Apparatus list:
Methanol Spirit burner
Ethanol Spirit burner
Propanol Spirit burner
Butanol Spirit burner
Pentanol Spirit burner
Tripod
Can
Burette
Top pan Balance (accurate to .00 of a gram)
Bunsen Burner
Wooden Blocks
Thermometer
Apparatus set up:
I need to do a few small trial experiments to make sure I get the best results possible. I need to find the temperature increase and water volume which will give me the best results. Having already done an experiment which featured burning alcohols, I do not need to test water volume, as 150ml work perfectly. I need to test the affect of temperature increase on my results. To do this, I will use pentanol. This is because it will heat up the water quickest (due the number of carbon atoms it contains). Here are my results:
I will measure a 15 degree Celsius, as the average mass loss figures it gives are closer together than for 10 degrees Celsius.
Control of variables is important to this experiment, as they can seriously affect trends in results. Temperature of each experiment is not a problem; they will all be done in a controlled environment. Positioning of the flame of the burner is crucial, as the temperature of a flame varies, so the same part of flame must be concentrated on the same part of the tin each time. The water must be constantly mixed to stand the best chance of having a constant temperature. Care must be taken not to read temperatures if the thermometer is on the can’s base, as this will be hotter than the water and therefore inaccurate. Water volume makes a lot of difference, as more energy is required to heat a larger volume of water. The temperature increase of the water is also important, because if I don’t measure a trend displaying increase my results will be poor.
Safety goggles must be worn as the experiment involves naked flames. Care must also be taken to not inhale the gases from each alcohol.
I predict that methanol will use the most energy to heat 150ml of water up 15 degrees from its initial temperature.
heat
CH3OH + 1.5O2 ---------> CO2 + 2H2O
Here is the displayed formula:
After finding the mass of the spirit burner before and after the experiment has taken place, I can calculate the energy/mole needed for each carbon atom bond to be broken. So:
Energy needed to break bonds when methanol burns in air = 2039kJ/mol
Energy needed to form new bonds after methanol has been burnt in air = 1528kJ/mol
Energy needed to break carbon-hydrogen bond = 413kJ/mol
Each successive alcohol contains one more carbon atom than the one before. It also has 2 more carbon-hydrogen bonds and 1 more carbon-carbon bond. So obviously as the amount of carbon atoms in an alcohol increases, so will the amount of energy released.
My results show that the average mass loss drops as the number of carbon atoms present increases. Consequently, this means the energy released drops as we descend down the alcohols. The greater the loss of mass, the longer the alcohol had to burn to heat up the water to 15 degrees Celsius more than it was. Methanol had the largest average mass loss. This is because it contained only 1 carbon atom, which when burnt in air gave out less energy than ethanol, for example.
My results point towards a rough trend of average mass loss decreasing as carbon atom content increases. However, due to inaccuracies in the experiment, the trend is very weak and has irregular spacings between alcohol groups. These inaccuracies could have been:
Height of the flame: Even with the two blocks in place, the height of the flame will vary, due to the height of the wick, and the amount of alcohol in the wick.
Heat loss: Heat from the flame can easily be lost in the air, and there was no system in place to control it.
Concentration of heat: Stirring the water to keep its temperature constant was not a very accurate method.
Evaporation of alcohol: The stronger alcohols evaporated quickly if the tops of the spirit burners were left off.
Ideally, if I was to repeat the experiment, I would insulate it to prevent excessive heat loss. I’d cap the spirit burners as quickly as possible, and try to heat the water evenly.
