6. Placing the alcohol lamp on a metal tray to prevent spillage of alcohol because of the highly flammable property of the alcohols.
7. The cap of the lamp should be used carefully to smother its flame.
8. Don’t contact the chemicals with skin.
Procedure
1. A data table for record the results of the experiment was design.
2. The temperature sensor was connect to the datalogger .The datalogger was connect to the computer .
3. Ensure the datalogging softwave is loaded and set to record the temperature of the sensor .Set the sampling rate to 1 Hz.
4. Weigh an aluminium can (a used Coke or 7-up can will do) empty.
5. Measure out 250 cm3 of water in the can. Take the temperature of the water.
6. Fill a bottle cap with methanol up to the rim of the cap.
7. Smear the whole length of about 4 cm of a thick string (not nylon or synthetic fibre) with methanol.
8. Push the string into the methanol leaving about 0.5 cm sticking out at the edge of the bottle cap and pointing up like a wick.
9. The alcohol lamp with methanol, including the cap were weighed and were recorded the mass.
10. The can was clamped in an upright position so that it will be just above the flame of the burner when lit.
11. The temperature was started recording.
12. Place the bottle cap and contents on top of a wooden block,
13. After 30 seconds, light the wick with matches and immediately place the can and its contents on a tripod
(do NOT add a wire gauze) to enable the burning wick to heat up the can of water directly. The wick should be positioned as to minimize heat loss
to surroundings and to ensure uninterrupted burning.
14. The water was gently stirred at frequent intervals using the temperature sensor. The temperature sensor should not be touched the bottom of the can.
15. When the temperature of the water has risen by 20 C, the flame was extinguishing and was immediately replaced the cap.
16. The temperature was continued stirring and was recorded for an additional 2 minutes. File of data was saved.
17. The alcohol lamp and cap as soon as possible were re-weighed after extinguishing the lamp.
18. Steps (6)-(17) were repeated using ethanol and propan-1-ol.
19. The heat capacity of the calorimeter and hence the heat of combustion of ethanol and propan-1-ol respectively were calculated.
Result
The graph showing the temperature change in certain time interval
Calculation
The change in weight of C-1 (Methanol) : 1.95g
Temperature change of water : 20oC
Specific heat capacity of water = 4.2 KJ Kg -1 K-1
The mass of the water = 250cm3 = 250g
Given: the standard heat of combustion of methanol = -715kJmol-1
The heat capacity of calorimeter = C
The mole of Methanol = 1.95g / 12 + (1)(4) + 16 = 0.060 mol
E=mc△T
(715)(0.060)=(250/1000 x 4.2+heat capacity of calorimeter)(20)
42.90=(1.05+C)(20)
C=1.095(kJ)
I.e. The specific heat capacity of calorimeter = 1.095kJ Kg -1 K-1
Ethanol
The change in weight of C-2 (Ethanol) : 1.87g
Temperature change of water : 20oC
Specific heat capacity of water = 4.2 KJ Kg -1 K-1
The mass of the water = 250cm3 = 250g
Heat evolved = MC△T
= MC△T of water + MC△T of calorimeter
= (250/1000x4.2+1.095)(20)
=42.90(kJ)
The mole of Ethanol = 1.87 / (12)(2) + (1)(6) + 16 = 0.041mol
Heat of combustion of Ethanol = -42.90/0.041
= -1046.34(KJmo-1)
Propan-1-ol
The change in weight of C-3 (Propan-1-ol) : 1.60g
Temperature change of water : 20oC
Specific heat capacity of water = 4.2 KJ Kg -1 K-1
The mass of the water = 250cm3 = 250g
Heat evolved = MC△T
= MC△T of water + MC△T of calorimeter
= - (250/1000 x 4.2+1.095)(20)
= 42.90KJ
The mole of Ethanol = 1.60 / (12)(3) + (1)(8) + 16 = 0.026mol
Heat of combustion of Ethanol = -42.90 / 0.026 = - 1650KJmol-1
Discussion
1. The slope of C1 > C3 > C2. C1 required the least time to rise temperature by 20oC.
2. The actual heat of combustion of Methanol, Ethanol and Propan-1-ol are -715 KJmol-1, -1370KJmol-1, -2010 KJmol-1 respectively.
That means a lot energies were lost. Therefore the accuracy of the result is quite low.
In order to improve the accuracy, two polystyrene board should be stand near the apparatus as shield. A cap of the soft drink can should be used for covering.
Also put the alcohol lamp closer to the can. All of them are used for preventing heat loss to the surrounding.
3. Using Coke can (made up of aluminium) in this experiment is that the heat can be transmitted ideally to water since aluminium is a good conductor of heat.
Besides, aluminium can is so light (relative to 250g of water, it's only 12g) that it's specific heat capacity may be neglected.
4. The initial temperature of water should not be much higher than room temperature and the distance between bottle cap & Coke can should not be
too long to reduce heat loss to surroundings.
5. In this experiment, simple calorimetry is used to calculate the energy content of alcohol per gram, no heat loss to surroundings is assumed.
However, the experimental value should be less than expected since there are some source of errors in this experiment:
(i) heat lost from top and sides of aluminium can
(ii) incomplete combustion of alcohol may occur
(iii) specific heat capacity of aluminium can is ignored
Some improvements may be done to minimize the errors:
(i) determine the specific heat capacity of Coke can and count it.
(ii) a lid with a hole (to insert the thermometer) to cover the Coke can and a draught screen surrounds the calorimeter.
Conclusion
After this experiment, it is found that there are some error occurred. The experiment was not carried out under standard conditions. Also between the can and alcohol lamp, there was a large separation which may affect the heat transfer to a calorimeter soft drink can form alcohol lamp and loss to the surrounding.
Percentage of experiment error:
Ethanol : [(1370-1046.34)/1370](100%) = 23.62%
Propan-1-ol : [(2010-1650)/2010](100%) = 17.91%
Simple calorimetry can be used to calculate the energy content of alcohol per gram. These values are important to scientists.
