The discussion focuses on the physical mechanism of oleum fuming, specifically the reaction of dissolved SO3 with air humidity, which occurs exothermically on the liquid's surface. The mist formation is attributed to the high concentration of SO3 above the oleum, leading to condensation in the gas phase rather than dissolving the produced H2SO4. The enthalpy of the reaction is approximately 160 kJ/mol, and the boiling point of sulfuric acid is around 300°C at atmospheric pressure. The conversation emphasizes the importance of understanding the "acid to water, never water to acid" rule in relation to this phenomenon.
PREREQUISITESChemists, chemical engineers, and safety professionals involved in handling oleum and sulfuric acid, as well as students studying physical chemistry and thermodynamics.
sludger13 said:Why does the mist occur instead of dissolving created H2SO4 in the solution?
You've pretty much answered your own question here.sludger13 said:SO3 reacts very exothermically with some air humidity
No, that is too simplified.Bystander said:You've pretty much answered your own question here.
Cp(H2SO4, liq) ~ 60 J/molK, and the enthalpy of the reaction between SO3 and water vapor at the surface is around 160 kJ/mol. Fume? You bet you --- that's the old "acid to water, never water to acid" rule from HS chemistry.sludger13 said:Or sulfuric acid with (~300°C, p(atm)) boiling point?
That makes me more sense, thank you.Borek said:My bet is that the concentration (partial pressure) of SO3 above oleum is high enough to start condensation in the presence of water vapor. That will happen above the surface, in the gas phase.
When water putted into acid or conversely acid putted into water, the liquid may spit around due to gas expansion, as I see. Also that is different physical process than oleum fuming.Bystander said:Fume? You bet you --- that's the old "acid to water, never water to acid" rule from HS chemistry.