Lesson 3 - part 9 - vid 3; p.79 Q12

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mcatacct1
Posts: 65
Joined: Sat Oct 26, 2019 4:35 pm

Lesson 3 - part 9 - vid 3; p.79 Q12

Post by mcatacct1 » Wed Jan 22, 2020 6:09 pm

We're finding Keq for the rxn provided.

So I had a couple issues with this one.

1. I thought the first Ksp provided was for calcium oxalate's formation, not dissociation, as formation of kidney stones necessitates calcium oxalate be present (i.e. Ca2+ + C2O4 --> CaC2O4; Ksp = 2.3 *10^-9). Brian uses the reverse rxn, but with the same Ksp. What am I not getting here that would allow me to say the Ksp for the dissociation, not formation, is 2.3*10^-9.

2. Second, I was not aware you can just multiply two Ksp values together to get a net Ksp. I can't find this in the chemistry review book. Could you elaborate on this? I initially thought we'd just go with the lower Ksp as the rate limiting step, but that wasn't even an answer choice.

Thanks in advance for the reply! :)
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: Lesson 3 - part 9 - vid 3; p.79 Q12

Post by NS_Tutor_Mathias » Thu Jan 23, 2020 12:47 am

A Ksp (a solubility product) is always expressed as the ratio of products over reactants for the dissociation of the solid into it's constituent solutes. Since the reactant is always a solid, it is never included in the ratio, and the resulting Ksp is always a simple product of the two solutes (the products).

The Keq of the final reaction is simply the net products over net reactants. It stands to reason that if I combine two reactions, their net equilibrium will be the net products over net reactants.

Consider just multiplying the concentrations that each Ksp and Keq represents:

Code: Select all

Ksp = [Ca2+][Oxalate]
Keq1 = [CaOxalate]/[Citrate][Ca2+]

Ksp * Keq1 = [Ca2+][Oxalate] * [CaOxalate]/[Citrate][Ca2+] = [Oxalate][CaOxalate]/[citrate] = Keqrxn
As you can see, we get exactly the products over reactants expression for the net reaction out of this. Since this holds true for the ratios of concentrations, this will also hold true for simply multiplying the equilibrium & solubility constants. Those two are equivalent after all, where the Ksp or Keq is simply a more compact way of expressing the product of those two concentrations at equilibrium.
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