Introduction: The aim of the experiment is to find out which fuel releases the greatest amount of heat. The fuels that will be used in this experiment are Methanol, Ethanol, and Propanol. To do this, we must find the enthalpy change when these fuels are combusted. Because the fuels burn in a powerful explosion when lighted up in a jar; they will be kept in a spirit lamp, the change in temperature will be recorded with the help of a second medium. Apparatus: Beaker, Thermometer, tripod, water, spirit lamp, ethanol, methanol, Propanol, draught shield. Procedure: The apparatus is setup as shown in the diagram,
There are certain variables that will be kept constant in this experiment: Mass of water Length of wick on burner Height of can above flame Type of can In the beaker, 250cm3 of water is taken. The temperature of the water is then recorded after 2 minutes. Now, a spirit lamp with a known mass of methanol/ethanol/propanol/butanol is lighted. The temperature rise in the water is then recorded after five minutes. This procedure is carried out 3 times for each fuel, and an average value is taken. A defined amount of alcohol will be burned using a spirit lamp, to measure the energy change by measuring the change in water temp.
As well as not wasting energy in the heating of the container, heat must be stopped from escaping around the edges of the container. To do this I would have to insulate the edge of the container by using the lid. I predict that there would be a change in the amount of energy given off, getting greater, the more carbon atoms in the fuel, because the more there are, the more bonds to be made, thus producing more energy. The reactions that will take place are strongly exothermic, that is why they produce a colored flame. All of the reactions involve the alcohol and oxygen in the air reacting together to form water vapor and carbon dioxide.
The formulae for the alcohols are below, with a general formula: METHANOL 2CH3OH + 3O2> 2C02 + 4H20 ETHANOL 2C2H5OH + 6O2 > 4C02 + 6H20 PROPANOL 2C3H7OH + 9O2 > 6C02 + 8H20 BUTANOL 2C4H9OH + 12O2 > 8C02 + 10H20 To deduce the amount of heat evolved the following formula will be used: Heat Evolved = Mass x Specific Heat Capacity x Temperature Rise The data collected in the experiment is given below: Fuel Avg. Initial Temperature in oC Avg. Final Temperature in oC Avg. Temperature rise in oC Methanol 23 50 27 Ethanol 23 57 34 Propanol 23 64 41 Butanol 23 67 44 Fuel Heat Evolved in Kilo Joules
Methanol 28. 215 Ethanol 35. 53 Propanol 42. 845 Butanol 45. 98 Evaluation: more accurate results could have been achieved by modifying the experiment. The main causes for error in this experiment were: 1. Heat, which never enters the water, because of draughts. 2. Heat loss from the top and sides of the beaker. 3. Heat that is not conducted well by the beaker. 4. Incomplete combustion – there is a restricted supply of oxygen, the alcohol was burning with an orange flame rather than blue. Some of the alcohol did not burn completely, giving carbon and carbon monoxide rather than carbon dioxide.
A carbon deposit (soot) on the bottom of the beaker indicated this. 5. Slight inaccuracies in the amount of water can greatly affect the end result. 6. Changes in the length of wick cause different amounts of alcohol burnt. Improvements can be made to this by using a better insulation on the sides of the beaker, such as thick expanded polystyrene. Using a different material for the beaker, such as copper, which conducts heat more efficiently. By using a better lid and providing a draught screen with no gaps. This experiment has many sources of error, as the results were not exactly what were predicted.
Things that can be improved to make a better investigation include using more results than just three to try to get rid of any uncommon results. Also I only used Alcohols that had the hydroxide group on the end, I could investigate using alcohols that have the hydroxide group in other places. I could use a much larger range of alcohols, going on to a higher range of carbon atoms, and investigate the differences between other organic substances such as hydrocarbons. There are also possibilities to investigate the burning of the alcohols under different temperatures or pressures.