Safety
I will make this experiment safe by wearing safety goggles at all times from before I get my equipment out until it has been put away. I will need to be careful when striking matches and extremely careful around the naked flame when the spirit burner is alight. I will need to be careful when handling the alcohols and walking across the laboratory with them. Also, I will be using a metal safety tray to do my experiment in, just in case any spirit burners are accidentally knocked over.
Preliminary work
Because this is to get a rough idea of what doing my real experiment will be like and to give me experience and contact with the experiment, as well as things that might need changing, I have decided to use the following alcohols. The first alcohol; ethanol, the third alcohol; butanol and the last I will be using; hexanol. Using the first, third and last will give me a decent range of alcohols and ideas about what I might need to change.
Results
What I have found out from my preliminary work
I have found out that the gaps between the results are too small because I need to be able to talk about trends when I do my real experiment so I will need my results quite far apart to allow for any experimental errors. To do this I need to burn more alcohol so I have decided to move the calorimeter to 10cm above the flame instead of 5cm. This isn’t the only reason for me doing this, however, I also found that the flame was leaving high quantities of soot on the calorimeter which is unburned carbon so by raising the height I can maybe force more carbon to be burned. Having soot on the calorimeter is not a good idea because it is an insulator and it could contaminate my results by heat-proofing the calorimeter to some extent. I am also going to raise my temperature by 60°c instead of raising the temperature to 60°c which will use up more alcohol still and give my more space between my results. I need more space between my results because I have to fit in another two alcohols, proponal and pentanol, between these three.
My results to my experiment
First set of results:
Second set of results:
Result Averages
I now need to work out the average mass used from each alcohol by adding them up and dividing them by two (the number of results I have for that alcohol.)
Below is my table, which calculates the averages:
Analysis of results
The energy transferred to the water.
I now have my results and I now need to calculate the energy transferred to the water by each alcohol. And also the energy transferred per gram of each alcohol. The energy transferred to the water can be calculated using the equation;
Energy transferred = mass of water (g) X Specific Heat Capacity X Rise in Temperature (°C)
This means that all of the results will be exactly the same because in each experiment I used the same mass of water (100g), the specific heat capacity of water which is always 4.2J/g and the temperature rise in each experiment which is always 60°C.
If I now put these numbers into the formula I get:
Energy transferred = 100g x 4.2J/g x 60°C = 25 200J
The answer is 25 200 joules but that number is a little big to work with so I can turn it into 25.2KJ instead.
Energy Transfer per gram of alcohol
I am now ready to work out my energy transfer per gram of alcohol. This is done by dividing the energy transferred to the water (25.2KJ) by the average mass of alcohol used to give the answer of the energy transfer per gram of alcohol in KJ/g.
The Heat of combustion per mole
I now need to calculate the heat of combustion per mole. This is done by multiplying the energy transfer per gram of alcohol by the relative molecular mass (RMM) of the five alcohols. Below is a table showing all of these calculations.
The heat of combustion in kJ per mole is now known and I can plot a graph, which will show the heat of combustion against the number of carbon atoms in one molecule. My graph is on the next page.
My Graph
Conclusion
My results came out quite well I think they followed a general trend – and coincided with my prediction. The results follow a general trend – except for the anomalous Hexanol result. As I predicted, the heat of combustion in kJ/mole increased proportionally with the number of Carbon atoms in that molecule. This is because, although Hexanol is a bigger molecule, it does use more energy to break it’s bonds during combustion, it gives out more energy when the bonds are made as it has more bonds to make.
My prediction was based on the theory of heat of combustion, I knew that in the reaction all the bonds of the reactants must be broken. This takes in energy. When the bonds of the products are made this gives out energy. Based on this, I knew that Hexanol must give out more energy because it has the most bonds.
During the combustion of alcohols the alcohol reacts with oxygen gas (from the air) to produce Carbon Dioxide and water. It doesn’t take too much energy to break the bonds up but carbon dioxide releases a massive amount of energy when creating it’s bonds because it is such a strong bond. This is why the alcohols give out so much more energy when compared to the energy they take in. it is logical therefore, for ethanol (at the lowest end) to give out much less energy because it makes less bonds than Hexanol. This is due to the amount of Carbon atoms in the molecule, the more Carbon atoms, the more Carbon Dioxide bonds are made and thus more energy is released.
Evaluation
I think that the procedure was suitable for my purposes. I was trying to find out the heat of combustion of alcohols and that is what I have done. It could definitely be done better and I will proceed to explain why throughout my evaluation.
How I could make my experiment more reliable.
My experiment could be changed to be much more reliable if I was allowed to eliminate certain factors. For example; the alcohol being burned is not at maximum efficiency because there were draughts which resulted in energy loss as heat and light energy to the surroundings. Erecting windshields to stop the draughts could solve this problem. Another problem was that not all of the Carbon atoms were used up in the combustion, some ended up as soot (unburned Carbon) on the bottom of the calorimeter, this soot acted as an insulator and minimised the energy heating up the water. Also, measuring volumes is difficult as it is hard to read from the meniscus of the water. Weighing the water on a set of digital scales could solve this problem, as 1cm3 of water is the same as 1g of water then for 100cm3 I could simply weigh out 100g’s of water. The last factor that could be mentioned was the wick length. Some of the spirit burners could have different wick lengths and, if a wick is higher, more energy can be transferred to the water, as less is lost to the surroundings. I couldn’t really solve this problem, as it would involve me touching an alcohol-soaked wick, which is very dangerous. I did, however, try and keep the distance between the wick and the calorimeter at a constant 10cm.
I took two sets or results in case some, or all, of these factors did interfere with my findings and working out the averages gave me a slightly more accurate set of results. If I had a wildly wrong result that was much higher than my first result and was clearly anomalous then being able to calculate an average from the two places the result in a more sensible position and will follow the trend much more suitably.
Are my results accurate?
I think that, under the circumstances, my results are as accurate as they could be. I think that I carried out my experiment precisely and followed my plan. I think that the very fact that they support my conclusion and follow a trend, not to mention agree with the theory of heat of combustion, shows that they are accurate. They produced a good graph and the line of best fit fits in nicely. The graph slopes up diagonally and I think that if I continued the line then I could accurately predict what the approximate heat of combustion would be for hectanol, which has 7 Carbon atoms in it. I could do this by drawing a vertical line from ‘7 carbon atoms’ to the line of best fit and then a horizontal one going across from the point where the vertical one touched the line of best fit. By doing this I would get an approximate idea of the heat of combustion.
There is one anomalous result and that is Hexanol. I think that the wick could have been slightly higher on the Hexanol spirit burner, which could have meant that less heat was lost to the surroundings and it was more efficient than the others. As more energy was out into the water in a shorter space of time, it reached the desired temperature rise of 60°C more quickly and used a lower mass to do this. Therefore giving a higher result. I think that Hexanol should have been at around the 500-kJ/mole mark for its heat of combustion. That is only an estimate and further results would have to be taken to support this.
I think that the evidence is definitely reliable enough to support the conclusion that, the more Carbon atoms in an alcohol, the higher the heat of combustion.
What I would change if I repeated this experiment.
This experiment could be improved but it would take better apparatus. I could try to eliminate most of the factors that I described that were disrupting my results. To do this I would need a large bell jar that I could set my experiment up in. The bell jar would stop draughts getting to the flame and prevent energy loss to the surroundings. I could also pump air into the bell jar, which would provide a constant supply of oxygen. This would encourage a more efficient burning process and prevent soot forming on the calorimeter and preventing heat conduction by being an insulator. I could also weigh my water to be more accurate and cut my wicks to the same size. All of this would improve the quality and reliability of my results and my experiment in general.
