Hexane
C6H14 + 9.5O2 6CO2 +7H2O
The Standard enthalpy change for is:
-4163 KJ Mol-1 for Hexane
-4816.9 KJ Mol-1 for Heptane.
These are the results we obtained from the experiment and used these formulas to work out the amounts of energy produce by these Alkanes.
E=MCDT and waters specific heat capacity as 4.18KJ Mol-1
Pentain C5H12+8O2 5CO2+6CO2+6H20
Net energy change =155983+-3270KJ/mol
Paraffin C10 H22+20CO2 +22H2O
Moles of Heptane = 1.46/100 = 0.0146 this is 1/68th of a mole.
Moles of Hexane = 1.43/86 = 0.0166 this is 1/60th of a mole.
Conclusion
My calculated values are well below that of the theoretical ones, this is party because of experimental errors that I will comment on later but the major factor was that the compounds did not combust completely, there was soot in the bottom of the conical flask therefore the alkenes when broken down did not form CO2 and H2O as expected therefore the carbon atoms did not covalently bond with the oxygen and not as much energy was released. Paraffin released the most energy this is because it has the most bonds. There was more energy produced because there are more bonds to reform.
The theoretical values of Heptane and hexane were quite close so I expected my results to reflect this, which they did; this was because the theoretical difference in energy released -674.55 KJ mol-1, which is a relatively small amount. This amount is almost identical to the difference in enthalpy change between these to alkenes so if I was to estimate the energy release by the next alkenes which is octane I would estimate it as –5254.25 KJ Mol-1 so I would expect is if repeated my experiment again under the same conditions octane would about –867 KJ Mol-1.
This is because hexane has one less carbon atom and two less hydrogen atoms than Heptane and, as this is a recurring pattern and that the energy to make and break any C-C bond is the same you would expect a predictable rise.
Evaluation
During this experiment no seriously hazardous materials were used so a minimum amount of safety equipment but what was used is listed in the equipment list. The procedure was seriously flawed as there was many ways for the heat energy to escape. There was nothing restricting wind therefore the flame was not concentrated on the centre of the conical flask which itself became covered in soot blocking some of the heat energy, due to the conical flask being made of glass this also absorbed some of the energy. Although we weighted the burners before and after the experiment the compounds evaporate very easily in air, which may have affected this weight as well as the five-second period in which the flame was still burning this would reduce the accuracy of the overall experiment. The easiest way to remove the limitations would be to use a sealed burner unfortunately we still have to use a spirit burner. There we two major anomalies in this experiment but both were due to human error more careful procedures would eliminate this, all in all with the improvements mentioned above and completing the experiment in standard conditions would the accuracy of these results would be greatly improved.
