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To investigate the combustion of alcohols.

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Introduction

To investigate the combustion of alcohols Planning I am going to investigate what happens when I burn specific alcohols. Alcohols are a series of related organic molecules of a homologous series, where each member varies from another by having a different number of carbon and hydrogen molecules. The general formula of an alcohol is: CnH2n+1OH, where n is the number of Carbon atoms. The first in the series has n=1 end the second has n=2 and so on. The similarities in molecular structure make alcohols have physically and chemically similar properties. Which change up the series because of the additional atoms. The table below shows a general increase in melting points and boiling points as the number of carbon atoms increases (from advanced chemistry) Name Molecular Formula Structural Formula Melting Point Boiling Point Methanol CH3OH ? ? Ethanol C2H5OH -117 78 (Propan-1-ol) C3H7OH -126 97 (Butan-1-ol) C4H9OH -90 117 The names like propan-1-ol refer to the position of the -OH group on the carbon chain, the OH groups above are on the first Carbon atom, the "1" position, there is also a "2" position and in larger molecules possible more. This table shows that I should investigate a series of different alcohols in my investigation to get an accurate and wide scope. Ideally I would need at least 5 alcohols for a good range of results for comparison, however we can only do 3. The complete combustion of an alcohol involves reaction with Oxygen to produce Carbon Dioxide and Water. The general formula for this reaction is: CnH2n+1OH + (n+1.5n)O2 (n+1)H2O + (n)CO2 Balanced equations for each of the available alcohols that I will use are as follows: Ethanol C2H5OH + 3O 2CO2 + 3H2O Propanol C3H7OH + 9O2 4.5CO2 + 4H2O Butanol C4H9OH + 6O2 4CO2 + 5H2O The combustion process involves the breaking and remaking of chemical bonds between the reactant elements. ...read more.

Middle

5. The same beaker will be used each time and it will be cleaned after each experiment. 6. Height of flame, this will depend on the length of exposed wick, this will be kept constant at 1 cm. 7. Each alcohol will be combusted three times so anomalies can be left out and identified. 8.The set up will be kept the same for each alcohol. 9. The spirit burners will be as identical as possible. 10. Stir the water constantly to keep the temperature universal. Calculating and predicting results Prediction. I predict that the higher the mass of the molecule then the higher the amount of energy given off. This is because there are more bonds to make and break, thus increasing the values involved. For example, you get more bonds made when you burn Butan1ol than when you burn ethanol: Adding each extra H-C-H to the molecule gives a theoretical 824 kJ/mole needed to beak them, but I know that the experiment is exothermic so the extra bonds will result in more energy given off. This gives a straight line graph. I will need to see if my results support this predicted constant increase in energy released. I also predict that the more the bonds the more efficient the reaction, this is because more energy will be released and the product will heat up quicker. I think this is a good way to do this reaction. I have used this set up before when I determined the enthalpy of combustion of some alcohols. Calculations By using the average bond energy values from calculations for GCSE chemistry I can predict the energies given off from burning the alchohols. ...read more.

Conclusion

My prediction was based on the fact that as more bonds are broken then remade with increasing alcohol molecule size then the more energy is released in the exothermic reaction. y overall prediction for this pattern of results is still valid so my results do support my prediction. The differences between alcohols in my investigation results do not show the predicted constant value. I could solve these problems by using a calorimeter which retains all substances inside a water jacket; to absorb all the energy produced. A bomb calorimeter. NB a calorimerter like this could be used to burn a specific amount of feul and the oxygen needed could be delivered via a tube. Also, rather than a measuring cylinder, I would choose to use a more accurate pipette, like we used for titration investigations. This will improve accuracy from 1 ml to 0.1 ml. I would use an extended range of alcohols, including Pentanol, hexanol, heptanol, octanol and decanol and methanol. The predicted and observed trend of increasing exothermic energy should continue with this series. Also I would use something other than spirit burners, with the alcohol at room temperature. I would have (if possible) an electic calorimeter which stirs, times and measures the temperature simultaneously. The effect of bond position can be investigated by using a range of isomers, e.g. butan-1-ol, butan-2-ol and butan-3-ol. These would show the differences in exothermic energy depending on the structure of the isomer. I am not sure how this would effect the experiment. According to chemistry in context the isomer means that there are different numbers of certain bonds, thus effecting the combustion. Also pressure could be investigated to see if the gasses burn better when in pressure. Other homologous series' could be investigated and their combustion compared to that of alcohols. They could be burnt the same way as this experiment (with the adjustments mentioned in the evaluation). ...read more.

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