Heat of Combustion Vs. Number of Carbon Atoms.

-1888 Butanol -2503 Pentanol -3118 APPARATUS >200ml measuring cylinder >Stand, boss and clamp >Spirit burners containing alcoholÆs- (methanol, ethanol, propanol, butanol, pentanol) >Bunsen >Thermometer (mercury)- nearest half a degree >Scales- electronic 2 decimal places >Heat proof mats >Goggles METHOD ? Measure out 200ml of water in the measuring cylinder. ? Pour into tin can. ? Record the temperature of the water-using thermometer. ? Chose a spirit burner. Record the name of the fuel in it and the mass of the whole burner (including lid). ? Clamp tin can and keep at same distance from the wick throughout whole experiment. ? Place heatproof mats around the experiment area. ? Light wick of spirit burner and place under tin can. ? Wait for temperature to change by 21oC and put the lid over the wick to put out flame. Record the new mass of the spirit burner (including the lid). ? Put fresh water into the tin can and wait for it to return to normal temperature. ? Scrape any carbon that has formed at the bottom of the can. ? Try to do each fuel three times following the same instructions above. To ensure that my experiment is as fair as possible I have decide to repeat the experiment on each alcohol three times. This is so that if there are any anomalous results then I can figure out an average and it wonÆt come into account. As you can see in my method I have also decide to place heat proof mats around my experimental area so as to minimise the heat lost into other parts of the room. The distance between the wick and the tin can will also need to be kept constant so as not to give any of the alcoholÆs a disadvantage. To ensure fair weights are recorded I am using an electronic scale and will record all of my results to two decimal places. The starting temperature of the water must always be the same so I am putting fresh cold water in the can after every experiment. As the experiment will be giving off carbon this may stick to the bottom of my tin can. By the time I get to my final tests there will be so much soot on the bottom of the tin that the heat will not be able to get through. This is why I purpose to scrape as much soot as possible off the tin can after each experiment. To ensure that my results are correct I will also need to keep the same variable through all the experiments and only one. If I had two variables then I would not know which one was effecting the results. My variable will be the alcohols. I will also need to ensure that I conduct the experiments safely. As alcohols are very dangerous and highly flammable I will wear my safety goggles at all times. I will need to keep all lose items off clothing tucked in. the lids on the alcoholÆs must be kept on at all times to prevent evaporation or any spillageÆs. My Bunsen burner will also need to be kept on a safety flame at all times. The experiment will be set up as shown below: PREDICTION For my prediction I have used lots of background knowledge and done some preliminary experiments. As you can see I have worked out the theoretical energy release for each alcohol and from these I would predict that as the number of carbon atoms go up so will the heat of combustion. I believe that the results will be proportional to the amount of carbon atoms. I believe this because everytime you add an extra carbon bond you also add two more hydrogen bonds, which means that the relative molecular mass will increase as well. I also predict that this will be a constant amount. When I achieve my results I will need to figure out the actual amount of energy given out and the molecular mass plays a big part in this. First I will need to use the formula Heat transferred = Mass of substance (g) temperature change (oC) specific heat capacity(j) The mass of the substance is the water as this is what I will be heating up. The specific heat capacity is the amount of energy it takes to raise the temperature of 1 gram of water (1cm3) by 1oC. This is why I chose water because its specific heat capacity is well known to be 4.2J. As you can see this number will be the same for all of my tests because none of these are variable in my experiment. Therefore I need to make it specific to each alcohol. This will be done because the number formed from the equation above will be the amount of energy produced by the amount of alcohol burnt. I will then divide the number formed by the equation above by the amount of alcohol burnt to find out the energy produced by 1 gram. I will then need to multiply this by the relative molecular mass of that particular alcohol. Then this will give me the amount of energy produce by 1 mole of that alcohol. I did a preliminary experiment with hexane to see if this formula worked. I chose hexane because it also produces carbon dioxide and water when combusted in air. The theoretical amount of energy produced for I mole is: 2C6H14 + 1902 12CO2 + 14H20 Results Temperature change oC Mass of burner at start (g) Mass of burner at end (g) Alcohol burnt (g) Average amount of alcohol burnt (g) Mass of H20 (g) Methanol 1. 21 2. 21 3. 21 156.65 155.37 175.86 155.50 154.22 174.44 1.15 1.15 1.12 1.14 200 200 200 Ethanol 1. 21 2. 21 3. 21 369.33 176.37 175.12 368.26 175.43 174.04 1.07 0.94 1.08 1.03 200 200 200 Propan-1-ol 1. 21 2. 21 3. 21 355.83 384.31 360.27 355.08 383.58 359.39 0.75 0.73 0.88 0.79 200 200 200 Butan-1-ol 1. 21 2. 21 3. 21 313.31 265.37 261.13 312.22 264.54 200.42 1.09 0.83 0.71 0.88 200 200 200 Pentan-1-ol 1. 21 2. 21 3. 21 170.62 170.00 289.35 169.88 169.22 288.87 0.74 0.78 0.48 0.67 200 200 200 Unfortunately we did not have time to repeat any of our experiments which I would have like to have done as you can see there are a few anomalous results. These results are not complete though I still need to calculate the amount of energy given out by 1 mole of an alcohol. I will use the same method as I said in my prediction to calculate this for each alcohol. Alcohol S.H.C (J) Temperature change (oC) Mass of substance (g) Energy produced for amount of alcohol burnt (J) Energy produced for 1 mole (kJ) Methanol 4.2 21 200 17640 495.16 ethanol 4.2 21 200 17640 787.81 Propan-1-ol 4.2 21 200 17640 1339.75 Butan-1-ol 4.2 21 200 17640 1483.36 Pentan-1-ol 4.2 21 200 17640 2316.90 Analysis I think my results tables and graphs clearly show the pattern that I have found in this experiment. That is that heat combustion does increase when the amount of carbon atoms increases. A table that answers the question more specifically and clearly is shown below: Number of carbon atoms Energy released in heat combustion (kJ) C1 495.16 C2 787.81 C3 1339.75 C4 1483.36 C5 2316.90 I believe that my results do show a positive correlation and do show that the more carbon atoms there are the heat of combustion goes up. In this case my prediction is right but not totally right. I said that it would be a constant amount. Another reason for these results is that the molecular length becomes longer in the bigger molecules increasing the surface area hence allowing more energy to be released. These results do support my initial prediction. Evaluation My results could never have been as accurate as the theoretical calculations because there were many experimental errors involved. As you can see from my results the theoretical calculations are way above the calculations that I received from my experiment one of the main reasons for this was the fact that I calculated all of the energy released and my experiments calculations are only the heat energy. Sound and light energy could have been lost into the room. Although I placed heatproof mats around my experimental area they could not have kept all of the heat in and much of this would have been taken away in the convection currents through the air. The beaker that the water was being held in would have used up some of the heat energy to heat itself up along with the rubber clamp. Also the size of the tin can was not exactly 200 ml so much of this space would have been wasted along with the fact that the alcohol containers had varying amounts of alcohol in them to start with along with varying sizes of wicks. This all contributed to the fact that the flame coming from the alcohol was varying in size so was sometimes not even touching the tin can. The room temperature would also have acted as a cooling agent. One of the less important factors could have been if there was a lacking of oxygen leading to incomplete combustion. Then the oxygen molecules would only form with one carbon molecule producing carbon monoxide but I doubt this actually happened. This was a very difficult experiment top conduct in a classroom because there are lots of potential ways of losing heat because everything likes to gain heat energy. I think the thing that hindered our results the most was the fact that gusts of air and convection currents were taking the heat away from the experimental area and there was no way to stop this. As you can see from my results and my graph I did have a few anomalous results and I believe this to be in the combustion of butanol. As I recorded the results three times and gained an average result I am a bit bemused by this as I can see no cause for this going more wrong than any of the other results. As you can see all the results were in fact wide ranging for this category. I think one of the things that is helping to this is that my propanol experiment could also have been a bit anomalous but with the two together my best fit line shows butanol to be the main culprit. If you loom at my results carefully for butanol you can tell that I did change my spirit holder after the first experiment. The next two results do drop drastically and this could have been because of the fact that the wick sizes were different on each spirit burner. If I could do the experiment again I would definitely improve the insulation around the experiment maybe using foil instead of heatproof mats. I would also ensure that there were no windows open because during our experiment there may have been and sudden gushes of wind could have taken some of the heat energy away. I would also try to find a better container for holding the water because tin might gain heat energy a lot more easily than other metals so this would cut down on my experimental errors. Conclusion After this I can conclude that my initial prediction was actually right but I didnÆt allow for all of the experimental errors. I conclude that carbon atoms in alcohols do have an effect on the heat of combustion. As the amount of carbon atoms go up the heat of combustion does. This is because everytime you add another carbon atom you are also adding 15 onto the relative atomic mass that plays a big part in calculating the end results.