[5601710051]

Hi
My experiment is investigating the difference in heat of combustion between alkanes and alcohols. I used hexane and ethanol.
Theoretically, hexane should have a higher heat of combustion than ethanol. However, my results show the opposite trend.

We used calorimetry and spirit burners to obtain the results. We tested 100% hexane and 100% ethanol, as well as a variety of different ratios of the two fuels. We had 2 trials of each fuel ratio, and averaged the results. (There were no anomalies in our trials - both trials always produced similar results).

Here's a graph of the results: https://imgur.com/gallery/f6AAvGT

Can you please help me to understand these results, and why the experimental trend does not match the theoretical trend? Is it just a human error/experimental design flaw? Or is there some 'chemistry' here that I don't understand?


Extra info: To calculate the results:
We used q=mc∆t, converted to kJ, divided by the mass of fuel used (initial weight of spirit burner - final weight) to obtain Heat of Combustion measured in kJ/g
Theoretical results were obtained online, and verified by a number of different websites.

(Also, I have already read lots of previous Heat of Combustion posts, including this excellent one: http://www.chemicalforums.com/index.php?PHPSESSID=dord2gjf6f2m1hf3rkmd0chll6&topic=77107.msg281123#msg281123
However, I still don't know)

[Enthalpy]

Experimental error. The heat of combustion is about 44MJ/kg for all non-cyclic alkanes except the very lightest.

[5601710051]

Yes, hexane's theoretical result is around 48kJ/g. We expected to get lower experimental results, as heat would be lost to the environment. But the percentage of heat lost should be pretty consistent across all ratio's tested, so the trend in our data should still be fairly reliable.

What has confused me is that there can't have been a massive error in the data collection, because it wasn't a one-off anomaly. All of our different ratios follow the same trend (more ethanol = more heat), but this is the opposite of the expected results (more hexane = more heat).

So I don't understand why our results have produced this trend.

[Borek]

Can you describe your procedure/experimental setup?

[Enthalpy]

Too little oxygen available?