Energy changes
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I have done a experiment to compare the enthalpy changes of combustion of different alcohols. However, my results are all lower than the data book values. I thought they were meant to be higher, due to heat loss, etc. Why is this?
Igloo writes
It is important to avoid ambiguous words such as “lower”, when describing exothermic energy changes. I presume by “lower” you mean less exothermic, i.e. less negative.
Heat loss to the surroundings will result in less energy being supplied to the water being heated up, and this in turn means that you will obtain less exothermic values for your enthalpy changes of combustion.
Another significant error in this experiment is that the alcohol being burnt does not combust completely, but tends to give products such as carbon monoxide and carbon, the latter causing your flame to be yellow and sometimes smoky. This becomes an increasingly serious problem as the molar mass of the alcohol increases and is a difficult one to overcome, but, once again, the consequence of incomplete combustion is that you will obtain an answer which is less exothermic than you expected.
In summary then, the two major errors in this experiment work in the same direction, and give you a less exothermic (less negative) answer than the theoretical answer taken from the Data Book. So your “lower” answers are to be expected.
Risk assessment
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For further guidance see our tutorial on Risk Assessment.
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updated: 29 February 2004
