How do I calculate Energy (kcal/g) if I know the Chemical formula, the kcal/mol, and the molar mass?

1 Answer
anor277
May 7, 2018

The question is a bit open-ended...

Explanation:

Thermochemical data are usually quoted #kJ*mol^-1#...and what do we mean by #mol^-1#...we mean #"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*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.

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