H H H H
H H
Heat of Combustion = +8568-11080
▲H = -2512 kJ mol –1
- Pentanol
H H H H H
1 H C C C C C O H + 6O=O 5=O=C=O + 6O
H H H H H
H H
Heat of Combustion = +10165-13618
▲H = -3453 kJ mol –1
- Pentanol
H H H H H H
1 H C C C C C C O H + 9O=O 6=O=C=O + 7O
H H H H H H
H H
Heat of Combustion = +11408-16156
▲H = -4748 kJ mol –1
Safety
- Safety goggles must be worn at all times to prevent the alcohol entering the eyes of the person using it.
- Any lose clothing must be tucked away from the experiment.
- Long hair must be tied back.
- The experiment planned must be carried out exactly as planned to prevent unexpected, dangerous, chemical reactions.
- No liquids should be drunk (especially “chemicals”), and no food should be eaten in the lab.
- Keep bags out of the way during the experiments so that fellow students do not trip over.
- Always walk in the lab, to avoid collisions.
The Experiment
Aim: To find the heat of combustion of five different alcohols.
Diagram:
Method: A tripod was taken and laced onto a bench mat. Then a large beaker was placed underneath the tripod. A small beaker was taken and filled with 50cm³ of tap water. The temperature of the water was then found with a thermometer, and recorded. An alcohol burner was then taken and weighed on a scale. The weight was recorded in a table, under the heading “starting weight”. A spill was taken and lit on a methane burner. The alcohol burner was then lit. While the burner was lit the water in the small beaker was stirred. When the temperature of the water had increased to between 15° to 20°, the alcohol burner was blown out. The final temperature of the water was then found, and the final weight of the alcohol was found.
To find the heat of combustion the heat change had to be found first. This found by using the equation…
Heat change = mass x temperature change x S.H.C
Once the heat change had been found the Heat of combustion could be found, and this was done with the equation…
Heat of Combustion = -heat change x Mr
Mass of alcohol burned
In the equations there were two constants. These were, the mass of water (50g), and the specific heat capacity (S.H.C)(4.2).
Results:
Skill Area “A” Analysing Evidence and Drawing Conclusions
Ethanol: Heat change = mass x temperature change x Specific Heat capacity (SHC)
= 50g x 15.5°C x 4.2J K-1g-1
= 3,255J
= 3.25kJ
▲H= Heat change x Mr
Mass of alcohol burned
= 3.25 x 46
0.72
= 207.64 kJ mol-1
Propanol: Heat change = mass x temperature change x SHC
= 50g x 16 °C x 4.2 J K-1g-1
= 3,360 J
= 3.36 kJ
▲H = Heat change x Mr
Mass of alcohol burned
= 3.36 x 60
0.802
= 251.37 kJ mol-1
Hexanol: Heat change = mass x temperature change x SHC
= 50g x 20°C x 4.2 J K-1g-1
= 4,200 J
= 4.2 kJ
▲H= Heat change x Mr
Mass of alcohol burned
= 4.2 x 74
0.871
= 356.831 kJ mol-1
Evaluation
This experiment has many sources of error, as the results were not what were predicted. The real results, predicted results and % error, had no anomalous results.
My results are highly inaccurate as they are all nearly half the predicted results. This is not due to the inaccuracy of not carrying out the experiment properly as the error bars on the graph were too small to be drawn accurately, as the two readings are almost the same.
These small inaccuracies are caused by slight differences in the values of the fixed variables, like the mass of water not being exactly 100g, due to incorrect reading of the measuring cylinder caused by a parallax error, caused when the scale is read at an angle to the eye, as the light is reflected through the glass, causing the reading to appear somewhere else. The same error could have been meant an error in the reading of the thermometer, causing there to be wrong temperature readings. The amount of wick on the burner will not have been exactly the same (6mm) on each burner as this was difficult to measure. This would have caused differences in the amount of alcohol burnt. The flame was not always just touching the can, as this again was difficult to measure accurately, and would have caused differences in the amount of heat given off, as the temperature of the flame is different and different heights. The can might not have been the same as this experiment was done over two lessons, and different cans have different conduction properties. Also the time for each burn might not have been three minutes exactly each time as it takes time for the final reading to be read and the flame to be extinguished after the time is up. The thermometer was not in the same place at each temperature recording, as even though the water was stirred, there would be differences in the temperature of the water at different depths.
To make the results slightly more accurate, I would use more accurate, maybe electronic ways of measuring temperature and volume, also measure the wick length and height of can above the flame. I would also have marked the can used, so that it could be used again, and had a more accurate timing system As these errors were only small, they could not have made the massive differences between the predicted results, so there must have been other factors that caused the heat from the burner not to reach the water, and cause these errors.
The whole experiment was surrounded in mats to stop a draught taking heat from the flame, however there were still gaps at the tops of the experiment where a draught could have caused heat to be lost. Also the can, although copper, would not have transferred all the heat across, some would be lost heating up the can. The can being copper meant that as well as heat being added easily, heat is lost just as easily, as the water heats up. There was a cardboard lid to prevent some of this heat lost, but the sides of the can were not insulated, the can gave of so huge amounts of heat. The clamp, which is metal, and was touching the can will have meant some of the heat, was transferred into the clamp and stand, causing more heat loss form the experiment. In order for this experiment to be more accurate, I would have to insulate the can and the clamps, completely exclude all draughts use a better conductor other than water to heat, and use a thinner can, made of a better heat conducting material.
Given that the range of the experiment was only 6 alcohols from methanol to hexanol and that the experiment was only 3 minutes, and the inaccuracies of the experiment, I would say that the evidence is not strong enough to draw firm conclusions from. If this experiment was to be done again, then all the possible sources of error mentioned would have to be counteracted and controlled, as well as using a much wider range of readings of many more alcohols, burn them for different periods of time, heat different substances other that water, investigate the other variables. I would also take many more readings so a much more accurate average could be taken. Other experiments could have been done investigating other organic compounds, such as hydrocarbons, to see if they behave similarly, and investigate them under different conditions, such as at extremes of temperature and pressure.