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Comparing the enthalpy change of combustion

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Introduction

Comparing the enthalpy changes of combustion of different alcohols The aim of this experiment is to investigate how the length of an alcohol molecule and the position of its OH group affect its enthalpy change of combustion. This will be done using the comparison method, using a known value for the enthalpy change of combustion for one fuel (here, propan-1-ol) and replicating the experiment for each alcohol, then accurately comparing the results. The 4 alcohols that will be compared to propan-1-ol (CH3CH2CH2OH) are: o methanol (CH3OH) o ethanol (CH3CH2OH) o propan-2-ol (CH3CH(OH)CH3) o butan-1-ol (CH3CH2CH2CH2OH) The propan-2-ol differs from propan-1-ol because its OH group is positioned on the 2nd carbon atom in the chain. Combustion reactions involve breaking the intra-molecular bonds (bonds between atoms, within the molecule) of a fuel, then making new bonds with oxygen. The formula for the combustion reaction is: Alcohol + Oxygen ==> Carbon Dioxide + Water e.g. CH3OH + 11/2O2 ==> CO2 + 2H2O Breaking bonds takes energy in from the surroundings, and making bonds gives energy back out. The enthalpy change is the net energy transfer of the reaction; it is negative if the reaction is exothermic, like combustion, and positive if the reaction is endothermic. ...read more.

Middle

The final temperature of the water, the temperature change, and the mass of alcohol used may differ. Methanol Ethanol Propan-2-ol Butan-1-ol Molar mass of alcohol g/mol Initial mass of spirit burner g Final mass of spirit burner g Mass of alcohol burned g Moles of alcohol burned mol Initial temperature of water �C Final temperature of water �C Temperature change K Enthalpy change of combustion kJ/mol If time allows, the experiments should be repeated for both the propan-1-ol (to ensure the value for the energy required for rise in water's temperature of 1K is reliable) and for the rest of the alcohols (to ensure the value for the enthalpy change of combustion is reliable). Results Alcohol Methanol Ethanol Propan-1-ol Propan-2-ol Butan-1-ol Initial temperature of water/�C 24 25 23 24 23 Final temperature of water/�C 42 44 44 45 43 Temperature change/�C 18 19 21 21 20 Initial mass of spirit burner/g 176.94 173.99 183.14 201.91 193.77 Final mass of spirit burner/g 175.73 172.84 181.94 200.81 193.01 Change in mass/g 1.21 1.15 1.20 1.10 0.76 The temperatures were measured using a 100�C thermometer, marked every 1�C. The masses were measured using a balance, reading to the nearest 0.1 grams. ...read more.

Conclusion

Although we tried to minimise this heat loss by using heatproof mats, this did not provide the closed environment which would be ideal. It should also be noted that all of the fuels burned with an orange/yellow flame, indicating incomplete combustion. This will have produced less energy than complete combustion, further decreasing the calculated enthalpy change of combustion. If I was to try and improve this experiment, I would use a calorimeter to reduce heat loss. I would also calibrate the equipment using a fuel with a known enthalpy change of combustion, to try and work out how much energy is lost to the equipment. I could then incorporate this into my calculations to give me a more accurate value for the enthalpy change. Sources Salters Advanced Chemistry, Chemical Storylines, Heinemann, 2000, pg 22. Salters Advanced Chemistry, sheets DF1.2 and DF1.3, 2000. Chemistry in Context, Nelson, 4th Edition, 1995, pg 450. These are reliable sources as they are respected and widely used textbooks and course materials. CLEAPSS Science Publications CD-ROM, 2005. CLEAPSS hazcards are usid in school laboratories. http://www.wpbschoolhouse.btinternet.co.uk/page18/02DFws.htm 6/11/2007 I believe this is reliable as it has been designed to help with the Salters AS course. http://www.chemicool.com/forum/post-177.html 11/11/2007 I believe this is reliable as there are other posts on the forum, by the same member, which I know are correct. ?? ?? ?? ?? Sophie Catt ...read more.

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