--------------------------------------POSSIBLE VARIABLES-----------------------------------
-- Type of Alcohol (Selected Variable)
-- Container size/Volume of water
-- Type of container
-- Volume/Mass of alcohol
-- Distance between wick and container
-- Starting temperature of water
FAIR TEST?
A fair test was maintained by keeping the above variables (apart from type of alcohol) constant in each experiment.
Altering the volume of water alters its mass, therefore affecting the equation: mass of water x rise in temp x SHC (specific heat capacity), and affecting the ability of the heat to be conducted. It is therefore easier to use the same amount of water in each experiment.
The same container will be used throughout to keep the volume and heat conduction constant. The chosen volume is 20ml, which will give the water a mass of 0.02kg.
The distance between the wick and the container will be maintained. The burner will be placed on a wooden block to reduce heat lost to the surroundings.
The starting temperature should remain constant because all experiments will be conducted at room temperature.
------------------------------------------HYPOTHESIS------------------------------------------
Increasing the number of carbon atoms in the molecule of the fuel will increase the energy released, as the bonds are broken. The total energy released will be increased by the amount of energy released in the breaking of a single c-c bond as each carbon atom is added. The energy released in the reaction will therefore be proportional to the number of carbon atoms in the ‘carbon chain’.
Obtaining Results
APPARATUS
- Beaker
- Tripod
- Spirit burners (containing ethanol, propanol and butanol)
- Thermometer
- Balance
- Gauze
- Wooden block
DIAGRAM
METHOD
The spirit burner containing ethanol was weighed on the balance and it’s mass recorded. The apparatus were then set up as shown in the diagram above, and the beaker filled with pure water. The temperature on the thermometer was recorded.
The burner was lit when it had been made sure that approximately 0.5cm of wick extruded the top. The burner was allowed to burn until the water reached a temperature of 60o C and was then put out. The burner was reweighed and the change in mass calculated.
The same was done for propanol and butanol. The same balance and thermometer were used and the beaker was allowed to cool before each experiment.
SAFETY
To insure that the experiments were conducted as safely as possible, all precautions required when working with fire were taken;
- Long hair was tied back
- The equipment was not touched when the burner was alight
- Equipment was allowed to cool before it was put away
- Goggles were worn
- The experiments were conducted at a safe distance from the edge of the lab table.
-
The burners were handled carefully when being carried and weighed to prevent them from shattering and spilling the flammable alcohols. Luke
Results
TABLULAR FORM
CALCULATIONS
The heat produced in these exothermic reactions has an energy value, which can be calculated through the formula:
Mass of water X temperature rise x Specific heat of water (kg)
Below are the ‘ideal energy values’ (J/mol). These were obtained using knowledge of the equation used to find the ‘heat of the reaction’ i.e. the difference in energy between the reactants and products. It is given the symbol ΔH. By definition,
Heat of reaction, ΔH = Energy of products – Energy of reactants
Using the balanced equations for each of the reactions I continued as follows:
Ethanol
C2H5OH + 3O2 ➔ 2CO2 + 3H2O
➔
5(C-H) = 5 X 412 = 2060
(C-C) = 348
(C-O) = 360
(O-H) = 463
3(O=O) = 3 X 496= 1488 +
4719
4(C=O) = 4 X 734 = 2936
6(O-H) = 6 X 463 = 2778 +
5714
5714
4719-
995 ←This is the difference in energy (ΔH). Therefore, 995KJ/mol is the energy released.
Propanol
2C3H7OH + 9O2 ➔ 6CO2 + 8H2O
So to work out the energy per mole I used:
C3H7OH + 4½O2 ➔ 3CO2 + 4H2O
➔
7(C-H) = 7 X 412 = 2884
2(C-C) = 2 X 348 = 696
(C-O) = 360
(O-H) = 463
4½(O=O)=4½X496 =2232 +
6635
6(C=O) = 6 X 734 = 4404
8(O-H) = 8 X 463 = 3704 +
8108
8108
6635-
1473 ←This is the difference in energy (ΔH). Therefore, 1473KJ/mol is the energy released.
Butanol
C4H9OH + 6O2 ➔ 4CO2 + 5H2O
➔
9(C-H) = 9 X 412 = 3708
3(C-C) = 3 X 348 = 1044
(C-O) = 360
(O-H) = 463
6(O=O) = 6 X 496= 2976 +
8551
8(C=O) = 8 X 734 = 5872
10(O-H) = 10 X 463 = 4630 +
10502
10502
8551-
1951 --This is the difference in energy (ΔH). Therefore, 1951KJ/mol is the energy released.
Analysis
The obtained results do not at all reflect the ideal results. They are much lower and conform to no visible line of best fit. It is not possible to single out a result as an anomaly, because there are only three. It can be accepted that within the obtained results, however, there is an anomaly - it is the result for butanol. Butanol should have had the highest energy released when combusted, but due to heat loss to the surroundings and incomplete combustion (see evaluation) the results for butanol were very poor. The most accurate result, if any, was that of the combustion of ethanol because there was complete combustion and all the product bonds were formed correctly.
Conclusion
Ethanol, propanol and butanol are alcohols. They react with oxygen in exothermic reactions, which means that energy is released in the form of heat and light. They have a ‘chain’ of carbon atoms that form the ‘backbone’. It is the length of this ‘carbon chain’, which affects the amount of bonds to be broken, and therefore, the amount of energy released. For every added carbon atom there are two C-H bonds and one C-C bond, which need to be broken, increasing the energy released by the total energy value of those bonds. In such conditions only a small amount of the energy released can be transferred to the water
Evaluation
The obtained results were much lower than the ideal results. This may be due to the lack of an efficient energy transfer from the wick to the water.
The greatest error, in this investigation was the amount of heat energy lost into the surroundings. Heat was continually being lost in all directions through conduction, convection and radiation. The heat loss would have greatly been reduced by the presence of some draught shields with reflective surfaces and a copper calorimeter could have been used so that the energy of the heat lost through its sides could be measured using knowledge of the specific heat capacity of copper.
For even more accurate results the experiment could be don in a bomb calorimeter: the sample to be burned is placed inside the bomb calorimeter, which is then completely sealed and the fuse lit. The material the calorimeter is made from, and the fact that it is completely sealed means that no heat that is evolved is lost, and so can be measured more accurately.
When heated to such a temperature the water in the beaker begins to evaporate decreasing the mass of the water, and altering another variable. At higher temperatures, heat is lost faster to the air and out of the calorimeter, due to the greater heat difference. It was therefore unnecessary to allow the water to reach temperatures above 40, that way results would be more accurate and easier to calculate and less heat would be lost this way.
There was heat conducted into the gauze and the beaker, this energy was not added to the equation because it could not be measured.
Another error is that of incomplete combustion. Complete combustion occurs if there are lots of oxygen atoms available when the fuel burns, then you get carbon dioxide (carbons atoms bond with two oxygen atoms).
A limited oxygen supply causes some carbon atoms to be released before they can bond with any oxygen atoms. When propanol and butanol were burned a fine layer of soot (carbon) was formed on the bottom of the beaker and there was an orange flame. These are indications of incomplete combustion. Since heat is given out when bonds form, less energy is given out by incomplete combustion, thus, it affects the outcome of the experiment. To overcome this problem, it would have to be made sure that a sufficient supply of oxygen was involved in the reaction.
Error could have also occurred where the temperature had to be observed. Instead of a thermometer a temperature probe connected to a computer could have been used, this way there would have been no error involved in reading the thermometer.
In further investigations, one could examine the combustion of fuels methanol, pentanol and hexanol. Investigations into the rate of reaction and the response to varying the material of which the beaker, or calorimeter is made are other ideas.
Overall it can be said that the chosen method was unsuccessful in transferring a sufficient amount of the energy from the wick, where it was released, to the water. Heat was lost into the atmosphere in a number of ways. This heat was not, or could not be measured or accounted for. The main problem that the method gave was that concerning the limited number of results (only three alcohols were tested). With only three results it is impossible to single out anomalies and a correct line of best fit cannot be drawn. It has been decided that al results obtained are ‘anomalies’ to an extent because they are so different to the ideal results because much of the energy was lost to the surroundings and incomplete combustion occurred.