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.