Merryp
Harmless
Posts: 29
Registered: 27-4-2019
Member Is Offline
|
|
Why is sodium carbonate not used as starting point for hypochlorite/chlorate production?
Someone suggested this equation:
3 Na2CO3 + 3 Cl2 → 5 NaCl + NaClO3 + 3 CO2
Looks reasonable on paper, and would give the same yield as NaOH.
Is there any particular reason why this reaction would be disfavoured?
|
|
|
Tsjerk
International Hazard
Posts: 3022
Registered: 20-4-2005
Location: Netherlands
Member Is Offline
Mood: Mood
|
|
Try to write down all reaction steps and see where the different equilibriums are and what the K values are. I have no clue but I guess at least one
is pretty far from your proposed product.
|
|
|
j_sum1
Administrator
Posts: 6218
Registered: 4-10-2014
Location: Unmoved
Member Is Offline
Mood: Organised
|
|
I think the process will be less efficient. Consider that you are bubbling Cl2 gas through a solution. At the reaction interface you will also have
CO2 diffusing back into the bubble. This is going to dilute the chlorine in the bubbles and the amount of unreacted chlorine will be greater.
It will also be more difficult to monitor the reaction. With NaOH you can observe visually how the bubble size diminishes and get an idea of how much
chlorine is reacting. If you have CO2 being produced you lose some of that visual feedback.
|
|
|
rockyit98
Hazard to Others
Posts: 283
Registered: 12-4-2019
Location: The Known Universe
Member Is Offline
Mood: no mood is a good mood
|
|
you can't. in fact adding soda water to bleach will make toxic Cl2 gas! more so with Ca(OCl)2 based ones because of insoluble CaCO3.
"A mind is a terrible thing to lose"-Meisner
|
|
|
Texium
|
Thread Moved
22-1-2020 at 08:16 |
clearly_not_atara
International Hazard
Posts: 2691
Registered: 3-11-2013
Member Is Offline
Mood: Big
|
|
Chlorine only dissolves well in water when hypochlorite is deprotonated, pKa 7.53, so you need pH > 9. A buffered sol'n of sodium sesquicarbonate
has a pH of 10.5 but a buffered sol'n of bicarbonate has a pKa of about 8. The latter is not basic enough to dissolve Cl2, so the conjugate acid for
this reaction is sesquicarbonate.
So your equation should really be:
4 Na2CO3 + Cl2 + H2O >> NaCl + NaOCl + 2 Na3H(CO3)2
Here it is evident that you need eight moles of sodium to get one mole of hypochlorite! What a waste!
[Edited on 04-20-1969 by clearly_not_atara]
|
|
|
Fulmen
International Hazard
Posts: 1693
Registered: 24-9-2005
Member Is Offline
Mood: Bored
|
|
Wait, what? Where does the Cl2 come from? This would assume you had an excess of Cl2 from another process, or that NaOH is far more valuable than
Na2CO3?
We're not banging rocks together here. We know how to put a man back together.
|
|
|
woelen
Super Administrator
Posts: 7976
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
Doesn't matter, the question is about making bleach from Cl2 and an alkaline solution. the Cl2 indeed comes from an external process/source.
The last answer of clearly_not_atara makes sense. Carbonate ion is just a weak base and a little added acidity (indirectly from the disproportionation
reaction of Cl2) quickly neutralizes it.
[Edited on 23-1-20 by woelen]
|
|
|
chornedsnorkack
National Hazard
Posts: 521
Registered: 16-2-2012
Member Is Offline
Mood: No Mood
|
|
Quote: Originally posted by clearly_not_atara  | Chlorine only dissolves well in water when hypochlorite is deprotonated, pKa 7.53, so you need pH > 9. A buffered sol'n of sodium sesquicarbonate
has a pH of 10.5 but a buffered sol'n of bicarbonate has a pKa of about 8. The latter is not basic enough to dissolve Cl2, so the conjugate acid for
this reaction is sesquicarbonate.
So your equation should really be:
4 Na2CO3 + Cl2 + H2O >> NaCl + NaOCl + 2 Na3H(CO3)2
Here it is evident that you need eight moles of sodium to get one mole of hypochlorite! What a waste! |
Yes, because HClO is a weak acid.
But HClO3 is a strong acid, like HCl.
At which pH does the reaction
3Cl2+3H2O<->5HCl+HClO3
have its equilibrium on the left?
|
|
|
clearly_not_atara
International Hazard
Posts: 2691
Registered: 3-11-2013
Member Is Offline
Mood: Big
|
|
Chloric acid is not stable with respect to decomposition:
2 HClO3 >> H2O + Cl2 + 2 O2 + ~275 kJ/mol
It is itself produced only by the disproportionation of hypochlorite. It cannot be produced at any pH where hypochlorite does not form.
[Edited on 04-20-1969 by clearly_not_atara]
|
|
|
AJKOER
Radically Dubious
Posts: 3026
Registered: 7-5-2011
Member Is Offline
Mood: No Mood
|
|
Per a source (see https://books.google.com/books?id=dXn3aB1DKk4C&pg=PA454&...), Cl2 first reacts to neutralize the alkaline aqueous Na2CO3 per the cited
reaction:
Cl2 + H2O + 2 Na2CO3 -> NaOCl + NaCl+ 2 NaHCO3
And then, neutralizes the more alkaline sodium hypochlorite:
Cl2 + H2O + NaOCl -> 2 HOCl + NaCl
where the two reactions above are stated by the reference. However, I would further argue even without more added chlorine, the bicarbonate can act on
hypochlorite also liberating hypochlorous acid in an equilibrium reaction (and, with an insoluble carbonate, like CaCO3, the reaction moves completely
to the right):
HCO3- + OCl- <--> HOCl + CO3(2-)
So on net, the action of chlorine on Na2CO3 is NOT a good path to NaOCl, but more powerful, at least in terms of disinfecting, or especially,
bleaching in the presence of ferrous ions (as are found in hot tap water) with the formation hypochlorous acid, and associated radicals created
therefrom. With respect to radicals, a Fenton-type reaction based on HOCl (in place of H2O2 with the classic Fenton reaction) proceeds as follows
leading to active radicals:
Fe(II) + HOCl -> Fe(lll) + •ClOH-
where at pH > 5:
•ClOH- -> Cl- + •OH (See cited sources at https://www.sciencemadness.org/whisper/viewthread.php?tid=15... )
where the hydroxyl radical can breakdown organics (like stains) or the DNA of bacteria,...
Also, in the presence of chloride:
•ClOH- + Cl- -> •Cl2- + OH-
Also importantly, created HOCl here is not stable so one cannot place it on a shelf at the supermarket (so, the action of chlorine on sodium carbonate
does NOT create even a stable bleach alternative).
[Edited on 24-2-2020 by AJKOER]
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
