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Enthalpy - investigate the difference in enthalpy of combustion for a number of alcohols

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Introduction

Enthalpy 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 will be attempting to find how the number of carbon atoms the alcohol contains effects the enthalpy change that occurs during the combustion of the alcohol. Method I plan to measure the enthalpy change by burning the alcohol, using a spirit burner, I will then use the heat produced during the combustion of the alcohol to heat 100ml of water that will be situated in a copper calorimeter directly above the burning alcohol. The calorimeter is made of copper as copper has a high thermal conduction value, this basically means that it is a good conductor of heat so a lot of the heat the copper receives will be passed on to the water which I am then able to measure. During the experiment I will be taking a number of measurements, I will firstly take the initial temperature of the water and initial mass of the alcohol I will then burn the alcohol until an increase in temperature of 20oc has occurred in the water I will then reweigh the alcohol. The measurements * Mass of alcohol burned (g) * Temperature increase (oc) will tell me what mass of alcohol is used during combustion to cause the temperature increase of 20oc in the water, I can then work out the energy released per mole and compare these values and see which has the highest enthalpy of combustion. I will need to repeat my experiment a number of times and take an average so I am sure of an accurate result. The set up of the apparatus that I plan to use is shown below The set up of the apparatus as you can see is very simple, the calorimeter, which contains the 100ml of water, is held directly above the spirit burner by a retort stand and clamp. ...read more.

Middle

0.5 19 39 20 278.34 277.75 0.59 19 38 19 277.47 277.00 0.47 18 38 20 276.57 276.07 0.5 19 40 21 276.03 275.50 0.53 17 37 20 275.56 275.09 0.47 I have drawn a graph, below, that shows the temperature increase per gram for each alcohol. The temperature increase per gram although can not be used to compare for the enthalpy of combustion, as that is a measure of the enthalpy change per mole it highlights a problem that I found with my results for butan-1-ol. I started to notice that the results that I got from the butan-1-ol experiments were not showing the decrease in mass of alcohol needed to causes the temperature rise almost straight away and could not understand why this was happening. I checked the apparatus and then I noticed that the bottom of the calorimeter had a black sooty type substance on it, which I knew 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, like soot in a chimney. 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. This can be explained by looking back to when I examined the bond energies, the smaller alcohol's needed less energy to break the bonds in the reactants and so less energy is needed to initiate the reaction. The results that I got from my experiments for butan-1-ol will have been affected in two ways *Firstly carbon is a better insulator than copper, it has a lower thermal conductivity value and so it will stop as much heat getting through to heat the water as there should be *Secondly the carbon has come from the spirit burner and for my results I am assuming that all the alcohol ...read more.

Conclusion

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. The graph below compares the enthalpy of combustion from the bomb calorimeter to the results that I got from my experiments. From the graph above it is possible to see why I have been pleased with the results that I have got from my experiments. The only 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 was able to improve my experiment after I initially completed it but I don't think I could get much more accurate results. I did though manage to meet the aims of the investigation by finding how the number of carbon atoms within the alcohol affects the enthalpy of combustion. I did have an idea on how to further increase the accuracy of my results but I did not have time to put in to practice. I thought that I could make something that directed more of the heat produced towards the apparatus. A sketch of it is shown below. This would keep more of the heat produced during combustion close to the calorimeter so more is absorbed. Lining the reflector with silver/ shiny surface would also mean a lot more of the heat is kept in the apparatus so that I am able to measure it. There are other aspects of the enthalpy of combustion of alcohols that I could have also investigated. Firstly I could have looked into whether the position of the OH group within the molecule effects the enthalpy change and also whether branching within the molecule also has any effect on the enthalpy of combustion. Sadly I didn't get time to do this. ...read more.

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