When an alcohol burns it reacts with Oxygen to produce CO2 and Water. For Example, Methanol:
Methanol + Oxygen Carbon Dioxide + Water
CH3OH + O2 CO2 + H2O
When this reaction occurs, chemical bonds are broken. Energy is needed for this to happen. New bonds are formed between the atoms in the products. When this happens, energy is released in the form of heat and light.
This table displays the total number of each bond type for each side of the reaction. The overall heat changes are negative numbers, this means that all these are exothermic reactions. This is because the products give out more energy than the reactants took in. The overall heat change shows the kJ of energy produced form burning one mole of the alcohol.
As the size of the alcohol molecule increases, the energy released also increases.
I predict that Hexanol would give out even more energy than Pentanol.
Hexanol + Oxygen Carbon Dioxide + Water
C6H13OH + 9 O2 6 CO2 + 7 H2O
This table displays the hypothetical details
of the combustion of Hexanol.
My prediction was correct. As the size of
the molecule increases, the energy produced
also increases.
This table shows exactly how many kJ of energy were produced during the experiment.
When we lit the spirit burner, we supplied the energy to start breaking bonds. As new bonds were formed between the products, heat was given out. The reaction of Pentanol yielded most energy, so it was able to heat the water most efficiently.
This graph shows the energy levels during the experiment.
- Reactants at constant temperature.
- Sudden increase in heat created by lighting the wick of the spirit burner.
- Energy level required to start reaction is reached.
- Reaction starts. Heat is given out resulting in a decrease of energy in the reactants.
- Reaction stops because the flame is blown out. Products at constant temperature.
There are some factors that may have made our results less accurate. Firstly, we didn’t always make sure that the test tube of water was the same distance from the flame.
Secondly, we didn’t always check that the water was at the same temperature every time we conducted the experiment.
These factors could have amounted to a poor set of results. If I repeated the experiment, I would make sure that there was plenty of oxygen under the sheet of aluminium foil because at one stage of the experiment there was some incomplete combustion. There wasn’t enough oxygen to make the carbon dioxide, resulting in carbon soot forming on the test tube.
I would also test substances, other than alcohols.
