I tested the burning efficiency of various alcohols, making the independent variable the molar mass of the alcohol. I did the experiment, and concluded that as the molar mass increases, so too does the enthalpy of combustion, and hence efficiency as a fuel. I was just wondering why the molar mass affected the enthalpy.

Unfortunately fuel efficiency is typically expressed as energy released per gram but you are reporting the energy released per mole (which is, after all, the definition of "enthalpy of combustion"). I think you will find, when you change units from "per mole" to "per gram," that your conclusions will reverse themselves.

As to why the molar enthalpy of combustion rises with molecular weight... common mono-alcohols (did you burn, for example, ethylene glycol or propylene glycol?) are entirely carbon and hydrogen other than the OH group. So the higher the molecular mass, the more carbon and hydrogen there is to combine with oxygen. The more carbon and hydrogen there is to combine with oxygen, the more energy will be released.

I hope this is helpful.

Dan Berger

P.S. Another way to check what I've been saying here is to restate your enthalpies as being per mole of oxygen. That controls for molecular mass the same way that stating "enthalpy per gram of fuel" does.