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Investigation to see the relationship between actual and theoretical energy released when burning different alcohols.

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Introduction

Investigation to see the relationship between actual and theoretical energy released when burning different alcohols Aim: Using the five alcohols in the Homologous series we will carry out a scientific experiment into the amount of energy released when alcohols are burned. Planning Bond breaking theory: As alcohols are burned in oxygen the alcohol molecules break and take energy to do this. When an alcohol's bonds are broken it takes energy to break them as they are strong ionic structures. When new bonds are made, such as Carbon Dioxide and water in this case, energy is released as it does not take as much energy to make new bonds as it does to break the old ones. The energy can be calculated in the formula Energy= products-reactants If the number is negative then it is an exothermic reaction as energy has been given off as heat as it takes less energy to make the new bonds as there are fewer bonds between the new structures. The energy not used is lost. The fewer bonds that are made in theory should mean more energy is given off. I know that water and carbon dioxide are made when an alcohol is burnt, so if I take the equation for oxidising Methanol and balance the equation, I can see the amount of energy to break the bonds in the reactants, and the amount of energy to make the bonds in the products and compare. I cans see that less energy is needed for the products to be made Methanol + Oxygen --> Water + Carbon Dioxide CH3OH + 11/2 O2 --> 2 H2O + CO2 2806.5 kJ/mol 3345 kJ/mol This reaction can be shown in an energy level diagram, the reactants are the substances we start with and the products are the substances made in the reaction. ?H is the symbol for the change in energy in a reaction. ...read more.

Middle

We decided that 2 minutes would be better. We also found that if the flame was too close to the beaker the experiment got too hot and the bottom of the beaker turned black, we therefore decided that the tip of the flame should be 2cm from the bottom of the beaker. We tested the experiment with 30ml of water and found there was not enough water as it got hot too quickly and started to evaporate. We found that 50ml of water was just right as there was just enough for the heat rises to show differences but not so much that the water started to evaporate. We found that 50ml of water, in a beaker 2cm from a flame on a wick of 1cm would be best, and timing the experiment for 2 minutes would be better to use in the experiment as there would be a smaller percentage of error due to evaporation and similar events. Diagram Precision and Reliability The key variable which I will be testing is the type of alcohol; I will use 5 different alcohols in this experiment. They are: Methanol, Ethanol, Propan-2-ol, Butan-1-ol and Pentan-1-ol. I will be measuring the change in mass of the alcohols in the experiment by weighing them before and after to see the amount of mass that is burnt. I will then use this information along with the rise in temperature to calculate the amount of energy given off. I will then compare this amount of energy given off found from my experiment, with the theoretical amount of energy given off that I calculated using bond energies, to see if there is a relationship between them, and if so, what is it and how big the gap between them is, this will show me how accurate the experiment was. I know there will be a difference in them as my experiment cannot be entirely reliable, but I will try to make it as accurate as possible. ...read more.

Conclusion

We should also have been more accurate when measuring the height of the wick and the distance between the beaker and flame, as although they are not used to calculate the energy given out, they would have still affected the results we measured. I think we could have also used the same piece of foil each time, and made sure it was placed on the top in the same way, as if it is not always as tight heat can escape. Because I had anomalous results for Propan-2-ol, due to a combination of its variation in physical properties from the other alcohols and human error when conducting the experiment, my results are not very reliable. I did think however that the method was quite suitable, as there didn't seem to be any big problems and it seemed fair, apart from the heat loss to the surroundings. I think that using 3 sets of results for each alcohol was good as it increased the reliability of the results, but in some cases more than others, I think that if I was to do this experiment again I would do at least one more repeat test for each other alcohols. I think this would make the average results a lot more reliable therefore making the average energy per mole lie closer to the line of best fit, as although most of the points were near, they could still be improved. I don't think the evidence I gained was sufficient enough to support a firm conclusion because of the anomalies within Propan-2-ol, if I was to extend the investigation I think I would be able to as at the moment there is not enough reliable evidence. To extend the investigation I could also test the other isomers for each alcohol, to see if they change the trend of the results and to see if there a big difference between the results of the other isomers for that alcohol. It would also be interesting to experiment with the next few alcohols in the series to see if the pattern continues. ...read more.

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