Fig1: Experiment setup.
Method
Safety must be ensured at all times, as highly flammable substances are being burned here. All students must wear eye protection during the experiment, stay standing during the experiment, and be aware of the flames around them.
- The apparatus is set up as shown in fig 1.
- The alcohol candle is weighed on the balance, and the mass recorded.
- The mass of water in the can measured and recorded
- The height between the candlewick, and the base of the can must be set and recorded, measuring the distance with a ruler.
- The temperature of the water is measured and recorded using the thermometer.
- The candle is lit and the thermometer monitored until the water has risen by a given temperature.
- The candle is blown out, the burnt wax on the base removed of the candle, and the candle is weighed.
The data produced by this method will then be calculated by the formulas:
∆ H (j) = mass of water (g) X specific heat capacity of water (S.H.C) X temperature rise (°C)
The result of this is found per gram of alcohol burnt by dividing it by the change of mass
∆ H per gram of alcohol burnt (J/g) = ∆ H (°C) / change in mass (g)
The results of this are the found per mole of alcohol burnt.
∆ H per gram of alcohol burnt per mole (J/mole) = ∆ H per gram of alcohol burnt (g) X RMM of alcohol
Variables
There are two types of variables: dependant variables and independent variables. Dependant variables are those that are kept constant at a set value, so that they cannot cause variation in the results, by changing throughout the experiments. There should only be one independent variable in a test, this is what is being tested. Therefore the dependent variables for this investigation are as follows (see fig 2):
- The distance of the wick from the base of the can
The distance of the flame from the base of the can may vary the results greatly, as it will decide how much of the flames heat is transferred directly to the can and to the water. The experiment should allow the flame to contact the can at its hottest point. The most accurate and safe way to measure flame distance from the can is to measure the distance of the wick. For the purposes of our experiment this should be accurate enough. Measuring the distance from the wick and the base of the can each time will control this variable.
- The mass of water heated.
The volume of water can vary the experiment greatly, as the greater the mass of water used, the more alcohol will be combusted to reach the target temperature. Also when a greater mass of water is used, the test will go on for longer, and thus a greater amount of energy will be transferred to the surroundings as wasted energy. Measuring the amount of water used each time will control this variable.
- The type of can used.
The type metal the can is made from is extremely important to the experiment, as different metals conduct heat with different efficiency and at different rates. Therefore the type of can used will affect the heat transfer to the water. Using the same can each time will control this variable.
- The heat increase
The heat increase can vary the experiment, as the greater the energy increase, the greater the mass of alcohol burnt, and the greater amount of energy transferred to the surroundings as waste. The start temperature may affect the results, as the higher the temperature the equipment reaches, the more heat will be wasted in the surroundings, according to the theory that heat energy moves from areas of high heat energy to areas of low energy. By allowing the equipment to cool after each experiment to a set temperature this variable may be controlled.
- Stirring of the water
Stirring the water will affect the results, as it means that the whole mass accounted for in the calculation will be being heated, not just one volume of water. The type of stirring in terms of frequency and power may affect the results also, and must be kept constant. To control this variable, the same type of stirring will be used each time.
The only variable in the experiment will be the changing type of alcohol being combusted, as this is what is being investigated.
PRELIMINARY INVESTIGATION:
In order to make the investigation as fair and accurate as possible, the method of testing must be tested by a series of preliminary tests. In the preliminary testing, the variables affecting the investigation are investigated so that they may be made constant and set at a suitable value to give the most accurate results. It is important to only vary one variable between two tests, to allow us to see the difference the change of just one variable has upon the experiment.
The preliminary tests were conducted as described in the method. Here different variables are tested and to be set at constant values for the main test.
PRELIMINARY RESULTS:
What is learnt?
I conducted only two preliminary tests where three or more are usually necessary, as I was confident that by the second test I had decided on dependent variable values that will serve me well in providing accurate and fair results in the main test. I felt that the wick should be kept at 2cm from the base of the can, as at this distance the flame produced was heating the can at its hottest point, therefore more heat is transferred directly to the water in the can. The type of can used was an aluminium “Dr Pepper” can, as they were readily available, and heated quickly providing fast and efficient heat transfer. For the preliminary investigations I set the start temperature at 20°C, as this was the temperature of the water straight from the tap. A temperature rise of 20°C will be a great enough raise to allow enough heat to transfer to the water to make an accurate calculation. It is also not a particularly high temperature thus the heat loss to the surroundings will be minimal. The mass of water used was the variable tested in the preliminary results. This can be seen to have on of the greatest affects on the results as it is used directly in the calculation. The water was stirred in both cases with the same frequency and force, however the lack of water in the first experiment meant that stirring was quite hard, and so the heat was not being transferred to the whole mass of water. This is reflected in the results, as we see a higher ‘MCDT / change in mass’ value for the experiment using only 100 ml of water. This is because the entire mass of water is not being heated as the stirring is inefficient, thus a lesser mass of candle is burnt producing a value too high. The second value is probably more accurate. However the length of time required to heat such a mass of water allows a greater value of heat energy to be ‘lost’ to the surroundings. The loss of heat energy to the surroundings may be limited by the use of draft shield, and so we find that the greater mass of water provides more accurate results. Water volume was the only variable tested, as it has the greatest effect on the results. Using my own scientific knowledge, reliable values may be set for the other variables.
The values are set as follows for the main testing:
- Distance from wick and base of can: 2cm
- Mass of water heated: 250g
- Type of can: Aluminium.
- Heat increase: 20°C
- Stirring of water: Frequent, and with mild force.
MAIN EXPERIMENT:
With the new values set for the dependant variables, we are ready to test the different types of alcohols as is required by the investigation. The experiment is conducted as stated in the method. In this testing it is important to maintain what is known as a fair test. A fair test was obtained by using the same equipment, and the same dependant variables throughout. Also we now conduct three experiments per alcohol to allow us to identify, eliminate and account for anomalies. This makes our investigation more accurate.
RESULTS:
The results of the experiment were recorded and tabulated as follows.
Anomalies
At this stage it is quite hard to identify anomalies, as the results are not yet calculated in a way that allow me to compare them to my hypothesis well. We do see a fluctuation in the results, however this can be expected due to dependant variables not controlled properly.
DATA ANALYSIS:
Now that we have got the data required for the investigation it is important to make accurate and relevant calculations in order to represent it well. Below I have demonstrated the equation (as stated in the method) used to calculate this data as an example. For this example I am using the first experiment conducted, which was for methanol.
- First I will find the heat transferred (∆ H (j)) during this reaction.
∆ H (j) = mass of water (g) X specific heat capacity of water (J/g) (S.H.C) X temperature rise (°C)
mass of water = 250g
S.H.C of water = 4.2 j/g
Temperature rise = 20°C
These figures are put into the equation to give
250 X 4.2 X 20 = ∆ H (j)
=21000 J
(These figures are all constants, and so the energy released will be the same for each experiment)
- Now I need to find the amount of energy per gram of alcohol burnt. This significance of this is that we are taking the amount of energy released overall, and looking at it in relation to the mass of the alcohol combusted.
∆ H per gram of alcohol burnt (j/g) = ∆ H (°C) / change in mass (g)
∆ H = 21000 kJ
Change in mass = 1.640 g
These figures are put into the equation to give
21000/1.640 = ∆ H per gram of alcohol burnt (j/g)
= 12.805j/g
- This figure must now be calculated in relation to the actual structure of the atom, so as to support our investigation. The current figure is useless by itself to use, as the number of methanol atoms per gram is different to the number of propanol atoms per gram. Therefore in relation to one another these results mean very little. So, we must find the energy released per mole of alcohol. This is found using the RMM of the alcohol
∆ H per gram of alcohol burnt per mole (j/mole) = ∆ H per gram of alcohol burnt (g) X RMM of alcohol
∆ H per gram of alcohol burnt = 12.805j/g
RMM of alcohol = 32
These figures are put into the equation to give
12.805 X 32 = ∆ H per gram of alcohol burnt per mole (j/mole)
= 409.756 j/mole
These calculations are made for each experiment using the equation functions of Microsoft Excel for speed and accuracy reasons. These calculations may be seen on the following table (over page):
Some features of this table should be noted:
- Results are given to three decimal places, as this gives us enough accuracy without using figures unnecessarily long.
- Averages are also given, not to better accuracy, eliminate anomalies.
- The results generally fluctuate, however the differences that are large enough to be classed as anomalous are highlighted in black.
DATA ANALYSIS:
To look at the accuracy of our data, we compare our own results to the ‘theoretical results’, which we may calculate using bond energy values. This is a system, looking at the structure of an alcohol molecule and the number and different types of bonds it has. We know the energy values of these bonds. Therefore we may calculate the energy released for each reaction. The process of doing this is as follows:
- Write out a balanced formula equation for the combustion of the alcohol tested.
- Using a table with bond types and energy values, look at the balanced equation and the structure of the molecule to record how many of each bond there is. Do this for both bonds broken and bonds made.
- Calculate the energy for the different bonds and add these up to give total energy used to make bonds and total energy released breaking bonds.
- Take away the total energy used to make bonds from the total energy released breaking bonds, to give the total energy for the reaction.
- To allow comparison to our own results calculate this figure to one mole of the alcohol.
The bond energy calculations for each alcohol may be seen below. These values were calculated using the equation function on Microsoft Excel.
Methanol
Balanced chemical equation: 2CH3OH + 302 → 2CO2 + 4H2O
Energy total (kJ): 5616 – 6932 = - 1316
Energy total per mole of alcohol (kJ/mole): -1316/2= - 658
Ethanol
Balanced chemical equation: C2H5OH → 2CO2 + 4H2O
Energy total (kJ): 4728 – 6004 = - 1276
Energy total per mole of alcohol (kJ/mole): - 1276/1= - 1276
Propanol
Balanced chemical equation: 2C3H7OH + 9O2 → 6CO2 + 8H2O
Energy total (kJ): 13296 – 17084 = - 3788
Energy total per mole of alcohol: -3788/2= - 1894
Butanol
Balanced chemical equation: C4H9OH + 6O2 → 4CO2 + 5H2O
Energy total (kJ): 8568 – 11080 = - 2512
Energy total per mole of alcohol: -2512/1 = -2512
Pentanol
Balanced chemical equation: 2C5H11OH + 15O2 → 10CO2 + 12H2O
Energy total (kJ): 20976 – 27236 = - 6260
Energy total per mole of alcohol: - 6260/2 = - 3130
Every result given is a minus figure, showing that this reaction is exothermic as energy is released in the reaction. The results of our own test are therefore minus figures in relation to the energy of the reaction, as the reaction is losing energy. The balanced formulas for methanol, propanol and pentanol all have 2 of the alcohol to balance the equation. Therefore to find the value per mole of the alcohol it is necessary to divide by 2.
An example of how I calculated these values is shown over the page, using methanol as an example.
Methanol
Balanced chemical equation: 2CH3OH + 3O2 → 2CO2 + 4H2O
The diagram shows the reaction as described by the formula.
At 1 energy is taken in during the reaction in order to break all the bonds. By calculating the number of each bond from these diagrams, we may then find out the total energy taken in using the bond energy values that we know already. There are:
-
6 C – H bonds broken. 6 X 413 = 2478J
-
2 C – 0 bonds broken. 2 X 358 = 716J
-
2 H – O bonds broken. 2 X 464 = 928J
-
3 O = O bonds broken. 3 X 498 = 1494J
This gives a total of 5616J taken in as bonds are broken.
At 2 energy is given out in during the reaction in order to create new bonds. By calculating the number of each bond from these diagrams, we may then find out the total energy given out using the bond energy values that we know already. There are:
-
8 H – O bonds broken. 8 X 928 = 3172J
-
4 C = 0 bonds broken. 4 X 805 = 3220J
This gives a total of 6932 given out as bonds are made.
The total energy given out is taken away from the total energy taken in to give the total energy exchange of the reaction. This gives – 1316J. To make this figure a representation of the alcohol being burnt, and allow comparison to our own results we must find this value per mole of alcohol. In our calculation two moles of alcohol are used, and thus the figure is divided by two. -1316/2= - 658J.
This may be seen on an energy level diagram, which shows the changes of energy during the reaction.
To assist summarizing the accuracy and reliability of my results, the theoretical results may be compared to my own results on a graph.
CONCLUSION:
In analyzing my findings it is important to restate the hypothesis:
I believe that the increased complexity of a molecule and the energy released by it are proportional.
When looking at the graphs we can see that my results support this hypothesis. This can be seen by the line of best fit, as it passes through the origin in a positive direction, denoting proportionality between energy released and the number of carbon atoms. The reason for this (as explained early with the bond energy calculations) is that each type of bond requires a certain amount of energy to break it, and the same amount of energy is released when this bond is formed. Thus as the molecule increases in complexity, the greater the number of bonds it will have, and the greater the amount of energy will be released when it is combusted. For example methanol has only one carbon atom, and according to the bond energy values produces only 658J of energy when combusted, whereas the more complex pentanol, with five carbon atoms produces 3130J of energy. As the reaction is releasing energy, we say the reaction is exothermic. The reason that a combustion reaction is exothermic relies on the principle that as bonds are broken energy is taken in and as bonds are made energy is given out. The reaction is exothermic because the amount of energy taken in by the reaction to break the bonds is less than the energy released when new bond are made. Thus more energy is released than is taken in, and the surroundings gain heat energy. Although the results are quite scattered and do not show perfect correlation, we are able to spot the trend that the increase of carbon atoms and energy released are proportional, thus proving the hypothesis correct.
The accuracy of my hypothesis may be seen by the similarity of the actual graphs and my hypothetical graph.
EVALUATION:
As the graphs show, the results of my experiment are not entirely accurate. They fluctuate in places, there are anomalies, and there is a significant difference between the theoretical results and the actual results. The theoretical results suggest that all the energy produced in the reaction is transferred to the water. In reality this is not the case as energy is transferred to other things other than just the water. Our results are lower than the theoretical results, thus not all the energy has been transferred to the water in the can. The results also fluctuate, suggesting that energy is not being transferred entirely to the can. Possible ways that this energy could have been otherwise transferred are as follows:
- As light energy in the flame
- Heating up the aluminium can
- Heating up the surrounding air
Although fluctuations did occur, there was only one fluctuation big enough to be counted as an anomaly. This anomaly was a value higher than the other results for that alcohol, suggesting an increased efficiency in the method. Other procedural problems were noticed. One major problem was the occurrence of incomplete combustion. This was noticed as a black layer of carbon was noticed to develop on the base of the can, thus the burner is losing mass that is not being used to heat the water. As a result my initial calculations are slightly inaccurate. It also meant that heat energy transfer from the flame to the water is less efficient, as it must heat the added carbon in the process of heating the water.
The dependent variables were controlled during the experiment by the following methods:
- The distance from the base of the can and the wick was kept constant by measuring this distance to a set value of 2cm each time.
- The mass of water heated was controlled by measuring the amount of water in a measuring cylinder to a set value of 250ml each time.
- The type of can used was kept constant by using the same can each time.
- The starting temperature of the equipment was kept constant by allowing the equipment to cool to a temperature of 20˚C each time.
- The stirring of the water was kept constant by stirring the water frequently with medium vigor for each experiment.
These were controlled quite well, as we see only minor fluctuations in the results. However the difference between the theoretical results and these results suggests that our method was not greatly efficient. The main areas that were ineffective in providing accurate results were:
- Although draft excluders minimized cold drafts entering the experiment area, it did not stop heat energy being transferred to the surroundings.
- For the water to be heated it was necessary for the can to be heated first, thus energy is “wasted” in heating the can.
- The methods of measuring may not always have been totally accurate. For example it was noticed on a few occasions, that although the burner had been blown out, the temperature continued to rise. Therefore it has taken time for the energy transferred to the water to be measured on the thermometer, and thus a smaller temperature recording of 20˚C has been used in a calculation with a mass loss that is relative to a greater temperature.
If able to repeat this investigation with better equipment it would be possible to improve the method so as to provide more accurate results. Some improvements might be:
- Electronic measuring equipment used to increase accuracy by eliminating human error.
- The energy taken to heat the can accounted for accurately in the calculations.
- The same type of burners used each time, so that the type of flame produced by each alcohol is similar.
- Somehow controlling the heat conditions around the experiment so that the surrounding temperature is higher, and thus less heat is transferred to the surroundings.
- Then use of a bomb calorimeter measures the energy transfer to a greater accuracy.
- Supply oxygen directly to prevent incomplete combustion.
Further investigation could include continuing to test different alcohols in the series to see if this trend continues. Also to look at the combustion of other fuels could aid investigating the relationship between complexity and energy produced.
EVALUATE….
Incomplete combus
Taking off blackness
Wax on base