# Calorific value of a gas in combustion

The Wikipedia article on heat of combustion says:

The calorific value [...] may be expressed with the quantities:

• energy/mole of fuel
• energy/mass of fuel
• energy/volume of the fuel

Is the energy obtained by the combustion mainly proportional to the mass of the gas, being then values in $$\pu{J/mol}$$ and $$\pu{J/kg}$$ a characteristic of the gas composition and basically independent of the pressure and temperature?

That is, for a specific gas, values in $$\pu{J/mol}$$ and $$\pu{J/kg}$$ will be mainly independent of the pressure/temperature of the gas in combustion, while value in $$\pu{J/m^3}$$ will be strongly dependent due to the relation between volume, pressure and mass.

In the latter case, given the heating value, $$f_0$$ (in $$\pu{J/m^3}$$) for a gas at $$\pu{°C}$$ and pressure $$p_0 = \pu{101 kPa} = \pu{1 atm}$$, we could (?) find new heating value $$f$$ (in $$\pu{J/m^3}$$) at some other temperature $$t$$ (in $$\pu{°C}$$) and pressure $$p$$ (in $$\pu{Pa}$$) applying the expression: $$f = f' \frac{p}{p_0} \frac{273}{273+t}$$

• Are you familiar with Hess' Law? Oct 11 '18 at 12:55
• @ChesterMiller: a few, but I do not see now the relation Oct 11 '18 at 13:44
• The heat of combustion changes with temperature because the specific heat of the products is not equal to the specific heat of the reactants. Google Hess' Law and see how it is applied to chemical reactions. So, yes, the heat of reaction does vary with temperature. Oct 11 '18 at 14:04
• “Energy/mole of fuel” is not a quantity; it’s a mixture of quantities and units. The correct quantity would be “energy per amount of fuel” or $E/n$, which could be expressed in J/mol.
– user7951
Oct 15 '18 at 10:58