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Comparing The Enthalpy Change OfCombustion Of Different Alcohols

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Introduction

Comparing The Enthalpy Change Of Combustion Of Different Alcohols The aim of my experiment is to investigate the enthalpy of combustion of a range of alcohols. The standard enthalpy of combustion is the enthalpy change that occurs when 1 mole of a fuel is burned completely in oxygen under standard conditions - 1 atmosphere pressure and 298K. All combustion reactions are exothermic which is why I am expecting all the values for the enthalpy change of combustion to always be negative. Prediction I predict that the greater the number of carbon atoms there are in the alcohol molecule, the greater the enthalpy change of combustion it will have. This is because in a chemical reaction energy is needed in order to break bonds and energy is released when bonds are made. If I were to look at these two values; the energy required to break the bonds of the reactants and the energy released when the bonds of the product are made then I am able to work out the enthalpy change of combustion of the alcohol. In order to do this we need to know the bong enthalpies of the bonds in the process. The quantity of energy needed to break a particular bond in a molecule is called the bond enthalpy. Below is a table containing the bond enthalpies of the bonds that occur in the combustion of an alcohol. Bonds Average bond enthalpy/kJ mol-1 C-C +347 C-H +413 C-O +358 O-H +464 O=O +498 C=O +805 By looking at the equation for the reaction that occurs when an alcohol burns, we realise that the reaction involves both breaking bonds and making new ones. From the equation of the process we can tell how many of the different number of bonds are broken and made and so work out the enthalpy change of combustion. We can work out the enthalpy of combustion of methanol by using the bond enthalpy values: We can use the enthalpy cycle above to work out the value for the enthalpy change of combustion of methanol, represented by ?H1. ...read more.

Middle

d) I will use 5 different alcohols, which is a sensible range in order to gather sufficient data to find a relationship between the enthalpy change of combustion and the number of carbon atoms in the alcohol. e) wait for the temperature to stay 5 alcohol which will give me a good range and so my results being more reliable Results Alcohol Temperature of water at start Temperature of water at end Temperature Change of water Mass of water (g) Mass of copper container and aluminium can (g) Total mass (g) Start mass of burner (g) End mass of burner (g) Mass of fuel used methanol 19 84 65 96.98 68.20 165.18 142.48 137.94 4.54 23 80 57 102.44 68.20 170.64 142.90 139.08 3.82 Ethanol 24 84 60 88.29 68.20 156.49 159.08 156.32 2.76 18 87 69 102.16 68.20 170.36 153.53 150.20 3.33 propan-1ol 21 83 62 103.47 68.20 171.67 141.85 139.97 1.88 15 79 64 109.5 68.20 177.70 170.39 167.55 2.84 butan-1-ol 18 84 66 94.45 68.20 162.65 135.35 132.47 2.88 16 92 76 90.76 68.20 158.96 170.69 167.50 3.19 Pentanol 27 86 59 103.17 68.20 171.37 143.41 140.90 2.51 24 82 58 100.58 68.20 168.78 138.15 135.71 2.44 I have chosen to repeat the experiments for each alcohol twice. Also I have taken results over a large temperature range. This means than all the values in my results will be greater and as values increase so does their accuracy and therefore I am expecting more reliable results. To work out the enthalpy change of combustion for the alcohols, I need to first work out the energy transferred to both the copper container and the water. I do this by using two constants; the specific heating capacity of water and copper. I also use the temperature rise of the water. So I use the relationship: Energy transferred = m1c1?T + m2c2?T Where c1 is the heating capacity of water (4.17Jg-1K-1) ...read more.

Conclusion

We can see that the error will be small in comparison to the whole value and so I do not feel that this error is significant enough to incorporate into my graph. Therefore the total error due to measurements was 3.09% which is not a significant amount to affect my results or my graph dramatically so I did not take into account theses errors in my calculations. But I could have reduced the error due to temperature change using the thermometer by using a data logger as it is more accurate and also gives almost instant temperature readings. There were also errors due to calculations of the change in enthalpy of combustion value. This error is shown below: The change in enthalpy of combustion of ethanol (CH3CH2OH) for my first experiment was -679KJmol-1 which is 600 plus or minus 10 because the value is in its hundreds. So the percentage error for each of these values is 1.7%, not a significant amount to change my graph by a large amount. Modifications of preliminary work justified: I used a cork lid on the can which greatly reduced the amount of energy loss through the top of the container and also kept in some evaporating vapour inside the container. This meant that my measurements of temperature could be more reliable. I used a tin foil draft excluder which reduced the draft from getting to the burner flame but I found that not enough oxygen was being supplied and so the flame went out a couple of times. To combat this problem I could have used an oxygen generator to supply to the flame. The tin foil resulted in my experiment to be less accurate because not enough oxygen was being supplied to the flame which in turn would have lead to a lower enthalpy change of combustion value. I used a tin can to insulate the copper container. This did not work and the heat energy from the flame was transferred to the water, then to the copper container and also then to the tin can. ...read more.

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