I can also see that still with 50ml of water and 20degree change in temp there was not a great change in mass. In conclusion to this preliminary I have decided on my variables and constants.
Variables: Constants:
Mass of the alcohol Volume of water
Type of alcohol distance water is from flame
Temperature change of water
I decided that 5 alcohols was sufficient if I repeated each alcohol 3 times to make sure that the results were accurate. I am going to burn 5 types of alcohol. The alcohol will heat a flask of water with a set temperature rise. From this I can tell how much alcohol was burnt for each.
I am going to test
1) Methanol – 1 carbon bond
2) Ethanol – 2 carbon bonds
3) Propanol – 3 carbon bonds
4) Butanol – 4 carbon bonds
5) Pentanol – 5 carbon bonds
My aim is to see how the alcohol ( the length of the carbon chain) affect the heat of combustion.
Prediction: using my scientific knowledge I believe that due to the theory that it becomes less flammable, there will be less energy and less heat given off when the length of the carbon bonds increase. This means that as the length of the carbon bonds increase, the longer the water will take to boil, forcing combustion to take longer.
To make my prediction valid I am going to calculate the predicted energy needed and the predicted energy provided using the chemical formula and the RHM.
EG: ETHANOL
C2H3OH + 3O2 → 2CO2 + 3H2O
Bonds:
C – H = 5
C – C = 1
C – O = 1
C – H = 1
O = O = 3
………… - energy needed
C = O = 2
O – H = 6
………… - energy provided
As I have stated, in theory the energy provided is greater than the energy needed.
Method: I am going to take a flask of alcohol and measure its mass. Once recorded I am going to use the same procedure as in my preliminary. The beaker with 50ml of water will be held by a tort stand 10cm above the floor. The thermometer is placed in the water and the initial temperature is recorded. The alcohol is placed under the beaker and lit. It is left there until the temperature of the water has changed 40 degrees. The flame is then put out and the final mass of the alcohol is measured. This experiment is repeated 3 times with 5 different alcohols.
I think that this is a relatively fair experiment. I decided to cover the experiment with books so that the wind or a gentle breeze would not affect the flame, disrupting the experiment. I also made sure that the beaker was attached to the tort stand in the same place so that the distance between the flame and the water was the same. I used the same beaker but cleaned it before each experiment. I used the same type of water (distilled) and all initial temperatures were relatively close. I did not use the same flask of alcohol as I did not feel this would affect the readings. From this I can conclude that my data is relatively accurate.
However if there isn’t enough oxygen present the combustion will be incomplete. This gives carbon monoxide and carbon as waste products. This can be very dangerous as carbon monoxide is colourless, odourless and poisonous. I believe that there will be enough oxygen at room temperature.
Safety: we are all required to wear safety goggles and use a safety mat under the Bunsen burner. The whole experiment was conducted under the supervision of a teacher.
Obtaining evidence:
Results for each alcohol repeated 3 times
Using the formula shown earlier I am going to find the energy of combustion of each alcohol.
M x S x T = mass x specific heat capacitiy x temp increase
= 50 grams of water x 4.2 x 40 degree increase
= 8400 (remains constant)
moles of alcohol burnt = average mass burnt
RHM of alcohol
Methanol = 1.520
32
= 0.0475
Ethanol = 0.860
46
= 0.0187
Propanol = 0.803
60
= 0.0133
Butanol = 0.676
74
= 0.0091
Pentanol = 0.703
94
= 0.0074
amount of alcohol burnt per mole:
Methanol = 8400
0.0475
= 176842 J
= 176.842 KJ
Ethanol = 8400
0.0187
= 449198 J
= 449.19 KJ
Propanol = 8400
0.0133
= 631579 J
= 631.579 KJ
Butanol = 8400
0.0091
= 923077 J
= 923.077 KJ
Pentanol = 8400
0.0074
= 1135135 J
= 1135.135 KJ
Analysis: from the graph in the previous page I can clearly see a very strong trend in the relationship between the KG / MOL of alcohol burnt and the carbon bonds. There is a strong positive correlation which means that when the carbon bonds increase the KG / MOL of alcohol burnt should increase.
This proves my prediction correct. My scientific knowledge was correct.
Calculating the predicted change showed me that there is more energy provided than needed. This is because some energy is lost in combustion as the combustion is not complete.
Evaluation:
I feel that this project gave me satisfactory results using a simple method. The experiment was quite limited with a lack of resources and a small time period, meaning that I could only do a number of experiments. This lowered the accuracy of the average, thus lowering the accuracy of the calculations.
The apparatus was vary simple using a tort stand and a ruler to judge the distance for the height of the beaker.
Because of the time limit I could only have a small amount of water and a small temperature change. This could have severely altered my results as the combustion may not have completed.
The alcohol was very limited, each had been used before. The flames were different sizes due to different size wicks in the alcohol. I made an effort to protect the flame but in some cases the flame had to be re-lit due to a draught in the classroom.
The only anomalie was using Pentanol. This was the longest carbon chain. I think this was the least accurate as combustion was not completed. This however did not affect my experiment greatly as I could still identify a strong positive correlation in my results, thus leaving my results reliable enough to come to my conclusion.
If I were to do the experiment again the main aspect needed to be improved would be the time period. If I had longer to do the experiment I could use a greater range of experiments thus making my average more accurate.
Christian Wimshurst UVD
