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Compare the enthalpy changes of combustion of different alcohols

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Introduction

Task and Aim: Compare the enthalpy changes of combustion of different alcohols. To find the enthalpy changes of combustion of different alcohols and draw conclusions on how these values are affected by their molecular structure. I shall experiment with a minimum of five alcohols. Each alcohol I use will be from the homologous series of primary alcohols, e.g. propan-1-ol or pentan-1-ol. By adding a hydroxyl group on the first carbon each time this ensures that there is only one variable between my 5 alcohols, the carbon chain length. Background Theory and Prediction: Energy changes occur whenever a chemical reaction takes place. A reaction that gives out energy and heats the surroundings is called an exothermic reaction; a reaction that takes in energy and cools the surroundings is called an endothermic reaction. When finding the enthalpy changes of combustion of different alcohols, the chemical reactions that take place will be exothermic. This is because ?H is always -ve and heat is given out to the surroundings. The standard enthalpy change of combustion, ?Hc is the enthalpy change that occurs when 1 mole of a fuel is burned completely in oxygen. In theory, the fuel needs to be burned under standard conditions, 1 atmosphere pressure and 298K. However, in practice this is impossible so I will burn the fuel in the normal way and make the necessary adjustments to allow for non-standard conditions. To measure the enthalpy change we use this relationship: Energy transferred=cm?T Where c is the specific heat capacity of water (4.2J/g) and m is the mass of water used. I predict that as the length of chain of carbons increase in an alcohol, the enthalpy change of combustion will increase e.g. heptan-1-ol will give out more energy than methan-1-ol. I predict this because as more bonds need to be broken and more bonds need to be formed. The intermolecular forces tend to be bigger as the chain increases. ...read more.

Middle

I must be extra careful incase the thermometer is filled with mercury as this is a poisonous substance. The calorimeter may also hit the spirit burner which would be extremely dangerous so I must be very sure that the clamps are stable. Bibliography -Teacher Support: Coursework Guidance AS/A Level GCE Chemistry (salters, blue sheet) -Chemical Ideas text book, chapter 4, and 13. -World Wide Web -Teacher support. Start Mass (g) End Mass (g) ? Temp (�C) ? Mass (g) Q (KJ) Mr (g/mol) Moles ? Enthalpy (KJ/mol) Actual ? Enthalpy (KJ/mol) Ratio Methanol 224.70 223.55 17 1.15 14.28 32 0.04 397.36 726 0.55 Repeat 1 223.55 222.28 16 1.27 13.44 32 0.04 338.65 726 0.47 Repeat 2 222.28 221.04 16 1.24 13.44 32 0.04 346.84 726 0.48 Repeat 3 221.04 219.66 16 1.38 13.44 32 0.04 311.65 726 0.43 Average 222.89 221.63 16.25 1.26 13.65 32.00 0.04 348.62 726.00 0.48 Ethanol 221.30 220.28 16 1.02 13.44 46 0.02 606.12 1367 0.44 Repeat 1 220.28 219.20 16 1.08 13.44 46 0.02 572.44 1367 0.42 Repeat 2 219.20 218.15 16 1.05 13.44 46 0.02 588.80 1367 0.43 Repeat 3 218.05 217.10 16 0.95 13.44 46 0.02 650.78 1367 0.48 Average 219.71 218.68 16.00 1.03 13.44 46.00 0.02 604.54 1367.00 0.44 Propan-1-ol 213.42 212.57 16 0.85 13.44 60 0.01 948.71 2021 0.47 Repeat 1 212.58 211.77 16 0.81 13.44 60 0.01 995.56 2021 0.49 Repeat 2 211.77 210.94 16 0.83 13.44 60 0.01 971.57 2021 0.48 Repeat 3 210.94 210.13 16 0.81 13.44 60 0.01 995.56 2021 0.49 Average 212.18 211.35 16.00 0.83 13.44 60.00 0.01 977.85 2021.00 0.48 Butan-1-ol 202.67 201.97 16 0.70 13.44 74 0.01 1420.80 2627 0.54 Repeat 1 201.97 201.24 16 0.73 13.44 74 0.01 1362.41 2627 0.52 Repeat 2 201.24 200.55 16 0.69 13.44 74 0.01 1441.39 2627 0.55 Repeat 3 200.55 199.87 16 0.68 13.44 74 0.01 1462.59 2627 0.56 Average 201.61 200.91 16.00 0.70 13.44 74.00 0.01 1421.80 2627.00 0.54 Pentan-1-ol 195.54 194.85 15 0.69 12.6 88 0.01 1606.96 3331 ...read more.

Conclusion

It was also because of lack of complete combustion that caused the most significant causes to this inaccuracy. This was due to the lack of proper draught exclusion and insulation. To have achieved higher accuracy and precision I might have taped aluminium foil to the heat proof mats. This would have allowed more heat radiation to be deflected onto the water which was where I was trying to divert all the energy towards. The container used to hold the water; the copper calorimeter was metal and would have radiated heat from its entire surface area. A better designed container or a dull outer surface would have minimized this. However when comparing my results to the text book values, I could see that although they were not the same, they were consistently wrong by about 50%. The lack of abnormalities in my results showed that my experiment was carried out using a consistant method which allowed me to better analyse my results. The measures that I kept the same during the experiment were the height of wick and the distance the flame was to the calorimeter. This measurement may not be completely accurate due to possible human error which is why not all my results were dead on 50% but were in the vicinity of this value. Other experimental techniques that I did for all my tests were stirring the water. This allowed the temperature to reach its highest peak using the particle collision theory. Once I stopped the burning, I carried on stirring the water to ensure that the temperature change was accurate in my results. However, as I explained the thermometers used were sometimes ineffective to 1�C. Things I would have changed had I done the experiment again, would be to use a digital thermometer as this would have reduced the possible human errors that I might have done. Also I would have used aluminium foil to reduce loss of heat radiation. ...read more.

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