dodgy or not dodgy? (1 Viewer)

[mitochondria]

mee found this on the CSU thing:

Sodium carbonate solution is basic, because the carbonate ion from the weak acid carbonic acid can hydrolyse.
CO3(2-) + H2 O ---> HCO3 - + OH-

but since HCO3 - is amphiprotic... wouldn't..

Na2 CO3 + H2O ---> H2CO3 + Na2O


and the solution becomes acidic rather than basic?

I think HCO3-, being amphiprotic is causing the problem here.. what does everyone else think?

There is a high possiblility that both of them are right.. if so.. how would you write an equation that includes the ionisation of Sodium Carbonate and water and the equilibrium of the two other reactions mentioned above?



Thank you...


aww.. just realised this after i've posted.. would it be possible that because Na comes form a strong base (such as NaOH) so it does not take part in the reaction and CO3(2-) can have it all?


ahh... just worked it out... i've just confused myself the only thing CO3(2-) can do is to produce a conjugate acid because it is an a base.. so the solution will be basic.. sorry.. sorry..

 

[ND]

Hmmm i remember reading about that equilibrium somewhere lately (maybe a trial?). I don't remember exactly, but i think it's a buffer solution in something, which keeps it slightly basic.

 

[ND]

Originally posted by mitochondria

ahh... just worked it out... i've just confused myself the only thing CO3(2-) can do is to produce a conjugate acid because it is an a base.. so the solution will be basic.. sorry.. sorry..

What do you mean by this?

 

[iambored]

yea, it's basic so it has to go to its conjugate acid

Originally posted by ND
What do you mean by this?

Na blag blah was a base, so it can't also be a base in solution, when combining with water it will form an acid, hence the H3O+ ion

 

[ND]

ohhh, sorry i misinterpreted your question. I was referring to the equilibrium: CO3 + H <--> HCO3 + H <--> H2CO3

Don't ask me why i thought your question had anything to do with this...

 

[Dangar]

how would you write an equation that includes the ionisation of Sodium Carbonate and water and the equilibrium of the two other reactions mentioned above?

Well i would have said that sodium carbonate doesn't ionise in water, it only dissociates.

i.e. Na2CO3 <---> 2Na(+) + CO3(2-)

 

[iambored]

Originally posted by ND
ohhh, sorry i misinterpreted your question. I was referring to the equilibrium: CO3 + H <--> HCO3 + H <--> H2CO3

Don't ask me why i thought your question had anything to do with this...

what is this equilibrium??

 

[ND]

I came across it again today looking through the CSU notes. It's a buffer in the blood, and keeps a pH of about 7.6.

 

[iambored]

k cool, i have never seen it before

 

[phenol]

but since HCO3 - is amphiprotic... wouldn't..

Na2 CO3 + H2O ---> H2CO3 + Na2O


and the solution becomes acidic rather than basic?

----------------------------------------------------------------------


Why is above equation suggesting the solution is going acidic?

First it is better to write your equation like this

CO3 (2-) + 2H2O <=> H2CO3 + 2OH-

as you can see, the forward reaction yields OH- therefore basic.

The key thing to think is the POSITION of the equilibrium.

CO3 (2-) + H2O <=> HCO3 - + OH-

this reaction is very much to the RHS (check an SI data book for the equilibrium constant). The reverse reaction (dissociation of HCO3-) is not as likely to occur (sorry about the dodgy wording).

So the equilibrium position of the above eqm (if you can imagine a number line with zero in the middle) lies closer to the RHS, i.e. the basic side.


The only equation needed to describe all these is

CO3 2- + 2H2O <=> HCO3 - + OH- + H2O <=> H2CO3 + 2OH-

Keep in mind the position of eqm

 

[mitochondria]

Originally posted by Dangar
Well i would have said that sodium carbonate doesn't ionise in water, it only dissociates.

i.e. Na2CO3 <---> 2Na(+) + CO3(2-)

they do ionise when dissolved in water, or else they wouldn't dissolve -- besides, the equation you have there is the ionisation of Na2CO3 when we refer to the dissociation of an acid in water, we are actually saying that the the substance dissociates in water (most of the time)

Note: you can use dissociate to replace ionise but not the other way around..

Originally posted by iambored
what is this equilibrium??

H2CO3 is a weak acid, so it will eventually reaches equilibrium

Originally posted by mitochondria
ahh... just worked it out... i've just confused myself the only thing CO3(2-) can do is to produce a conjugate acid because it is an a base.. so the solution will be basic.. sorry.. sorry..

sorry about confusing everyone


Na+ comes from a strong base, therefore it does not tend be "reversed" and remain as ions in the solution, that's why they usually only consider the ions come from a weak acid because for a basic or acidic salt you there must be one strong acid/base involved, so the part that it contributes will not matter (a only have neglectable effect) when the salt is dissolved..

sorry about the confusion