Timeline for Why is octane more volatile than water while having a higher boiling point?
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19 events
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| Oct 10, 2023 at 9:43 | history | edited | Poutnik | CC BY-SA 4.0 |
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| Oct 10, 2023 at 9:36 | comment | added | Poutnik | That too, it is quite high for water. Like 2 effects of 1 cause ( pM/RT and ΔH_evap,spec) of the molecular mass. | |
| Oct 10, 2023 at 9:35 | comment | added | Buck Thorn♦ | Ok, then a specific enthalpy of evaporation seems the right thing to compare, since intermolecular interactions (vdW specifically) strengthen with molecular size in absence of H bonds. | |
| Oct 10, 2023 at 9:32 | comment | added | Poutnik | @BuckThorn I mean, you see 2 comparable drops of water and n-octane and you do not say "Look, the n-octane drop contains 8 times less molecules.". You say "Look, the n-octane drop has similar volume and mass as the water one.". // I also assume that in pseudostatic scenario is not the determining factor the rate of bidirectional mass transfer between phases, but mass concentration of saturated vapor. | |
| Oct 10, 2023 at 9:25 | comment | added | Buck Thorn♦ | I don't see where perception comes in. The two drops being compared containing the same volume, and roughly same mass and density? Is that the idea? And, evaporation of a drop with the same surface area, ie volume, but bigger (heavier) molecules is expected to be faster? | |
| Oct 10, 2023 at 7:35 | comment | added | Poutnik | @BuckThorn But our perception is based on volume and mass, not amount. | |
| Oct 10, 2023 at 7:27 | comment | added | Buck Thorn♦ | I took another look at the answer and have trouble understanding this: "The liquid with higher molar mass evaporates faster than a liquid with the same vapor pressure but lower molar mass." The mass in the vapor is greater for a larger Mw, but the amount, as described by the molar density (or concentration), is the same at a given pressure. | |
| Oct 7, 2023 at 5:15 | comment | added | Buck Thorn♦ | Also, other posts refer to area or number of molecules. This is also a kinetic effect. I think your thermodynamic description is the right one. | |
| Oct 7, 2023 at 5:14 | comment | added | Buck Thorn♦ | The OP does not refer to volatility in terms of a kinetic effect (maybe a shortcoming with the q in not being specific): "Volatile substances have higher saturated vapor pressure at a given temperature". Therefore the existing concentration of water in the vapor should be irrelevant to whether it is more or less volatile. | |
| Oct 6, 2023 at 16:10 | comment | added | Poutnik | @jimchmst Sure, I have let it there for completeness. I have originally thought there is bigger difference until I have compared the values. I know there isTrouton's_rule of about constant molar entropy of evaporation 95-88 J/k/mol, but I have thought there are deviations for highly polar liquids with hydrogen bonds like water. | |
| Oct 6, 2023 at 15:58 | comment | added | jimchmst | Since molar enthalpies are similar it is the Number o molecules that is pertinent | |
| Oct 6, 2023 at 11:00 | history | edited | Poutnik | CC BY-SA 4.0 |
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| Oct 6, 2023 at 10:51 | comment | added | Poutnik | @NicolauSakerNeto I do agree, assuming the effect is on the order molar mass > vapor saturation > evaporation enthalpy. The mentioning order was not meant as order of importance. I sil reorder that. | |
| Oct 6, 2023 at 10:40 | comment | added | Nicolau Saker Neto | I suspect the molar mass difference really is the biggest factor at work here. Assuming we're talking about the vapour phases as ideal gasses (which we are), the natural quantity to talk about is number of molecules. Two puddles of water and octane with the same volume (what "feels intuitively right" to compare) actually refers to amounts of molecules which differ by a factor of $\mathrm{\frac{114.23}{18.02}\frac{0.997}{0.703}=8.99}$ times - there is simply just much less octane to evaporate. | |
| Oct 6, 2023 at 6:41 | history | edited | Poutnik | CC BY-SA 4.0 |
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| Oct 5, 2023 at 16:51 | history | undeleted | Poutnik | ||
| Oct 5, 2023 at 16:51 | history | edited | Poutnik | CC BY-SA 4.0 |
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| Oct 5, 2023 at 16:45 | history | deleted | Poutnik | via Vote | |
| Oct 5, 2023 at 16:42 | history | answered | Poutnik | CC BY-SA 4.0 |