Timeline for Why does the standard enthalpy of formation diverge so far from the standard Gibbs free energy of formation for some substances?
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| Jun 10, 2019 at 5:56 | history | edited | mhchem | CC BY-SA 4.0 |
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| May 7, 2017 at 17:38 | answer | added | porphyrin | timeline score: 1 | |
| May 7, 2017 at 13:34 | comment | added | Maximus1115 | I realize I'm being rather stubborn, but I'd like a conceptual explanation for why some substances have widely varying degrees of ΔH∘f and ΔG∘f. Since the difference reduces to −TΔS, what is it about some substances that makes the −TΔS special as compared to 95+% of other substances? Merely citing equations is one thing, but bereft of any conceptual explanation to illuminate these anomalies and give flesh to the mathematical skeleton, I still feel the need to understand why. If there truly is no conceptual explanation that exists, I guess it's my loss. Thanks for your patience. | |
| May 7, 2017 at 9:29 | comment | added | porphyrin | It is not due to any reactivity a molecule may have but just the entropy. The absolute entropy $S^{\mathrm O}$ is normally obtained from calorimetry as the sum of the entropy contributions from $0$ K to $298.16$ K and this means integrating as $\int C_P/T$ between phases plus $\Delta H/T$ for any solid phase transitions as well as melting and vaporisation transitions as necessary. | |
| May 7, 2017 at 2:18 | comment | added | Maximus1115 | I think I didn't say that right, but is the overall idea correct? | |
| May 7, 2017 at 2:04 | comment | added | Maximus1115 | Thanks. However, saying that H and G measure different quantities doesn't explain why they are so extremely similar for most substances on the standard data tables. Thinking about it more myself, this is what I have: given that the system in question is at constant temperature, where does the difference in $\Delta S$ come from? Mostly, the work done by a chemical reaction is minimal, but some nitrates are used in explosives, which do work by expanding rapidly. This is what separates some nitrogen compounds from other substances and leads to differences in H and G on thermodynamic tables. | |
| May 5, 2017 at 15:49 | comment | added | porphyrin | Welcome to Chemistry Stack Exchange. The derived quantities H and G measure different things; have a look at their definitions. The difference is the energy $-T\Delta S$ , when this is large so is the difference between enthalpy and Gibbs energy. Nothing particular to nitrogen. | |
| May 5, 2017 at 15:38 | review | First posts | |||
| May 5, 2017 at 16:45 | |||||
| May 5, 2017 at 15:34 | history | asked | Maximus1115 | CC BY-SA 3.0 |