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Comparison of Enthalpy Changes of Combustion of Different Alcohols.

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Introduction

Emma Powditch 12RP Chemistry Coursework Comparison of Enthalpy Changes of Combustion of Different Alcohols Aim: To investigate how and why the enthalpy of combustion of varying alcohols is affected by molecular structure. The enthalpy change of combustion refers to the amount of energy transferred when one mole of fuel burns completely. This value was obtained by using various fuels to heat water, knowing 4.2J of energy are required to raise the temp of 1g of water by 1oC, using the following method. The enthalpy change of one mole of alcohol can be calculated from the amount of energy the water has absorbed. Apparatus Fuel burners and the following alcohols Methanol (CH3OH) Ethanol (C2H5OH) Propan-1-ol (C3H7OH) Butan-1-ol (C4H9OH) Octan-1-ol (C8H17OH) Copper Calorimeter Thermometer 250cm3 measuring cylinder Goggles Draft excluder Electronic scales (to 3dp for accuracy purposes) Stirring rod Diagram Method 1. Pour 200cm3 of water into a copper calorimeter and record the temperature. 2. Using a clamp stand support the calorimeter over your chosen spirit burner. (At this point it is useful to erect some kind of draft excluder, for increased accuracy, in my case I used a tin can.) Any disturbances could affect the infra red radiation being produced by the burning alcohol and help direct it towards the calorimeter. 3. Weigh the burner and alcohol including lid, otherwise the alcohol will evaporate and disrupt the results, record the result in a suitable table. ...read more.

Middle

The amount of energy taken in by the water = 200 x 4.2 x 15 = 12600J (12.6 KJ) To calculate the amount of moles of propan-1-ol were used in the experiment we first need to calculate it's molecular mass. C3H7OH = (12 x 3) + 7 + 16 + 1 = 60g 1.191g / 60g = 0.01985 moles Therefore 1 mole of propan-1-ol will release: 12600J/ 0.01985moles = 634.707053. 634.7 to 1 (dp) As with all exothermic reactions heat gain by the surroundings must have come form some energy loss in the reaction making the enthalpy a negative figure. Propan-1-ol in this case is calculated to have an enthalpy of combustion of - 635 (3sf) KJ mol -1. The following table represents all the relevant enthalpies derived from my experiment, using the same method of calculation. There are two results for each alcohol, which I attempted to average to arrive at a more accurate result. Alcohol Energy of combustion KJ mol-1 Average energy of combustion KJ mol-1 Standard Enthalpies kjmol-1 Difference in KJ mol -1 Percentage Error Methanol A -334 -343 -726 32.86 53 B -353 Ethanol A -583 -595.5 -1367.3 228.2 56 B -608 Propan-1-ol A -635 -1167 -2021.0 854 43 B -1167 Butan-1-ol A -20 -1869 -2675.6 806.6 31 B -1869 Octan-1-ol A -18 -24.86 -5293.6 212.6 99.7 B -32 The next columns compare my results to the standard enthalpies of combustion from each alcohol. ...read more.

Conclusion

Take the complete combustion of ethanol for example, where all the C-H and C-C bonds are broken to form the new products carbon- dioxide and water. C2H5OH(l) + 3O2(g) 2CO2 (g) + 3H2O(l) The bonds are broken when there is sufficient energy (activation energy) in the alcohol this process takes in energy. New bonds form giving out energy, this exchange of energy determines the final enthalpy, as all the bonds have specific bond enthalpies. The difference between the sum of bonds broken and the sum of new bonds formed is the enthalpy of combustion. Therefore it is logical to suggest that as you lengthen the carbon chain and add a two more hydrogen molecules each time you will increase the negativity of the enthalpy, within a linear relationship. Ignoring the obvious anomaly of the octan-1-ol result I plotted a graph (see back page) to represent the enthalpy of combustion for my chosen alcohols. Comparing the two you can see that the theoretical values are more linear which is more expected. To predict the trend for the larger molecules I would suggest that the line would get increasingly steeper as it begins to do in both cases. Sources I researched the properties and combustion of alcohols, from a variety of sources including books and the Internet. Named sources Ratcliff Brian, Eccles Helen, Johnson David, Nicholson John, Raffan John (2000) Chemistry 1, Cambridge University Press Ritchie Rob (2000) Letts Education Revise AS Chemistry, Letts education Chemistry data Booklet (used to obtain the theoretical enthalpies) 1 ...read more.

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