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COMPARING THE ENTHALPY CHANGE OF COMBUSTION OF DIFFERENT FUELS.

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Introduction

COMPARING THE ENTHALPY CHANGE OF COMBUSTION OF DIFFERENT ALCHOHOLS. Aim The aim of my experiment will be to find out which alcohols have a higher enthalpy change of combustion. The comparison of the enthalpy changes of these fuels will then determine the alcohol efficiency and effectiveness. I will experiment on the first 5 consecutive primary alcohols. These are; methanol, ethanol, propan-1-ol, butan-1-ol, pentan-1-ol and hexan-1-ol. The reason why I chose these fuels is because they are the most reliable and accurate fuels to compare within the group; which also have the smallest variable, 'add one carbon' each time, to the aliphatic chain. All combustion reactions are exothermic which is why I am expecting all the values for the enthalpy change of combustion to always be negative. CH3OH + 1.5O2 CO2 + 2H2O METHANOL CH3CH 2OH+ 3O2 2CO2 + 3H2O ETHANOL CH3CH 2 CH 2OH+ 4.5O2 3CO2 + 4H2O PROPAN-1-OL CH3CH 2 CH 2 CH 2OH+ 6O2 4CO2 + 5H2O BUTAN-1-OL CH3CH 2 CH 2 CH 2 CH 2OH+ 7.5O2 5CO2 + 6H2O PENTAN-1-OL CH3CH 2 CH 2 CH 2 CH 2 CH 2OH+ 9O2 6CO2 + 7H2O HEXAN-1-OL Prediction I believe that hexan-1-ol will have the highest enthalpy of combustion because there are more bonds to break and form and the energy released when bonds form is greater than energy needed to break bonds, this means that the reaction is exothermic and there is a higher temperature rise. The molecular formulas are shown below. The general formula for alcohols series are CnH2n+1OH.This means you move across the series the amount of carbon and hydrogen atoms increase. This also means that the relative atomic mass increases. Although more energy is needed to break up hexan-1-ol than that of methanol, the energy released when new bond form is greater due to its higher mass and higher length of aliphatic chain, therefore it's more exothermic. The stored energy of the reactants is higher than the stored energy of the reaction. ...read more.

Middle

180.83 179.36 1.47 24 44 20 -685.71 BUTAN-1-OL 1.) 143.83 142.49 1.34 20 40 20 -927.76 -916.44 2.) 142.83 141.13 1.36 40 60 20 -914.12 3.) 141.13 139.76 1.37 18 38 20 -907.45 PENTAN-1-OL 1.) 190.95 189.65 1.30 27 47 20 -1137.23 -1155.28 2.) 189.65 187.39 1.26 47 67 20 -1173.33 3.) 187.65 185.55 1.84 18 38 20 -803.48 HEXAN-1-OL 1.) 178.10 176.92 1.18 42 62 20 -1452.20 -1520.48 2.) 176.92 175.87 1.05 18 38 20 -1632.00 3.) 175.87 174.71 1.16 41 61 20 -1477.24 Using the information I have gathered from my results, I have worked out the enthalpy change of combustion for the six alcohols. Using the formula's I have mentioned previously, these were; o Heat Transferred= CxMx?T (X) C is equal to the specific heat capacity of water it is equal to 4.18(J), which will raise the waters temperature by one Celsius. M is the mass of water used; in this case it was always 200cm3. I have collected three sets of results for each fuel to get reliable and accurate results. I will run through the calculations for one of the fuels. The fuel I will use as an example is Methanol. I have kept the temp rise variable constant meaning my temperature change was always 20�C. I will use this information and put it into formula to find the ?Hc for methanol. Energy > 200 x 4.18 x 20 M x C x ?T = 16800J So I can say that the energy transferred was 16800J. To find out how much energy should have been released for one mole of the fuel; I will firstly get my 1st 2nd and 3rd trials for methanol. FUEL START MASS (g) END MASS (g) MASS CHANGE(g) START TEMP (�C) END TEMP (�C) TEMP CHANGE(�C) METHANOL 1.) 185.26 183.41 1.85 22 42 20 2.) 183.41 181.01 2.4 43 63 20 3.) 181.01 178.87 2.14 20 40 20 I know that methanol; CH3OH has 1carbon, 1oxygen and 4hydrogen's present in one mole. ...read more.

Conclusion

This will cause heat loss to the surroundings as the fuel is burning when no access to the water is enabled. This means a mass loss of spirit burner is the case which will result in a higher enthalpy change of combustion which will skew my results. However despite all errors I could still make a valuable comparison about the enthalpy change for the homologous series of alcohols. I could see a rise in enthalpy as the number of carbons increase. However the smaller errors may have built up into a large error and may have biased the overall results of my experiment. Nevertheless I believe my results were accurate and useful in identifying the highest enthalpy within the alcohols. I believe that they were as accurate as I could possibly do them, most errors were inevitable and due to human error. If I was to redo the experiment I would focus on using better draught excluders and a lid for my copper calorimeter, in order to prevent heat loss as best as I could. I would also consider using another technique to wait for the water to boil, in this case I was waiting for the water to reach 20?C and then extinguished the wick, whereas if I was to use a set time, e.g. waiting for 1 minute. It would ensure my results were unbiased and more precise. It is also a lot easier to make mistakes using the thermometer; to know when to stop, as it has to rely on human judgement, whereas using a stopwatch would be easier and more accurate as thermometer had the highest percentage error. I will also make sure that I check the percentage errors on my equipment and decide on the ones which have the smallest percentage error and most suitable. Overall I believe that my experiment succeeded in finding out which alcohol out of the six provided had the highest enthalpy of combustion. ...read more.

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