THERMODYNAMICS is a subject that makes smart men quail and lesser men run to the nearest exit.
However, we live in a world increasingly dominated by energy use and creation, so that I have invented a simpler calculation not requiring more than a page to explain.
The energy of any fuel or Cc-Hh-Oo compound (where c, h and o are the numbers in the compound formula) can be calculated using the following rules and exceptions:
Carbon, C= 420kJ/mole
Hydrogen= 120 kJ/mole
Oxygen = – 90 kJ/ mole (minus, since it represents a partial combustion).
These numbers will predict the heat of combustion of all “normal” C-H-O compounds with better than 15% accuracy! And that is about the limit to which many of them are known.
EXAMPLES: Methane, CH4, = 420 + 4(120) = 900 kJ/ mole. (889) Methanol CH4O = 420 + 4(120) – 180 = 720 kJ/mole (726) (The numbers in () are the official vales.).
The above applies to all “normal” CHO compounds and fuels.
It does NOT apply to compounds like acetylene, (super unstable) or carbon monoxide or hydrogen gas, H2, which I call “extra energetic (“hyperenthalpic”). They are actually 280 kJ/mole, rather than the 240 predicted by the above rules.
It does NOT apply to CO2 and H2O.
SUPER stable compounds. They would predict the heat of combustion as 60 kJ/ mole, but they are the “Zero” energy of our chosen scale. I call them “hypoenthalpic”.