These bond energy values in the above table are only averages. This is because not all atoms the same they contain the same number of protons but some contain different numbers of neutrons. These atoms with different numbers of neutrons are called isotopes. These isotopes will exert a slightly higher or lower gravitation attraction on their electrons. The molecular shape also affects bond energies. In higher length carbon chain molecules isomers are formed. Isomers are compounds that have the same chemical formula but have a different structural formula. It is the different structure that affects the bond energies.
The alcohols chosen for this experiment are the first four in the functional group. These alcohols starting with the shortest length chain are Methanol, Ethanol, Propanol and Butanol. The first three alcohols only exist in one form and the fourth only exists in two. These substances are extreamly flammable therefore have to be treated with caution. Methanol is also toxic so its use will have to be limited to yhe fume cupboard. This makes doing the experiment more difficult but safety must come first.
Another reason why these alcohols were chosen over higher carbon chain length alcohols is because as the carbon chain length increases the oxygen demand for its complete combustion also increases. Each mole of oxygen required is 24dm3. This is the standard volume that all gasses occupy at room temperature and pressure (R.T.P)
It was decided that the first four alcohols in the organic family would be used as the volume of oxygen required by the fifth to completely combust is more than the air can supply. This means that another source of oxygen would be needed for example a liquid oxygen tank. This is too expensive to be carried out in a school laboratory.
A theoretical value for the enthalpy change of combustion of each of the four alcohols can be worked out. This involves using the number and type of bonds and their corresponding bond energies. First the broken bonds are taken and worked out then the bonds that are made. As the bonds that are made give out energy this value is taken away from the value of the breaking bonds. This results in a negative number if the reaction is exothermic or a possitive number if it is endothermic.
The energy released, as the carbon chain gets longer should increse. This is because there are more bonds to break and therefore more are made, this releases more energy. Between each alcohol and next the same number and type af atoms are added, this results in the same number and type of new bonds being formed. From this a prediction could be arrived at, that the energy released increases by the same amount between each alcohol.
Methanol
2786.45 – 3466 = -679.55 KJ/Mol-1
Ethanol
- - 6004 = -1297.1 KJ/Mol-1
Propanol
6627.35 – 8542 = -1914.65 KJMol –1
Butanol
8547 –11080 = -2532.2 KJ Mol –1
These theoretical values for the enthalpy change of the selected alcohols are shown in the table below.
The difference in the values shown, shows that the bond energy values used were only averages.
An initial experiment was set up to investigate ways in which the experiment could be carried out in a more accurate and fair way. From this preliminary experiment it was determined that water would be placed in a copper calorimeter and placed above the spirit burner, ceramic heat mats would be used as draught excluders. It was also decided that the thermometer would be clamped with the bottom in the middle of the water and a stiring rod would be used to stir the water. This was so that the thermometer was not in contact with the calorimeter because if it were it would give the temperature of the calorimeter not the water. A copper calorimeter was chosen over a glass one because it conducted the heat through and into the water quicker and better. This was found when the two systems were tested. The possitioning of the copper calorimeter was chosen to be above the spirit burner instead of aside it, because heat rises so there would be a greater degree of error if it were placed to the side.