1/2 O = O H ----- O ----- H
1 O = C = O
H
|
H ----- C ----- O ----- H
|
H
- C --- H 1236 KJ 2 C = O 1468 KJ
- C --- O 360 KJ 4 O --- H + 1852 KJ
-
O --- H 463 KJ
- O = O 496 KJ Total Energy to
1/2 O = O + 248 KJ form new bonds: 3338 KJ
Total Energy to
break bonds: 2803 KJ
Above, it can be seen that the actual ΔH figure is a negative figure; this means that the reaction is an exothermic reaction as it releases more energy meaning it releases heat; if it were to be a positive number i.e. the release of more input energy than output energy then it would be an endothermic reaction. To work out all of the ΔH values individually is a long and time consuming job, therefore, I have worked out an easier route to take in order to find the ΔH values. This route is derived from the RMM table where I have stated that there is an increase of one carbon atom and two hydrogen atoms from the previous example, for example, there is an increase of one carbon atom and two hydrogen atoms from methanol to ethanol. Therefore, if the bond energies for these extra atoms are added to the previous alcohol, the ΔH can be found out for the next alcohol. Below are the bond energies that are inputted into the previous alcohols to work out the ΔH of the next alcohol.
Addition of: H There is an increase of:
|
C 2 C = O 1486 KJ
| 2 O --- H + 926 KJ
H Total energy
Output: 2412 KJ
∴This includes the addition of:
- O = O 496 KJ
1/2 O = O 248 KJ
- C --- H 824 KJ
1 C --- C + 348 KJ
Total energy
Inputted: 1916 KJ
I have worked out as shown above the extra figures that need to be added to the previous alcohol to find the new ΔH of the alcohol that comes next in the series.
Overleaf is a table showing the ΔH data that I have worked out for all of the alcohols.
Through the form of research, I have found out that there are different ΔH values for the family of alcohols; these are results that have been worked out scientifically and are known as the ‘True Values’. A table showing the true values are shown below:
The above table therefore means that when I plot a graph to show my results I can compare them with the True Values and the Worked out values.
PLANNING
Equipment
In this experiment, I am going to find out the ΔH for each of the alcohols to see if they have a correlation with the amount of carbon atoms. In order for me to carry on with this experiment, I shall need the use of various equipment, these equipment pieces are:
Alcohol burners + lid
Water (300ml)
Scales
Drought excluder (tin can)
Water tin
Thermometer
Bunsen Burner
Independent Variable
This is the item that I am going to change for each alcohol, in this experiment only one factor will be changed, this is the alcohol used. There will be eight alcohols used.
Dependant Variable
These are the items that I will keep the same, in this experiment; the items are all of the rest of the factors will stay the same, such as equipment, amount of water, same scales.
Fair Test
In order to keep this experiment fair, and as accurate as possible, I shall have to make sure that all of the equipment used is the same for all of the alcohols. This includes items such as the same scales, the same thermometer, the same amount of water and so on. It would be much better if I were able to use the same size burner. If time permits, I shall repeat each of the experiments.
Prediction
My prediction is the fact that there will be a strong, if not definite correlation between the amounts of carbon atoms there are in the alcohol and its enthalpy change of combustion.
Hypothesis
I stated in my prediction above that I think there will be a correlation between the amount of carbon atoms in an alcohol and its enthalpy change of combustion due to the fact that as we travel further down the family of alcohols, like for methanol, the reactant bond energies are greater than the products. I think that this will carry on through the whole of the family of alcohols. Also, due to the fact that carbon burns easily and more intensely, so therefore the more carbon atoms there are the higher the ΔH value would be. Furthermore, the product ΔH value will be greater each time as there are more bond energies to be formed in the products section.
Diagram
The diagram below shows the set-up for the equipment in the experiment:
Safety
In this experiment, safety plays a big role as inhalation or the swallowing of the alcohols can have dreadful side effects. Also, there is the use of fire and also hot water, improper uses of these may lead to burning or scalding. Safety goggles and a laboratory coats must be worn.
Method
- Pick one of the eight alcohols.
- Weigh the alcohol burner including the lid and record the weight reading.
- Measure 300ml of water and put it into a tin can.
- Measure the temperature of the water.
- Put the alcohol burner in a drought excluder.
- Take of the lid and light the alcohol burner wick.
- Balance the water tin can on the drought excluder.
-
Wait until the temperature of the water has risen by 15° to 18°.
- Take off the water can from the drought excluder and replace lid quickly.
- Re-weigh burner and the lid and record the weight.
- Repeat the experiment.
-
Find out the ΔH value by the two equations shown below:
4.2 * Δ°c * Mass of H2O = XJ = X KJ
1000
Moles = Mass burnt = Y
RMM of alcohol
ΔH = X KJ KJ/Mol-1
Y
Multiply Result by –1 to give total in KJ/Mol
OBTAINING EVIDENCE
Obtaining Evidence
Below is the table I am going to use in order to record my results while carrying out the experiment.
ANALYSIS
The table below shows the results of my experiment through the calculations.
*Note: Due to the fact that there are two results for some of the experiments as shown above.
Having two different results in a graph for the same alcohol would prove difficult.
Therefore, I am going to use the average for the two results as shown in the table below. *
GRAPHS
*Note: It is evident that the graphs should be slanted on a downwards slope, but for the ease of analysing,
the figures for ΔH has been multiplied by –1 in order to give a positive figure. *
CONCLUSION
The aim of this experiment was to try and discover if there was a correlation or any common pattern between the amount of carbon atoms in an alcohol and its standard enthalpy change of combustion (ΔH). According to the aim and title, I have been able to successfully carry out the experiment and have gained satisfying results. As can be seen from the previous graphs, Graphs 1, 2, 3and especially on graph 4, there seems to be a good correlation between the amount of carbon atoms and the ΔH of the alcohol. The general trend is the fact that when the amount of carbons increases, then generally the ΔH of that alcohol also increases (meaning the negative ΔH number increases). This means that the more carbons there are in the alcohol the greater the output energy of that alcohol is. Hence, the more carbons, the more of an exothermic output the reaction is.
The lowest ΔH values were that of Methanol at 311.75257732. The highest value was with Octan-1-ol where the ΔH value was 3448.421053. This shows the relationship from only these two figures, it can be seen that Octan-1-ol has eight carbons and Methanol has one carbon atom so therefore a small correlation can be seen from the basic output.
At the starting stages of the experiment, I made the prediction that there would be a correlation between the two factors, the amount of carbon atoms and the ΔH of the family of alcohols. This prediction that I stated is now confirmed through completing the experiment and I am able to use the experiment to support my first prediction. However, the hypothesis is correct only to a certain extent. I stated that the enthalpy of the products would decrease but in fact, they increase. The fact that I stated carbon burns easily is a true fact and has been shown by the experiment.
The basic fact for this experiment is the fact that the creation of the products for each of the alcohols in increasing number of carbon atoms is harder to bond together and more energy is needed for the manufacture of the products as they have high bond energy figures. The bond energies for the reactants are proportionate to the amount of carbon atoms in the alcohol.
Overall, I am able to use the experiment to support my prediction as both of them underline the same phrase:
‘An increase in the amount carbon atoms in an alcohol means an
increase in the ΔH of the alcohol.’
Even though this experiment could be derived from looking at figure books, the act of doing the experiment helps to understand the reason why there is a correlation between the ΔH and the amount of Carbon atoms in the alcohol.
EVALUATION
Due to the fact that the experiment was carried out in a school laboratory, the entire experiment was only a rough estimate to what the actual results would be. The coursework experiment was in my view done very well. I think that for the standard of equipment I had, the experiment was completed very well indeed. The reason why I would say that this experiment was successful is because of the fact that the two requirements for the experiment i.e. the enthalpy change of combustion and also the number of carbon atoms in the alcohol have been available and satisfying rational results have been produced from the experiment. However, there were some faults with the experiment, the most obvious fault is that of the one shown on Graph 3, the result for the ΔH value for Butan-1-ol. This result is out of place with the rest of the results and is probably caused by a fault in the apparatus. However, even though this experiment gave results, I do not think that the evidence gained is enough for a firm conclusion as there are anomalous results within the experiment.
This brings the section onto limitations and problems. There were very many limitations to this experiment mainly due to the fact that the experiment was not as much of a fair test as it should have been. The first problem is the fact there were different burners for different alcohols. This meant that the same burner could not be used for the experiment for the different alcohols. Furthermore, the burners were all different sizes so therefore the test was unfair. This also meant that different sized drought excluders had to be used having a knock on effect to mean different sized water containers had to be used. This means that even though the same water volume is used, there are different amounts of surface area meaning one can lose heat more quickly than the other. Another problem was the fact that different sized wicks were also evident meaning again an unfair experiment. Within the experiment there were two losses of heat, these were at the points of the period when the burner is alighted and is transferred to under the drought excluder and also at the point where it has been covered by the water that is being heated, there are gaps in the drought excluder. There is also a loss of weight at the point where the lid is being taken off from the burner and the wick is being a flamed.
As I have said before, I think that I have successfully completed the coursework for the conditions that I was working under. If I did this experiment under stricter conditions with better equipment, I would be able to produce better and more accurate results. The best improvements for this experiment would be to change the experiment in order to satisfy the faults or limitations that I have expressed above.