[breanainneire]

Hi. My Chem 12 class just did a qualitative analysis lab (adding various solutions to other solutions to see if they would precipitate). In one case, I added Ba(NO₃)₂ to a solutions of Na₂SO₄ and Na₂CO₃ and precipitates formed. In another, AgNO₃ was added to solutions of Na₂CO₃ and NaCl and also formed precipitates. We were told to add nitric acid to the solutions that formed precipitates. The BaCO₃ dissolved completely, and the BaSO₄, Ag₂CO₃ and AgCl dissolved a bit. What I’m wondering is is why do the precipitates redissolve? What reaction occurs (I’m looking for some kind of equation)? My teacher didn’t explain why and neither did the text. I’m kind of stumped.
Thanks

[xiankai]

i think it is called the common ion effect.

let's use the case of  Ba(NO₃)₂ and Na₂SO₄

say that all the ions are at equilibrium, and u add an excess of one ion (nitrate in this case). by le chatelier's principle, the system will act to reverse the effects of the excess. therefore take the following equation:

Ba²⁺ (aq) + 2NO₃^- (aq) --> Ba(NO₃)₂ (aq)

(or replace Ba²⁺ with Na⁺, but that isnt the reaction we're concerned about)

as u can see, the system will try to form more Ba(NO₃)₂ in order to 'remove' the excess nitrate. in doing so it uses up Ba²⁺.

but what happens to the BaSO₄? its the precipitate, and it still is insoluble right? how does it redissolve?

Ba²⁺ (aq) + SO₄^2- (aq) --> BaSO₄ (s)

this is where the loss of Ba²⁺ comes in. in the first equation, we lost Ba²⁺ and nitrate to form barium nitrate. now the Ba²⁺ is in a shortage. by bla bla bla's principle, the equation will shift to the left = to produce more Ba²⁺ . and thus you witness the redissolution.

at least, that's what i think... 

[breanainneire]

I guess that kind of makes sense. I've learned about the common ion effect before, but I don't really get why the Ba²⁺ would still be associating with the NO₃^-. Isn't the molecular formula when you add Ba(NO₃)_2(aq) and Na₂SO_4(aq):

Ba(NO₃)_2(aq) + Na₂SO_4(aq) --> NaNO_3(aq) + BaSO_4(s)  ?

[Donaldson Tan]

Ba²⁺ (aq) + 2NO₃^- (aq) --> Ba(NO₃)₂ (aq)

Ba²⁺ (aq) + 2NO₃^- (aq) = Ba(NO₃)₂ (aq)

I hate to spoil the fun here but the only re-dissolution of the precipitate upon addition of HNO₃ should only happen to BaCO₃ and Ag₂CO₃. This is purely an acid-base reaction.

[xiankai]

I don't really get why the Ba²⁺ would still be associating with the NO₃^-.

the reaction does not progress to completion, and hence there is tiny bit remaining.

Ba²⁺ (aq) + 2NO₃^- (aq) Ba(NO₃)₂ (aq)

i was trying to illustrate the equilibrium of the two ions among the others but i had no idea to go about it i guess...

This is purely an acid-base reaction.

perhaps u can explain further?

[Borek]

Every precipitate is in equilibrium with some small amount of dissolved salt. Think what happens to CO₃^2- anions when you add strong acid to the solution containing precipitate. What do you know about carbonic acid?

Note: IMHO Ag₂CO₃ should dissolve as well for exactly the same reasons.

[breanainneire]

Sorry for my ignorance (i'm just in grade 12), but what should I know about carbonic acid?

[Borek]

I am more then sure that you will find the necessary information in your textbook.

What happens when you add strong acid to solid sodium bicarbonate? Or carbonate?

[vhpk]

Yeah, of course, the strong acid will react with them and there is CO₂, because, H₂CO₃ is a very weak acid, it'll be decomposited into CO₂ and HO.
Another question: do you think that the precipitation of ion sulfur(like PbS or CuS) won't soluble in any acids

[Borek]

because, H₂CO₃ is a very weak acid, it'll be decomposited into CO₂ and HO.

It will decomposoe not because it is weak acid, but because it is unstable.

Another question: do you think that the precipitation of ion sulfur(like PbS or CuS) won't soluble in any acids

Please stop adding new questions to old threads.

[AWK]

It will decomposoe not because it is weak acid, but because it is unstable.

Stability of H₂CO₃ depends on conditions.

Dry ether solution of carbonic acid can be evaporated at normal pressure, then carbonic acid sublimes, solidifies and as solid is stable at room temperature at prolonged time, but without a traces of water. water catalytically decomposes this acid.

[Borek]

Yep. It can be also prepared by freezing water with carbon dioxide, radiating the mixture and then letting CO₂ and H₂O to sublimate. But for most practical purposes it is unstable.

Hi kids, please remember, that on the HS level carbonic acid is unstable always