We were shown a preliminary experiment which we then used to develop our own methods. In the preliminary experiment a burner of hexane was weighed then placed under a can containing 100ml of water. The water was heated by 10ºc then the burner was weighed again. This provided the basis for my method.
Prediction and Theory
The alcohols are a series of organic homologous compounds, with the general formula Cn + H(2n+1) + (OH). Alcohols react with oxygen in the air to form water and carbon dioxide. The reaction that is involved in burning alcohols is exothermic so heat is given out.
The energy is given out when forming the bonds between the new water and carbon dioxide molecules. The amount of energy produced by such exothermic reactions can be calculated by using the formula:
Mass of the substance x rise in temp x SHC (specific heat capacity)
The specific heat capacity is the number of joules required to heat one gram of a substance by 1ºC. I will be using water which has an SHC of 4.2j/g/ºc. this means that for water, the amount of energy required to raise 1ooml of it by 20ºc will be 8400.
I predict that the more bonds there are holding the carbon, oxygen and hydrogen atoms together, the more energy that will be required to break them apart. For example Ethanol has the formula C2H5(OH). In this formula you have five C-H bonds, one C-C bond, one C-O bond and one O-H bond. To separate these types of bonds you require a certain amount of energy for each different type of bond.
To separate C-H bond you need to apply 410 joules of energy. There are five such bonds in ethanol so you multiply 410 by five to get 2050 joules. You do these calculations for all the other types of bonds that make up ethanol, add them all together and you get 3270 joules. All of the other alcohols can be broken up in this way. Below is a table showing the energy required to break up the bonds in each alcohol.
As you can see a longer molecule is, the more energy it takes to break its bonds. I can come to predict that the longer the molecular structure in the alcohol the more energy it will take to break the bonds.
I also predict that the increase in energy required will be proportional to the number of extra carbon atoms. With each new carbon atoms that you add to the chain, you also add two new hydrogen atoms. Apart from this, the chain remains unchanged. I therefore predict that the energy required to break the bonds will increase by 1170 joules for each new carbon atom.
Obtaining Evidence
Summary of Results (First Repeat)
Summary of Results (Second Repeat)
Summary of Results (Third Repeat)
Average of Repeats
Heats of Combustion
Analysing Evidence and Drawing Conclusions
Graph
Conclusion and Theory
As the graph shows, my prediction that the heat of combustion would increase with the number of carbon atoms was correct. I also did not appear to have any anomalous results in the final graph. This suggests that the results are accurate as does the smoothness of the increase in heat of combustion
This therefore confirms the theory that if there are more carbon atoms( and the associated hydrogen atoms), then there are more bonds to be broken so more energy is given out.
Evaluation
I am pleased with my experiment as it returned results to confirm my prediction. I did not observe any anomalies which suggests that the results were accurate in relation to each other (suggesting that there were no human errors) but I can be sure that they are not accurate illustrations of the amount of energy given off as the experiment had no hope of accurately measuring this. This is due to several factors:
1. Energy given off through sound and light.
2. Radiation of heat out into the atmosphere.
3. The fact that the tin can gets hot.
4. The rubber clamp transferred heat way.
5. By incomplete combustion
6. The activation energy to get the alcohol burning.
7. The availability of alcohol for the wick to burn, if not enough then the wick would burn not the alcohol which would give an inaccurate result.
8. Evaporation of water so there will be less water to heat, making the water hotter.
9. The size of the wick.
10. Not all of the water was the same temperature.
11. The flame size changed due to the type of alcohol, hence it was a different distance away from the beaker each time.
I could obtain a better picture of how the number of carbon atoms affects the heat of combustion in two ways. 1. I could extend my current experiment by testing other alcohols such as heptanol or octanol as well as by doing more repeats to increase the accuracy of my results. 2. I could conduct a similar experiment with other homologous series of hydrocarbons such as the alkanes or the alkenes. This would allow me to compare the results from each experiment and compare how the number of carbon atoms in the chain affects the heat of combustion.
