Thermochemical data are usually quoted kJ*mol^-1kJ⋅mol−1...and what do we mean by mol^-1mol−1...we mean "per mole of reaction as written"per mole of reaction as written
And so for a simple combustion reaction....we might write...
CH_4(g) + 2O_2(g) rarr 2CO_2(g)+2H_2O(l)+890*kJCH4(g)+2O2(g)→2CO2(g)+2H2O(l)+890⋅kJ
(of course I did not know the value off the top of my head!)
And so HERE DeltaH_"combustion"^@"(methane)"=-890*kJ*mol^-1
By which we MEAN PRECISELY THAT the "COMPLETE COMBUSTION" of one MOLE of methane with two mole of dioxygen gives TWO MOLE of carbon dioxide gas, and TWO MOLE of liquid water PLUS AN ENERGY of 890*kJ. Do you see from where I am coming?
Now of course, since we know the molar masses of products and reactants, I could convert that kJ*mol^-1 into kJ*g^-1 of fuel, or if I am a real masochist into kcal*mol^-1, or kcal*g^-1.
So the take home message is that kJ*mol^-1 implies a "MOLE QUANTITY" of a specific reaction of reference. You have to be sufficiently flexible to recognize WTF that reaction is, and write it out.
