DAT Destroyer 2014 2Gen Chem #268?

[BrazilianRider]

Hey guys, quick question about #268 on Gen Chem section of Destroyer 2014...

It was my understanding that cations/anions of strong acids and strong bases could not be hydrolyzed? Does this mean they won't react with water at all? In this problem, it says that the results of sodium and excess water would be NaOH and H2... However, wouldn't NaOH just then dissociate into -OH and NaH in the excess water?

Am I just reading way into this? Am I wrong in any of my concepts?

Thanks a ton guys!

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[Triflate]

not sure I understand your question....but i think this is what you're talking about:

2Na(s) + 2H2O(l) ----> H2(g) + 2NaOH(aq) + heat

Remember, sodium (and all Group I metals) is extremely reactive in water. It is very exothermic and releases a ton of heat. In this case, some of the heat evolves in the form of H2(g). The hydroxide anion isn't strong enough to deprotonate molecular hydrogen, so no, you do not form NaH. NaOH would dissociate into 2Na+ and 2OH- in water, but it really doesn't matter how you write it, unless they were asking for the net ionic equation.

 

[BrazilianRider]

Yeah, that's what I'm talking about... However, there was another question on Destroyer that said anions/cations of strong acids and strong bases couldn't be hydrolyzed... I thought hydrolyzing something was reacting it with H2O? Or is hydrolyzing adding H2O to the molecule?

The later explanation would make sense in the context of the answer!

 

[Hailstorm]

Treat the anion/cation as a lewis base/acid. Also you might want to review the solubility product Ksp.

Na+ + H2O is in equilibrium with NaOH and H+, but the equilibrium lies essentially entirely on the reactants.

Conversely if you have Mg2+ in a highly basic solution you have (reversing the equation to how you're likely used to seeing it):

Mg(OH2)->Mg2+ + 2OH-

Ksp=[Mg2+][OH-]^2 / [Mg(OH)2 -> which is a pure solid and therefore 1 regardless of concentration]

In this situation you're likely going to see the equilibrium entirely on the side of the solid rather than the aq ions.

 

[BrazilianRider]

Treat the anion/cation as a lewis base/acid. Also you might want to review the solubility product Ksp.

Na+ + H2O is in equilibrium with NaOH and H+, but the equilibrium lies essentially entirely on the reactants.

Conversely if you have Mg2+ in a highly basic solution you have (reversing the equation to how you're likely used to seeing it):

Mg(OH2)->Mg2+ + 2OH-

Ksp=[Mg2+][OH-]^2 / [Mg(OH)2 -> which is a pure solid and therefore 1 regardless of concentration]

In this situation you're likely going to see the equilibrium entirely on the side of the solid rather than the aq ions.

Gooootcha! That makes sense. So the cations/anions of strong bases/acids DO have a reaction with H2O, but the equilibrium lies to the left not the right, so they don't "technically" hydrolyze.

 

[biomolecmed]

Yes. Now you got it. Hydrolyze, kind of means to break down using water. Na is not broken down here, it is actually hydrated, not hydrolyzed.