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Combustion of Alcohols Investigation.

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Introduction

Combustion of Alcohols Investigation PLAN Aim: To find out how much energy is required to burn enough of the following alcohols: methanol, ethanol, propanol and pentanol, in order to heat a beaker of water by 40�. The combustion of an alcohol is an exothermic reaction, meaning that more energy is given off during the process than is being taken in. by knowing the individual bond energies for each bond, we can calculate what the energy values obtained from our investigation should aspire to to be accurate. Each bond (eg. C-H, where the '-' signifies a single bond, or O=O, where the '=' denotes a double bond), have different energies. They are as follows: Bond Bond energy (kJmol-1) C-H O=O C=O H-O C-C C-O 435 497 803 464 347 358 MOLECULE DIAGRAMS The following are the reactions which occur when each alcohol combusts. METHANOL: CH3OH + 11/2 O2 CO2 + 2H2O ETHANOL: CH3CH2OH + 3O2 2CO2 + 3H2O PROPANOL: CH3CH2CH2OH + 41/2 O2 3CO2 + 4H2O PENTANOL: CH3CH2CH2CH2CH2OH + 71/2 O2 5CO2 + 6H2O Given this knowledge we can calculate how much energy should be given out per mole of alcohol that is combusted. To do this we add up the bond energies for each bond that is either broken or made during the combustion process. We then simply subtract the total energy given out from the total energy used up. We should in theory obtain a negative number from this, as energy is lost in the reaction. CALCULATIONS: METHANOL: (C-H) ...read more.

Middle

The following diagram shows the new set-up: As before 100cm3 of water was put in the beaker. This time, the bottom of the beaker was cleaned for soot before each reading was taken. The thermometer was placed in the same way as before, we again lit the burner until the water had changed temperature from 20 to 60 degrees C. Again, the burner was weighed before and after heating the water, but this time the lid was kept on to make sure no alcohol evaporated on the way to the scales, which would compromise the integrity of the readings. Where each alcohol had two readings taken from it in the trial run, this time we would take three readings from each to give a better average. RESULTS: alcohol mass at 20 mass at 60 mass change temp change mass of water (g) (g) (g) (degrees C) (cm3) meth1 111.7 109.4 2.3 40 100 meth2 109.3 106.9 2.4 40 100 meth3 110.6 108.5 2.1 40 100 eth1 112.6 110.6 2 40 100 eth2 157.6 155.9 1.7 40 100 eth3 116.1 114.4 1.7 40 100 prop1 126.1 124.5 1.6 40 100 prop2 168.1 166.8 1.3 40 100 prop3 125.1 123.8 1.3 40 100 pent1 159.8 158.8 1 40 100 pent2 162.8 161.9 0.9 40 100 pent3 116.1 115.1 1 40 100 From this table of results we can take the averages, to give a more accurate final set of values. ...read more.

Conclusion

As stated previously, it would be unfeasible to create such measures within a school environment to effectively minimise heat loss to the air. IMPROVEMENTS: To increase accuracy, more measurements could have been taken per alcohol, even though three, as was undertaken, could easily be accepted as sufficient. The more readings taken, the bigger the chance of spotting any anomalies, even though there were no glaring ones in this particular investigation. No matter how much the bottom of the vessel was cleaned, there would still be some dirt on the bottom which could hinder the energy transfer, so an easier to clean vessel could be utilised. The only way to maximise energy transfer to the water and eradicate heat loss to the air, would be to use a bomb calorimeter, as shown below. However this is highly unfeasible for such a small-scale experiment and contains too much water, so heating it all by 40 degrees would take a lot longer, almost certainly not enough to be completed in the time given. the bomb calorimeter is a thermally sealed environment, meaning that there is no way for heat energy to go anywhere except the surrounding water. EXTENSION TO EXPERIMENT: As an extension to the experiment, we could investigate more alcohols, even though the outcome is a forgone conclusion. Also, we could use other substances than water. They would give different results altogether, as their specific heat capacity is different. Acknowledgement: Figure 1 from www.innovescent.com Steve Smith 11SB 11P Chemistry coursework 08/05/2007 1 ...read more.

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