Solubility of PbSO4: Uncovering the Mystery

[Kqwert]

Homework Statement

The solubility of PbSO4(s) increase with the addition of H2SO4. Why?

Relevant Equations

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I don't quite understand this. When solving PbSO₄(s) we get the equilibrium equation:

PbSO₄(s) <--> Pb²⁺ + SO₄^2-.

I know H2SO4 is a strong acid, that will dissociate completely (or almost completely), Like this:

H₂SO₄ -> H⁺ + HSO₄^-
HSO₄^- -> H⁺ + SO₄^2-So as far as I can see, the concentration of SO₄^2- increases, which should give a lower solubility of PbSO4(s) (Common ion effect). What am I missing?

 

[Borek]

Check the value of Ka2.

 

[Kqwert]

It's 10^-1.99? I.e. high? What does that tell us?

 

[Borek]

It's 10^-1.99?

Yes. Use this number to calculate what part of HSO₄^- is dissociated at pH 1.

 

[Kqwert]

What do you mean by "what part"?

I don't understand what this is leading to, and why exactly pH 1?

 

[Borek]

"Part" as in dissociation percentage (fraction) - ratio of dissociated to the analytical concentration.

pH of 1 because that makes calculations easy, it could be as well pH 0. Mainly it is about low pH (which is typical of sulfuric acid solutions). Conclusion (once we reach it) is the same in both cases.

I don't understand what this is leading to

When we get there you will either wonder why you have seen it from the very beginning, or it will broaden your understanding of strong/weak acids

 

[Kqwert]

Ok,

so we have the equilibrium eq. :

10^-2 = [10^-1][SO4^2-] / [HSO4^-].

This results in:
0.1 = [SO4^2-] / [HSO4^-]

I.e. concentration of HSO4^- is significantly higher than SO4^2-. But what exactly does this tell us?

 

[Borek]

Hint: you have already mentioned common ion effect.

 

[Kqwert]

I assume it's something like [H^+] from the acid reacting with [SO4^2-] from the dissolution of our compound (which reduces the total concentration of [SO4^2-] in solution), which again will increase the amount of [Pb^2+] in solution..?

 

[Borek]

Yes. Sulfuric acid, H₂SO₄, is a strong one, but HSO₄^- is a much weaker one (still quite strong though). At low pH solutions are dominated by the HSO₄^- form. That in turn means at low pH solubility of sulfates goes up.

 

[Kqwert]

Solubility of SO4^2- goes up and not down?

 

[Borek]

No such thing as solubility of SO₄^2-. You can't dissolve single anion. What changes is the solubility of sulfates - salts containing SO₄^2-.

 

[Ygggdrasil]

It is worth noting that while PbSO4 is insoluble in water, Pb(HSO4)2 is soluble. Given this information and the information about the equilibrium between H⁺ + SO4 $\rightleftharpoons$ HSO4, can you explain how the addition of H2SO4 to PbSO4 causes the Pb²⁺ ions to go into solution.

 

[Kqwert]

Something like the H+ ions will "force" Pb2+ ions into solution as it wants to form HSO4-? (and therefore needs the SO₄^2- from PbSO4)..??

 

[epenguin]

Isn't this the same sort of thing as blowing into lime water turns it milky with CaCO₃ precipitate, but blowing further makes it turn clear as you get the more soluble Ca(HCO₃)₂? Something that took me several years to get my head around at school, the apparently two-way reaction to varying one parameter seeming paradoxical and I was tendentially incredulous like Kqwert. But at least I had seen it really happen. Hopefully there is a you-tube demonstration for lead sulphate/bisulphate to be found.

I think one difficulty for students is that this explanation needs an explanation. I.e. Why are the bicarbonates and bisulphates more soluble than the carbonates or sulphates? Considering how much solubility comes into chemistry it tends not to be proportionately treated in chemical education. Er, Madelung, Born-Landé, waves hands, buut crudely A more concentrated double negative charge on the molecule is effectively lmore attractive to positive charges surrounding than is two separated single charges, giving more stability to the solid. I am sure somebody can improve on this