[tal]

Hey guys

We've been going over polyprotic acids in class, but there was one thing which was not clear to me and I couldn't find an answer for online.
How do I know how the ionized acid will react after the first ionization? I mean, how is it decided whether the ionized acid will react with hydronium as a conjugate base, or keep on ionizing again(only if it can donate another proton of course, talking about H2PO4- for example)?

Cheers

[docnet]

Polyprotic acids have ionization constants Ka1, Ka2, Ka3... for each ionization (where the numbers are the subscripts). Usually the ionization constants get significantly smaller with each subscripts, meaning the second ionization will happen to a much smaller extent.
Each ionization is represented by a reaction equation and a Ka value. the ionization of a weak acid always exists in equilibrium with the reverse reaction, meaning the resulting conjugate bases react with hydronium ions to form the acid. All but the strong acids such as HCl and HBr ionize incompletely and the same idea applies to weak bases and strong bases.
 
in a solution, H2PO4,  HPO4-, (PO4)-2 and the H+/H3O+ ions would exist together in equilibrium. hope others can correct me if I'm wrong.

[Borek]

How do I know how the ionized acid will react after the first ionization?

Matter of pH and pKa value. If pH is high enough (a lot of H⁺ present) and the original acid was weak enough, ionized acid will get protonated back. If the pH is high (low concentration of H⁺) and the next Ka value is high enough, acid will dissociate.

As docnet wrote, it is an equilibrium reaction.