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Comparing the enthalpy change of combustion of alcohols down a homologous series.

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Introduction

Comparing the enthalpy change of combustion of alcohols down a homologous series Aim The aim of this investigation is to discover what effect of increasing the length of the carbon chain in an alcohol has on the energy produced. To investigate this, the enthalpy of combustion will be calculated, by burning the fuels and heating a measured volume of water. Preliminary Investigation A preliminary investigation was carried out in order to show any weaknesses in the method that is used for this investigation. Diagram of Apparatus Method The apparatus was assembled as shown in the diagram above. 100cm3 of cold water was measured out into a calorimeter. The temperature of the water was recorded. The spirit burner was weighed and recorded, then placed under the calorimeter. The wick was then lit and left until the water had reached a temperature 15�C higher than the original recorded temperature. When this temperature was reached the flame was put out and the spirit burner reweighed and the final weight recorded. Results Methanol -274.67Kjmoldm-3 Ethanol -475.38 Kjmoldm-3 Propanol -515.00 Kjmoldm-3 Butanol -1172.6 Kjmoldm-3 Pentanol -1437.2 Kjmoldm-3 Prediction Combustion occurs due to oxidation; in this case the oxidation is of alcohols. When combustion of alcohols occurs an exothermic reaction takes place. An exothermic reaction releases energy in the form of mainly heat but also light. ...read more.

Middle

The wick length must be 1.5cm long and this length must be the same for each investigation. This ensures that the flame is the same distance from the bottom of the calorimeter in each case. Once the burner is weighed and replaced the wick was ignited. When the temperature is 15degrees higher than the original temperature the flame was extinguished. The burner must now be reweighed and recorded. This method must be repeated three times for each fuel. The method must be exactly the same in each case. For each different fuel the same spirit burner is to be used, changing only the wicks. This ensures that there are no differences in the methods providing accurate results. The following equations will be used to calculate the energy produced per mole The first E = MC x T Where M = the mass of water heated C = the specific heat capacity of water which is 4.17Kj-1g-1 T = the temperature change. This provides the energy transferred to the water provided by the fuel. The next equation is used to work out the energy produced per mole for each fuel. This is so a fair comparison can be made between the values. Moles = Mass Mr Moles = energy per mole. Energy Results table Alcohol Original Weight (grams) ...read more.

Conclusion

* The accuracy of the equipment used was calculated and accounted for. Better equipment would have provided better results. The lack of equipment in the laboratory had a major effect on the results. Better equipment would have allowed much better results that were closer to the text book results to be obtained. However the required equipment for much more accurate results was not available. However the results are precise in that they all, except butanol are on the line of best fit. Overall the experiment was successful. The results obtained give a clear conclusion, that the larger the alcohol chain the higher the enthalpy change. Improvements The improvements that can be made to this particular experiment are: * The specific heat capacity of copper and the mass of copper can by multiplied by the temperature rise. This will give the value for the energy transferred to the copper calorimeter. This would account for a lot of the energy lost from heating the copper calorimeter. * A broader base copper calorimeter would prevent the flames from flaring out around the base. This would reduce the heat lost heating the surrounding area of the calorimeter. To improve the experiment all together a bomb calorimeter can be used. In a bomb calorimeter, a large voltage is required initiate a reaction. The calorimeter is surrounded by an excess of oxygen to ensure complete combustion. A high pressure of 25 atmospheres ensures that there is an excess of oxygen and no air or moisture. http://www.acornuser.org/education/thesis/bombcalorimetry. ...read more.

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