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Comparing the enthalpy changes of different alcohols

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Stephanie Wickers 8th November 2004 Comparing the enthalpy changes of different alcohols In this coursework, I am going to find the enthalpy change of combustion of a number of different alcohols so that I can investigate why and how the enthalpy change is affected by the molecular structure of the alcohol. The enthalpy change of combustion is the energy given off when one mole of a fuel is completely burned in oxygen, under standard conditions. I carried out a practical trial experiment to find out the enthalpy changes of hexane and methanol. I calculated the enthalpy changes of combustion of the two fuels and found that they were completely different to the data book values. This would probably have mainly been down to the heat loss to the surroundings. I have learned from the trial experiment that I need to improve my method of preventing this. If I use the same equipment, set up in a slightly different way I should be able to achieve more accurate results, and make fairly accurate comparisons between the enthalpies of the fuels. This is only true if we ensure that the temperature losses are the same. This is achievable by using the same starting temperature and heating the water to the same temperature in each experiment. The same quantities of energy must have been released if they heated the same volume of water through the same temperature rise and so it should be simple to calculate the ?Hc� in KJmol-1. The alcohols that I will be using and comparing are: * Methanol (CH3OH) * Ethanol (CH3CH2OH) * Propan-1-ol (CH3CH2CH2OH) * Butan-1-ol (CH3CH2CH2CH2OH) These alcohols are from the same homologous series and are all straight chain alkanes, this means that it will be a fairer test as all of the structural formulae are the same apart from the number of carbon atoms and the number of hydrogen atoms. ...read more.


Use the measuring cylinder to accurately measure out 200cm3 of cold water. Pour this into the copper calorimeter and take the temperature of the water. Next, the mass of the first fuel needs to be recorded. In an attempt to reduce the error of the mass, three readings need to be taken and the average mass calculated. Then the spirit burner will need to be positioned under the calorimeter at a height of 10cm above the top, and the remainder of the draft exclusion system needs to be arranged. Balance a heat proof mat on the top of the two side ones ensuring that there is enough space left for the thermometer to move in the calorimeter. Carefully light the spirit burner, and place the final heat proof mat in front of the spirit burner. Monitor the temperature of the water, as it gradually increases. Once the temperature has increased by 17�C snuff out the spirit burner and continue to stir the water in the calorimeter. Record the highest temperature that the water reaches after heating. Once this has been recorded, reweigh the spirit burner (three times and calculate the average). Empty the water from the calorimeter and rinse with cold water and make a note of any incomplete combustion (soot) around the base of the can. Repeat the experiment for all four fuels and record all results. Results Mass tables Fuel used Initial mass of Fuel (g) Average Final Mass of Fuel (g) Average 1 2 3 1 2 3 Ethanol 217.9 217.92 217.9 217.9 215.3 215.31 215.33 215.3 Methanol 215.43 215.44 215.44 215.44 213.28 213.28 213.29 213.29 Propan-1-ol 222.23 222.21 222.22 222.21 219.97 219.99 219.98 219.98 Butan-1-ol 218.03 218.05 218.05 218.05 216.4 216.39 216.41 216.4 Final results table Fuel Initial temp of water �C Final temp of water �C Temp rise of water �C Mass of water used (g) Initial Mass of fuel (g) ...read more.


The molecular structures of the compounds are as follows: Ethanol: H H | | H-C-C-O-H | | H H Methanol: H | H-C-O-H | H Propan-1-ol: H H H | | | H - C - C - C - O - H | | | H H H Butan-1-ol: H H H H | | | | H -C -C -C -C -O -H | | | | H H H H The enthalpy of combustion of alcohols, within the same homologous series is affected by the molecular structure of the compound. The numbers of CH2 groups have an effect on the enthalpy of the molecule, because it increases the number of bonds that will need to be broken and reformed within the compound during combustion. For example, in Methanol, there is a CH3 group and an OH group. These bonds, when formed are what determines the enthalpy change of combustion of the alcohol. When Methanol is burned in oxygen the thermo chemical equation is: CH3OH + 2/3 O2 -->-->--> Co2 + 2H2O This means that when Methanol is burned the following bonds need to be broken. C -H C -H C -H C -O O -H These bonds all absorb energy when they are broken and therefore endothermic giving a positive value. The enthalpy of the bond breaking al ads up to: 413 x 3 = 1239 KJ 358 x 1 = 358 KJ 464 x 1 = 464 KJ Total: + 2061 KJ When bonds are formed in a compound the enthalpies will be negative because they release energy into the surroundings. The bonds that will be formed in the CO2 and H2O are: C - O C - O O - H O - H O - H O - H 358 x 2 = - 716 464 x 4 = - 1856 Total: - 1140 Therefore the total bond enthalpy in the combustion of methanol = 921 ...read more.

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