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Author: Subject: Equilibrium nitric acid concentration
chornedsnorkack
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[*] posted on 6-5-2016 at 01:56
Equilibrium nitric acid concentration


Nitric acid is known to decompose:
4HNO3<->4NO2+O2+2H2O

But this reaction, leftward, is also standard reaction for HNO3 production, because it is an equilibrium reaction.

Precisely where is the equilibrium of that reaction?
At 20 Celsius and 1 bar, with gas phase being 80 % NO2 and 20 % O2, with no inert gases like N2 present and the only impurities in gas being vapour H2O and NO2, what is the HNO3 concentration at equilibrium?
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hissingnoise
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[*] posted on 6-5-2016 at 11:06


Decomposition of HNO3 is not reversible so no equilibrium exists!

NO2, being a mixed anhydride, produces nitric and nitrous acid with water ─ the nitrous acid may be oxidised by using an efficient tower but the oxidation is slow at NTP!

And the azeotrope at NTP is ~68% . . .

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clearly_not_atara
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[*] posted on 6-5-2016 at 11:52


In a sealed vessel containing NO2, H2O, and O2, it should be possible to calculate the equilibrium of the reactions:

4NO2 + O2 + 2H2O <> 4HNO3

2NO2 <> 2NO + O2

NO + NO2 + H2O <> 2HNO2

However, this is only a metastable equilibrium, as the true equilibrium features only N2, O2, and H2O, IIRC.
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hissingnoise
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[*] posted on 7-5-2016 at 01:37


The decomposition products of HNO3 do not spontaneously recombine to any but a negligible extent so there is no equilibrium to calculate . . .

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clearly_not_atara
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[*] posted on 7-5-2016 at 07:15


I'm pretty sure they do, considering this spontaneous combination is the basis of HNO3 production, and I literally wrote the equation right there.

(Also, quantum mechanics is unitary yadda yadda)

[Edited on 7-5-2016 by clearly_not_atara]
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hissingnoise
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[*] posted on 7-5-2016 at 09:51


Quote:
I'm pretty sure they do, considering this spontaneous combination is the basis of HNO3 production

No! Spontaneous means unforced ─ in the large absorption towers essential to NA production the 'combination' is certainly not unforced . . .

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clearly_not_atara
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[*] posted on 7-5-2016 at 11:38


No, spontaneous means "not driven by nonthermal forces". Electrolytic and photolytic reactions are truly not spontaneous. HNO3 production just happens when the concentration of NO2 vs the concentration of HNO3 is such that the conditions favor HNO3 production. These are unnatural conditions, and the NO2 must be confined in a specialized vessel, but the reaction fits the thermodynamical definition of "spontaneous" fine.

NO2 and O2 react with water to give HNO3 pretty reliably, if Wiki isn't a giant mess of twisted lies...

http://en.wikipedia.org/wiki/Nitric_acid#Laboratory_producti...

These same chemicals are the primary breakdown products of HNO3 so this is not too surprising.
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hissingnoise
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[*] posted on 7-5-2016 at 12:15


Quote:
No, spontaneous means "not driven by nonthermal forces".

We're getting into semantics here ─ do you think that paper, say, bursting into flame when thrown on a fire is an example of "spontaneous cumbustion" . . . ?

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clearly_not_atara
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[*] posted on 7-5-2016 at 13:04


Yes, since the reaction between cellulose and oxygen certainly fits the thermodynamical definition of "spontaneous".

I don't understand why you want to use "spontaneous" as a technical term when you're making a prediction about HNO3, and colloquially when paper is burning. It's just equivocation. Certainly nobody believes the activation energy for the half-reaction:

N2O4 + OH- >> NO3- + HNO2

is particularly high.
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hissingnoise
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[*] posted on 8-5-2016 at 03:48


Yeah, I'm just bullshitting you, at this stage . . . :D

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AJKOER
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[*] posted on 8-5-2016 at 11:26


More precisely, to quote:

"When boiling in light, even at room temperature, there is a partial decomposition with the formation of nitrogen dioxide following the reaction:

4HNO3 → 2H2O + 4NO2 + O2 (72°C)

which means that anhydrous nitric acid should be stored below 0°C to avoid decomposition."
Source: http://www.newworldencyclopedia.org/entry/Nitric_acid

Also, in aqueous solution, it readily ionizes into the nitrate ion NO3− and a hydrated proton (hydronium ion) H3O+ :

HNO3 + H2O → H3O+ + NO3-

Now, combine the above reaction with some radical based chemistry introduced by exposure to heat, light, solvated electrons, radiation,... could proceed as follows:

HNO3 + M ⇌ ·OH + ·NO2 + M (see http://pubs.acs.org/doi/abs/10.1021/j100488a024 )
NO3− + ·OH = ·NO3 + OH-
·NO3 + ·NO2 = N2O5 (see http://scitation.aip.org/content/aip/journal/jcp/88/8/10.106... )
N2O5 → N2O4 + 1/2 O2 (assumes full completion of action of 1/2 O2 + NO to NO2 whose reaction speed declines with increasing temperature)

Reference: See http://pubs.acs.org/doi/abs/10.1021/ja00872a004

The approximate implied net reaction by direct summation of the above:

2 HNO3 + H2O → H2O + H+ + OH- + N2O4 + 1/2 O2

Or equivalently:

2 HNO3 → H2O + N2O4 + 1/2 O2

As was required, which actually has been confirmed by investigations of the self decomposition reaction of HNO3 in the liquid phase between room temperature and 80 C and also in the gas phase to 475 C. Reference: https://www.google.com/url?sa=t&source=web&rct=j&...

Interestingly, the author of the cited work on the reaction notes that the reversal of the (net) reaction is not observed to appreciably occur under the specified range of conditions.

[Edited on 8-5-2016 by AJKOER]
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Eosin Y
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[*] posted on 8-5-2016 at 11:40


So what is the white fuming that comes off WFNA? The moisture in the air must catalyse some kind of reaction, but I can't see what.
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aithecomputerguy
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[*] posted on 8-5-2016 at 12:09


Quote: Originally posted by Eosin Y  
So what is the white fuming that comes off WFNA? The moisture in the air must catalyse some kind of reaction, but I can't see what.


There is no reaction. What happens is the vapors absorb and condense water from the air to form tiny droplets of extremely concentrated acid.




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hissingnoise
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[*] posted on 8-5-2016 at 12:35


Quote:
"When boiling in light, even at room temperature, there is a partial decomposition with the formation of nitrogen dioxide

There's a fairly important "and" missing above! :D

But if nitrogen oxyanions were stable, nitro compounds would be rather less interesting . . . and have far fewer uses?

Quote:
So what is the white fuming that comes off WFNA?

The fuming is nitric acid vapour and should be avoided, if possible, but quantities are very small at ordinary temp. and the occasional whiff is unlikely to do much harm . . .

RFNA gives off NO2 in addition and so is more noxious than WFNA!

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