Apparatus
- Retort Stand
- Clamp
- Copper container
- 25ml measuring cylinder
- Selection of fuels, methanol, butanol, ethanol and pentanol
- Thermometer
- Water
- Heat proof mat
Method
- Collect and set up apparatus as shown in the diagram
- Measure 20ml of water in the measuring cylinder
- Pour into copper container
- Place thermometer in the water and measure the starting temperature
- Weigh fuel container, record weight for before the experiment
- Make sure all safety precautions are in place (safety goggles, heat proof mat)
- Ignite fuel containers wick
- When the temperature reaches 60°C put out the flame
- Weigh the fuel container and record weight after experiment
- Repeat 3 times for each fuel
Fair Test
- The Mass of water must stay the same each time the experiment is repeated. Too much water will take longer to heat, and too little water will take quicker to heat, and so the volume of water is clearly going to have an impact on the results.
- The height of the beaker from the wick of the spirit burner mustn't vary. If this distance varies then the amount of heat reaching the test tube will vary each time the experiment is repeated.
- Scales used to mass the fuels should be the same for each weighing.
- The way the spirit burner is weighed should remain constant. This means that if the spirit burner was weighed with the lid on in the first experiment, then it should be weighed with the lid on in the next experiment. Not doing so will lead to drastic errors in the results.
Prediction
I predict that the more bonds there are holding the carbon, oxygen and hydrogen atoms together; more energy will be required to break them apart, and therefore the faster the water will be heated. For example Ethanol has the formula C H OH. In this formula you have five C-H bonds, one C-C bond, one C-O bond and one O-H bond (all of which are single bonds). To separate these types of bonds you require a certain amount of energy which I will show in a table.
TYPE OF BOND ENERGY REQUIRED BREAKING THE BOND (kJ)
C-H 410
C-O 360
O=O 496
O-H 510
C-C 350
C=O 740
To separate C-H bond you need to apply 410 joules of energy. There are five of these 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.
Type of alcohol Energy required to break the bonds in the alcohol (KJ)
Methanol 2100
Ethanol 3270
Butanol 4560
Pentanol 6420
As you can see, a longer molecule takes more energy to break its bonds, in this case Butanol, compared to a smaller molecule, methanol, which requires less energy to do so. I can come to predict that the longer the molecular structure in the alcohol the more energy it will take to remove the bonds. So when I come to predicting results I can safely say that Butanol will evolve more energy than methanol simply because it has more bonds to break.
Methanol
Pentanol
Butanol
Ethanol
Molecular Weight
1) ΔH = M x 4.2 x ΔT 2) ΔH = ΧKj per gram of alcohol burnt
1000
3) XKj = Kjg-of alcohol burnt 4) Kjg- X RMM
Mass of alcohol burnt
Anomalies are apparant at Butanol and Pentanol. Looking at my spreadsheet that is attached I am able to identify the results that seem to be creating these anomalies, and so I can now subtract them from the averages that are plotted on the graph.
Conclusion:
From the graph I can come to conclude that the longer the molecular structure in the alcohol the more energy that is released. For example Pentanol released more energy than methanol simply because it has more bonds to break and form. The energy released was also directly proportional to the number of molecules bonded together to form each alcohol. As the molecular structure increased by one carbon and two hydrogen’s the energy released increased by around 415 KJ a mole from the previous fuel. This tells us that there is a direct link between the number of carbon and hydrogen atoms, and the amount of energy released by each fuel. The results therefore justify my written prediction which states that the longer the molecular structure in the alcohol the more energy that is released.
If the reaction of an alcohol and oxygen only goes as far as producing carbon and water, then the energy released is going to be far less than if carbon dioxide and water was produced, as shown in the diagram below. As some soot was evident in the experiment then it is clear that complete combustion had not entirely taken place.
Evaluation
In general I believe that the experiment was carried out carefully and accurately, and the apparatus was set up efficiently ensuring a good set of results.
Although my results were extremely far out in comparison to my theoretical values, this does not mean that my results were inaccurate as all the results were out by a constant value.
Below are some possible reasons why my results were much lower than the theoretical values:
- Some of the apparatus may have absorbed some of the heat energy given off by the fuel. Such as the beaker getting hot.
- Some of the heat energy may have been taken away by gusts of winds or may have escaped into its surroundings.
- Some of the water may have evaporated as a result of the high temperatures. This would mean that there would be less water to heat, making the water get hotter quicker.
- The flame size changed due to the type of alcohol; hence it was a different distance away from the beaker each time.
- And lastly incomplete combustion would have acted as a major factor into reducing the amount of energy release.
I would not extend this investigation by experimenting with more alcohols as it is clear that the trend identified is likely to continue as long as the chain of molecules for each alcohol is increasing.
I would extend this investigation by investigating other organic compounds, such as hydrocarbons, to see if they behave similarly. Likewise experimenting with the calorific values of different foods would also make good experiments. This is where the numbers of calories for each type of food will be measured and investigated.