Except in the case of ionic reactions in solution, the enthalpy changes accompanying
mixing and separation are insignificant in comparison with the contribution from the
reaction itself. For the combustion of methane, the standard value refers to the reaction
in which 1 mol CH4 in the form of pure methane gas at 1 bar reacts completely
with 2 mol O2 in the form of pure oxygen gas at 1 bar to produce 1 mol CO2 as pure
carbon dioxide gas at 1 bar and 2 mol H2O as pure liquid water at 1 bar; the numerical
value is for the reaction at 298.15 K.