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# Molar Heat of Combustion of Alcohols

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Introduction

Molar Heat of Combustion of Alcohols Task: To find the molar heat of combustion of propanol, ethanol and butanol and to calculate the theoretical values using average bond enthalpies. Then to compare these to the practical values obtained from the experiments, also to predict the theoretical and practical molar heat of combustion of pentanol. Planning Background Information Alcohols: Alcohols are part of a family of compounds that are all based on a chain of carbon atoms. They can be considered derivatives of water in which the hydrocarbon atom has been replaced by a hydrocarbon chain. Alcohols all have the general formula CnH2n+1OH and which burn cleanly in oxygen or a plentiful supply of air to produce carbon dioxide and water vapour. If the air supply is reduced then they will burn to produce carbon monoxide, soot and water vapour. All Alcohols contain an oxygen atom in a hydroxyl group, meaning that the oxygen atom is joined to the hydrogen atom as well as the carbon atom. All alcohols contain at least one oxygen atom joined to a carbon atom by a single bond. Equations for the Three Alcohols: This is the formula for all alcohols: Cn H2n+1 OH The molar heat of combustion is the heat liberated when one mole of alcohol is burnt in a plentiful supply of air. These are the formulas for the combustion of the three alcohols. Butanol + Oxygen � Carbon Dioxide + Water C4H9OH + 602 � 4CO2 + 5H2O (One Mole) Propanol + Oxygen � Carbon Dioxide + Water C3H7OH + 4.5O2 � 3CO2 + 4H2O (One Mole) Ethanol + Oxygen � Carbon Dioxide + Water C2H5OH + 3O2 � 2CO2 + 3H2O (One Mole) Bond Enthalpies: Bond enthalpy is the amount of energy needed to break a chemical bond. Bond enthalpies differ with each type of bond and the exact enthalpy for a particular bond can differ. ...read more.

Middle

= Energy used by 1g. 15269.331/0.42=36355J One Mole of Butanol weighs 74g so we times the energy used by 1g by 74. 36355.55*74=2690310J Actual Experiment: We weighed the burner and the container before and after testing and recorded the temperature change. We heated the container with 25ml of water for two minutes. We weighed the water before and after so we could make the test fairer by averaging the two, the average gives a more accurate picture of the mass of water used. We used heatproof mats to stop any drafts getting to the burner as this would have made our results less accurate. We used a digital thermometer to measure the temperature at the end of the test because they are more accurate. We tried to make sure that the wick on each of the burners was the same length to make the experiments more accurate. The container is held at a set position above the burner as to make the experiment more accurate. To keep our tests fair we will have a set of variables which will remain constant throughout the test, these variables are; * The mass of water (25cm3). * The container, (different containers would have different thicknesses and sizes, therefore possibly being more or less accurate than others). * The length of the wick (0.5cm). * The distance between the container and the flame must stay the same because if there was any difference in this then the experiment would be inaccurate. * The conditions in the room should stay the same, the temperature should be around 23OC and there should be hardly any breeze as this would move the flame around which would stop the heat getting to the container. The variables that must be changed throughout the experiment are only the alcohols (and therefore the burner). The measurements we will use in our experiment are as follows: Moles (mol) ...read more.

Conclusion

The fact that we also changed containers throughout also means that we have no way of telling the weight because the ones we used are not marked. There is also the possibility that the calorimeter may have been hotter or colder than the temperature of the water. Evaluation: I was expecting our test to be very inaccurate but having looked at the different possibilities for error and at the results when including the heating of the calorimeter then I realized that it was a lot more accurate than I was expecting. Including the heating of the calorimeter the accuracy of the test peaked at 72% of the theoretical value that we worked out using the bond enthalpies. This was for butanol. We could have easily improved our test but this may not have yielded more accurate results. The biggest mistake we made throughout was not to mark our containers, this means that we may well have used containers with different masses throughout. We also did not thoroughly clean out our apparatus and containers beforehand. This means that the containers we used could well have had a covering of dirt and carbon therefore reducing conduction. Overall I think that we could have improved the accuracy of the test by reducing human error but this may not have increased the accuracy of our data. The climate and outside conditions played a large part in all of our results and there is no way to control this in the circumstances. The electrons within the alcohol molecule are shared therefore it has covalent bonds as opposed to ionic ones. Each atom within the molecule has a full outer shell of electrons from sharing with other atoms; this makes the molecule very stable. Each step-up in the group of alcohols adds two carbon-hydrogen bonds, a carbon-carbon bond, an oxygen=oxygen bond and half a carbon=carbon bond that are available for combustion. Therefore, there is a net energy gain every time. This means that the additional energy added will be the same each time. ...read more.

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