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Comparing the enthalpy changes of combustion of different alcohols

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Introduction

Comparing the enthalpy changes of combustion of different alcohols Altaf Korimbocus Aim I am going to investigate the difference in enthalpy of combustion for a number of alcohols, the enthalpy of combustion being the 'enthalpy change when one mole of any substance is completely burnt in oxygen under the stated conditions'. I plan to find out which alcohol, out of methanol, ethanol, propanol and butanol, produces the most energy when burned in air Background and other information Methanol is the smallest alcohol, it releases 524.5 kJ mol-1, the next largest, Ethanol is one carbon and two hydrogen's larger, It release 1013 kJ mol-1 this is a difference of 488.5 kJ mol-1 this trend continues as the alcohols get larger. This is effectively because the only difference between the alcohols is a increase in size by one carbon and two hydrogen's each time. For my investigation I am going to use propan-1-ol and butan-1-ol as representatives of propanol and butanol. Propanol and butanol are large enough molecules to form isomers etc, I have decided to use propan-1-ol and butan-1-ol because they have the closest structural arrangement to the other alcohols that I am going to be testing. Using butan-1-ol and propan-1-ol means all the alcohols that I am comparing have their OH group joined onto an end carbon and they are all straight chain alcohols. ...read more.

Middle

Temp. difference (�C) Initial alcohol mass (g) Final alcohol mass (g) Difference in mass (g) 1 21.5 43.0 21.5 213.18 212.06 1.12 2 21.0 43.5 21.5 216.06 211.08 0.98 3 21.0 41.0 21.0 211.08 210.05 1.03 Butan-1-ol Exp. No. Initial temp. of water (�C) Final temp. of water (�C) Temp. difference (�C) Initial alcohol mass (g) Final alcohol mass (g) Difference in mass (g) 1 20.5 39.0 18.5 280.08 280.08 0.5 2 19.0 93.0 20.0 278.34 277.75 0.59 3 19.0 38.0 19.0 277.47 277.0 0.47 From looking at my results the butan-1-ol experiments were not showing the decrease in mass of alcohol needed to causes the temperature rise almost straight away. I checked the apparatus and then I noticed that the bottom of the calorimeter had a black sooty type substance on it, which I thought must have been affecting my results for butan-1-ol. I came to the conclusion that the black substance was carbon from the alcohol that had not been completely combusted in the reaction and was being deposited on the calorimeter. Although this may have also happened on the smaller alcohols it will not have been as severe as the activation energy for the combustion of the alcohols as they get smaller decreases. ...read more.

Conclusion

My results are still not very accurate, but that is because a lot of the heat that is produced does not even enter the apparatus, it is aloud to escape immediately. This is an area where the accuracy of my experiment could have been improved a lot, by not allowing any heat escape, but to do this I would have to use a bomb calorimeter, which was not available to me. If I were to do this experiment again I would have more time so that I could collect a better set of results and also I might have changed the way my apparatus was set out to increase the accuracy and reliability of my experiment, a possible improvement could be that I use a lid on the cup I was using to help decrease heat loss The only other thing that could have been improved would be to have been to be able to measure all the heat energy that had been released by the combustion of the alcohol's, increasing the accuracy which was not possible with the equipment available to me. I do have a number of ideas on how to further increase the accuracy of my results but I did not have time to put in to practice, I think that the fact that I was not able to fully control the heat loss had the most confounding effects on my results 1 ...read more.

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