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Investigate the enthalpy change of different alcohol

Extracts from this document...

Introduction

Investigate the enthalpy change of different alcohol My aim is to compare the enthalpy change of combustion of different alcohols in relation to the structure of each molecule. The enthalpy change of combustion of a fuel is a measure of the energy transferred when one mole of the fuel burns completely. In a chemical reaction, bonds must either be made or broken, this involves an enthalpy change. The formation of bonds is exothermic, energy is lost to the surrounding; on the other hand, breaking bonds is endothermic, energy is taken in. I obtain the value for the enthalpy change of each fuel by using the formula: Energy transferred from the fuel=cm?T where c is the specific heating capacity of water (4.17 Jg-1K-1) -m is the mass of water, in g -?T is the change of temperature of the water Apparatus Apparatus I will use Size of the apparatus Value /quantity distill water / >3 litres, as much as possible* thermometer 0-110C thermometer 1 measuring cylinder 100 cm3 1 electronic balance correct to 2 decimal places 1 Bunsen burner / 1 draught shielding each approx. 20cm x 20cm 5 metal calorimeter *** 4 clamp about 1 meter 6 spirit burner with wick / 4 match/wooden stick / as many as possible burner cap / 4 tile as big as possible 1 Propan-1-ol / half filled the spirit burner * Butan-1-ol / half filled the spirit burner * Butan-2-ol / half filled the spirit burne * Cyclohexanol / half filled the spirit burner * ***-I will find out in preliminary test *-Half filled the spirit burner because alcohol should be away from the flame, they are very flammable. Filling up the burner would mean that direct contact would be possible. Some spared alcohol is needed because just in case the water or alcohol got evaporated or being used up in an unsuccessful experiment. Method 1. ...read more.

Middle

Systematic errors are bad if you are interested to find accurate results, but here I am just trying to look out for a pattern, same amount of heat is lost to the air, table, me, except for the cans in 'Testing with the size and height of can', but only only a small amount is considered so it can be neglected. **-I used ethanol fuel throughout so that it is a fair test Enthalpy change from the data sheet Alcohol Enthalpy change from data sheet (J/g/K) Propan-1-ol -2021 Butan-1-ol -2676 Butan-2-ol Cyclohexanol -3728 From the table above, I can see that increasing the hydrocarbon chain has an effect of increasing the energy given out (comparing between propan-1-ol and butan-1-ol). 'Burying' the OH group in between chain (comparing between butan-1-ol and butan-2-ol) has also an effect of increasing the energy. While cyclohexanol gives out most energy. The values are all negative because the reaction is exothermic, that is why there will be a rise in temperature in the distill water in the can, energy (mainly in form of heat) is lost from the molecule to the surrounding. Prediction My prediction would be similar to the results I got from the data sheet, Errors These are factors that leaded to inaccurate results: 1. Although the draught system is designed, there is still some heat lost to the surrounding that is unable to being measured up. 2. Some of the heat is used to heat you and the equipment up. 3. Limitation in equipments. E.g. If I have used a more accurate electronic balance, one that is corrected to more decimal places; and if I have used a more accurate thermometer. Then I should get more accurate results. 4. Unequal tilting of the base of the can (as explained in 'Testing with the position of flame' in preliminary test), the trial with the flame closer to the base of the can, the temperature would increase quicker. ...read more.

Conclusion

0.25% I think the overall % uncertainty is very difficult to calculate because the fact that most of the heat is lost to the surrounding, there are so many ways and factors that heat can be lost. The only way I think the most accurate to calculate the % uncertainty is to compare the true reading from the data sheet: Propan-1-ol: Enthalpy from my experiment: -1066 Actual enthalpy change: -2021 % uncertainty = [-2021-(-1066)]/-1066 x 100% �90% Butan-1-oll: Enthalpy from my experiment: -1520 Actual enthalpy change: -2676 % uncertainty = [-2676-(-1502)]/-1502 x 100% �78% Butan-2-oll: Enthalpy from my experiment: Actual enthalpy change: % uncertainty = Cyclohexanoll: Enthalpy from my experiment: -2278 Actual enthalpy change: -3728 % uncertainty = [-3728-(-2278)]/-2278 x 100% �64% The % uncertainty overall of all the alcohol fuel is more than 50%, in reality it seems that the results are rubbish, but again looking for pattern in this experiment is more important than caring for the actual, true results. The smaller the result, the bigger the % uncertainty. The procedural errors accumulated and affect the results. Improvement If I am to do the experiment again, I would: > use a more accurate thermometer showing more decimal places > use a more accurate electronic balance showing more decimal places > a more suitable lid that can fit perfectly onto the can without any chances that water molecule can escape > make sure the wick length is exactly the same > make sure the flame position is the same > make sure the base of the can is horizontal > use a more accurate measuring cylinder showing more decimal places, I don't think using a pipette or burette is ideal here because we are dealing with 200g of water > same kind of burner every trial > stir gently and more evenly > carry out the experiment in a bomb calorimeter, this is done at constant volume in a close container so not heat can escape, in fact they can all be measured. ...read more.

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