The Combustion Of Alcohols Investgation: Investigate The Heat Energy In A Range Of Alcohol's Used As Fuels
Combustion of Alcohols
THE COMBUSTION OF ALCOHOLS
Investigate the heat energy in a range of alcohol's used as fuels.
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
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.
2CH3OH + 3O2(r) 2C02 + 4H20
2C2H5OH + 6O2(r) 4C02 + 6H20
2C3H5OH + 9O2(r) 6C02 + 8H20
2C4H7OH + 12O2(r) 8C02 + 10H20
2C5H9OH + 15O2(r) 10C02 + 12H20
2C6H11OH + 18O2(r) 12C02 + 14H20
2C7H13OH + 21O2(r) 14C02 + 16H20
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
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.
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Combustion Of Alcohols Essay
Combustion Of Alcohols
This investigation involves burning alcohol in the air. Key science- Chemistry by Eileen Ramsden says that " an alcohol is a series of organic, homologous compounds, with the general formula Cn H2n+1OH". The alcohol reacts with the oxygen in the air to form the products water and carbon dioxide:
Cn H2n+1OH +(n+n/2)-1O2 ? nH2O + nCO2
The structure of the molecules in this reaction is:
H H | | H - C - C - O- H + 3[O=O] ? 1/2[O=C=O] + 3[H-O-H] | | H H
This reaction is exothermic, as heat is given out. This is because the amount reactant energy is more than the product energy the difference between this is ?H, therefore some energy has been given out in the form of heat.The energy is given out when forming the bonds between the new water and carbon dioxide molecules. This can be shown in an energy level diagram: Reaction co-ordinate ?H is the heat content, which is the enthalpy, which is negative in exothermic reactions as the diagram shows that energy is 'lost' as heat. Enthalpy is defined as the energy of reaction, or the heat energy associated with a chemical change. Chemical Principles By Master & Slowinski says that "For any reaction carried out directly at a constant pressure, the heat flow is exactly equal to the difference between enthalpy of products and that of the reactants", or: Qp = Hp - Hr = ?H Where Qp is the heat flow at constant pressure, Hp is heat energy of products, and Hr is the heat energy of the reactants.
To measure ?H given off, we must use this energy to heat something, this will be water. This is assuming that all the heat produced by combustion of fuel (?H) will equal the amount of heat absorbed by the water (q). So I will measure the amount of energy required to do so. This can be worked out by useing the formula: q = mass X specific heat capacity X temperature rise 1000 Where q is the quantity of heat. The specific heat capacity is the amount of energy required to heat the substance, and is calculated using the formula q = MC?, where q is the enthalpy, is the specific heat capacity and ? is the temperature rise. I chose to use water as it is safe, easily obtainable, and has a constant, reliable specific heat capacity of 4.2J/?C.
The bonds which are made in a exothermic reaction "are forces of attraction between the atoms or ions in a substance" according to Key science- Chemistry by Eileen Ramsden. These can be of two types: covalent, in which the atoms share electrons. Examples of this are water and carbon dioxide, which has a double covalent bond because it shares two pairs of electrons are shared. The other type of bonds are ionic, where a metal is involved. This is where electrons are transferred from one ion to another, so there is an electrostatic force between the ions.
The variables that must be controlled are:
* Mass of water
* Amount of wick...
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