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I know enthalpy is a component of the Gibb's Free Energy (along with entropy). For practical purposes, though, isn't Gibb's Free Energy what a chemist is actually interested in, as that determines the overall equilibrium of a reaction? What important detail does enthalpy inform us of and what applicable function does that detail serve?

I am asking this because I have a DFT calculation that yields both the enthalpy and free energy, and I see a paper citing the enthalpy, so I am wondering why it is being preferred over the free energy.

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While a detailed answer to this question could go on at length, the major practical distinction is the following:

  • The enthalpy change of a reaction/process indicates how much heat input or output is involved with the reaction, with a negative $\Delta H_\mathrm{rxn}$ indicating heat output.

  • As you note, the Gibbs free energy change of a reaction/process indicates whether a reaction is thermodynamically favored or disfavored, with a negative $\Delta G_\mathrm{rxn}$ indicating a favorable reaction.

Note that, e.g., a reaction/process can have $\Delta H_\mathrm{rxn} > 0$ (endothermic), but also $\Delta G_\mathrm{rxn} < 0$ (thermodynamically favored). That is, despite the reaction requiring heat input, it is still favored to occur. This is because the change in entropy of the process, $\Delta S_\mathrm{rxn}$, is sufficiently large that the $T\Delta S_\mathrm{rxn}$ term is able to overcome the positive $\Delta H_\mathrm{rxn}$. A common example is the endothermic dissolution of various salts, such as ammonium nitrate $\left(\ce{NH4NO3}\right)$.

Thus, the paper probably used the enthalpy results because they were comparing their calculations to experimentally-measured enthalpies (heats) of their process(es) of interest.

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  • $\begingroup$ Hi, thank you very much for the response! Would you mind telling me why the heat of a reaction is useful? For example, the Gibb's Free Energy tells us whether a reaction is favorable or not. We can draw conclusions about the usability of the reaction compared to an alternative reaction, as we can derive the reaction metric K. What application does the enthalpy, or rather the heat, have, that is useful to a chemist in the lab? It seems like any experimentally useful conclusion we'd like to draw from it is unreliable because the other half of the picture, entropy, could alter the interpretation. $\endgroup$ – Blaise Jun 11 '16 at 20:17
  • $\begingroup$ If you are designing a chemical reactor for a chemical processing plant, you need to perform a heat balance on the reactor (and all other equipment in the plant) so that the temperatures into and out of the various pieces of equipment are satisfactory and so that you control the energy budget of the plant. The energy budget is related to $. Also, for the reactor, you need to know how much heat you need to supply or remove. Plus, the temperature in the reactor controls the kinetics, which, in turn, determines the size of the reactor (equipment cost). $\endgroup$ – Chet Miller Jun 11 '16 at 20:27
  • $\begingroup$ I see. That's exactly the kind of direction I'm going for with my question. So just to be clear, enthalpy is a metric more useful on an industrial scale than laboratory? Any idea why a paper would cite the enthalpy over the free energy for a lab-scale reaction? I guess if someone told me the enthalpy for a lab reaction, I wouldn't know what to make of it and would prefer to know the free energy instead. I strongly suspect I am looking at this the wrong way somehow. $\endgroup$ – Blaise Jun 11 '16 at 20:34
  • $\begingroup$ @Blaisem "Enthalpy is a metric more useful on an industrial scale than laboratory." No, this is incorrect; enthalpy and free energy are useful on both scales, but the manner in which each is useful at each scale may or may not be different, depending on the application. "Useful" itself is a problematically vague term here; without knowing the specific context of interest, it's difficult to say a priori which is more "useful". $\endgroup$ – hBy2Py Jun 11 '16 at 22:34
  • $\begingroup$ @Blaisem "Would you mind telling me why the heat of a reaction is useful?" This is a fairly fundamental chemical concept; if you're unclear on it, you should consult an introductory chemistry textbook. It's certainly too broad to answer in these comments, and it may be too broad for Chem.SE in general. $\endgroup$ – hBy2Py Jun 11 '16 at 22:37

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