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# Enthalpies of combustion.

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Introduction

Enthalpies of combustion In this experiment we are trying to see how the enthalpies of combustion change with 4 different alcohols. The alcohols we will be using are. ?Methanol (CH3OH) ?Ethanol (C2H5OH) ?Propanol (C3H7OH) ?Butanol (C4H9OH) As you can see all of these alcohols are made out of the same compounds. What we want to know is how the number of Carbon atoms (C) affects the enthalpy of combustion. We set up the apparatus as shown above. We put a thermometer in and measured the temperature of the water. The alcohol was weighed and put under the beaker. The wick was set alight and we watched until the temperature had gone up approximately 10oC, extinguished the alcohol with the lid of the spirit lamp, took it to the front and weighed it again. ...read more.

Middle

29.09090909 Propanol 20 33 13 191.63 191.42 0.21 61.9047619 Propanol 20 32 12 191.42 191.21 0.21 57.14285714 Butanol 20 34 14 174.98 174.68 0.3 46.66666667 Butanol 20 34 14 174.68 174.52 0.16 87.5 We then did a table which showed the averages of all the alcohols. It looked like this: Alcohol Initial Temp. (oC) Final temp. (oC) Temp. Change (oC) Initial weight (g) Final weight (g) Mass used (g) Temp. rise/gm used Methanol 20 34 14 182.285 181.74 0.545 25.71186441 Ethanol 19.5 34 14.5 193.025 192.635 0.39 42.80632411 Propanol 20 32.5 12.5 191.525 191.315 0.21 59.52380952 Butanol 20 34 14 174.83 174.6 0.23 67.08333333 Next we worked out the heat absorbed by the H2O. To do this we had to calculate it using this method: (Mass of H2O*4.2*temp. ...read more.

Conclusion

This is because it takes more of the heat to break down the carbon atoms. We also found that the butanol left a sooty residue on the beaker. This happens because there is not enough oxygen in the air to react with the carbon and break it down. This could also explain why the graph stops sloping in a straight line after propanol, because there aren't enough oxygen atoms to react with the carbon atoms therefore slowing the reaction down and causing the dip in the graph. If we had done it in a controlled environment the graph might have looked like this: As you can see the graph has a constant slope now. This would be how it would look if the experiment was done in a controlled environment. Philip Pearson 11WG ...read more.

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