Saerynide
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Soda and NaOH
Im confused. If:
NaOH + NaHCO3 --> Na2CO3 + H2O
Na2CO3 + 2H2O --> 2NaOH + H2CO3 -> H2O + CO2
Then, why does NaOH get ruined if left exposed to air, when the reverse reaction can occur in water?  What am I not getting?
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TheBear
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Those reactions are equilibrium reactions. And I'm guessing we are talking about aquaeous reactions?
Then you have the following reactions happening continiously:
H2O + CO2 <---> H2CO3
H2CO3 + H20 <---> H3O+ + HCO3-
HCO3- + H2O <---> H3O+ + CO3--
OH- + H3O+ <---> 2H2O
Because OH- is a strong base and HCO3+ is an ampholyte (can act as both an acid as well as a base) The first reaction will be strongly shifted to the
right. Though I really do feel you should write it as this:
OH- + HCO3- <---> H2O + CO3--
To know what concentrations you will have in solution. Use pKa values. It was too long I calculated anything using pKa when having several kinds of
ions. Perhaps someone with fresher memory would like to enlighten us?
/TheBear
[Edited on 27-5-2004 by TheBear]
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unionised
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The decomposition of sodium carbonate to CO2 and NaOH (the second reaction) doesn't happen (or, at least, it happens to a vanishingly small
extent).
Washing soda doesn't fizz in water.
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Saerynide
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Ah, I see. Cause I never remembered washing soda fizzing in water either. I dont remeber which site I read it from, but this problem really drove me
crazy 
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ilwdx
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why it can't happen?
| Quote: | Originally posted by unionised
The decomposition of sodium carbonate to CO2 and NaOH (the second reaction) doesn't happen (or, at least, it happens to a vanishingly small
extent).
Washing soda doesn't fizz in water. |
isn't it hydrolysis?
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darkflame89
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But due to equilibrium, hydrolysis can only occur to a certain extent..
Ignis ubique latet, naturam amplectitur omnem.
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ilwdx
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| Quote: | Originally posted by darkflame89
But due to equilibrium, hydrolysis can only occur to a certain extent.. |
if continue to heat the solution and therefore make the CO2 fly away, will the hrolysis reacts completely for this reaction?
Na2CO3 + 2H2O --> 2NaOH + H2CO3
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Ium
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Most alkali metal Carbonates decompose in solution when heated to give a Hydroxide and gaseous CO2. Carbonates of K and Na are an exception to this.
It takes much higher temperatures (higher than the BP of H2O) for them to decompose to their former Oxides/Hydroxides.
Evidently some Carbonates are produced from bubbling CO2 through a solution of the Hydroxide. Eg. CaCO3.
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BromicAcid
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| Quote: | | Most alkali metal Carbonates decompose in solution when heated to give a Hydroxide and gaseous CO2. Carbonates of K and Na are an exception to this.
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From Descriptive Inorganic Chemistry 3rd edition:
| Quote: | | The alkali metal carbonates (except that of lithum) do not decompose when heated. Lithium carbonate and carbonates of other moderately
electropositive metals such as calcium give the metal oxide when heated |
According to my chemical dictionary lithum carbonate begins to decompose around 1200C.
And as for the production of a carbonate by bubbling CO2 though the hydroxide, as long as the hydroxide is moderately soluble it will work as the
reaction is favored by two factors. 1)Most carbonates are insoluble (except ammonum carbonate and the alkali metal carbonates) 2) CO2 in solution
produces in situ carbonic acid which will react with a hydroxide in an acid-base reaction.
Getting back to aqueous solutions though:
| Quote: |
if continue to heat the solution and therefore make the CO2 fly away, will the hrolysis reacts completely for this reaction?
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I don't think it would go to completion but the reaction would be more noticeable at the boiling point of water.
Na2CO3 + H2O ----> H2CO3 + 2NaOH
And because it is at the boiling point you're going to shift the equilibrium
H2CO3 ----> H2O + CO2(g)
[Edited on 5/31/2004 by BromicAcid]
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unionised
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The originlal question was "if
NaOH + NaHCO3 --> Na2CO3 + H2O
Na2CO3 + 2H2O --> 2NaOH + H2CO3 -> H2O + CO2
then why....."
It's quite simple; the second reaction, as written, doesn't happen (to any meaningful extent).
Boiling the solution would drive off the water much faster than CO2 simply because there is vastly more water present.
Worse than that, you really don't drive a reaction by removing one of the reactants. So you won't force the reaction of water with a
carbonate by boiling off the water.
You really don't need to boil a solution to get H2CO3 to decompose, you can just open a bottle of fizzy drink.
How long can this thread get?
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darkflame89
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The opening of fizzy drinks to decompose carbonic acid to carbon dioxide is probably due to pressure release. So, to shift the equilibrium to produce
carbon dioxide, add pressure! 
Ignis ubique latet, naturam amplectitur omnem.
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unionised
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I hope you are kidding.
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Saerynide
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Ouch.. thats harsh
To shift the equilibrium to produce carbon dioxide, one should decrease pressure. Gases are less soluble in lower pressure. Thats why when you open
a can of pop, you release the pressure inside the container, the H2CO3 decomposes and the CO2 comes out of the solution.
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vulture
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My washbottle for nitric acid destillation filled with a oversaturated K2CO3 starts to fizz violently when 100mbar vacuum is applied.
Thank me for making this thread even longer!
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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darkflame89
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my mistake..
Ignis ubique latet, naturam amplectitur omnem.
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