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Topic: Why is this not an acid-base reaction?  (Read 651 times)

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Offline INeedSerotonin

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Why is this not an acid-base reaction?
« on: February 03, 2020, 01:18:46 PM »
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+ + CO32- --> 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 CO32- 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

Offline mjc123

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Re: Why is this not an acid-base reaction?
« Reply #1 on: February 05, 2020, 04:41:36 AM »
Calcium carbonate is an ionic solid containing Ca2+ and CO32- ions. No Lewis acid-base reaction occurs.

Offline INeedSerotonin

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Re: Why is this not an acid-base reaction?
« Reply #2 on: February 05, 2020, 11:05:20 AM »
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+ + CO32-) 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?

Offline mjc123

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Re: Why is this not an acid-base reaction?
« Reply #3 on: February 05, 2020, 01:02:16 PM »
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.

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