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The Combustion of Alcohols and the factors affecting these reactions

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Introduction

The Combustion of Alcohols and the factors affecting these reactions Planning Aim: The aim of this experiment is to investigate the combustion of alcohols and how the different structures of alcohols affect the amount of energy given off. Alcohols: The general formula is CnH2n+1OH and n representing the number of carbon atoms in an alcohol molecule. So if I know the number of carbon atoms present this formula allows me to find the formula for every alcohol. From the simplest alcohol methanol, that has one carbon molecule, to pentanol with five carbon atoms. Throughout the combustion of the alcohols, the alcohols will react with the oxygen in the air. The word equation below represents this: Alcohol + Oxygen Carbon Dioxide + Water The table below shows the alcohols I will be using in this investigation: Alcohol No. of Carbons Formula Equation Methanol 1 CH3OH 2CH3OH(l) + 3O2(g) 2CO2(g) + 4H2O(g) Ethanol 2 C2H5OH C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(g) Propanol 3 C3H7OH 2C3H7OH(l) + 9O2(g) 6CO2(g) + 8H2O(g) Butanol 4 C4H9OH C4H9OH(l) + 6O2(g) 4CO2(g) + 5H2O(g) Pentanol 5 C5H11OH 2C5H11OH(l) + 15O2(g) 10CO2(g) + 12H2O(g) To show the structure of these alcohols I am using the following diagrams illustrate this: Methanol CH3OH Ethanol C2H5OH Propanol C3H7OH Butanol C4H9OH Pentanol C5H11OH Variables: There are a number of variables that I have to consider in this investigation. Variables are factors which will have an effect on the results of the experiment. It is important to keep most of the variables constant or "controlled", they do not change, to ensure a fair test. The controlled variables are: the height of the calorimeter above the spirit burner, the material of the calorimeter, the change in temperature, the amount of water, the size of the copper calorimeter and the size of the wick, which was easily chosen as only one size was available. The rise in temperature was decided to be a 15�C increase from the difference of the initial and final temperature, because it did not ...read more.

Middle

8 � O- -C: 8 � 805 = 6440 10 � O-H: 10 � 464 = 4640 Total: - 11080 kJ/mole ?H = + 8559 - 11080 = - 2521 kJ/mole Pentanol Equation: Structure: Bonds Breaking (+) 11 � C-H: 11 � 412 = 4532 4 � C-C: 4 � 347 = 1388 1 � C-O: 1 � 358 = 358 1 � O-H: 1 � 464 = 464 7.5 � O=O: 7.5 � 498 = 3735 Total: + 10477 kJ/mole Bonds Making (-) 10 � O=C: 10 � 805 = 8050 12 � O-H: 12 � 464 = 5568 Total: - 13618 kJ/mole ?H = + 10477 - 13618 = - 3141 kJ/mole CH4O (l) + 1.5O2 (g) CO2 (g) + 2H2O (l) C2H6O (l) + 3O2 (g) 2CO2 (g) + 3H2O (l) C3H8O (l) + 4.5O2 (g) 3CO2 (g) + 4H2O (l) C4H10O (l) + 6O2 (g) 4CO2 (g) + 5H2O (l) C5H12O (l) + 7.5O2 (g) 5CO2 (g) + 6H2O (l) When I group all the equations together a pattern emerges: Every time n increases by 1. In the alcohol molecule, H increases by 2n. the number of; O2 molecules increases by 1.5, CO2 increases by 1 and H2O molecules increase by 1. therefore the general formula of this equation is : CnH2n+1OH + 1.5nO2 nCO2 + (n+1)H2O I have already discussed what I will do after the experiment, and now I will be doing that. I will work out the energy released per mole, also known as enthalpy change (?H). I have just worked out the enthalpy change for each alcohol I will be using, by scientific methods. Therefore I predict the energies released per mole for the different alcohols to be similar to the results shown in the table below. Pentanol will give off more heat than methanol and I predict that less pentanol will need to react to raise the temperature of the water than methanol. ...read more.

Conclusion

However, the length of the wick did vary sometimes, a centimetre at most; this did affect the results though. To improve the experiment the wicks should all be the same length to make sure of a fair test. The distance between the calorimeter and the flame should be constant at 2cm for example. Draught shields (excluders) could be put up to reduce draught and insulate the heat loss escaping into the environment. The experiment could take place in a fume cupboard to minimize the heat loss. But the only realistic method could be the draught excluders. Further work could include using more alcohols and more repeats of them this would give clear evidence and increase the reliability of the results. I would say that the evidence is strong enough to draw conclusions from, because three repeats were just enough to maintain a strong average. My graph shows that the numbers of carbon atoms are directly proportional to the enthalpy change. This shows that the more carbon atoms the higher the level of enthalpy change. Overall my experiment was quite reliable considering all the sources of error. My prediction was not close to the results, the graph displayed the line of best fit was close to all the points so there was not a wider scatter of results. Except for my anomalies all the repeats were very close. This indicates that the results are reliable. Although the anomalies suggest they were not entirely reliable. If this experiment was to be done again, then all the possible sources of error mentioned would have to be counteracted and controlled, as well as using a wider range alcohols. I would also take more repeats so that a much more accurate average could be taken and the results would be more accurate and reliable. I suggest 6 or 7 repeats for each alcohol. More results would make a far conclusive and reliable and accurate. More repeats would show that the trend line stays the same and that the enthalpy change is directly proportional to the number of carbons atoms present. ?? ?? ?? ?? 1 ...read more.

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