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Experiment to investigate the heat of combustion of alcohols.

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Chemistry AT1 - Experiment to investigate the heat of combustion of alcohols. Introduction. The heat of combustion of alcohols is the change in kJ/mol when 1 mole of the alcohol is burnt in excess oxygen (O2). I will be investigating 6 alcohols, using predictions and a practical to guide me through this experiment and form an overall conclusion. Using formulas and calculations, I can show how much energy is released from these substances, and work out their heat of combustion. From the homologous series of alcohols using the general formula of CnH2n+1OH, I have chosen 6 alcohols, giving the Mr for calculation purposes later on: Mr * Methanol - CH3OH 32 * Ethanol - C2H5OH 46 * Propanol - C3H7OH 60 * Butanol - C4H9OH 74 * Pentanol - C5H11OH 88 * Hexanol - C6H13OH 102 The reactants will produce the products of carbon dioxide and water (CO2): Alcohol + Oxygen Carbon Dioxide + Water I.E: C2 H5 OH + 3O2 2CO2 + 3H2O Apparatus List/Safety Precautions. * Tin can - For holding the water * 200 cm3 water - being heated by burning the alcohol. * Thermometer - measuring temperature accurately. * Heat-Proof Mat - keeping heat loss to minimum and safety against fires. * Spirit Burners - hold the alcohol that is going to be burnt. * Clamp Stand - hold the tin can in place. * Measuring Cylinders - Measure the amount of water accurately. * Mass balance - Accurate weight measuring equipment. N.B: As you are dealing with fire, it should be noted that you must wear goggles and use the heat-proof mat to put out any flames from using a splint or a match. Diagram. Fair Test. There are certain factors that could be considered to affect the results in any way, to keep a fair test: * Room Temperature - A high temperature could heat up the beaker of water, thus pushing up the thermometer reading. * Heat Loss - Heat is always lost during this reaction. ...read more.


and place it under the can. * It is important that the lid of the wick is kept on unless it is being burned because of the possibility that some of the mass of alcohol may be lost. * Light the spirit burner, heating directly under the tin can. * Watch the thermometer carefully and replace the lid on the burner, stopping the fire from burning, 20o C more than the starting temperature. * Reweigh the burner (+lid), recording the mass of alcohol burnt by subtracting this result from value recorded before the burning. * Repeat these steps for each other alcohol and tabulate them as done in the following tables. * I will repeat this experiment to produce 2 sets of results for more accurate values with more variance. * I then averaged the mass of alcohol burnt from both sets of results, producing a more general graph. Results. Set 1 Methanol Ethanol Propanol Butanol Pentanol Hexanol Mass (g) of burner + lid (before) 168.68 160.17 157.00 198.15 192.79 245.35 Mass (g) of burner + lid (after) 166.34 158.61 155.44 197.14 191.05 244.35 Mass of alcohol burnt (g) 2.34 1.56 1.56 1.01 1.74 0.85 Initial Temperature oC 14 20 14 13 13.5 14 Final Temperature oC 34 40 34 33 33.5 34 Temperature oC change 20 20 20 20 20 20 Set 2 Methanol Ethanol Propanol Butanol Pentanol Hexanol Mass (g) of burner + lid (before) 174.91 241.18 188.80 222.48 200.42 266.53 Mass (g) of burner + lid (after) 171.23 238.84 186.93 221.23 198.83 265.55 Mass of alcohol burnt (g) 3.68 1.34 1.87 1.25 1.59 0.98 Initial Temperature oC 17.5 13.5 15 14 13 14 Final Temperature oC 37.5 33.5 35 34 33 34 Temperature oC change 20 20 20 20 20 20 Methanol Ethanol Propanol Butanol Pentanol Hexanol Average Value for alcohol burnt (g) 3.01 1.45 1.715 1.13 1.665 0.915 Calculation. To work out the heat of combustion of the 6 alcohols, I will have to work out the amount of energy ...read more.


Or by having a larger variety of alcohols, extending your range of results or repeating the experiment more than 4 times to secure extremely good readings, giving a more reliable conclusion. Overall, this experiment has proven that it is able to record very reliable and accurate results but there an alternate method of measuring the heat of combustion, as the diagram shows below: This diagram illustrates the use of a calorimeter, a container especially designed for this purpose to obtain very accurate results, restricting heat loss from its shape and size. The burner is filled two-thirds of the way with the alcohol and the starting temperature is recorded. A small current of air is drawn through the spiral, to provide a steady supply of oxygen to the wick, with the water constantly being stirred by the stirrer. After a rise of a certain temperature (20oC), the maximum temperature is recorded and the burner is reweighed. Calculating the heat capacity is difficult because there are different materials (copper/water and glass) that have different heat capacities. To get round this obstacle, an electric heater is placed in the water, and then connected to a joulemeter (measures the number of joules supplied to the calorimeter). It is kept on until the apparatus temperature reaches the equivalent of the temperature in the experiment with the alcohol. Here are some sample results: Starting Temperature = 19.4oC Final Temperature = 39.4 oC Mass of Methanol burned = 1.02g Starting reading on Joulemeter = 7030 J Final reading on Joulemeter = 29020 J Difference in Readings = 21990 J Therefore it takes 21990 J, to increase the temperature of the calorimeter from 19.4oC to 39.4 oC, and that 1.02g of methanol releases 21990 J: Energy released when 1 mole of Methanol is burned = 21990 x 32g/1.02g = 689, 882 J Thus the heat of combustion is -689.88 kJ (negative for exothermic reacting), being a lot closer then my experimental result of 178 kJ. This just shows how effective this method is to getting closer to the extremely accurate result. ...read more.

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Good calculation methods used and an excellent hypothesis, however the portrayal of the results could be improved. There are also mistakes in the units used with numerical answers.

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Marked by teacher Brady Smith 02/06/2012

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