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# Comparing the Enthalpy changes of combustion of different alcohols

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Introduction

Comparing the Enthalpy changes of combustion of different alcohols Aim: The aim of this experiment is to find out how the enthalpy change (total energy released when the alcohols are completely combusted in a plentiful supply of air) for 5 different alcohols is affected by the number of carbon atoms in the alcohol and other factors contributing to the molecular structure. Prediction: I predict that as the amount of carbon atoms in the alcohol increases, the higher the enthalpy of combustion will be. I have made this prediction, using the values for the enthalpy change of combustion for each alcohol, calculated using bond enthalpies and Hess' law. Methanol's molecular formula is CH3OH. This is the basic structure for all the alcohols, then to make the larger ones an extra carbon is added to the existing carbon each time and the oxygen-hydrogen molecule gets added to the atoms added to the new carbon atom When methanol combusts in air, it reacts with oxygen molecules to from water and carbon dioxide. The balanced equation fort this is: CH3OH (l) + 1.5O2 (g) CO2 (g) + 2H2O (l) This means that the bonds broken are; 3 carbon- hydrogen, 1 carbon-oxygen, 1 oxygen-hydrogen and 1.5 oxygen- oxygen (double bond) and the bonds broken are; 2 carbon- oxygen (double bond) and 4 oxygen-hydrogen. Constructing a Hess' law cycle will show how these are linked together: CH3OH (l) + 1.5O2 (g) CO2 (g) + 2H2O (l) C (g) + 4H (g) + 4O (g) N.B. bond enthalpies are for elements in their gaseous states If a calculation for the amount of energy needed to break the bonds is made and then the amount of energy given out from bond formation, the resultant energy difference (negative because the reaction is exothermic) ...read more.

Middle

This has sufficient measuring accuracy, as the amount of water is quite high, so using a more accurate measuring device will not significantly increase the accuracy of my experiment. I have used 100cm� of water because it is enough to have a low percentage error when it is measured, but it will reach the temperature rise quicker than a larger amount of water, leading to a lower amount of burning time, which means less heat energy is lost to the surroundings and a more precise result is achieved. I will then pour the water into a small copper calorimeter. I am using a copper calorimeter because copper is a good conductor of heat, so a higher percentage of the heat from the burning fuel will be transferred to the water to be measured. It is small to reduce the amount of energy that the calorimeter absorbs itself. I shall then set up the apparatus as shown in the diagram, with a safety mat, the calorimeter held by a clamp stand, with the thermometer inside and draught shields set up all around the experiment. The draught shields have reflective surfaces and so will reduce the amount of energy lost to the surroundings by convection and radiation by reflecting the heat back in on the experiment. I will make sure that the spirit burner is filled with methanol to start off and that the wick is about 1cm long so that for each experiment the flame will be about the same height, leading to more precise results. The wicks will also be of the same thickness, so that the alcohols have the same surface area to burn from so that the experiment ...read more.

Conclusion

molar mass of methanol = 12 + 16 + 4(1) = 22. I can then work out the amount of energy given out per mole of fuel burnt by dividing the energy given out by the amount of moles burnt. I will then divide this by 1000 to change the final units to kJmol-1 and I will have the enthalpy change of combustion for the fuel. I will do this for each alcohol and record the results in a table and display them as a graph, ready for analysis. Safety-Hazard assessment: These are the safety considerations I will take into account when doing my experiments- * The alcohols are flammable, so they need to be handled carefully and never exposed to a naked flame * The experiment produces heat, so I need to take care when handling hot equipment i.e. wait until they've cooled down * Alcohol vapours can catch fire at very low temperatures, so whenever possible keep the lid on the spirit burners and re-fill them in a fume cupboard * The fumes have a toxic effect if inhaled-make sure the room is well ventilated. Seek medical attention if they are inhaled * They are irritating to skin so if split on skin or eyes, rinse with water and seek medical attention * If they are any spillages, I will clear them up as I go to avoid any fire hazard, and will remove any clothes that have spillages * I will wear safety goggles at all times to protect my eyes and will cover my skin where possible * I will take care with other people's experiments as well, trying not to expose their alcohols to a naked flame Sources used in plan: Chemical Ideas - Salters advanced chemistry Activity book - Developing fuels Marc Duxbury ...read more.

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x 12600 = -281250 j - 281250/1000 = 281.25 kJ mol-1 I repeated the same steps for working out the enthalpy for ethanol trial 2. Ethanol trial 2 standard enthalpy of combustion = -294.39 kJ mol-1 The average for these 2 results is = (-294.39 + -281.25)/2 = - 287.82

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+ 4(H-O) =3(410) + 360 + 460 + 1.5(496) =2(740) + 4(460) =1230 + 360 + 460 + 744 =1480 + 1840 =+2794KJ/mol (bonds broken) =-3320KJ/mol (bonds formed) H =+2794 -3320 =-526KJ/mol A Hess cycle could also be used to calculate the enthalpy change in combustion.

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One hydrogen from the hydroxyl group (-OH) and one hydrogen atom is removed from the carbon where hydroxyl functional group is attached and replaced by a carbon-hydrogen double bond. Since the carbon atom (where the hydrogen from ?OH group and the hydrogen atom is removed)

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Final mass of alcohol +spirit burner (g) ± 0.01 (M2) Mass of alcohol used up (M2 –M1) (g) ± 0.01 Initial temp.of H20 (t1) (0C) ± 0.1 Final temp. of H2O (t2) ± 0.1 (0C) Temp. Change of H2O (t2-t1) ± 0.1 (0C) Mass of H2O (g)

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