[INeedSerotonin]

Hello

I found this exercise:

Shellfish have a shell made of calcium carbonate, which is formed when calcium ions, secreted from shellfish cells, find sea water, which is rich in dissolved carbon dioxide.

One can say that

I - one of the reactions is Ca²⁺ + CO₃^2- --> CaCO₃
II - The reaction which involves the calcium ions while forming the shell is an acid-base reaction
III - the product is classified as an basic oxide

Which ones of these are true? Answer: only (I)

I think it should be (I) and (II), because CO₃^2- has electrons to give, and Ca²⁺ misses electrons. So one donates to the other, making it a Lewis acid-base reaction.

Can you guys shed some light here?

Thank you

[mjc123]

Calcium carbonate is an ionic solid containing Ca²⁺ and CO₃^2- ions. No Lewis acid-base reaction occurs.

[INeedSerotonin]

Thanks. I'm still confused, though.

Isn't AlCl₃ + Cl^- <--> AlCl₄^- an acid-base reaction too? Cl^- is a base, right?

I don't get why my first reaction (Ca²⁺ + CO₃^2-) cannot be an acid-base reaction too.

Perhaps it has something to do with having only ions, which makes it not be an acid-base reaction?

[mjc123]

The Cl^- donates a pair of electrons to form an Al-Cl covalent bond. That does not happen in CaCO₃. The ions are held together by coulombic attraction, not by electron donation and bond forming.