Investigation of Combustion of Alcohols Essay
The aim of this experiment is to investigate the combustion of various alcohols. The alcohols will be burnt to heat up a test tube of water, I aim is to find out how much energy is produced when burning the following alcohols: Methanol, Ethanol, Propanol, Butanol, and Pentanol. Any form of burning is an exothermic reaction (heat is given out), this means that the reactants energy is higher than that of the product.
Alcohols react with oxygen in the air to form water and carbon dioxide. Prediction: When bonds are broken, energy is absorbed by the system (endothermic) and when bonds are formed, energy is released (exothermic). When alcohols are burnt, the reaction is always overall exothermic, therefore, more energy is being released than is being consumed. Thus, my prediction is that each alcohol will burn exothermally and therefore the temperature of the water will rise.
The larger the alcohol molecule, the more bonds will be broken and formed, and therefore the more heat will be produced, so I think that Methanol will be the alcohol to combust the least, followed by Ethanol, Propanol and Butanol, then finally Pentanol, combusting the most. Preliminary Work Before carrying out my final experiment, I have to carry out a preliminary experiment, to determine the most efficient method to use, and on what scale. Preliminary Results
My preliminary results enabled me to decide on 30 degrees as a suitable water temperature change, as this allowed a large enough change in mass of the alcohol, but was not too time consuming. It also helped me decide to experiment with each alcohol twice, this will ensure accurate results just one could be unreliable. Apparatus * 5 Alcohols, Methanol, Ethanol, Propanol, Butanol and Pentanol. * Test tube * Thermometer * Water * Draught preventers (chemistry books) * Clamp * Heat proof mat * Measuring cylinder Method Collect and set up apparatus as shown in the diagram.
Measure __ml of water in the measuring cylinder. Pour into test tube. Place thermometer in the water and record the starting temperature. Weigh alcohol container, record weight for before the experiment. Make sure all safety precautions are in place (safety goggles, heat proof mat). Ignite alcohol containers wick. When the temperature has rose 30 degrees, put out flame using lid. Weigh the alcohol container and record weight after experiment. Repeat twice for each alcohol. Variables to be kept the same * Height above the mat at which the test tube is clamped (__cm)
We keep this the same because if the flame was closer to the test tubes on some, than on others, it would not be a fair test, some would obtain more heat than others. * Temperature change Changing this would leave nothing to measure how long to burn the alcohol. Results To work out the results, averages of the masses were taken from both the experiments, however, some of the end results proved to be inaccurate, therefore, only the results from the first experiment were used. Changes in mass Before (g) After (g) Before (g) After (g) Average Change/ Change in Mass (g) Methanol 181. 05 178. 63 169. 07 168. 06 1. 715
Ethanol 163. 80 161. 84 N / A N / A 1. 96 Propanol 171. 80 171. 07 N / A N / A 0. 73 Butanol 192. 78 192. 19 216. 05 215. 19 0. 775 Pentanol 174. 34 173. 69 N / A N / A 0. 65 Changes in temperature All increase by 30°C, except for Pentanol, the results for which were given to us, as the experiment would be too dangerous to conduct in the lesson. Before (°C ) After (°C ) Before (°C) After (°C) Change in temp (°C) Methanol 20 50 18 48 30 Ethanol 20 50 20 50 30 Propanol 20 50 20 50 30 Butanol 20 50 20 50 30 Pentanol 23 37 N / A N / A 14 Relative Molecular Mass Methanol CH3OH = 32 Ethanol C2H5OH = 46 Propanol C3H7OH = 60
Butanol C4H9OH = 74 Pentanol C5H11OH = 88 Fraction of moles used Change in mass/relative molecular mass Fraction of mole Methanol 1. 715 / 32 0. 053594 Ethanol 1. 96 / 46 0. 042609 Propanol 0. 73 / 60 0. 012167 Butanol 0. 775 / 74 0. 010473 Pentanol 0. 65 / 88 0. 007386 Temperature Change per mole Temp change/fraction of mole Temp change per mole (°C) Results I was aiming for Methanol 30 / 0. 053594 559. 764143 680 Ethanol 30 / 0. 042609 704. 076604 1298 Propanol 30 / 0. 012167 2465. 685872 1916 Butanol 30 / 0. 010473 2864. 508734 2152 Pentanol 14 / 0. 007386 1895. 477931 2770 Here is a graph to show my results
Conclusions Methanol The experimental value obtained was relatively close to the ideal temperature change per mole, suggesting that the technique was followed accurately, and all went well. Ethanol This end result followed my prediction in being a larger increase in temperature than Methanol, although it wasn’t quite as close to the ideal value as Methanol was. Propanol This result was unsatisfactory, it was higher than the expected (‘ideal’) result, (an anomaly) this could be because I didn’t stir the water, so the thermometer only gave the temperature at the bottom of the test tube, the nearest to the flame.
Butanol This end value was also incorrect in that it was way to high, if I was to repeat this experiment, I would be sure to stir the water before taking my reading. Pentanol This was the result given to me by the teacher, it is satisfactory and seems to follow my prediction. Evaluation What can be said about this experiment is that we could have lost a lot of energy while doing it. My results aren’t very accurate, even the ones that were lower than my ideal results, were not all very close. This suggests that the experiment design was not suitable for measuring accurate values.
This experiment had several potential flaws that effected the end results, for example the difficulty in monitoring heat loss accurately. This effected my results because energy was wasted, the water did not get all the heat from the flame, some of the heat was lost to our surroundings and so measurements and recordings could have been off. It might be an idea to have burnt our alcohol in pure oxygen to make sure that there was a complete reaction. An incomplete combustion is where there isn’t enough oxygen to release the energy.
If I was to repeat this experiment there are several changes I would make, I would have large pieces of foil rapped cardboard, to prevent draught, rather than using exercise books, and as I mentioned earlier, I would have stirred the water before taking my reading. In conclusion, the experiment was worthwhile as it showed the relationship between different sized alcohol molecules in a combustion reaction. However, the evidence is not accurate enough to show any other information, other than the fact that the larger the alcohol molecule the more heat is produced during combustion.