Specific Heat Capacity of Water
It takes exactly 4.2 joules of energy to heat 1ml of water by 1oC. This is called the specific heat capacity of water.
This formula uses the specific heat capacity of water to work out the energy supplied to an amount of water:
Energy Supplied (J) = Mass of Water (g) * Specific Heat Capacity of Water (J/gOC) * Temp Change (OC)
If we say that the energy supplied is A and the amount of alcohol burnt is B then we can use the formula A/B to work out the amount of energy liberated for 1g of alcohol.
We can use the formula to work out how much 1 mole of the alcohol would give out by adding up all of the atomic masses in the compound and then multiplying that by the energy given out by 1g of the alcohol. (This is shown in detail a little later.
Safety
The experiment we are going to carry out is relatively safe but obvious precautions such as not standing whilst doing the experiment will have to be followed. Goggles and gloves will be worn at all times both when conducting and following the test. Alcohols are highly flammable and there is always the possibility that the glass alcohol burner may explode, for this reason, we will use goggles at all times when using the burners. We will use gloves when moving the container, also any metallic equipment used will have heated up after using the burner so any transportation of these items would have to be done with caution to avoid being burnt or scolded by the water. We will always carry and store the burners and containers carefully to avoid any risk of breaking them and spilling the contents.
Equipment:
Burners for all three alcohols
A copper calorimeter
A test-tube
A beaker
A clamp stand
Heatproof mats (draft excluder)
Heatproof gloves
Goggles
Digital Thermometer
A stopwatch
Matches/ Lighter
Electronic Scales
Prediction:
I think that the alcohols with the most carbon atoms will have higher amounts of energy released from them. As you ascend the alcohols methanol → ethanol, ethanol → propanol, and propanol → butanol each step up adds two hydrogen atoms, three carbon atoms, and three oxygen atoms. These are available for combustion to form one water molecule and one carbon dioxide molecule. Therefore there is a net energy gain every time.
Preliminary Experiment:
We heated 25ml of water for two minutes in a copper calorimeter, a test-tube and a beaker, we took the temperature and the weight of the burner before and after testing because we can work out which container is the most effective from that.
Trial Experiment Results
First Experiment
Mass of Water (g)* Specific Heat Capacity of Water (4.2)* Temp Change (OC) = Energy Supplied (j)
Energy Supplied/ Amount of Alcohol Burnt= Energy Supplied for 1g
Energy Supplied By One Gram* Weight of One Mole of Alcohol= Molar Heat of Combustion for One Mole of Alcohol
For The Boiling Tube
This is how we work out the energy released from the combustion of the alcohol.
Take the mass of water (25) multiply it by the specific heat capacity of water (4.2) multiply that by the temperature change (13) → this gives the energy absorbed by the water.
25*4.2*13 → 1365J
We then take the energy supplied to the water (1365J) and then divide this by the amount of alcohol used (0.42) → this gives us the energy absorbed by the container when one 1g of alcohol was burnt.
1365/0.42=3250J
One mole of butanol weighs 74g so we times the energy used by 1g by 74. This will therefore give us the energy absorbed by the water when one mole of alcohol is burnt.
3250*74=240500J
Boiling Tube
25*4.2*13=1365
1365/0.42=3250
3250*74=240500
One Mole of Butanol: 241kJ
Beaker
25*4.2*46=4830
4830/0.45=10733.3
10733.3*74=794264.2
One Mole of Butanol: 794kJ
Copper Calorimeter
25*4.2*49=5145
5145/0.41=12548.8
12548.8*74=928611.2
One Mole of Butanol: 929kJ
Second Experiment
Boiling Tube
25*4.2*16=1680
1680/0.40=4200
4200*74=193200
One Mole of Butanol: 311kJ
Beaker
25*4.2*41=4305
4305/0.53=8122.6
8122.6*74=373639.6
One Mole of Butanol: 601kJ
Copper Calorimeter
25*4.2*28=2940
2940/0.28=10500
10500*74=483000
One Mole of Butanol: 777kJ
A Table Showing the Heat of Combustion for Each Container
From our original results it may have seemed like the beaker had performed the best but the beaker also used a lot of fuel. From both the graph and the table (above), we can see that the copper calorimeter performed clearly better than the other two. This is because the base of the copper calorimeter is larger (therefore having a larger area of conduction) than that of either the beaker or the test-tube and copper is a much better conductor than glass.
Plan:
For our real experiment, we are going to record the weight of the burner and the water before and after the experiment and record the temperature of the water at the beginning and end. We use the temperature of the water and the weight of the burner to work out the molar heat of combustion of the alcohols. We average the mass of the water for each test and then use the more accurate averaged values to work out the molar heat more accurately. We will use gloves to carry the hot containers and wear goggles when using the burners
Method:
The method is very similar for both the preliminary tests and our real experiment. We used a draft excluder for our entire real experiment but in the preliminary tests we only used them for the second test.
- Firstly, I collected all of the necessary equipment and set it up as shown above.
- I measured out 25 ml of water and weighed the burner and set them both up in the test noting the weight of the burner.
- I wrote down the initial temperature of the water and set the clamp stand 5cm above the end of the wick on the burner. I set the wick on the burner to 0.5cm to keep the experiment fair. I have gone into detail about why I chose these variables already.
- I set up the draft excluder and then lit the wick while simultaneously starting the timer.
- I timed for two minutes continuously stirring for even heat distribution before extinguishing the flame and quickly recording the maximum temperature.
- When necessary I took the draft excluder apart and weighed the burner on a set of electronic scales.
- Finally I calculated the practical heat of combustion using the formula:
Energy Supplied to Water (J)=Specific Heat Capacity (J/gOC)*Mass (g)*Temperature Increase(OC)
We then use the energy supplied to water to work out the energy for one Mole of the substance.
- I repeated steps 1-7 for each alcohol and container.
The heat transferred to the water is the heat of combustion. It is essential that all the heat liberated from the combustion is transferred to the water and not wasted.
For example:
For butanol, (this is just one of the results we obtained from the real data):
The mass of water found by averaging the mass of water before and after testing, (74.195) * the specific heat capacity of water, (4.2) * the temperature change, (49) = the energy supplied to water.
74.195*4.2*49 = 15269J
The energy supplied to water (15269.331) / The amount of alcohol used (0.42) = Energy used by 1g.
15269.331/0.42=36355J
One Mole of Butanol weighs 74g so we times the energy used by 1g by 74.
36355.55*74=2690310J
Actual Experiment:
We weighed the burner and the container before and after testing and recorded the temperature change. We heated the container with 25ml of water for two minutes. We weighed the water before and after so we could make the test fairer by averaging the two, the average gives a more accurate picture of the mass of water used. We used heatproof mats to stop any drafts getting to the burner as this would have made our results less accurate. We used a digital thermometer to measure the temperature at the end of the test because they are more accurate. We tried to make sure that the wick on each of the burners was the same length to make the experiments more accurate. The container is held at a set position above the burner as to make the experiment more accurate.
To keep our tests fair we will have a set of variables which will remain constant throughout the test, these variables are;
-
The mass of water (25cm3).
- The container, (different containers would have different thicknesses and sizes, therefore possibly being more or less accurate than others).
- The length of the wick (0.5cm).
- The distance between the container and the flame must stay the same because if there was any difference in this then the experiment would be inaccurate.
-
The conditions in the room should stay the same, the temperature should be around 23OC and there should be hardly any breeze as this would move the flame around which would stop the heat getting to the container.
The variables that must be changed throughout the experiment are only the alcohols (and therefore the burner).
The measurements we will use in our experiment are as follows:
Moles (mol) we will only use this in calculations to find out the molar heat of combustion.
Millilitres (ml) we will use this when measuring the amount of water to heat.
Degrees Centigrade (C) we will use this to measure the temperature of the water after the experiment.
Grams (g) we will use Grams to weigh the alcohol before and after burning.
Real Results:
Ethanol
50*4.2*46=9660
9660/0.73=13233
13233*46
Heat transferred to the copper calorimeter by burning one mole of ethanol.
608718J
50*4.2*57=11970
11970/0.86=13919
13919*46
Heat transferred to the copper calorimeter by burning one mole of ethanol.
640274J
50*4.2*47=9870
9870/0.93=10613
10613*46
Heat transferred to the copper calorimeter by burning one mole of ethanol.
488198J
Butanol
50*4.2*49=10290
10290/0.42=24500
24500*74
Heat transferred to the copper calorimeter by burning one mole of butanol.
1813000J
50*4.2*57=11970
11970/0.56=21375
21375*74
Heat transferred to the copper calorimeter by burning one mole of butanol.
1581750J
50*4.2*48=10080
10080/0.56=18000
18000*74
Heat transferred to the copper calorimeter by burning one mole of butanol.
1332000J
Propanol
50*4.2*48=10080
10080/1=10080
10080*60
Heat transferred to the copper calorimeter by burning one mole of propanol.
604800J
50*4.2*44=9240
9240/1=9240
9240*60
Heat transferred to the copper calorimeter by burning one mole of propanol.
554400J
50*4.2*47=9870
9870/0.58=17017
17017*60
Heat transferred to the copper calorimeter by burning one mole of propanol.
1021020J
Table to show our results more clearly
From this table we can see that the best performing container was in fact the Beaker. The Copper Calorimeter came second and the Test-Tube did not perform nearly as well as either of these. The table shows that the alcohols come in the order Butanol-Propanol-Ethanol from best to worst.
We can work out the average enthalpies by dividing each result by three:
1736/3=579
2179/3=726
4726/3=1575
Bond Enthalpy Values for the Three Alcohols
Ethanol
Ethanol + Oxygen = Carbon Dioxide + Water
C2H5OH + 3O2 = 2CO2 + 3H2O
The Bonds Broken In the Reaction:
3*O=O 3*498=1494
5*C-H 5*413=2065
1*C-C 1*347=347
1*C-O 1*336=336
1*O-H 1*464=464
1494+2065+347+336+464=
+4706kJ
The Bonds Made In the Reaction:
4*C=O 4*805=3220
6*O-H 6*454=2724
3220+2724=
5944kJ
4706-5944=-1238
The theoretical molar heat of combustion of ethanol is 1238kJ
Propanol
Propanol + Oxygen = Carbon Dioxide + Water
2C3H7OH + 9O2 = 6CO2 + 8H2O
The Bonds Broken In the Reaction:
9*O=O 9*498=4482
14*C-H 14*413=5782
4*C-C 4*347=1388
2*C-O 2*336=672
2*O-H 2*464=928
4482+5782+1388+672+928=
+13252
The Bonds Made In the Reaction:
12*C=O 12*805=9660
16*O-H 16*454=7264
9660+7264=
16924
13252-6924=-3672
3672
3672/2=
The theoretical molar heat of combustion of propanol is 1836kJ
Butanol
Butanol + Oxygen = Carbon Dioxide + Water
C4H9OH + 6O2 = 4CO2 + 5H2O
The Bonds Broken In the Reaction:
6*O=O 6*498=2988
9*C-H 9*413=3717
3*C-C 3*347=1041
1*C-O 1*336=336
1*O-H 1*464=464
2988+3717+1041+336+464=
+8546
The Bonds Made In the Reaction:
8*C=O 8*805=6440
10*O-H 10*454=4540
6440+4540=
10980
8546-10980=-2434
The theoretical molar heat of combustion of butanol is 2434
A Table To Show Our Bond Enthalpy Results Compared To The Theoretical Values.
I can use these figures to predict the molar heat of combustion of pentanol and to work out the percentage of accuracy of my experiment. We can see that on the graph (below) the results rise at a set rate. This is because there are patterns in the amounts of bonds made and broken throughout the experiment. The results should rise by a similar amount each time and mine rise by 598J exactly each time. This shows me that my prediction is right and that the alcohols with more carbons burn more effectively producing more heat. In the larger molecules there are a bigger ratio of C-H bonds to O-H bonds, this may well affect how the energy produced rises each time.
Ethanol:
579/1238*100=51.5
47%
Propanol:
726/1836*100=35.5
40%
Butanol:
1575/2434*100=79.5
65%
Butanol was by far the best 65% accuracy compared to 40% accuracy by Propanol.
All of the percentages we have worked out are about what I was expecting from the results. The distances between each result are expected in the test and highlight well how the differences in both the uncontrollable aspects and human error can mould the accuracy of the data.
Both the table and the graph (below) show that our experiment was relatively accurate, both Ethanol and Butanol were relatively similar but for Propanol our result was just over a third of the theoretical value. Inconsistencies like this are expected in this type of test because of the amount of energy being lost all of the time.
Analysis of My Results:
For this type of test huge inconsistencies are expected throughout the data, our results reflect this. The inaccuracy of the method we used and the margin for human error can easily make up for these inconsistencies.
The fact that all of the results were out by so much shows that it is not human error actually making the data so poor, the main reasons for heat loss would be conduction, convection and radiation. Nearly all of the heat would have been carried straight past the beaker by the rising air, the shelter created by the heatproof mats would stop a small amount of heat escaping by trapping it but the shelter is not airtight and to big to be effective. Energy is radiated from the apparatus and beaker, the beaker could have been covered with insulation around the sides and tin foil to cover the top but most of the heat would still have escaped. It is possible that energy was transferred to the apparatus via conduction but this would be a tiny problem compared to radiation and convection, this could be easily stopped anyway by covering the ends of the clamp with an insulator like rubber.
Lining the inside of the cover made with heatproof mats would also have helped by reflecting the heat back towards the test and not absorbing it. One of the mistake we made whilst performing the experiment was not to label the calorimeter we used as to make sure we used the same one again, this may have cause large inaccuracies because there is the possibility the copper may be thicker on one than the other. In addition, the different calorimeter had different sized bases, if one of the calorimeters had a smaller base than the other then the one with the smaller base would have less material near the flame to conduct the heat energy, therefore having a smaller area of conduction. Other problem such as weather conditions would have affected the test so in a perfect world we would have conducted it in a controlled environment where uncontrollable conditions such the humidity and temperature could be kept constant. There are other reasons why this test would have been inaccurate:
-
The thermometer can only be read to 1OC and even then could be read inaccurately.
- Impurities in water (may change the specific heat capacity of water).
- All results were conducted under different conditions.
Not all of the alcohol was burnt effectively in the test. We can tell this because of the build up of soot on the base of the container; this build up of soot will have affected our test. Carbon is a much better insulator than copper and may have stopped the heat getting through to the water efficiently. Using a skirt with the container would help to trap the heat and would therefore reduce the heat lost by convection. Even though all our results were inaccurate they are still far more precise than I was expecting so I was generally pleased with the outcome of the test. Using a skirt and insulation would have defiantly improved the consistency of our data, also using an electronic thermometer would also have increased the accuracy of the results. The experiment could have easily been improved but we would have needed to use equipment that was not available to us at the time. In a perfect world I would have liked to redo the test using insulation, a skirt and a more accurate electronic thermometer but we unfortunately did not have time to do this.
Heat Absorption in the Copper Calorimeter
In our results we did not take into account the energy used to heat the container. We can do this for the calorimeter by taking the specific heat capacity of copper (0.39jOC-1g-1) multiplying it by the amount heated (OC) and multiplying that by the weight of the calorimeter (g). We do not know the weight of the calorimeter so we will suppose it is 100g.
I have constructed the following graphs to help me with the formulae:
This is to show me the energy absorbed by each container for 1g of alcohol each.
This graph shows me the averages I require to do my formulae.
To find out how much energy was used in each test we use a simple formula:
0.39jOC-1g-1 * 44OC * 100g = the energy used by the calorimeter
We the divide this by the amount of alcohol used in the test to find out how much energy the calorimeter absorbs for 1g of the alcohol.
Propanol:
0.39 * 50 * 100 = 1950j
1950/0.47=4149
Butanol:
0.39 * 51 * 100 = 1989j
1989/0.86=2313
Ethanol:
0.39 * 46 * 100 = 1794j
1794/0.84=2136
We can add each of these values to the original values for the calorimeter and use this to work out the percentage of the written values and the percent increase.
For the accuracy increase I am going to work out the percentage for both the calorimeter before and after adding the extra energy absorption. This means that I will be able to work out the increase more accurately.
Propanol
Percent Increase
12112+4149=16261
16261/12112=1.34
1.34*100=134%
Accuracy Increase In Per Cent:
Without Container Heat Absorption
To find out the percentage of the written value we have to find out what the result would be if we burned exactly one mole of the alcohol:
12112*60=726720
We convert this figure to kilojoules by dividing it by 1000
726720/1000=727
We then the divide this by the theoretical value:
727/1836=0.40
This is the percentage of accuracy for just the calorimeter without adding the extra energy used by the calorimeter.
38%
With Container Heat Absorption
We then do the exact same process with the result including the heating of the Calorimeter:
16261*60=975660
975660/1000=976
976/1836=0.53
53%
The test for the Propanol Calorimeter is 15% more accurate with the inclusion of the energy used in the heating of the Calorimeter.
Ethanol
Heating Increase
12588+2136=14724
14724/12588=1.17
1.17*100=117%
Accuracy Increase
Without Container Heat Absorption
12588*46=579048
579048/1000=579.048
579.048/1238=0.47
47%
With Container Heat Absorption
14724*46=677304
677304/1000=677.304
677.304/1238=0.55
55%
For ethanol there is an accuracy increase of 8%.
Butanol
Heating Increase
21292+2313=23605
23605/21292=1.11
1.11*100=111%
Accuracy Increase
Without Container Heat Absorption
21292*74=1575608
1575608/1000=1575.608
1575.608/2434=0.65
65%
With Container Heat Absorption
23605*74=1746770
1746770/1000=1746.77
1746.77/2434=0.72
72%
The percentage of accuracy for butanol has increased by 8%.
All the alcohols have a significant rise in the amount of energy used in heating the beaker, this shows that the experiment is a lot more accurate than we originally thought.
We can compare the accuracies for the calorimeter before and after.
This table above expresses the differences most accurately.
The tables above all show that taking into account the energy use of the calorimeter decreases the error throughout the test a lot, the calculations we have done are obviously not very accurate because we do not know the weight of the calorimeter. The fact that we also changed containers throughout also means that we have no way of telling the weight because the ones we used are not marked. There is also the possibility that the calorimeter may have been hotter or colder than the temperature of the water.
Evaluation:
I was expecting our test to be very inaccurate but having looked at the different possibilities for error and at the results when including the heating of the calorimeter then I realized that it was a lot more accurate than I was expecting. Including the heating of the calorimeter the accuracy of the test peaked at 72% of the theoretical value that we worked out using the bond enthalpies. This was for butanol. We could have easily improved our test but this may not have yielded more accurate results. The biggest mistake we made throughout was not to mark our containers, this means that we may well have used containers with different masses throughout. We also did not thoroughly clean out our apparatus and containers beforehand. This means that the containers we used could well have had a covering of dirt and carbon therefore reducing conduction. Overall I think that we could have improved the accuracy of the test by reducing human error but this may not have increased the accuracy of our data. The climate and outside conditions played a large part in all of our results and there is no way to control this in the circumstances.
The electrons within the alcohol molecule are shared therefore it has covalent bonds as opposed to ionic ones. Each atom within the molecule has a full outer shell of electrons from sharing with other atoms; this makes the molecule very stable. Each step-up in the group of alcohols adds two carbon-hydrogen bonds, a carbon-carbon bond, an oxygen=oxygen bond and half a carbon=carbon bond that are available for combustion. Therefore, there is a net energy gain every time. This means that the additional energy added will be the same each time.
