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Investigate the heat energy in a range of alcohol's used as fuels.

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Introduction

THE COMBUSTION OF ALCOHOLS

Aim:
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

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Middle


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. The equation for working out the energy released is:

Heat released = (-574 x No. of carbon atoms) x 2024

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.

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Conclusion

EVALUATION
The results I got from my experiment were accurate enough to give me results I can relie on. I could have got more accurate results by modifying my plan for my experiment. When I was carrying out my experiment I saw four main things that could be improved to make the results more accurate.
1. Heat which never enters the water, because of draughts, for example.
2. Heat loss from the top and sides of the beaker.
3. Heat which is not conducted by the beaker.
4. Incomplete combustion - there is a restricted supply of oxygen, the alcohol was burning with an orange flame rather than blue. Some of the alcohol did not burn completely, giving carbon and carbon monoxide rather than carbon dioxide. A carbon deposit (soot) on the bottom of the beaker indicated this.

Improvements can be made to this by insulating the sides of the beaker, using a different material for the beaker, using a lid a providing a draught screen as shown below.

Alternatively we can remove all faults in planning by using an advanced technique such as a bomb calorimeter. This is the most accurate way of measuring bond energies and this will be as accurate as we can get in our results.

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