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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: $$f = f' \frac{p}{p_0} \frac{273}{273+t}$$

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  • $\begingroup$ Are you familiar with Hess' Law? $\endgroup$
    – Chet Miller
    Oct 11 '18 at 12:55
  • $\begingroup$ @ChesterMiller: a few, but I do not see now the relation $\endgroup$
    – pasaba por aqui
    Oct 11 '18 at 13:44
  • $\begingroup$ 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. $\endgroup$
    – Chet Miller
    Oct 11 '18 at 14:04
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    $\begingroup$ “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. $\endgroup$
    – user7951
    Oct 15 '18 at 10:58
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Calorific values are usually quoted under standard conditions making the distinctions a matter of preference and convenience

The apparently different units used in quoting heat of combustion are not as different as they appear. We need to know heat of combustion for many reasons (eg how to bill domestic customers for the correct amount of heating or cooking their gas supply delivers). But the units used are all equivalent once we define a standard set of conditions when measuring them.

For example, a mole of pure methane at standard temperature and pressure (STP) occupies 22.4 L and will weigh about 16g and will burn in air releasing about 890kJ of energy. This means that, if we stick to the same conditions (STP in this case) we can interconvert the energy/mol, energy/mass and energy/volume with a simple calculation.

Which one we choose to use is a matter of convenience. Domestic gas meters might choose the energy/volume as they measure volume flow at a standard pressure and temperature. Liquefied gas ships may choose to measure by mass as the methane is mostly liquefied and the mass may be easier to measure than the volume, temperature or pressure (but, knowing the mass, we can work out the volume at STP to work out how much domestic gas can be produced). If a chemist is working on some reaction scheme, then they will probably prefer molar metrics as they make balancing the equations easier.

But the essential point is that a given amount of substance always has the same heat of combustion. How you measure the amount is a matter of convenience.

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