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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 By Steven Duong Background Information The enthalpy change of the combustion of a fuel depends on two things. First, there is the number of bonds to be broken and made - and that depends on the size of the molecule involved. The ?H?c also depends on the type of bonds involved: e.g. combustion of a) methane and b) methanol a) CH4 + 2O2 CO2 + 2H2O H O=O O=C=O H-O-H O=O H-O-H H C H H b) CH3OH + 11/2O2 CO2 + 2H2O H O=O O=C=O H-O-H O H-O-H H C O - H H From the examples shown above, the products are the same, but the key difference is that methanol already has an O-H bond; one of the bonds to oxygen is already made, unlike with methane, where all the new bonds have been made. The energy released during combustion comes from the making of bonds to oxygen. If methanol already has one bond made, it will give out less energy when it burns. The enthalpy change of a fuel is a measure of the energy transferred when one mole of the fuel burns completely. A value for the enthalpy change can be obtained by using the burning fuel to heat water and using the fact that 4.17J of energy are required to raise the temperature of 1g of water by 1�C. So, the aim of this investigation is to find the enthalpy change of combustion of a number of alcohols so that you can investigate how and why the enthalpy change is affected by the molecular structure of the alcohol. Preliminary Investigation To give a clear indication of how to conduct the experiment precisely and properly, preliminary tests may be required. The results from these particular tests may give you an indication as to what type of range and temperature difference you may wish to use in your actual experiment. ...read more.

Middle

To calculate the expected value, you need to be familiar with bond energy calculations. To do this, simply follow the diagram below: ?H reaction Reactants Products ?H1 ?H2 Gaseous atoms The diagram (above) refers to Hess' law; according to law, it states that the enthalpy change for any chemical reaction is independent of the intermediate stages, provided the initial and final conditions are the same for each route. An example of how to calculate the expected value of the enthalpy changes of combustions of an alcohol can be shown in the previous pages. To calculate your experimental value, simply get the average enthalpy change from your results and replace that figure with the 'actual' figure from the data book, of the intended alcohol. An example of how to calculate the experimental value can be shown in the 'analysis' section. Analysis Steps in calculating the experimental value (?H from bond enthalpy): 1. Write a balanced equation of reactants and products involved. 2. Split the reaction up into categories of "breaking bonds" and "making new bonds", so that the reactants go under the breaking bonds category and the products go under the making new bonds category. 3. Calculate the total energy of the reactants. To do so, add the average enthalpy change of that specific alcohol to the average bond enthalpy of the molecule the alcohol reacts with. 4. Calculate the total energy of the products. To do so, add the average bond enthalpies of the two products. E.g. when a hydrocarbon combusts, it usually produces carbon dioxide and water. The value you end up with when calculating the energy of the products, is always negative because making bonds release energy; it is an exothermic reaction. 5. To calculate the total enthalpy of combustion from bond enthalpies, add the value you calculated from breaking bonds with the value from making new bonds. e.g. calculating the experimental value (?H from bond enthalpy) for methanol... Breaking bonds Making new bonds CH3OH + 11/2O2 CO2 + 2H2O (146 KJ) ...read more.

Conclusion

Justifying the attempts of modifying the method of conduction to improve both reliability and accuracy of results, the reason why the standard thermometer should be replaced with an electronic thermometer, is because it is highly accurate in its measurements and it enables you to set it to the desired temperature difference, so you need not worry about temperature lagging. By varying the variety (greater range) and type of alcohol, it would enable you to clarify and justify to a greater extent that alcohols release indeed more energy with the most bonds in their molecular structure. Also, by increasing the number of repeats on each test, it would allow you to obtain results of an even greater degree of accuracy simply because you would have a wider collection of results which can be averaged. By following such modifications, it may actually help to minimise the significant sources of error. Calculating the percentage error: Measurement Value Error No. of measurements taken % error Equipment used Volume of water 50g +/- 0.5 1 1 100cm3 measuring cylinder Temp. of water 20�C +/- 0.1 2 1 Thermometer (-10 - 110�C) Mass of calorimeter 52g +/- 0.05 2 0.1 Balance Mass of alcohols: methanol 1g +/- 0.05 2 10 Balance Ethanol 0.5g +/- 0.05 2 20 Balance Propan-1-ol 0.37g +/- 0.05 2 27 Balance Butan-1-ol 0.3g +/- 0.05 2 33 Balance Pentan-1-ol 0.26g +/- 0.05 2 38.5 Balance So, the total percentage error per test on each alcohol is: * Methanol = 12.1% * Ethanol = 22.1% * Propan-1-ol = 29.1% * Butan-1-ol = 35.1% * Pentan-1-ol = 40.6% Having calculated the percentage error of the tests on each of the alcohols, you are able to see how accurate the results actually are. Judging from the percentage errors, the results were fairly accurate and they are reliable in the sense that a firm conclusion is produced. Again, as previously stated, more energy is released by the alcohols with the most bonds in their molecular structure, because breaking those bonds by burning; it produces more bonds, which initially release more energy. ...read more.

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