question for anyone really good at chemistry

[Dani9]

hey,

In my chemistry exam there was this question

what two anions are formed when carbon dioxide dissolves in water?

Now i can safely say that I have NEVER heard of this in my chemistry classes so i just guessed the answer. I put HCO3 negative and the other one i cant remember. Then the next question said what happens to these anions when calcium ions are present, I said that calcium hydrogencarbonate is formed (i put this at it reminded me of what happens in caves

is this right? and if its not what is the right answer??

Dani

[Dave22]

Well when carbon dioxide dissolves in water, it forms carbonic acid with the formular H2CO3, which then dissociates partially (as its a weak acid) into hydrogen ions and HCO3(minus) ions.

I'm guessing when calcium ions are added, which have a 2+ charge, calcium hydrogencarbonate is formed, but which 2 hydrogencarbonates on the calcium. sounds like you got it right!

[Dani9]

Hey,

thanks for that, I just took an educated guess to be honest with you! maybe I should trust my judgement more!

cheers for that!

Dani

[Dave22]

I may be wrong :P, but I'm pretty sure thats right.

[AdamP]

Ah you took the WJEC exam! I did that too.

The two anions I'm guessing were hydrogencarbonate ions and carbonate ions, the carbonate ions would produce a white precipitate with the calcium ions (group II) whereas calcium hydrogencarbonate is soluble in water, so a [colourless] solution would form. I think this is what I wrote anyway, the exam was over a month ago.

[pete1184]

I do WJEC chem, I put the same answer as you Dani9. The exam wasn't over a month ago it was on the 6th of june

[lincolnshire]

CO2(aq) + H2O(l) ----> H2CO3(aq)

Only about 1% of the dissolved CO2 exists as H2CO3. Carbonic acid is a weak acid which dissociates in two steps:

H2CO3 <-----> H+ + (HCO3)¯
(HCO3)¯ <-----> H+ + (CO3)2¯

Answer is: (HCO3)¯ and (CO3)2¯

If you add Ca2+ :

Ca2+(aq) + 2 (HCO3)¯(aq) <-----> CO2(aq) + H2O(l) + CaCO3(s)

and

Ca2+(aq) + (CO3)2¯(aq) <------> CaCO3(s)

Also
CO2(g) + H2O(l) + CaCO3(s)<------> Ca(HCO3)2(aq)

Answer: CaCO3(s) and Ca(HCO3)2(aq)

I hope this helps.

[AdamP]

The exam wasn't over a month ago it was on the 6th of june

I was thinking of CH3a then, my mistake

[eminem_89]

caco3+H2

[AdamP]

caco3+H2

Not only is that the wrong answer, but this thread is almost 2 years old