Fair Test:-
I will have to ensure that only the alcohol in the burners changes. However this may be hard because we will have to do the experiment over a range of lessons meaning that some of the equipment and conditions of the environment may change.
These are the unfortunate imperfections of performing the experiment in a school lab, so we will have to obtain the best results possible with the given equipment.
Safe Test:-
I must follow all of the regular safety precautions such as :-
- Tuck your tie in
- Take your blazer off
- Tie back long hair
- Push stools under work benches
- Stand at all times
- Wear safety goggles
- Handle the alcohols with care some are extremely strong and harmful
Prediction:-
Considering my Research into covalent bonds I believe that ethanol will burn the quickest and lose most mass in the time. This is because it goes through the following reaction:
2C2H5OH + 7O2 4CO2 + 6H2O
This requires 1 carbon to carbon bond, 6 carbons to hydrogen bonds and 1 carbon to oxygen to be broken. The formation of the reaction products require that eight carbons to oxygen bonds and twelve oxygen to hydrogen bonds are made. Octanol has more bonds to be broken so will burn slower, but more bonds are made therefore releasing more energy. Octanol will burn the slowest and loose the least amount of mass. This is because Octanol has a substantially higher density, more bonds, and so there for will need longer exposure to a sustained heat or need to burn as much as ethanol however more energy will be released in the same amount of time by Octanol than Ethanol.
Apparatus:-
- Thermometer
- Clamp
- Spirit lamp containing alcohol
- Calorimeter
- 100ml of water for each test
- Retort Stand
- Weighing Scales
Diagram
Method:-
- Set up equipment as above
- Weigh the Spirit Burner.
- Measure 100ml of water into a measuring cylinder and then pour into calorimeter.
- Measure the temperature of the water to begin with.
- Light the lamp under the tin can, making sure the tin can is 12 cm from surface.
- Take Readings
- Dampen the flame to put it out because blowing it out may not work first time and may waste energy and fuel thus making the test unfair.
- Measure the temperature of water and write it down.
- Weigh lamp on weighing scales
- Collect data into a table.
- Perform the above test for each alcohol.
- Repeat each measurement to improve accuracy thus giving two results.
Preliminary Results
We took a few preliminary results to decide a few minor adjustments to the method
-The Calorimeter should suspended at 12cm from the surface. This is tall enough for the fame just to touch in the centre of heat concentration but not too tall so that lots of heat is lost to the surroundings.
-The time scale should be two minutes because after that some of the stronger alcohols can boil the water and then some of the heat is lost in the process of it boiling and releasing gas.
-The thermometer should be suspended in the water in some way because otherwise the thermometer could gain heat from the bottom of the calorimeter and produce biased results.
Results:-
Run 1
Run 2
Now that I have collected the results I must find out for each run and alcohol the energy released in joules per gram.
I can use a standard formula to find out energy in joules/gram
This is:-
Mass of water (100g) x 4.2 (Specific heat capacity of water) x Temperature change ºC
Mass of alcohol burnt (initial mass of alcohol – final mass of alcohol)
Table to show average energy released from alcohol
We averaged our findings of energy released and plotted it against the expected results.
Table to show average energy released against expected results
Conclusions:-
As you can see from the first graph, Runs 1 and 2 both bear little resemblance to each other although when put together as an average they show a trend. This tells us that because the same experiment was carried out for both runs, the conditions of which the experiment was done in changed, beyond our control. From the above graph we can see that our results were not very close to our expected results. However my prediction was accurate in thinking that the more bonds the fuel had the more energy it would produce when burnt. The expected results are in a steady gradient, this is because that all the fuels had the same type of bonds in them, they were just in a higher ratio for the longer molecules. The first 5 substances show a clear trend in the increased number of carbon bonds the higher the amount of energy released. e.g.… The results that we collected for Octanol was 6,429 joules released per gram, the expected results show that it was supposed to be nearer 27,660 joules per gram. However I do not know why the results took a massive dip at the end. I can find no scientific research to show why it should, so I must declare the dip at the end of all of the runs anomalous results.
These could be explained that the heptanol and octanol burners had slightly smaller wicks than the rest of the alcohols.
Evaluation:-
I think the reason we did not get the expected results is due to the environment surrounding the experiment and the apparatus used. This would have caused the following errors:-
- The equipment was not of a high standard therefore measuring the alcohols and temperature may have been inaccurate making our results flawed.
- An optical mercury thermometer was used.
- The flame was burning the wick as well as the alcohol.
- The heat was absorbed by some of the apparatus.
- All spirit burners had different shapes and sizes of wicks.
- Spirit burners had varying amounts of alcohol in them so different percentage of the wick inside the burner was absorbing the alcohol.
- The conversion of the alcohol into heat energy may have been incomplete combustion, not burning as efficiently.
- Heat from the alcohol could have been wasted because of surrounding interference.
- The room temperature could have changed making the results not reliable.
- Since the room was not airtight there could have been gusts of wind which meant that not all the energy was transferred to the tin can then to the water.
- The tin can itself heated up not transferring its energy completely to the water.
- There was a thick layer of soot on the tin can
In order to improve the results the following changes could be made to the experiment:-
- Measuring instruments could be electronic making the results substantially more accurate.
- By making sure the environment remains constant e.g. on the same day, same place same room etc.
- An oxygenated environment would ensure complete combustion so energy would be more efficiently transferred.
- Use different materials therefore reducing the amount of heat energy taken up by equipment.
- Makes sure that the room is airtight to avoid gusts of wind.
- Do more measurements or repeat experiment several times to avert anomalous reading contaminating the results.
- Make the alcohol pressurised so that is burns quicker, therefore loosing less heat.
However, you can never have perfect conditions in a school science lab. Professionally a Bomb Calorimeter would be used. It works in the following way.
The bomb calorimeter consists primarily of the sample, oxygen, the stainless steel bomb, and water.
The Dewar prevents heat flow from the calorimeter to the rest of the universe
Since the bomb is made from stainless steel, the combustion reaction occurs at constant volume and there is no work
Thus, the change in internal energy for the calorimeter is zero
The thermodynamic interpretation of this equation is that the calorimeter is isolated from the rest of the universe.
This would be the perfect as all of the problems that I identified in the original experiment are eliminated. This would mean that your collected results should be exactly the same as the expected result where no energy is wasted and complete combustion occurs.
Unfortunately there is no such thing as a perfect experiment so I think our experiment was sufficient enough to deduce simple conclusions from even if the results were biased, but I had enough scientific evidence and research to understand why I was getting the anomalies and so this made me aware of how many imperfections an experiment like this can have.
