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Topic: Complex ion formation's effect on solubility  (Read 4293 times)

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Offline pear

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Complex ion formation's effect on solubility
« on: February 25, 2010, 01:09:50 AM »
Hi,

So I'm trying to understand what happens when excess I- is added to the equilibrium  PbI2(s)  ::equil::  Pb2+(aq) + 2I-(aq)

What I'm thinking is that first the reaction is governed by solubility product.  And eventually, [I-] is so high that Q >>>> Ksp and the lead re-dissolves to instantly form covalent bonds with the two outer electrons && the two electrons from the unfilled d shell, thus removing twice the [I-] as the PbI2 molecule.

Am I thinking logically/correctly?

Offline pear

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Re: Complex ion formation's effect on solubility
« Reply #1 on: February 26, 2010, 11:11:56 AM »
So here's the answer, for anyone else wondering..

Eventually the amount of energy possessed by a solution with excess negatively charged ions will overcome the high energy input required for the 2 d-shell electrons of Pb to create covalent bonds alongside the 2 outer p-shell electrons.  So instead of the solid PbI2, which only takes care of 2 I- ions, the covalent, aqueous molecule PbI42- forms and lessens the amount of excess energy in the solution.

Offline zeoblade

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Re: Complex ion formation's effect on solubility
« Reply #2 on: February 26, 2010, 08:06:04 PM »
Isn't this an application of Le Chatelier's principle where if you add excess product, it drives the reaction to favour the reverse reaction and you will precipitate the reactant?

Offline Borek

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Re: Complex ion formation's effect on solubility
« Reply #3 on: February 26, 2010, 09:12:16 PM »
PbI42- formation is a separate reaction, just applying LeChetelier's principle to half of the system won't get you a correct result.
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Offline pear

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Re: Complex ion formation's effect on solubility
« Reply #4 on: March 02, 2010, 12:05:06 PM »
PbI42- formation is a separate reaction, just applying LeChetelier's principle to half of the system won't get you a correct result.


But the explanation I posted is correct, right?

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