Comparing the Enthalpy Changes of Combustion of Different Alcohols
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Chemistry Coursework: Comparing the Enthalpy Changes of Combustion of Different Alcohols Background The enthalpy (energy) change of combustion, or as it is known in its shortened form ?Hc is the enthalpy change that occurs when 1 mole of a fuel is burned completely in oxygen. For standard enthalpy change of combustion to take place the fuel needs to be burned in standard conditions i.e. 1 atmosphere pressure and 298K. Of course this is more theoretical as carrying out a combustion experiment without any heat loss is very difficult. In the experiment I am carrying out I will not be able to achieve standard conditions for combustion due to limitations with equipment. A way of minimising the heat loss during an experiment to measure the enthalpy change of combustion is by using a bomb calorimeter to maximise the amount of heat trapped during the combustion and afterwards. Below: Diagram of a bomb calorimeter: 1) Stirrer 2) Calorimeter bomb 3) Jacket 4) Calorimeter vessel 5) Thermometer 6) Ignition lead 7) Jacket lid By using a bomb calorimeter it allows for accurate measurements of energy changes of combustion. Fuel inside a bomb calorimeter is ignited electrically and burns the oxygen inside a pressurised container, as can be seen in the diagram. The energy from the combustion of the fuel is transferred to water surrounding the pressurised container and the temperature of this water is measured. The bomb calorimeter allows for a constant volume and pressure and also since a closed container is used with a vacuum jacket insulating and trapping the water, heat loss is thus minimised. Using the bomb calorimeter we can measure (in the same way as in the experiment I carried out) the energy transferred from the combustion of the fuel by the total amount of heat transferred to the water. Enthalpy (H) of a substance cannot be measured on its own. We can only measure the change in enthalpy when a reaction occurs by the equation: ?H = H products - H reactants There are two types of energy change in a chemical reaction.
* Once again weigh the spirit burner with its lid on and record this reading in the results. * Repeat with each alcohol at least 3 times to give reliable results. * Perform the calculations above to find out the ?Hc of each alcohol. Risk Assessment In order to maintain safety always ensure a lab coat and safety goggles are worn at all times in the lab to help prevent injuries. Make sure that bags and other loose items are stored under the desk to prevent people tripping over them. Methanol is toxic by inhalation, if swallowed and by skin absorption. If swallowed wash out the mouth and give a glass or two of water. Seek medical attention if victim shows drunken symptoms or if methanol is involved. If vapour is inhaled remove the victim to fresh air to rest and keep warm. If liquid is splashed in the eyes then flood the eye with gently running tap water for 10 minutes. Seek medical attention. If liquid is spilt on skin or clothes then remove contaminated clothing. Wash affected area thoroughly with cold water. Soak contaminated clothing to reduce fire risk if more than 10ml is involved. If the alcohol involved is spilt in the laboratory then shut of all sources of ignition, open all windows and apply mineral absorbent to the spill. Scoop up into a bucket and add water. Note that the calorimeter, draught excluder and other metal instruments that have been in contact or near to the flame may still be hot for long after combustion is finished, so do not handle until sufficiently cooled. Results Table of results for the enthalpy change of the combustion of methanol: Methanol Initial Final ? In Mass/Temp ?Hc (kJmol-1) Average ?Hc (kJmol-1) 1 Mass (g) 198.15 197.15 1.00 -201.60 -186.95 Temp (?C) 19 34 15 2 Mass (g) 197.15 195.97 1.18 -170.85 Temp (?C)
Another problem with having an unsteady oxygen supply is that there is not a consistent supply of heat to the calorimeter. Breeze around the lab also affects the reliability of my experiment as, even with the draught excluder in place, it causes the flame to waver and thus not always heat the base of the calorimeter but to also heat the surroundings. Heat loss is present through the gap between the draught excluder and the calorimeter. It is also present through the walls of the metal draught excluder and also from the surface of the water in the open topped calorimeter through heat transfer to the air and also through evaporation of the heated water. All of these factors combine to make the experiment less reliable (less accurate too?) in its results. The scale of the thermometer can affect the accuracy of the temperature readings as the greater the length of the scale for the same maximum temperature reading, the greater its accuracy. Temperature readings can also be inaccurate as when I am taking results I can only read off the scale provided on the thermometer and no more accurate than 0.5 of a ?C. The weighing scales I used can also cause inaccuracies as they only read to 0.01 of a gram. This means that uncertainties are present throughout my results. An example of an uncertainty in my results for mass is that of a mass reading for the change in mass of ethanol: Initial Mass: (219.42 ? 0.005) grams Final Mass: (218.63 ? 0.005) grams Change in Mass (Initial - Final): ((210.42 - 218.63) ? (0.005+0.005)) = (0.79 ? 0.01) grams The value for the 'change in mass' is particularly important to be accurate as it is the main variable factor in the ?Hc calculation. This value is used when calculating the moles of the alcohol and if this value is wrong by just ?0.01 of a gram then the final output value for the ?Hc calculation could be very far off, giving unreliable and inaccurate data.
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