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Author: Subject: Confusion regarding the phosphoric acid reaction
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[*] posted on 11-7-2022 at 02:42
Confusion regarding the phosphoric acid reaction

Wikipedia gives the following reaction as an industrial route to producing phosphoric acid from apatite using sulphuric acid:

1. Ca₅(PO₄)₃OH(s) + 5H₂SO₄(aq) -> 3H₃PO₄(aq) + 5CaSO₄(s/aq) + H₂O(l)

I don't really understand how this reaction is supposed to work. Why doesn't it proceed further like so:

2. 2H₃PO₄(aq) + 3CaSO₄(s/aq) -> Ca₃(PO₄)₂(s) + 3H₂SO₄(aq)

Reaction 1 is basically a sort of dehydration reaction whereas reaction 2 is the salt metathesis reaction that I think ought to happen.

Calcium phosphate is way less soluble than calcium sulphate, so that would seem to be the preferred reaction product unless the presence of sulphuric acid somehow drives reaction 2 in the opposite direction.

How are all these different chemical species (H₃PO₄, CaSO₄, Ca₃(PO₄)₂, H₂SO₄) separated?
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[*] posted on 11-7-2022 at 03:41

As far as I know, the first reaction is a displacement reaction, more precisely it is an acid-base reaction, it isn't a dehydration, the water is produced by neutralization of the basic calcium hydroxyapatite.

The analgalous reaction using calcium phosphate would also work, if that makes it clearer:

Ca3(PO4)2 (s) + 3H2SO4 (aq) ---> 3CaSO4 (s) + 2H3PO4 (aq)

The calcium sulphate would be removed by filtration, as it is practically insoluble.

The reverse reaction won't happen to any significant extent because the sulphate ion is a weaker base than the phosphate ion, so when the above reaction is written showing the acid-base reactions, the equilibrium will lie heavily to the right:

PO4 (3-) (stronger base) + 3H2SO4 (stronger acid) ---> 2H3PO4 (weaker acid) + 3SO4 (2-) (weaker base)

The hydroxide ion in your equation seems to complicate things but actually its just another acid-base reaction, forming water.

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