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To See If the Size of the Molecule Affects the Heat of Combustion

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Aim: to see if the size of the molecule affects the heat of combustion Hypothesis: I predict that the more bonds there are holding the carbon, oxygen and hydrogen atoms together, the more energy will be required to break them apart. To be able to make this statement we can break down each alcohol, (methanol, ethanol, Propanol, butanol and pentanol) and work out the amount of energy required to break them. From this we can see a larger molecule, such as pentanol, takes more energy to break its bonds, 5380J. Compared to a smaller molecule, methanol, which requires less energy to do so, 2100J. I can thus come to predict that the longer the molecular structure in the alcohol, the more energy it will take to remove the bonds. So when I come to predicting results I can safely say that the larger molecule, in my case hexanol, will evolve more energy than ethanol simply because it has more bonds to break. ...read more.


0.76 Hexanol 20 50 152.32 151.72 0.60 We repeated the experiment for propanol again as we realised that we must have made a mistake somewhere along the line. The second sets of results were much more what we expected to get. Alcohol Start temp. water (?C) End temp. water (?C) Mass of alcohol at start(g) Mass of alcohol at end(g) Mass of alcohol used (g) Propanol 19 49 166.40 165.54 0.86 Energy Transferred - 0.1 x 30 x 4.2 = 12.6KJ Energy per gram of fuel: ethanol - 12.60/0.90 = 14KJ/g propanol - 12.60/0.86 = 14.65KJ/g butanol - 12.60/0.84 = 15KJ/g pentanol - 12.60/0.76 = 16.58KJ/g Heat of combustion ethanol - 14 x 46 = 644KJ/mol propanol - 14.65 x 60 = 879KJ/mol butanol - 15 x 74 = 1110KJ/mol pentanol - 16.58 x 88 = 1459KJ/mol hexanol - 21 x 102 = 2142KJ/mol Graph to show the relationship between the molecular mass and the heat of combustion Analysis: In this experiment I proved that my hypothesis was correct and that as the molecular structure increases, so does the heat of combustion, the energy required to break the molecule. ...read more.


To ensure this experiment was as reliable as possible we then decided to repeat the experiment for the alcohol propanol. Repeating this alcohol, the result produced was much more like what we had expected the first time and so the first result was disregarded for the rest of the experiment. If I were to repeat the experiment again I would perhaps use different temperatures, raising the original temperature of the water by a different amount. With these other results I would be able to compare them with the first set of results and record any change. Also, to perhaps make it a fairer test and therefore more accurate, I would use the same amount of alcohol each time and see if this would cause a change in my original results. Based on my prediction I believe this was a suitable procedure to use to get the appropriate results I wanted. It was simple to understand and easy to undertake and my results were accurate as possible for our environment, which proves the suitability of the experiment. Overall, I enjoyed this experiment and believe it went well. Katrina Joseph Chemistry coursework - 1 - ...read more.

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