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(Note I've refocused a previous unanswered question into a more general question. You can find the original much more specific question here) There seems to be two main ways of looking at the enthalpy of substances: molar enthalpy and heat of formation. What are the differences/advantages and disadvantages of each and are either one or more or less suitable for different purposes?

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Molar enthalpy is the energy released when the substance undergoes combustion. It is essentially the energy required to fragment the substance into its component sub-compounds.

Heat of formation is the theoretical difference between the energy each atom typically has at STP and the total energy that the compound actually has. Essentially, it is the energy that goes into formulating the bonds between the atoms.

Molar enthalpy is generally used a lot more in experimental chemistry because it can be measured directly, and it gives useful values on the energy that can be extracted from a substance.

Heat of formation is preferred in theoretical chemistry, as it is the theoretical maximum amount of energy able to be extracted in perfect conditions.

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  • $\begingroup$ This confuses me because for example the CHEMKIN thermodynamic data base (www3.nd.edu/~powers/ame.60636/kee2000.pdf) for many things that don't seem 'combustible' in that there is no real obvious 'sub-components' for them to break into. $\endgroup$ – J-S Jul 15 '15 at 23:07
  • $\begingroup$ For example, KCl, Na+, O+, O-, O etc.. Many of these things literally can't break down so I don't see how this can be right.. $\endgroup$ – J-S Jul 15 '15 at 23:09
  • $\begingroup$ The document refers to these values as: 'Standard State Enthalpies and Entropies at 298 K.' $\endgroup$ – J-S Jul 15 '15 at 23:09

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