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To investigate the relationship between the structure and heat produced by combustion for a range of alcohols

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Introduction

CHEMISTRY COURSEWORK: COMBUSTION OF ALCOHOLS AIM To investigate the relationship between the structure and heat produced by combustion for a range of alcohols THEORY ALCOHOLS Alcohols are a homologous series of organic compounds containing the functional group -OH known as a hydroxyl group. A functional group is a group of atoms in a structure that determines the characteristic reactions of a compound. In an alcohol, the hydroxyl group is bonded to an alkyl group. This is a chain of methyl groups with the formula CnH2n+1. The hydroxyl and methyl groups are covalently bonded. Bonds are the attraction of atoms or ions that hold a compound together. The bonds in an alcohol are always covalent. These are chemical bonds formed by the sharing of one or more pairs of electrons between two atoms. Covalent bonds are relatively weak bonds. The alcohol series are compounds that contain atoms of carbon, hydrogen and oxygen. Where n is the constant, the general formula for alcohols are CnH2nOH. This is very similar to another series of compounds the alkanes. The formula for alkanes is CnH2n+1. Because the alcohols and alkanes are very similar, alcohols are sometimes known as alkanols. The homologous series of alkanols contains the following isomers (that will take part in the experiment). Alkanol Molecular Formula RMM Methanol CH3OH 32 Ethanol C2H5OH 46 Propan-1-ol C3H7OH 60 Butan-1-ol C4H9OH 74 Pentan-1-ol C5H11OH 88 Notice how we are using propan-1-ol, butan-1-ol and pentan-1-ol. The -1- denotes that the hydroxyl group is bonded to the alkyl group on the first carbon atom. The structure of these alkanols is shown below: Methanol Ethanol Propan-1-ol H H H H H H ? ? ? ? ? ? H - C - O - H H - C - C - O - H H - C - C - C - O - H ? ? ? ? ? ? H H H H H H Butan-1-ol Pentan-1-ol H H H H H H H H H ? ? ? ? ...read more.

Middle

Methanol 20.00 4.20 250.00 1.49 21000.00 14093.96 14.09 32.00 451.01 Methanol 20.00 4.20 250.00 1.51 21000.00 13907.28 13.91 32.00 445.03 Methanol 20.00 4.20 250.00 1.56 21000.00 13461.54 13.46 32.00 430.77 Ethanol 20.00 4.20 250.00 2.03 21000.00 10344.83 10.34 46.00 475.86 Ethanol 20.00 4.20 250.00 1.89 21000.00 11111.11 11.11 46.00 511.11 Ethanol 20.00 4.20 250.00 1.83 21000.00 11475.41 11.48 46.00 527.87 Propan-1-ol 20.00 4.20 250.00 1.47 21000.00 14285.71 14.29 60.00 857.14 Propan-1-ol 20.00 4.20 250.00 1.45 21000.00 14482.76 14.48 60.00 868.97 Propan-1-ol 20.00 4.20 250.00 1.38 21000.00 15217.39 15.22 60.00 913.04 Butan-1-ol 20.00 4.20 250.00 1.47 21000.00 14285.71 14.29 74.00 1057.14 Butan-1-ol 20.00 4.20 250.00 1.49 21000.00 14093.96 14.09 74.00 1042.95 Butan-1-ol 20.00 4.20 250.00 1.46 21000.00 14383.56 14.38 74.00 1064.38 Pentan-1-ol 20.00 4.20 250.00 1.01 21000.00 20792.08 20.79 88.00 1829.70 Pentan-1-ol 20.00 4.20 250.00 1.06 21000.00 19811.32 19.81 88.00 1743.40 Pentan-1-ol 20.00 4.20 250.00 1.07 21000.00 19626.17 19.63 88.00 1727.10 I have produced a graph that plots energy transfer per mole against the relative molecular mass (RMM) of the alkanol. This graph shows the trend that the results give. The graph shows that as the RMM increases, the energy transfer per mole of alkanol also increases. There is a line of best fit which gives evidence that the increase is a regular increase. This is shown by how the line is a straight line that is close to many of the points. Each result is not on the line but is very close, especially in relative terms i.e. the scale is quite big. This gives further evidence to suggest that the stated trend is accurate. There is one anomalous result or rather there is a set. Pentan-1-ol doesn't fit into the line. As mentioned, the scale is quite large so the result for pentan-1-ol is not as bad as it seems, however, it doesn't fit into the trend directly supported by the other alkanols. ...read more.

Conclusion

Surrounding temperature is the problem here. One improvement would be to have tried to keep the surrounding temperature constant. Secondly, heat would have been lossed to the can itself. To reduce heat loss more efficiently, a better system than a can could have been used such as a container which covers the flame (still allowing oxygen in), but also reducing heat loss from the water. If we again consider the similarity between the results of the investigation, and the bond energies, we see that the pattern of the lines is the same in that it is a straight line with a positive gradient. This is the most important thing as it shows the results are accurate, as the relationship between the results is the same. There is one clear anomaly in pentanol. This may have been due to the difference in spirit burner, or the certain volume of methanol used. GENERAL IMPROVEMENTS AND EXPANDING There is one broad improvement that could be made to increase the accuracy, and the aspect of as fair test. This lies in the spirit burner and alcohol used. Different spirit burners and different mass of alcohols were used. To make the test much fairer, the same spirit burner should have been used for each experiment. This would mean cleaning the spirit burner out after each experiment, and also putting in a new wick which would be exactly the same for each experiment. Also the mass of alcohol should always be the same. This may have made a small difference in the result as a larger mass of alcohol may have meant an increased rate of reaction (combustion), as there would be more alcohol to combust. There are many ways in which the experiment could be expanded. Firstly would be to increase the number of replicates to increase the accuracy, after all, the more replicates, the higher the accuracy. Another method would be to test more different alkanols to create more results to secure a firm conclusion including hexanol, heptanol and octanol. My results, however, definitely support a firm conclusion as they are. ...read more.

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