using it repeatedly. After cleansing, measure the mass of the washed can to see if it coheres with the initial one to reduce error.
-Ignorance of Heat Capacity of the Aluminium Can
The heat capacity of the aluminium cannot be ignored.
Heat capacity of water = (250 x 4.2)1000
= 1.05 kJoC-1
Heat capacity of aluminium can = 1.84-1.05
= 0.794 kJoC-1
The heat capacity of the aluminium can is quite significant when compared with water. Large proportion of heat from combustion is absorbed by the aluminium can. If the heat capacity of the aluminium can is ignored, the error in calculation would be very significant. The percentage error of the experimental result will be more than 30% which is very inaccurate.
-Choice of Container
In the experiment, aluminium can is used instead of glass or other materials. This is because aluminium is a good thermal conductor. Also, it has high melting point which can withstand with burning. Moreover, it’s light and can be obtained easily. However, it is not transparent so we cannot observe if convection currents affect the measurement of temperature by data-logger.
In the experiment, the heat capacity of the can is 1.84 kJoC-1. Also, the enthalpy change of combustion for ethanol and propan-1-ol are -1414.9 kJmol-1 and -1926.8 kJmol-1 respectively.
The objective of the experiment has been fulfilled as the enthalpy change of combustion of alcohols is successfully determined.