The combustion of pentanol is shown as:
pentanol + oxygen carbon dioxide + water + energy
C5H11OH + 15/2O2 (g) 5CO2 (g) + 6H2O (g) + energy
Once again using bond energies, I can work out how much energy is given off during the combustion of pentanol. The energy required to break the existing bonds is 21445kJ and the energy produced when new bonds are made is27196kJ. This means that 2875.5kJ of energy are given out on the combustion of pentanol.
The above values are my predictions for the experiment however I may find that my results are not entirely accurate or close to these values. This could be because some energy is lost to the surroundings and some energy is also taken in by the copper calorimeter. I must consider these factors in my conclusion.
Apparatus: Retort stand
Clamp
Boss
Methanol
Ethanol
Propanol
Pentanol
Spirit burner
Copper calorimeter
Water
Thermometer
Draught shield
Measuring cylinder
Splinter
Electronic scales
Diagram:
Variables: Alcohol used
Volume of water being heated
Distance of copper calorimeter from the flame
Length of time water is heated for
Temperature change
Key Variable: Alcohol used
Method:
The apparatus will be set up as shown in the diagram above. We must ensure that the boss and clamp are tightened properly to ensure that no apparatus is in danger of getting knocked over or falling.
Once I have set up the apparatus, we collected a spirit burner with a specific alcohol in it ( methanol, ethanol, propanol and pentanol). We then weighed the spirit burner with the alcohol in it on the electric scales to see the total weight of the spirit burner and the alcohol in it. We then cleaned the copper calorimeter of the soot that had collected on it from any previous experiments. The reason we used a copper calorimeter instead of a beaker was because a beaker, which is made out of glass, is a poor conductor and therefore will take in some of the heat. This will reduce the accuracy of our experiment. The copper calorimeter absorbs very little heat since it is a good conductor of heat. The soot was cleaned of after every test to ensure that heat was not absorbed by the soot thus making the experiment an unfair one. We then measured 50mm of water in the measuring cylinder and then put this into the copper calorimeter. Once everything was set, we put up our books as draught shields. We then noted down the temperature shown on the thermometer and then lit the spirit burner. We waited till there was an increase of 20oC and then put the spirit burner out. The temperature was watched until it reached its peak and then noted down. The spirit burner was taken to the electric scales and weighed to see how much of its mass had been used up. We then repeated this procedure for all four alcohols, doing each alcohol twice. After each test the water was tipped away and we measured out 50mm of water from the tap. The copper calorimeter was also cleaned to ensure all the soot was removed making the test as fair as possible.
Once we have measured the decrease in mass, we can use this to work out how many moles were used to make a specific temperature change. This can then be used to work out how much energy is given off by each mole of that specific alcohol.
Fair Test:
To make sure that this experiment is as accurate as possible we must keep all factors constant except the KEY VARIABLE which in this case is the alcohol burnt to heat the water. As I have mentioned earlier there are a variety of different variables that can be changed. These include the volume of water being heated, the length of time the water is heated for, temperature change, distance of copper calorimeter from the flame and the volume of water that is being heated. As well as these variables there are also certain factors that need to be controlled. These are any draughts near the experiment that may cool the temperature, how much soot is left at the base of the copper calorimeter, the length of wick that is produced by the spirit burner.
Such factors as time will not be controlled. This is because we are measuring the temperature change of the water, therefore each alcohol may take different times to increase the temperature of the water by 20 oC. However all the other factors will be attempted to be controlled to the best of our abilities.
VOLUME OF WATER-
To make sure that the volume of water is constant throughout the experiment, we will accurately measure out 50mm of water in the measuring cylinder making sure the same person measures out the water each time.
LENGTH OF WICK-
This is difficult to control since the volume of alcohol in each spirit burner will decrease as the experiment goes on since other groups will be using the spirit burner. This means that the length of the wick may vary. This may cause anomalies in our experiment but hopefully we can still obtain as accurate results as possible.
BUILD UP OF SOOT-
To make sure that there is as little amount of soot on the copper calorimeter as possible we will clean it after every test thoroughly. Water will be run over the copper calorimeter and will be dried before we test the next alcohol. This is to ensure that as much heat energy as possible heats the water to ensure our experiment is as fair as possible.
DISTANCE OF COPPER CALORIMETER-
We must keep the distance of the copper calorimeter from the spirit burner the same after each experiment. This is to ensure that as little heat energy as possible is lost or not used to heat the can. This also ensures that all the alcohols have equal distance from the copper calorimeter so the same amount of heat energy is lost to the surroundings. I will keep the same distance by keeping the boss in the same place on the retort stand at all times and only loosening the clamp to take out the copper calorimeter to clean. The can will also be kept as horizontal as possible so that equal surface area of the water receives the same amount of heat.
THERMOMETER-
We must make sure that the thermometer is not touching any part of the copper calorimeter. This is because then the increase of temperature shown will be the heat from the copper calorimeter. This makes the experiment unfair and does not tell us accurately the increase of temperature of the water.
DRAUGHTS-
Even though we will be holding up our text books as draught shields, we do not want to take the chance of wind interfering with our experiment. To prevent this, all windows are closed as well as the door. When we move to get certain apparatus we must make sure we do not rush past anyone's experiment as this may create a draught and thus making our results inaccurate. We must also make sure that we do not blow or breathe too heavily onto our experiment.
Apart from all these factors we must also make sure that the same copper calorimeter is used. This is because a different copper calorimeter may have more soot on it that may be difficult to clean of or it may not be able to conduct heat as well as the copper calorimeter we initially use. We have chosen a COPPER calorimeter because it is a good conductor of heat and does not take away any of the heat thus most of the heat energy produced by the spirit burner will be used to heat the water. A fresh sample of water will also be used after each experiment because the water needs to start at around about the same temperature. We should also use the thermometer to stir the water to ensure there is an equal temperature in the body of water. If we used the same sample of water for each experiment, then the starting temperature would be quite a bit away from each other and the volume of the water may start to vary making the experiment unfair. We have also decided to do each experiment twice to make sure that the chance of getting any anomalous results is low.
Safety:
We must make sure we wear safety goggles. We must be very careful around the spirit burners since they contain alcohols which should not be let to touch our hands or any part of our body since they can damage the skin. We must not sit down whilst the spirit burner is near us. We should not rush through the experiment in case we do not secure the apparatus safely. The flame must not come in contact with open alcohol since alcohol is extremely flammable this causes a risk to us and other members of the set.
When we did our experiment we recorded several things. We first took down the mass of the spirit burner before the water was heated and then the mass after the water was heated. We also noted the temperature before the water was heated and when it got to its peak. These results were then interpreted and made into a simple, easy to read table shown below:
This information can now be used to find out how much energy was given of by one mole of a particular alcohol. I must initially find out how much heat energy or enthalpy energy was produced by each alcohol. From that I can work out how much heat was produced when 1g of each alcohol burns. This then leads on to how much heat is produced when 1 mole of each alcohol burns.
The formula to work out heat energy is:
H = m * c * T
Where H is heat energy, m is the mass of water, c is the heat capacity of water and T is the temperature change. We kept the mass of water at cm3 for every experiment therefore m = 50. From research I know that the heat capacity of water is 4.2. Now I can work out the heat energy produced by the certain amount of alcohol used. I will first use this equation for methanol.
H = 50 * 4.2 * 22
H = 4620J
0.59g of methanol produced 4620 joules of heat energy. Therefore to work out how much one gram produced we must divide 4620J by 0.59g.
energy produced by 1g of methanol = 4620/0.59
= 7830J
To convert this into moles we must multiply the amount of energy produced by one gram by the relative molecular mass of methanol. The R.M.M. of methanol is 32.
Energy produced by 1 mole of methanol = 7830 * 32
= 251000J
= 251kJ
So one mole of methanol produces 251kJ of energy according to my results. I will now repeat this process for the second experiment I did with methanol. Then with the two values I get, I will work out an average.
Heat energy produced = 50 * 4.2 * 24
= 5040J
Mass of methanol used was 0.7g.
energy produced by 1g of methanol = 5040/0.7
= 7200J
Energy produced by 1 mole of methanol = 7200 * 32
= 230000J
= 230kJ
So in the second experiment, one mole of methanol produced 230kJ of energy. Between the two values I have got, the average amount of energy produced by one gram of methanol is 7250kJ. The average amount of energy produced by one mole is 240.5kJ. I will now repeat this process with the other 3 alcohols too see if there are any trends.
For ethanol I must make sure that I use the correct molecular mass number. The R.M.M of ethanol is 46g. I will now go through the same steps I used for methanol.
H = m * c * T
= 50 * 4.2 * 24
= 5040J
Mass of ethanol used is 0.55g.
heat produced by 1g of ethanol = 5040/0.55
=9160J
R.M.M. of ethanol is 46g.
heat produced by one mole of ethanol = 9160 * 46
= 422000J
= 422kJ
Now I will repeat these steps for the second experiment conducted with ethanol.
H = 50 * 4.2 * 24
= 5040J
We can see that there was the same temperature change in both experiments with ethanol however the mass change was different. This is to be considered in the conclusion.
Mass of ethanol used is 0.62g.
heat produced by 1g of ethanol = 5040/0.62
= 8130J
R.M.M. of ethanol is 46g
heat produced by one mole of ethanol = 8130 * 46
= 374000J
= 374kJ
We can see there is a huge gap between the values. There is a 1030J difference between the energy produced by one gram of ethanol and there is a 48J difference between the energy produced per mole of ethanol. However I must still follow my method and this gives an average for ethanol as 8650J/g and 398kJ/mole. I must make sure I take into account the great difference in my conclusion.
I will now use these same steps to find out how much energy produced per mole for propanol. I must once again make sure I use the correct relative molecular mass.
H = m * c * T
= 50 * 4.2 * 27
= 5670J
Mass of propanol used was 0.53g
energy produced by propanol = 5670/0.53
= 10700J/g
R.M.M. of propanol is 60
energy produced by propanol = 10700J * 60
= 642000J/mole
= 642kJ/mole
I will now repeat these steps for the second experiment conducted with propanol.
H = 50 * 4.2 * 24
= 5040J
Mass of propanol used was 0.39g
Energy produced by propanol = 5040/0.39
= 12900J/g
R.M.M. of propanol is 60
Energy produced by propanol = 12900 * 60
= 775000J/mole
= 775kJ/mole
We can see that from our results that once again there is a large difference between the energy produced per gram in the two experiments. This has then lead to a gap between energy produced per mole for both experiments. The average between both experiments for energy produced per gram is 11800kJ/g. The average between both experiments for energy produced per mole is 709kJ/mole.
I will now repeat these steps for my final experiment. This was the experiment with pentanol. Once I have recorded this last bit of data I will put the results into a table and then draw a graph showing both energy produced per gram and energy produced per mole.
H = m * c * T
= 50 * 4.2 * 26
= 5460J
The mass pentanol used was 0.37g
energy produced by pentanol = 5460/0.37
= 14800J/g
The R.M.M. of pentanol is 88
energy produced by pentanol = 14800 * 88
= 1300000J/g
= 1300kJ/mole
I will now repeat these steps for the second experiment conducted with pentanol.
H = 50 * 4.2 * 25
= 5250J
The mass of pentanol used was 0.36g
Energy produced by pentanol = 5250/0.36
= 14600J/g
The R.M.M. of pentanol is 88
Energy produced by pentanol = 14600 * 88
= 1280000J/mole
= 1280kJ/mole
The average amount of energy produced by pentanol was 14700kJ/g and 1290kJ/mole. I now have an average for energy per mole and gram for each alcohol. I will now present this data in a form of a table. This table will then be inputted into Excel to produce two graphs that will show the relationship between the number of carbon atoms in each alcohol and the amount of energy produced.
This data will now be used to produce a graph to show the relationship between the alcohol and energy produced.
Now that I have presented my results in graphs, I will present my predictions in a graph format as well. This allows me to compare my predictions with my results and see if my predictions in the hypothesis are accurate. My graph will show my predictions against the actual results that I got. Below is a table showing my predictions against my results.
The graph can be used to deduce that with butanol ( 4 carbon atoms), I would expect that if I had done the experiment that 1000kJ/mole would have been produced. I can also see from the graph that as the number of carbon atoms increases, so does the amount of energy produced per mole. The increase is also at a constant rate. With our predictions the rate was at about 600kJ/mole. With our results however it was at about 240kJ/mole.
Conclusion:
We can see from the graph above that my results are a considerable amount lower than my predicted results. I must now consider why there is a large difference. My first reason is that the energy given of by the alcohol was not all used to heat the water. Some energy would have been absorbed by the copper calorimeter. Even though the copper calorimeter is a good conductor, it would have taken in some energy and given it out to the surroundings. This energy would not have been used to heat the water and thus there would not have been as much an increase in the temperature as there should have been.
Heat energy may have also been lost due to draughts. Even though we had draught shields to prevent this from happening, we didn't have the shield all the way around the experiment. This means that some energy would have been lost to the surroundings directly from the flame. There was a certain gap between the flame and the copper calorimeter containing water. This gap could have allowed heat from the flame to be lost to the surrounding.
Heat energy would have been lost to the surrounding by convection, conduction and evaporation. As the water was getting heated, convection currents would have taken some of the heat away. The conduction through the copper calorimeter would have lost some heat and some of the water may have evaporated.
Both our predicted results and actual results have an increase of energy produced at a constant rate, hence both have a best-fit line that is straight. The reason that the increase of energy produced per mole is constant is because as we increase the carbon atoms in the alcohol, we also add 2 hydrogen atoms. This then produces an extra carbon dioxide molecule and an extra water molecule. So therefore as we go from one alcohol to the next we are only adding an extra CH2 and an extra 1.5 mole of oxygen to get a balanced equation and making and extra CO2 and H2O. Therefore we are only breaking 2 more C-H bonds, one more C-C bond, and an extra 3/2 O-O double bond. From this we are making two extra C-O double bonds and 2 extra H-O bonds. When all these bond energies have been calculated we get an extra 600kJ/mole produced, hence the constant increase. However with our actual results the increase is less since energy is lost to the surroundings. However our results show that all the tests were done in similar conditions since the increase is constant. We have not got any anomalous results hence our experiment was made fair.
By studying the graph, we can see that the gap between my results and the results I predicted increases as the number of carbon atoms increases. This means that more energy was lost as the number of carbon atoms increased. This is because the constant increase in my results is lower than the constant increase in my predictions. Some of the energy produced by adding an extra CH2 and 3/2 O2 and making and extra CO2 and H2O is lost to the surrounding or absorbed by the copper calorimeter. However this amount of energy lost is constant since we made our experiment fair and therefore the constant is lower and therefore as we go further up the graph, the gap between the predicted results and the actual results increase.
Evaluation:
We can see that the heat energy produced in my experiments and the values I worked out using bond energies are not the same. The values in my experiment are lower than what I predicted. I have concluded that this was because heat energy was lost to the surrounding and a lot of energy was absorbed by the copper calorimeter.
Our experiment however was done with the same conditions for each of the experiments since on our graphs we can not see any anomalies. This could also have happened since we repeated each experiment twice. This reduced that chance of us having any anomalous results. We did get some anomalies in one or two of our experiments. This could have been because there was a sudden draught or we may have accidentally blown onto the apparatus. It may have also been that we didn't record the temperature change correctly. We may have read the mass incorrectly of the weighing scale or we may not have cleaned of all the soot of the copper calorimeter.
We may have lost heat energy through heat loss, which cannot be prevented. There is bound to be heat loss in any experiment concerning heat. Therefore it was expected that our values from our experiments would be lower than our predictions.
Improvements:
We could have improved the experiment by concentrating on how to lower the heat loss. Having a lower temperature change could have done this. This would have lowered the heat loss since heat loss is proportional to temperature change. This means that the higher the temperature change, the more heat lost to surroundings. Therefore if we had a lower temperature change, less heat energy would have been lost to the surrounding.
If we had found out the heat capacity of the copper calorimeter, we could have used this value to work in the formula to find out the amount of heat absorbed by it. This value would have been part of the heat energy given of by the alcohol even though some of it would have been passed on to the water. If we had taken this into account, our results would have been a lot more closer to out predicted values.
We could have also had an evaporation lid with a hole in it to allow the thermometer to go through it. This would have lowered heat escaping through convection or evaporation lid.
We could have also made sure that we have draught shield all the way round the experiment. This would have made sure that no draughts can affect our experiment and thus this would lower heat loss.
The experiment could have been repeated more times to ensure that we obtain a better average and thus we would obtain a smooth straight line.
Extension:
We could have taken the experiment further in a variety of ways. Below I have listed a few ways:
- Compare the energy produced by alcohols to energy produced by alkanes.
- Do the experiment with a larger variety of alcohols, with more carbon atoms.
- Tested the alcohols against different volumes of water.
