To investigate the varying energy outputs when different alcohols are combusted
Chemistry Coursework
Planning
Aim
I aim through this experiment to investigate the varying energy outputs when different alcohols are combusted.
Prediction
This was made with a computer program- as quantities of bonds and alcohols' relative molecular masses were put in, and the theoretical heat to be given out was calculated. This gave us the amount of heat that would be produced by the different alcohols if no heat were lost during the experiment.
I predict that the larger the number of carbon atoms in the alcohol, the more heat will be produced, but the less efficient it will get, as more carbon is incompletely combusted through lack of oxygen.
Diagrams
Methanol
Ethanol
Butanol
Preliminary results
This experiment was done in order to find out which parts would need to be altered in the plan. I changed the amount of water to 100ml, from 80ml as it had heated to 80 degrees to quickly. I also found that the wick length made a huge difference to the results and so in the next experiment I tried to make this constant.
I also found that a lot of heat was lost to the surrounding air through the sides- not all heat went straight up to the water can. This meant that I introduced heat- proof mats to the sides- around the alcohol burner. I also covered there with aluminium foil to try to reflect most of the heat back to the water can.
I found that different cans had different qualities eg; thickness, so I took all the readings at one time to avoid any changes in equipment.
Safety
Safety goggles were worn at all times, lab rules were obeyed and since the experiment involved the heating of water, I stood at all times in case of spillage- in which case I would be able to move quickly away. Since the beaker grew hot during the experiment, it was handled with tongs and cooled with water from a tap.
Plan ?
The prediction told me that the reaction would be exothermic. This, I know, is because the reactant energy produced by the bonds being broken is more than the amount of product energy ...
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Safety
Safety goggles were worn at all times, lab rules were obeyed and since the experiment involved the heating of water, I stood at all times in case of spillage- in which case I would be able to move quickly away. Since the beaker grew hot during the experiment, it was handled with tongs and cooled with water from a tap.
Plan ?
The prediction told me that the reaction would be exothermic. This, I know, is because the reactant energy produced by the bonds being broken is more than the amount of product energy needed to break those bonds. The bonds being the forces of attraction that occur between the Hydrogen and Carbon for example. The difference between the reactant and the product energy is the heat.
The energy liberated per gram of alcohol is negative for an exothermic reaction. This is because the energy is lost- in the form of heat energy.
I will measure the heat lost during the combustion of the alcohol by heating water. I will use 100ml of water and this will be kept constant in order to keep it a fair test. Water is used because it heats sufficiently for readings to be taken, but does not heat up too quickly- which could make the results taken, inaccurate. It is also cheap.
In the experiment, I will weigh the alcohol before and after combustion. I will heat the water to 80 degrees centigrade and so be able to work out how much alcohol needed to be used to increase the temperature of the water from room temperature to this new reading.
The theoretical results rely on all the water going into the water. My results will probably show that the alcohol has taken longer to heat the water (with more alcohol therefore required), as heat loss to the air etc will be incurred. Because of this, I will try to stop any heat loss through the equipment to the air. I will also have to stop any variables other than the temperature of the water from changing between the alcohols eg: the wick length.
The variables which I will try to control during the experiment are as follows: wick length, mass of the water, evaporation of alcohol after heating is completed- before the alcohol is weighed, temperature of the water before and after the experiment, distance between flame and can, and the can- each is different in weight etc. The incomplete burning of the carbon is a factor that I cannot control. The variables that will change are: the length of time that the alcohols burn for and type of alcohol. I will use the alcohols: Methanol, ethanol and Butanol. The predicted heat given out by each of the alcohols is shown on a later page.
Equipment
Method
Measure out- using a measuring cylinder for accuracy, 100ml of water and put this in the metal can. Attempt to keep the wick lengths approximately the same (1/2 an inch). Record the temperature of the water with a thermometer and weigh the alcohol. Put up the heat- proof sides, and light the wick of the alcohol burner. Use a retort stand to hold the can so that the bottom just touches the end of the flame. Stir the water to keep the water temperature even and extinguish the flame when the water reaches 80 degrees.
Analysis ?
On the graph are two sets of results: my results and the theoretical results calculated by the computer. If my results were perfect- had no heat loss or variables, the increase in heat given out by the combustion of the alcohol would double each time.
I found that as the number of carbon bonds increased, for the alcohol, they produced more heat and so lost less mass while raising the temperature of the water to 80 degrees.
My results support my hypothesis that heat produced increases with the number of Carbon bonds and that thus it would take a shorter time for example for Methanol to raise the temperature to 80 degrees than it would for Butanol.
So though my results do not directly match the predicted ones, they do have the same trends, and my knowledge of the other variables and heat loss would support this.
To calculate the amount of energy I used the equation: mass x Rise in temperature x joules needed to raise water temperature by 1 degree.
?
To calculate the amount of energy produced per gram of alcohol, I used the equation: energy x mass of fuel combusted
?
Evaluation
My results were very inaccurate. This was mainly because of the enormous heat losses that were incurred during the experiments. Energy was lost by the light produced from the flame, the convection currents in the air and the conduction of heat away from the can through the retort stand etc. Heat is also lost as the actual beaker heated up, by the incomplete combustion of carbon atoms in the alcohols, the length and girth of the wick- which couldn't be kept constant. The flame size was also varied by size of the wick and by the type of alcohol. This made it difficult to decide whether to move the beaker up or down as the flame size could have been because of the alcohol- in which case the test would be fair, OR by the wick, in which case a different flame size would make the results unreliable. Evaporation of the water as the temperature rose to 80 degrees would also mean that as the water increased temperature, the speed that it increased temperature would also increase as there would be less water to heat.
Human error would also have made a difference to the results. Even if the alcohol was stopped from burning at the right time, and no alcohol was lost due to evaporation, measurements can be different if read from a scale seen from above of below eg on the measuring cylinder.
If more alcohols were used, such as Propanol, this would also give more evidence to the conclusions that we are lead to by the 3 alcohols. Another alteration that I would make is to have an oxygen supply to the alcohol flame. This would make the combustion of the alcohol much more complete and so the black smoke would not be produced. I would also increase the measures to prevent hat loss by surrounding the flame more completely and using insulators to hold the can etc instead of conductors of metal.
