I also noticed that the hexane and pentane bottles are not all the same height. I will have to measure 1cm from the base of the flame to the base of the copper tin. In order to make sure the flame lengths are the same, I will tweak the wick until both flames are roughly 1.5cm each.
Plan
I will set up apparatus as I did in my preliminary experiment, and make sure I accurately measure 50ml of water for each test. I will also make sure in each test the water is at 20°C. This was the temperature of 50ml of water left in a room for 15 minutes. Next I will take two spirit burners, one filled with hexane, the other with pentane. I will weigh each spirit burner and record their weights in grams before and after the experiment. This will allow me to see how much of each fuel was burned in the separate tests in order to boil the water. This means I am going to burn each fuel under the copper tin until the 50ml of water is boiling (100°C).
The two spirit burners must be the same distance underneath the copper tin, and they must both have the same flame size. I can make sure they are both tested the same distance away from the copper tin by measuring 2cm from the top of the wick on each spirit burner to the base of the copper tin. I thought a good flame height would be 1.5cm, as there is then 0.5cm of flame ‘touching’ the copper tin. I can adjust the flame height by trimming the wick. This involves moving the wick up and down in the narrow wick grasps. Before I start testing a fuel, I will also set up a windshield around the apparatus. This will stop draughts from ‘interrupting’ and moving the flame. This will cause heat coming from the flame miss the copper tin and be less concentrated. By using a windshield I will keep the two tests fair, as whilst testing hexane the flame may be interrupted lot of times and then pentane may not be moved at all or visa-versa, so a wind shield will keep another possible variable the same.
I shall firstly test hexane. When I light the spirit burner I will keep an eye on the thermometer and blow out the flame when the water is at boiling point. At this point I will re-weigh hexane and record the new weight in order to calculate the weight loss and the efficiency. Once I have tested with hexane I will do the same for pentane, and weigh it before and after the test just like I did with hexane. Before I light the pentane spirit burner, I will cool the apparatus down and re-fill the copper tin with another 50ml of water, and wait till it settles at 20°C. This way I am keeping the starting temperature the same and therefore a fairer test.
Safety Equipment
The experiment I will be doing involves chemicals, fire and hot apparatus. Chemicals can damage the eyes, so I will wear safety goggles when handling and testing the two fuels, hexane and pentane. As there is fire involved in the experiment, I will use a heat mat so as not to burn and catch the experiment work surface on fire. I will also tuck my tie into my shirt, and clear the work area of bags etc. Lastly, whilst working around the hot apparatus, I will take care to avoid touching it, and make people around my work area aware that some of the apparatus is hot. Whilst moving the hot apparatus i.e. the copper tin, I will use re-tractable tongs.
The Combustion of Pentane and Hexane
The word equation for Pentane combusting
Pentane (l) + Oxygen (g) Carbon Dioxide (g) + water (l) + Heat Energy
The chemical equation for Pentane combusting
C5H12 (l) + 8O2 (g) 5CO2 (g) + 6H2O (l) + Heat Energy
The word equation for Hexane combusting
Hexane (l) + Oxygen (g) Carbon Dioxide (g) + Water (l) + Heat Energy
The chemical equation for Hexane combusting
1C6H14 (l) + 9.5O2 (g) 6CO2 (g) + 7H2O (l) + Heat Energy
Pentane Structure H H H H H
C5H12 H C C C C C H
H H H H H
Hexane Structure H H H H H H
C6H14 H C C C C C C H
H H H H H H
Prediction
In order for me to predict which fuel will be more efficient, I have calculated the amount of energy released by each one when burning (combusting). From the two energy-calculation tables below you can see that pentane released more energy than hexane. This makes pentane more efficient than hexane, as when say boiling water, pentane will break bonds quicker than hexane at any one point, and will therefore be producing more heat. (When a bond is broken heat is produced. This is called an exothermic reaction). Therefore I predict pentane will have a smaller weight loss than hexane, once used to boil water from 20°C.
Results
Conclusion
From my results I can see that pentane had less weight loss than hexane. This proves that pentane is more efficient at boiling water than hexane, as it uses less of itself to get the same overall result; boiled water. I have also proven my prediction correct, and I can see that pentane had a smaller weight loss than hexane. I have already shown that pentane is better at breaking bonds and is able to start a reaction (EA-activation energy) quicker than hexane in the energy calculation tables.
I think pentane is able to release more energy than pentane at any one time because it is better at breaking bonds. When a chemical is breaking bonds in an exothermic reaction, heat is produced. If a chemical is good at breaking bonds, it will be able to break more at any one time, and therefore be producing more heat. Hexane however is not as good at breaking bonds, and therefore needs to burn a lot longer in order to produce the same amount of energy pentane produces in a short amount of time.
Evaluation
My method worked very well, and was very simple to follow. Nothing went wrong whilst doing my experiment, which was mainly down to careful planning and safety. If I was to improve my experiment, I would have taken more results in order to get a more accurate average weight loss of the fuels. To extend the investigation, I would simply test more fuels in the same way as I did with pentane and hexane, and compare the results and draw a wider conclusion about the efficiency of fuels and exothermic reactions.
Accuracy
In the experiment my results were quite accurate. This is mainly because of the good quality lab equipment and apparatus I used. When checking the temperature of the water I was boiling in each test, I used a thermometer which measured in °C to the nearest degree. The water I boiled separately with the two fuels started at 20°C and I stopped heating at 100°C. The 1 degree accurate thermometer I used meant there was only a 1 degree margin of error. This is very accurate, and means I managed to keep the test fair by only heating water from 20°C-100°C with both fuels.
For both the tests (hexane and pentane) I heated 50ml of water. I used a good measuring cylinder which measured the volume of water to the nearest 1ml. Again, this is very accurate, and helped me get accurate results.
Reliability
I think my results are very reliable. I have bullet pointed a list of things I made sure I did in the experiment to make sure my results are as reliable as I can make them.
- I blew out the flame of each spirit burner at the correct moment
- I used exactly the same amount of water for each test
- I trimmed each spirit burners wick so a 2cm flame would burn from each. This keeps the experiment fair
- I used a wind shield to act as a draught exclusion for each test. This again ensured a fair test for each fuel
- I knew that if I left a spirit burner burning for longer than it should be (still burning after water has reached 100°C) it will weigh less than it should