Prediction
I predict that the more bonds there are holding the carbon, oxygen and hydrogen atoms together; more energy will be given out as new bonds are formed. For example Ethanol has the formula C2H5OH. In this formula you have five C-H bonds, one C-C bond, one C-O bond and one O-H bond. To separate these types of bonds you require a certain amount of energy shown below in the table.
To separate C-H bond you need to apply 410 joules of energy. There are five such bonds in ethanol so you multiply 410 by five to get 2050 joules. You do these calculations for all the other types of bonds that make up ethanol, add them all together and you get 3270 joules. All of the other alcohols can be broken up in this way. Below is a table showing the energy required to break up the bonds in each alcohol.
As the size of the molecule increases, so does the amount of energy needed to break its bonds, in this case Hexanol. This then leads to the obvious prediction that the longer the molecular structure in the alcohol the more energy it will take to remove the bonds. When predicting results, I can therefore predict that Hexanol will evolve more energy than methanol simply because it has more bonds to break.
The alcohols used in this experiment will be from methanol, to hexanol, their formulas and predicted enthalpy changes are:
The method that I will use is as follows…
- Measure 250cm of water into a metal tin.
- Place the tin into the grasp of the clamp stand.
- Record the starting temperature of the water
- Weigh the spirit burner with the lid on.
- Put the chosen alcohol burner under the beaker allowing the flame to just touch the beaker.
- Leave to heat up until the temperature of the water is exactly 30ºC more than the original temperature.
- Weigh the spirit burner
- Record all results
The variables that must remain constant throughout the experiment are…
- Mass of the water 250cm
- Tin
- Temperature rise of 50ºC.
- Surrounding temperature of around 23ºC
- The height of the beaker from the wick
- Same set of scales
- Weigh the spirit burner with the lid on.
The variable that must be changed is:
The type of alcohol used
Results
Analysis
In order to calculate the amount of energy evolved, I will use the formula
Energy evolved = Mass x Rise in temperature x SHC
Energy evolved = 250g x 30ºC x 4.2
Energy evolved = 31500
As you can see all of the alcohols will have the same amount energy evolved because all the numbers that are filled in the formula remain constant for each alcohol and the same numbers are applied for each individual alcohol. Below is a table showing the amount of energy evolved in each case…
To find out how much energy is produced per gram we use the formula…
Energy per gram of fuel = Energy evolved x Mass of fuel burnt
Energy per gram of fuel = 31.5kj x ?
Below is a table showing how much energy is produced per gram when burning the alcohols in question…
As you can see the energy per gram decreases as the length of the molecule increases. This is because more fuel is burnt so there is more of it to be filled with the energy.
To find out how much energy is produced per mole you have to use this formula.
Energy per mole = Energy per gram x Formula mass
Below is a table showing the energy produced per mole…
I did predict that the energy per gram would decrease as the molecular length increases. I think this because the alcohols with more carbon atoms in like pentanol burnt more fuel so there would be less energy per gram because more fuel has been burnt. The reason why more fuel has been burnt is because of the large number of carbon atoms and large molecular length; hence the surface area is large allowing more energy to be released.
My graph shows that the results were fairly consistent, apart from the slightly anomalous result for Butanol. The graph does not pass through the origin however, because the smallest alcohol molecule possible, Methanol, contains one carbon atom, and hence three hydrogen atoms, and so that is the minimum energy possible given out. The graph and table do show that the results are roughly proportional to the number of carbon atoms, and so therefore my prediction is correct.
Evaluation
My results are highly inaccurate as they are all nearly half the predicted results. The equipment that I used in this experiment was very inaccurate because heat is a bad way of transferring energy without any loss of it. However, the fact that the graph is a consistent line shows that roughly the same amount of heat must have been lost each time, and this still allowed valid conclusions to be drawn from the graph.
Some of the energy was given off as sound and light, so not all of the energy went into heating the water. The whole experiment was surrounded in cardboard to prevent draughts, and to prevent heat loss. However there were still gaps at the tops of the experiment where heat could have been lost. Also the can would not have transferred all the heat across, and some would have been lost heating up the can. The can was made from aluminium, which is a good conductor of heat, but this meant that as well as heat being added easily, heat is lost just as easily, as the water heats up. Also the sides of the can were not insulated, so a large amount of heat was lost from the can. The clamp was also touching the can, and this will have meant that some of the heat was transferred into the clamp and stand, causing more heat loss form the experiment.
Also, at higher temperatures, heat is lost faster to the surrounding air due to the bigger heat difference. This could be combated in future by measuring a smaller heat increase than the 30 degrees I investigated. Incomplete combustion may also have been a factor. Complete combustion occurs if there are lots of oxygen atoms available when the fuel burns, then carbon dioxide is formed. However, if there is a limited supply of oxygen then carbon monoxide is formed. This is incomplete combustion has occurred. If the oxygen supply is very limited, some atoms of carbon are released before they can bond with any oxygen atoms, this is often known as soot. Since heat is given out when bonds form, less energy is therefore given out by incomplete combustion. To overcome this problem, I would have to make sure a sufficient supply of oxygen was involved in the reaction.
Evaporation of water may also have had an effect, therefore there would be less water to heat, making the water hotter, but also some of the energy would be lost during the evaporation process. Also, depending on the alcohol, the flame size changed and therefore it was a different distance away from the beaker each time.
Given that only 5 alcohols were tested, and the inaccuracies of the experiment, I would say that the evidence is not strong enough to draw firm conclusions from, however, as stated earlier, the consistency of the results seems to imply that roughly the same amount of heat was lost each time. Therefore, this experiment is valid when considering the proportionality of the ratio of molecular size to energy per mole, however, it cannot be used to accurately indicate the amount of energy given off. If this experiment was to be done again, then all the possible sources of error mentioned would have to be counteracted and controlled, as well as using a much wider range of readings of many more alcohols, burn them for different periods of time, heat different substances other that water, investigate the other variables. I would also take many more readings so that a more accurate average could be taken. Next time reducing heat lost would be my main priority. Improving insulation techniques would be a valuable asset in obtaining the most reliable data I could.
