The extra enthalpy change per extra carbon atom in the molecule is 1920-2538=-618. This shows how as the molecule gets longer, the enthalpy change of combustion will increase.
Both of the propanol isomers have the same bonds to break and make, but the position of the OH group affects the stability of the molecule due to dipoles. When the OH group is at the end of the molecule, it is more unstable than when it is bonded to the 2nd carbon atom. An unstable molecule has more energy contained in its bonds than a stable molecule, so will have a slightly greater enthalpy change of combustion when the bonds are broken.
Equipment
5 spirit burners, containing methanol, ethanol, propan-1-ol, propan-2-ol and butan-1-ol.
Copper can with a 100cm3 capacity. This will be filled with water to act as a calorimeter. 100cm3 is an appropriate volume as it will not take too long to be heated.
0-50°C thermometer. Markings on a thermometer this size are to the nearest 0.1°C, compared to the nearest whole degree on a 100°C thermometer. This allows my results to be more precise, as the water will not be heated above 50°C anyway.
100cm3 measuring cylinder, marked every whole cm3. This is the most precisely marked measuring cylinder available to measure out 100cm3 of water.
Balance measuring in grams to 2 decimal places. This will be used to weigh the spirit burner before and after the experiment to find out the mass of alcohol burned in the experiment. 2 decimal places give an appropriate degree of precision.
Tripod, clay pipe triangle and 250cm3 beaker. These will be used to support the copper can and the spirit burner at the correct height.
The equipment should be set up as shown:
Method
Measure out 100cm3 cold tap water in a 100cm3 measuring cylinder and pour into the copper can. Stir and note its temperature.
Weigh the spirit burner containing propan-1-ol, including its lid. Record the mass. Replace the spirit burner under the copper can.
Remove the lid of the spirit burner and light the wick.
Carefully stir the water using the thermometer until the water’s temperature has risen by 10°C. Cover the spirit burner’s flame with its lid.
Continue stirring the water until its temperature reaches a maximum. Record this temperature.
Weigh the spirit burner with lid.
From these measurements (and the known standard reference value for the enthalpy change of combustion of propan-1-ol) you can work out the mass of alcohol burned, and therefore the number of moles of alcohol burned, and therefore the amount of energy released by the burning of the fuel.
Repeat the experiment with each alcohol, using the same apparatus, the same mass of water, and the same starting temperature of water. The final temperature of the water, the temperature change, and the mass of alcohol used may differ.
If time allows, the experiments should be repeated for both the propan-1-ol (to ensure the value for the energy required for rise in water’s temperature of 1K is reliable) and for the rest of the alcohols (to ensure the value for the enthalpy change of combustion is reliable).
Results
The temperatures were measured using a 100°C thermometer, marked every 1°C.
The masses were measured using a balance, reading to the nearest 0.1 grams.
The wick of the spirit burner was measured each time and adjusted to be 5mm, and the distance between the wick and the base of the copper can was 30mm.
Analysis
To work out the energy transferred to the water, the mass of water is multiplied by its specific heat capacity multiplied by the temperature rise. In a combustion reaction, this will be a negative value as the reaction is exothermic.
Energy transferred to water/J = -(mass of water/g x temperature rise/°C x 4.2)
E.g. for methanol, energy transferred to water = -(100g x 18°C x 4.2) = -7560J
The number of moles of fuel burnt is the mass of fuel used divided by the mass of 1 mole of the fuel.
Moles of fuel/mol = mass of fuel used/g
molar mass of fuel/g
E.g. for methanol, moles of fuel = 1.21g = 0.04 moles
32g
To calculate the enthalpy change of combustion, the energy transferred to the water is divided by the number of moles of fuel used, then the result is divided by 1000 to give an answer in kJmol-1.
ΔHc/kJmol-1 = energy transferred to water/J
1000 x moles of fuel/mol
E.g. for methanol, ΔHc = -7560J _ = -202 kJmol-1
1000 x 0.04mol
As the graph on the next page shows, a longer chain alcohol has a greater enthalpy change of combustion than a shorter one. Methanol has the lowest enthalpy change of combustion, and butan-1-ol has the highest. As the chain gets longer, the graph gets steeper, which seems to show that the bond enthalpies change as the size of the molecule increases.
As the size of the molecule increases, there are more intramolecular bonds to be broken (which takes energy in from the surroundings) and more intramolecular bonds to be made (which gives energy out in the form of heat).
With each extra carbon atom in the chain, these are the extra bonds which must be broken and made.
The enthalpy change of combustion becomes greater when the molecule is larger as more energy is given out than taken in for each carbon atom. There is theoretically an extra 618 kJmol-1 given out for each extra carbon atom in the chain. However, this is not shown in my results. My data show that there is between an extra 117 to 377 kJmol-1 given out per carbon atom. This is partly due to the fact that bond enthalpies vary depending on the size of the molecule and the position of the bond, but it is mainly due to inaccuracies in the procedure.
Although propan-1-ol and propan-2-ol have the same molecular formula, propan-2-ol has a higher enthalpy change of combustion. This is because the position of the –OH group in propan-2-ol makes it more stable, so it requires more energy to break the intramolecular bonds.
Evaluation
There are always errors associated with pieces of equipment used for measuring, usually half of the smallest division marked on it.
The balance and thermometer have an error of two times half the smallest division because the values used in calculations are obtained by subtracted the measured values. However, the ruler’s error is multiplied by two because there is an error at both ends of it: when positioning it at 0 and when reading the distance between the wick and the calorimeter.
The greatest percentage error is associated with measuring the height of the wick with the ruler, giving a 1/5 = 20% error. This is unlikely to have heavily influenced my results as the size of the wick will only have slightly changed the size of the flame.
The smallest percentage error is associated with the balance, when calculating the changes in mass of fuel. The percentage error ranges from 0.8% for methanol to 1.3% for butan-1-ol. This is unlikely to have heavily influence my results as it is such a small error.
The biggest source of error was probably heat loss to the surroundings. In my calculations, I assumed that all of the heat from the flame was transferred to the water. In fact, the energy also heated the equipment and the air, as well as being lost as light. Although we tried to minimise this heat loss by using heatproof mats, this did not provide the closed environment which would be ideal.
It should also be noted that all of the fuels burned with an orange/yellow flame, indicating incomplete combustion. This will have produced less energy than complete combustion, further decreasing the calculated enthalpy change of combustion.
If I was to try and improve this experiment, I would use a calorimeter to reduce heat loss. I would also calibrate the equipment using a fuel with a known enthalpy change of combustion, to try and work out how much energy is lost to the equipment. I could then incorporate this into my calculations to give me a more accurate value for the enthalpy change.
Sources
Salters Advanced Chemistry, Chemical Storylines, Heinemann, 2000, pg 22.
Salters Advanced Chemistry, sheets DF1.2 and DF1.3, 2000.
Chemistry in Context, Nelson, 4th Edition, 1995, pg 450.
These are reliable sources as they are respected and widely used textbooks and course materials.
CLEAPSS Science Publications CD-ROM, 2005.
CLEAPSS hazcards are usid in school laboratories.
http://www.wpbschoolhouse.btinternet.co.uk/page18/02DFws.htm 6/11/2007
I believe this is reliable as it has been designed to help with the Salters AS course.
http://www.chemicool.com/forum/post-177.html 11/11/2007
I believe this is reliable as there are other posts on the forum, by the same member, which I know are correct.