Newsgroup: it.scienza.chimica
Titolo: Formazione di CH4

Ho due quesiti da porre al NG; il primo è questo, che è più che altro
una curiosità.
L'altro, più serio, lo scriverò in altro post.
 
Vorrei sapere come può formarsi in ambiente non biotico (in realtà
extraterrestre) del metano dagli elementi C e H.
Provo a dire le ipotesi che mi riesce di fare, per essere corretto,
smentito o (molto meno probabile) confermato: -)
 
In ambiente interstellare c'è abbondanza d'idrogeno, credo sia in forma
atomica che molecolare.
Il carbonio è meno abbondante, ma non manca.
 
Tuttavia mi pare si possa escludere un reazione diretta del tipo
C + 2 H2 --> CH4
su atomi di C isolati, perché non vedo come potrebbe essere smaltita
l'energia che si libera.
(Tra parentesi: quanta è? Non ho cercato ma ritengo si tratti di
alcuni eV. Per i chimici, che usano meno gli eV: 1 eV per molecola =
100 kJ/mol all'incirca.)
 
Però è probabile che il C elementare sia già aggregato in microgranuli
(che però non so come si possano formare...).
In questo caso la reazione di cui sopra non presenterebbe problemi,
anche se credo avverrebbe in passi successivi di idrogenazione degli
atomi di C alla superficie del granulo.
L'energia di cui sopra si distribuirebbe fra tutti gli atomi del
granulo come vibrazione e quindi in ultima analisi andrebbe solo ad
aumentare leggermente la sua temperatura.
 
Quante ne ho dette? :-)
Thx
-- 
Elio Fabri
 

 100 kJ/mol all'incirca.)
 
L'entalpia standard di formazione del metano è -74.81 kJ/mol. Per 
confronto entalpia di combustione è -891 kJ/mol.
 
-- 
Saluti
VITRIOL

 Thx
 
 
-- 
1) Resistere, resistere, resistere.
2) Se tutti pagano le tasse, le tasse le pagano tutti
Soviet_Mario - (aka Gatto_Vizzato)
 
 
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 qualche caso meno generale alla volta
Scusate entrambi il grande ritardo, ma sono sempre affogato.
Non saprei circoscrivere: era una domanda generica, solo per capire
dove e come può formarsi metano da processi abiologici.
Comunque su questo argomento non insisto; era più che altro una
curiosità.
Il prossimo mi preme di più.
 
 
-- 
Elio Fabri
 

 dove e come può formarsi metano da processi abiologici.
 
Sono ancora molte le cose che non si sanno. Prova a leggere:
 
 
https://en.wikipedia.org/wiki/Atmosphere_of_Titan#History
 
 
   The presence of a significant atmosphere was first suspected by
   Spanish astronomer Josep Comas i Solà, who observed distinct limb
   darkening on Titan in 1903,[1] and confirmed by Gerard P. Kuiper in
   1944 using a spectroscopic technique that yielded an estimate of an
   atmospheric partial pressure of methane of the order of 100 millibars
   (10 kPa).[2] Subsequent observations in the 1970s showed that Kuiper's
   figures had been significant underestimates; methane abundances in
   Titan's atmosphere were ten times higher, and the surface pressure
   was at least double what he had predicted. The high surface pressure
   meant that methane could only form a small fraction of Titan's
   atmosphere.[3] In 1980, Voyager 1 made the first detailed observations
   of Titan's atmosphere, revealing that its surface pressure was higher
   than Earth's, at 1.5 bars.[4] On June 23, 2014, NASA claimed to have
   strong evidence[which?] that nitrogen in the atmosphere of Titan came
   from materials in the Oort cloud, associated with comets, and not
   from the materials that formed Saturn in earlier times.[5]
 
 
https://en.wikipedia.org/wiki/Atmosphere_of_Titan#Composition
 
 
   The atmospheric composition in the stratosphere is 98.4% nitrogen—the
   only dense, nitrogen-rich atmosphere in the Solar System aside from
   Earth's—with the remaining 1.6% composed of mostly of methane (1.4%)
   and hydrogen (0.1–0.2%).[11] Because methane condenses out of Titan's
   atmosphere at high altitudes, its abundance increases as one descends
   below the tropopause at an altitude of 32 km, leveling off at a value
   of 4.9% between 8 km and the surface.[11][12] There are trace amounts
   of other hydrocarbons, such as ethane, diacetylene, methylacetylene,
   acetylene and propane, and of other gases, such as cyanoacetylene,
   hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, argon
   and helium.[12] The orange color as seen from space must be produced
   by other more complex chemicals in small quantities, possibly tholins,
   tar-like organic precipitates.[13] The hydrocarbons are thought to
   form in Titan's upper atmosphere in reactions resulting from the
   breakup of methane by the Sun's ultraviolet light, producing a thick
   orange smog.[14] Titan has no magnetic field, although studies in
   2008 showed that Titan retains remnants of Saturn's magnetic field on
   the brief occasions when it passes outside Saturn's magnetosphere and
   is directly exposed to the solar wind.[15] This may ionize and carry
   away some molecules from the top of the atmosphere. In November 2007,
   scientists uncovered evidence of negative ions with roughly 10 000
   times the mass of hydrogen in Titan's ionosphere, which are thought
   to fall into the lower regions to form the orange haze which obscures
   Titan's surface. Their structure is not currently known, but they are
   thought to be tholins, and may form the basis for the formation of
   more complex molecules, such as polycyclic aromatic hydrocarbons.[16]
 
   Energy from the Sun should have converted all traces of methane in
   Titan's atmosphere into more complex hydrocarbons within 50 million
   years — a short time compared to the age of the Solar System. This
   suggests that methane must be somehow replenished by a reservoir on
   or within Titan itself. That Titan's atmosphere contains over a
   thousand times more methane than carbon monoxide would appear to rule
   out significant contributions from cometary impacts, because comets
   are composed of more carbon monoxide than methane. That Titan might
   have accreted an atmosphere from the early Saturnian nebula at the
   time of formation also seems unlikely; in such a case, it ought to
   have atmospheric abundances similar to the solar nebula, including
   hydrogen and neon.[17] Many astronomers have suggested that the
   ultimate origin for the methane in Titan's atmosphere is from within
   Titan itself, released via eruptions from cryovolcanoes.[18][19][20]
   A possible biological origin for the methane has not been discounted
   (see Life on Titan).
 
Bye,
 
   *GB*