Comparing the enthalpy changes of combustion of different alcohols.

Mass of water (x) Rise in temp (x) Specific heat capacity

Energy transferred to water        = 200 x 15 x 4.2

                                = 12600 J

I now need to calculate the number of moles in each alcohol so that I can find out the enthalpy change of combustion for the alcohols using this formula:

Enthalpy = Energy transferred to water (/) Moles of fuel burnt

To do this I will use the formula:

Number of moles = Mass Burnt (/) Relative molecular mass

Example of Complete Alcohol Calculation

Eg. Methanol

Enthalpy Change of Combustion        = Energy transferred to water / Moles of fuel burnt

                                        = 12600 J / 0.06125

                                        = 205714.29 (2.d.p)

                                        = / 1000 for kJ = 205.71 kJ mol-1 (2.d.p)

Table for all enthalpy changes

Conclusion

        As my graph shows, as the number of carbon atoms in the alcohol molecules increase, so does the enthalpy change of combustion. This is because the more carbon atoms, and therefore hydrogen bonds, that the molecule possesses, the more energy is needed to break the larger amount of bonds.

Evalutation

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.

Bibliography

Developing Fuels (Salters) – Enthalpy Change of Combustion Experiment.

Chemical Ideas (Salters) – Enthalpy and Mole Calculations.

Coursework Guidance – OCR.