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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 Plan Aim: To compare the enthalpy changes of combustion of different alcohols. Apparatus and Quantities: � 4 Spirit Burners containing - methanol, ethanol, propan-1-ol, butan-1-ol � Small copper can to act as a calorimeter � 0-110�C thermometer to measure temperature � 100 cm^3 measuring cylinder � Aluminium shield to reduce heat loss to surroundings � Digital Balance � Retort Stand, Boss and clamp (x2) � Water (800 cm^3) Method: 1) Using a 100 cm^3-measuring cylinder, measure 100 cm^3 of water and this water in the copper can. 100 cm^3 of water should be used, as this is an appropriate amount of water as this fills � of the can, so it is just enough water to give a sufficient temperature rise. The copper can is used as copper is a good conductor and therefore it will distribute the heat equally. 2) Take the first spirit burner with methanol and weigh this using the digital balance as it is more accurate. Record this as the initial mass. This will then be used to calculate the amount of mass used during the combustion. 3) ...read more.

Middle

This formula will then calculate the energy transferred to the water by the fuel burned assuming that all the energy from the burning fuel is transferred to the water. The energy that would be transferred to the water by burning 1 mole of fuel then needs to be calculated following these stages: Formula of fuel = Mass of 1 mole of fuel = (RMM) Number of moles of fuel used = (mass/RMM) Energy transferred by this number of moles of fuel = (cmDT) Energy transferred by 1 mole of fuel = (cmDT)/(mass/RMM) Enthalpy change of combustion = Risk Assessment: Alcohol is being used, therefore, it is important to take care when handling them. Lab coats and goggles should be worn at all times to ensure safety in the event of an accident. All alcohol's being used are highly flammable, therefore, the lids of the spirit burners must be kept on when not experimenting as the alcohol can vaporise and the vapour will catch fire at temperatures above 13�C. Methanol is also toxic by inhalation, if swallowed and by skin absorption. Therefore, wear eye protection and lab coats at all times. Butan-1-ol is also an irritant, so contact with eyes and skin should be avoided. ...read more.

Conclusion

of moles used = 6.657 kJ Energy transferred by 1 mole of Butan-1-ol = 6.657/0.006 = 1109.5 kJmol^-1 Enthalpy change = -1110 kJmol^-1 Using these formulae, I calculated the results for the enthalpy changes for different fuels. These are the results I found: Fuel Enthalpy Changes (Kjmol^-1) CH[3]OH- methanol -352 C[2]H[5]OH- ethanol -687 C[3]H[7]OH-propan-1-ol -840 C[4]H[9]OH -butan-1-ol -1110 [image002.gif] From the results, it is clear that the enthalpy changes increase as the number of carbon atoms gets bigger. It is clear that, more energy is needed to break the bonds in the fuel butan-1-ol than needed to break the bonds in methanol as the table shows that the enthalpy change for methanol is -352 Kjmol^-1 and that for Butan-1-ol is -1110 Kjmol^-1. This is also evident on the graph where the descending order of the enthalpy changes is shown. Methanol needs the least amount of energy to break its bonds, ethanol needs -687 Kjmol^-1 to break its bonds, which is less than propan-1-ol which needs -840 Kjmol^-1. However butan-1-ol needs the most energy to break its bond. This trend can be explained by using the structural formulae of these fuels. [image003.gif] As you can see, methanol has fewer bonds that need to be broken in order for combustion to take place. Evaluation ...read more.

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