- The amount of water (200ml) to be kept the same throughout the investigation. I have decided to keep this constant as the more water you have the more accurate your results are, as there is less evaporation however I will have to keep the constant time higher to account to allow for this.
Variables
Throughout this experiment I am going to vary the following things:
- The alcohols which I am using. This is so I am able to see the difference in the heat of combustion having increased the length of the chain in the alcohol.
Apparatus
- Spirit Burners
- Retort Stand
- Splint
- Lighter
- Metal Beaker (Copper Can)
- Measuring Cylinder
- Thermometer
- Wooden lid for metal beaker
- Measuring Balance
- Water
- Draft Excluder (3 sided)
Following alcohols:
- Methanol
- Ethanol
- Propan-1-ol
- Butan-1-ol
- Pentan-1-ol
I have realised that the use of a draft excluder would ensure that the flame was more concentrated to the copper can. I also thought that it was possible for heat to be lost, by having an open lid to the copper can. This is why I have decided to use a wooden lid. There is a wooden hole in the centre, which enable the thermometer to be placed through and kept in the same place.
Diagram
Retort Stand
Thermometer
Lid for copper can
Copper Can
Water
5cm Draft Excluder (3 sided)
Spirit Burner
Alcohol
Heat proof mat
Method
- Set up diagram as seen above
-
Using a measuring cylinder, measure and then pour 200cm2 of water into the metal beaker
- Affix the lid to the metal beaker and record the temperature of the water
- Fill spirit burner with alcohol and weigh it on measuring balance
- Place burner under metal beaker so that the wick is 5cm below beaker
- Place draft excluder (three sided) into position
- Light spirit burner, having removed the lid of the spirit burner
- Leave the spirit burner burning for 2 minutes (120secs)
- Quickly replace the lid of the burner
- Record the new temperature of the water
- Weigh the sprit burner again
To increase the accuracy of my results repeat three times for each alcohol. I must then repeat with the following alcohols:
- Methanol
- Ethanol
- Propan-1-ol
- Butan-1-ol
- Pentan-1-ol
Prediction
My prediction is that
‘The amount of energy released per mole of any certain alcohol is linearly proportional to the number of CH2 sections in one of mole of that certain alcohol.’
Scientific Reasoning for Prediction
I have made this prediction with help from my preliminary work that I carried out using Focus Science Investigations 2. I carried out a similair experiment to the one which I have now planned and will carry out. In my preliminary work although I only carried each experiment out once but I don’t believe that I have anomalies. I am only changing the quantities however I believe that there will be linear relationship between the number of CH2 sections and the amount of energy released per mole of a certain alcohol.
The same bonds will release the same amount of energy, this means that each CH2 section releases the same amount of energy. However it will not be directly proportional as it to go throguh zero on the graph. If it was zero it would mean that no energy was given off from the reaction.
OBTAINING EVIDENCE SECTION
Changes to Plan
There are no parts of the plan that I think that are necessary to change, as I am satisfied with my plan.
Results
I have put my results into a table, which can be seen overleaf. I have also calculated the enthalpy and added this column into the table. I have put all my results to 3.s.f.
I have shown an example of how I worked out the combustion of heat.
I am going to take the first result for methanol.
Temperature change (oC) = 14
Change in mass of alcohol (g) = 1.10
Specific heat of water = 4.18
Heat evolved (J) = Temperature change (oC) x Mass of water heated (g) x Specific heat of water
Heat evolved (Q) = 14 x 200 x 4.18
= 11704J
Q is the quantity of heat evolved on burning Δ m grams of methanol (Mr=32)
The amount of heat released when 1g of methanol burns = Q / Δm x 32J
= 340480
ΔHc = ΔH for combustion of one mole of methanol = - 340kJ/mole
This is what I have done for each experiment.
ANALYSIS SECTION
Processing my results
I have already processed my results, which can be seen in the table in the previous section. I have also drawn a graph, which can be seen overleaf. This graph is labelled G2.
What my results show
My results show a negative correlation, which thus shows a linear relationship. The results steadily decreasing (excluding anomalous results), constantly increasing. I would say that this was directly proportional (i.e. the graph shows a straight line through the origin) however this is not possible as some heat has to taking in or given off in a reaction. In the combustion of alcohols heat is always given off so therefore it is inappropriate and incorrect for this graph to pass through the origin.
Comparison with Prediction
In my prediction I stated:
‘The amount of energy released per mole of any certain alcohol is linearly proportional to the number of CH2 sections in one of mole of that certain alcohol.’
All my results (except Propan-1-ol) show exactly that this that there is a relationship which is linearly proportional.
Scientific Explanation
Excluding anomalous results my graph can be explained scientifically by exactly the same as in my planning section. This is that each CH2 chain gives off the same amount of energy. Therefore every time an extra chain is added the amount of energy given off extra will be the same. This therefore means, that will be a linear relationship.
EVALUATING EVIDENCE SECTION
The quality of my evidence
I think my evidence is of a high quality. I can tell this from looking at my results. On the whole my results followed a clear trend, except one result. The majority of my results are close to my line of best fit, which shows that they are quite accurately.
I have also observed that except for my results for Propan-1ol all of my repeats for the same alcohols were similar. This set of anomalous results for Propan-1-ol are most likely to have been caused due to the surface area of the wick or errors such as measuring the distance between the wick and copper can accurately as having done in previous experiments.
The largest difference between an average temperature and the values used to make up the average is found in the alcohol Butan-1-ol. The average heat of combustion of one mole of alcohol is –1090, with a deviation of from this of –1010. This is a percentage difference of 7%. This is not bad given it is the furthest out of all my results (excluding anomalies).
Sources of error
Any measurement is subject to a random error, so my temperatures and masses will not be exact. However I carried out each the experiment out three times for each alcohol, averaging the results which should have reduced this error.
Each time I used the measuring cylinder, some of its contents would have remained in the cylinder, meaning that not exactly 200ml could have gone into the copper can.
Although I had a draft excluder, this didn’t stop all the effect of a draft on the flame. This means that the flame wasn’t concentrated at the same point throughout.
At the end of each experiment soot (carbon) could be seen on the base of the copper can. This means that not all the energy from the alcohol went to heat the water. Also if there were a gradual build up of soot on the copper can, it would mean that it the thickness between the water and the flame was increasing. This could again have affected the results.
The spirit burners were all slightly different. This could not be helped due to the equipment available to me. The surface area of the wicks was slightly different which meant that on occasions the flame was spread out of a larger area.
Possible ways to minimise errors
Taking more results before averaging them can reduce random errors such as volume measurement and mass.
I could have used a more accurate measuring device such as a burette.
I could construct something out of tin foil that would cover the copper can and the spirit burner, to help concentrate the flame even more than it was previously.
There is little I can do to stop the soot forming, however at the end of each experiment I could wipe off the soot which had formed with a damp cloth to reduce the possibility of this effecting my results adversely.
If I were caring this out again, I would just have to make sure that the spirit burners’ wick were all the same surface area.
Further Work
To provide further evidence about the link between concentration and the enthalpy of alcohols I could do the following.
I could use different alcohols. Instead of investigating the affect of increasing the length of the chain, I could investigate the position of the O-H section. I could do this by using other alcohols such as Propan-2-ol, Butan-2-ol and Pentan-2-ol comparing them to Propan-1-ol, Butan-1ol and Pentan-1-ol.
I could calculate the heat of reaction, and then compare it with data book values. This would allow me to calculate the percentage errors.
I could change the constants and variables in such as seeing the effect by varying the distance between the spirit burner and copper can or volume of water. I could even investigate this experiment with another substance and not water and compare the results.
I could also look up more information from the Internet. There are websites that have sample data that could be useful towards my investigation.
