# Combustion of Alcohols

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• Published : September 7, 2011

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PROBLEM
Investigate the heat energy in a range of alcohol's used as fuels. HYPOTHESIS
I predict that octanol will release the most heat energy. This is because there is more bond energy in that molecule than the other alcohols. Within a molecule there are bond energies that are holding the atoms together. When the fuel combusts a chemical reaction takes place, this breaks the bonds, this requires energy, and makes new bonds this gives out energy. The energy differences between the two tell us how much energy was given out or taken in. We can show this on a graph. ENERGY CHANGES DURING A REACTION

To find the bond energies in the molecule of the alcohol we have to look at the how much energy is in the separate bonds of the molecule. Below we have a table of bond energies. Bond Bond Energy (kj/mol)

C - OH 402
C - H 435
C - C 347
H - O 464
C = O 805
If we draw out the structure of each molecule involved in the chemical reaction we can easily find out how much energy is in that molecule. STRUCTURE OF THE MOLECULES INVOLVED
METHANOL

ETHANOL
PROPANOL
BUTANOL
PENTANOL
HEXANOL
HEPTANOL

OCTANOL
CARBON DIOXIDE

WATER

BALANCED EQUATIONS
If we work in the bond energies into these balanced equations, and we subtract the energy taken in by the breaking of the bonds from the energy given out by the formation of new bonds we will get the total energy released. METHANOL

2CH3OH + 3O2(r) 2C02 + 4H20
ETHANOL
2C2H5OH + 6O2(r) 4C02 + 6H20
PROPANOL
2C3H5OH + 9O2(r) 6C02 + 8H20
BUTANOL
2C4H7OH + 12O2(r) 8C02 + 10H20
PENTANOL
2C5H9OH + 15O2(r) 10C02 + 12H20
HEXANOL
2C6H11OH + 18O2(r) 12C02 + 14H20
HEPTANOL
2C7H13OH + 21O2(r) 14C02 + 16H20
OCTANOL
2C8H15OH + 24O2(r) 16C02 + 18H20

We can see from the table above Octanol releases the most heat energy. This clearly shows that there is a correlation between the number of bonds in the molecules and how much total energy was released. I also predict that the amount of heat energy released will increase with the number of carbon atoms in the alcohol. I can show this on a graph. This graph shows us that the number of carbon atoms in the alcohol is directly proportional to the heat release. The line is a straight line and we can make a liner equation for this that links the number of carbon atoms to the heat released. This is, Heat released = (-574 x No. of carbon atoms) x 2024

VARIABLES
My variables in my experiment will be the temperature of the water in the beaker and the mass of alcohol burnt. Things such as amount of water and how much the alcohol raises the water temperature must be controlled. FAIR TEST

To keep this a fair test we have to bear certain aspects in mind. The beaker the water is contained in must be the same shape because if it is not the flame may have more surface area of where to heat the water. The alcohol must be weighed accurately with scales that weigh up to, least, one decimal point. During weighing the spirit lamp must be covered to avoid and evaporation of the alcohol. The alcohol has to be weighed accurately before and after the experiment. The alcohol has to be blown out immediately when the water temperature has been raised 30 degrees; it must be covered after the experiment to avoid evaporation. The thermometer must be swirled around the water before a reading can be taken, this insures that you are measuring the temperature of the whole water not just the bottom of the beaker. The shape of the spirit lamp must stay the same and so must the wick length. If all this is done we can ensure that we will get an accurate reading.

DIAGRAM

METHOD
1. Set up apparatus as diagram.
2. Weigh the spirit lamp + alcohol on scale note down weight. 3. Note down the water temperature.
4. Light the wick and let the alcohol heat up the water until it is raised by 30 degrees. 5. Blow out the flame and weigh the alcohol + spirit lamp immediately. Note down weight. 6. Work out the mass of alcohol burnt by subtracting the weight before from...