My question is where do the values of the std heats of formation come from? My book emphasizes that the "reaction" to form a substance from its constituent elements can be (and often is) hypothetical. "We can't combine carbon and hydrogen in the laboratory to make methane, for instance, yet the heat of formation of methane is still defined as the enthalpy change for the hypothetical rxn:
C(s)+2H2 (g)==CH4(g) delta H= -74.8 kJ
If we can't perform this reaction in the laboratory, where does the -74.8 kJ come from?

Question

My question is where do the values of the std heats of formation come from?  My book emphasizes that the "reaction" to form a substance from its constituent elements can be (and often is) hypothetical.  "We can't combine carbon and hydrogen in the laboratory to make methane, for instance, yet the heat of formation of methane is still defined as the enthalpy change for the hypothetical rxn:
C(s)+2H2 (g)==>CH4(g)   delta H= -74.8 kJ
If we can't perform this reaction in the laboratory, where does the -74.8 kJ come from?

anonymous · Answer

You can do several other reactions which are possible and which add up to the formation reaction.  Remember Hess's Law: you can add and subtract chemical equations to get a net reaction, and equivalently add and subtract their heats of reaction to get the heat of reaction of the net reaction -- whether you can run that reaction or not.

anonymous · Answer

thanks!