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Investigating the enthalpy change of different fuels when combusted.

Extracts from this document...

Introduction

Investigating the enthalpy change of different fuels when combusted. Aim: The aim of this experiment is to see how the enthalpy changes vary when different alcohols are combusted in standard conditions. The alcohols used will be: Ethanol, Propan-1-ol, Butan-1-ol, Pentan-1-ol and Hexan-1-ol. These alcohols all have the OH functional group on the first Carbon atom of the molecule. Background Knowledge The enthalpy change of combustion (also shown as ?Hc) is a measure of energy when one mole of a fuel burns completely in air, at a standard temperature and pressure. This standard temperature is 298 Kelvin and 1ATM pressure. Maintaining these conditions would be complicated; therefore the experiment will be carried out in normal conditions. Any drastic alterations to these conditions will need to be taken care of to maintain a degree of accuracy. As combustion is exothermic (heat is transferred to the environment) all of the enthalpy changes will be negative. The formula for enthalpy change is: Energy Transferred= Heat capacity of water x Change in water temp x mass of water. Or E=MC?T Hess's Law. "the enthalpy change for any chemical reaction is independent of the intermediate stages, provided the initial and final conditions are the saeme for each route." This basically means that in an enthalpy cycle, detours can be made to reach the final products and the total energy required for the detour will be equal to the direct route ie. The enthalpy change for ?H1 = ?H2 + ?H3. as we cannot go directly along ?H1, we must take the detour and from this we can calculate the enthalpy change. This can be calculated by the enthalpy changes of combustion for carbon, hydrogen and the alcohol in question. Bond Enthalpies: The amount of energy required to break the bonds between atoms is called the disassociation energy. The higher the disassociation energy, the shorter the bond is in length and the stronger the bond is. Single bonds are relatively easy to break as opposed to double or triple bonds. ...read more.

Middle

fill a copper calorimeter with 100 cm^3 of water 4. note the temperature of the water 5. ignite the spirit burner and wait for temperature to rise by 20�C 6. extinguish the spirit burner once the temperature has risen by 20�C 7. Measure the mass of the spirit burner with lid again. Note mass. 8. Empty and Clean the base of the calorimeter to clear up any soot due to incomplete combustion. 9. repeat the experiment for all alcohols 10. repeat experiment 3x for each alcohol. Safety All of the alcohols used in this experiment are harmful to human health if ingested or if contact with eyes occurs. Ethanol vapours are known to have narcotic properties and could be dangerous if inhaled in moderate volumes. All of the alcohols are irritant to the skin and eyes. To overcome this problem, gloves must be worn when handling alcohols out of the spirit burners and goggles should be worn at all times to prevent irreversible damage to the eyes. Alcohol vapours may have narcotic effects so they should not be inhaled or ingested. All of the alcohols are flammable so should be kept away from sources of ignition and incandescent materials. Ignition of the alcohols should only be completed when all of the equipment is set up and there is no risk of any materials catching fire or melting. Measurements All temperature measurements will be undertaken with either a thermometer or a data logger. The data logger is digital and therefore more accurate. These can show how much the temperature has changed and can provide a signal to extinguish the flame of the spirit burner. To maintain a degree of accuracy, all experiments should be carried out at least three times and an average should then be taken. If the mass changes are more than 5% away from each other, the experiment should be retaken to get more accurate results. ...read more.

Conclusion

This layer of soot acts as an insulator to the heat and increases the number of moles of fuel required to heat the water. The above factors all significantly affected the results due to heat loss. There were other contributing factors to the large difference between predicted and actual results. The distances that the wick stuck out from the top of the burner were all equal, but some of the wicks had differing diameters and amount of fraying at the end of the wick. Both of the factors caused there to be differing surface areas that could contain fuel for combusting. The diameter of the wick caused there to be larger or smaller surface areas through which the fuel could be combusted and the fraying increased the surface area drastically, as each fibre from the wick was fully exposed at the end. This cased there to be more or less fuel burned across the different fuels. The larger flames were in contact with the base of the calorimeter and on one occasion were large enough to ignite the tin foil covered card that was insulating the top of the calorimeter, whereas the smaller flames were barely reaching 5cm from the tip of the wick. This caused there to be more fuel burnt and more heat lost due to flames escaping from the insulation for the larger flames, and more fuel being burnt due to the distance between tip of flame and calorimeter. There is also a degree of uncertainty. This is calculated by using the following formula: Predicted - actual Predicted x100 = uncertainty. Here are the uncertainty values for my results: Predicted (KJ/mol) Actual (KJ/mol) Uncertainty Ethanol 1249 200 83.98719 Propanol 1867 316 83.074451 Butanol 2485 483 80.56338 Pentanol 3103 559 81.985176 Hexanol 3721 594 84.036549 There is a large degree of uncertainty in this experiment caused mostly be the lack of sufficient heat insulation. These figures show that the results are very inaccurate and are relatively useless to science. ?? ?? ?? ?? Luke Harlow Candidate No.: 1109 Centre No.: 20806 1 ...read more.

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