The value of the enthalpy change is obtained by heating water using the fuel which is to be tested. Then by using the specific heat capacity of water (4.2 J/G/ º C), which is the energy required to raise the temperature of 1g of water by 1 º C.
Therefore there several equations are needed to work out the enthalpy change of combustion for a fuel:
Energy transferred = Mass of water X Specific heat capacity X Temperature change
To water (g) (4.2 J/G/ ºC) (ºC)
(J)
Then to find the amount of energy transferred to the water by burning 1 mole of fuel, another equation can be used:
Moles burnt = Mass of fuel used (g) / Molecular mass of fuel (g)
Enthalpy change of combustion (KJ/mole) = Energy transferred to water (J) X Moles of fuel burnt (divided by 1000 to get kilojoules per mole)
I plan to use equipment that is as reliable as I can get and I also plan to take a number of repeats to ensure accuracy and precision in my readings.
Health and Safety
Risk assessment of fuels for chain length investigation
Ethanol- Highly flammable- Will catch on fire at temperatures above 13ºC or 286.15 Kelvin and therefore vapour and air should not be mixed if above these temperatures. Do not inhale as it is intoxicating, and it also irritates the eyes if any contact is made with them. If it is inhaled then it is best to get away from the source, rest and keep warm. If it makes contact with the eyes, it is best to rinse eyes thoroughly with water for about 10 minutes. If the problem persists then it is best to contact a doctor. If it is spilt everyone that could possibly make contact with this should be warned and also it should be mopped up by a person wearing protective clothing.
Methanol – Highly flammable and Toxic as vapour will catch on fire at temperatures of above 12ºC or 285.15 Kelvin. Do not inhale, swallow or make contact with skin- Danger of serious irreversible affects. Higher concentrations may cause dizziness, stupor and digestive disturbances.
Propan-1-ol – Highly Flammable- Has a flash point below 21ºC and may catch fire easily. It has the same health and safety precautions and spillage procedures as methanol
Butan-1-ol and other Higher Alcohols– Irritant and Harmful- irritates the respiratory system and there is a risk of damage to the eyes. Also irritates the skin and must not be swallowed. Do not inhale- vapours may cause drowsiness or dizziness and narcosis. If it does catch on fire, then it can be put out with water sprays, dry powder sprays and vaporising liquids.
For safety reasons:
Goggles must always be worn, all the way through the experiment. A fire extinguisher should be kept near-by and the room must be well ventilated.
Methanol CH3OH
Ethanol C2H5OH
Propan-1-ol C3H7OH
Butan-1-ol C4H9OH
For the extension experiment I will be using Propan-1-ol, Propan-2-ol, Butan-1-ol and Butan-2-ol.
Propan-2-ol C2H8O- Safety glasses must be worn. Same measures of first aid as ethanol.
Butan-2-ol C4H10O- To use this fuel safety glasses must also be worn and it must be used in a well ventilated area. Same measures of first aid as methanol.
Equipment
Copper Calorimeter
Heatproof mat
Accurate balance (2.d.p)
Funnel – to avoid spillages which may be dangerous when close to lit fuel burners
Thermometer (0.1 ºC divisions)
Heat shield
Clamp stand
Measuring Cylinder
Glass rod
Alcohol (fuels in burners, at least 200g of each)
-Methanol
-Ethanol
-Propan-1-ol
-Butan-1-ol
Bunsen burner
Splint
Lab coat
Distilled Water
Goggles
Diagram
Precision and Accuracy
I will work carefully as I do these investigations, not only for health and safety issues, but also for accuracy and precision. I will use all apparatus correctly as far as my understanding of them allows me to, which will make my measurements as precise as possible. This means that my repeats should almost be the same as my results, and if not they should be very close. They should also be to such an extent of accuracy that anyone else using the same procedure was to do it then results should virtually be identical. Therefore this will indicate the reliability of my measurements. The degree of precision will be limited by the apparatus I will be using using, such as the balance and the thermometer.
I will also face problems with precision when measuring temperature change. This is because there can be heat loss when heating the water with the fuel. This problem can be overcome if a heat shield is used around the calorimeter. Therefore this will ensure more precision and accuracy in my results.
In my investigation, the balance I am using to weight the masses of water and fuels has a precision error of ± 0.005g. This will affect the final results however the importance of this experimental error does depend on how large the mass of the object being weighed is. The larger the mass of the object being weighted the smaller the percentage error.
Percentage error is worked out using the following equation:
Percentage error = (error x 100) / reading
A smaller percentage error will also come from a reading with more significant figures/decimal places.
Due to these reasons I have decided to have 200g of water as this mass will have a reduced precision error than 100g.
Percentage error (%) = (0.005 x 100)/100
= 0.005%
Whereas
Percentage error (%) = (0.005 x 100)/200
= 0.0025%
Thermometers also have some inaccuracy. The thermometer that I will be using has a precision error of ± 0.05 and this is due to the fact that they have 0.1 ºC divisions. To overcome this problem I have decided to measure the temperature of the water after a 20ºC increase in the temperature, as this is a sufficient measurement and should also minimise percentage error associated with the particular thermometer.
Method
- Measure 200g of distilled water into a copper calorimeter, ensuring that the balance is zeroed before placing the calorimeter on it. (I will make sure that I start my experiment straight after weighing the fuel, as this will ensure minimum fuel vapour lost, especially for more volatile fuels.)
- Measure the initial highest temperature of the water before the experiment and record.
- Get the fuel needed in a fuel burner and find its mass on the scales, making sure that the scale is zeroed before beginning. Note down the fuel and fuel burners mass.
- After setting up the equipment as shown in the diagram (including ensuring that the draught shield is in a suitable place and minimising heat loss), light the fuel burner using a splint from the Bunsen burner and get the flame of the fuel as close as possible to the bottom of the copper calorimeter (ideally the hottest part of the flame should be touching the calorimeter). This helps reduce excess energy loss making the results more accurate. For extra precision, make sure that the wick of the burner is always the same length for each experiment (ideally 5mm).
- Stir the water continuously throughout the experiment using a glass rod so that there is no uneven heat distribution.
- After the temperature has risen by 20 ºC (this allows for a sufficient amount of fuel to be burnt) extinguish the flame using the burner lid (which will safely extinguish the fire- cuts the oxygen supply).
- When the highest temperature is reached take it’s reading looking at in from a position which ensures that your eyes are parallel to the end of the mark on the thermometer, record and work out the rise in temperature.
- Now measure the mass of the fuel again to find out how much fuel has been burnt and record all results onto a table like the one below.
- Use the formulas stated above to find out the enthalpy change of that fuel.
- Repeat this method as many times as necessary (ideally this should be done in the next two times if precision steps are followed) in order to get consistent results.
- Finally repeat the whole procedure again using the remaining fuels and if there is sufficient time left over repeat the same procedures on the isomers of propan-1-ol and Butan-1-ol.
Layout of results table
Theoretical calculations of Enthalpy Change
By using the bond enthalpy values (from the data tables), the enthalpy change of combustion can be calculated theoretically.
For example
Methanol + Oxygen Carbon dioxide + Water
CH2OH (g) + 1.5 O2 CO2 + 2H2O
C-H 413 kJmol-1
C-O 336 kJmol-1 2 x C=O 2 x O-H
O-H 464 kJmol-1
O=O 498.3 kJmol –1
C=O 805 kJmol-1
3x C-H
1x C-O + 1.5 x O=O
1x O-H
2039 kJ + 747.45 kJ 1610 kJ + 1856 kJ
2786.45 - 3466.00
Enthalpy change of combustion of methanol = -679.55 kJmol-1
It is apparent that my result differs to that of the data sheets, which shows that the Enthalpy change of combustion of methanol is -726 kJmol-1. This is because the value given in the data sheets is the enthalpy change of combustion of methanol in standard conditions, but also the fact that the exact bond enthalpies depend on the different compounds used to obtain the bond enthalpy.
After having worked out all the calculations, I can then produce my final results for my experiment like this:
Bibliography
The Enthalpy level diagram is from
Salters Advanced Chemistry, Chemical Ideas, Second Edition, Heinemann, Chapter 4; Energy Out, Energy in page 57.
The diagram of the apparatus is from the website below:
I also referred to the
Data Sheets provided for the Salters Advanced Chemistry Course
AS level Chemistry Introductory Booklet- Syllabus and Coursework Information.
Developing Fuels Activity Sheets- In particular DF1.3 comparing the enthalpy changes of combustion of different alcohols.
I gathered information about the risk assessment of different fuels appropriate to my experiment from the Laboratory Hazcards booklet.
I used this website to obtain further information about hazards of fuels I used:
