Why won't water molecules dissociate?

[Lim Y K]

Pure water is not an electrolyte. This is because the hydrogen atoms and oxygen atom will not dissociate into individual charged particles. Why won't they dissociate?

 

[Borek]

They do dissociate, as described by a water ion product. However, concentrations of H⁺ and OH^- thus produced are quite low, so the specific resistance of a pure water is quite high (18 MΩ).

 

[tommyxu3]

Besides, that's not two atoms'( $\text{H}$ and $\text{O}$ ) dissociation, but $\text{H}^+$ and $\text{OH}^-.$

 

[PWiz]

Besides, that's not two atoms'( $\text{H}$ and $\text{O}$ ) dissociation, but $\text{H}^+$ and $\text{OH}^-.$

Isn't the actual ionic equilibrium $2H_2 O (l) ⇔ H_3 O^{+} (aq)+ OH^{-} (aq)$ ? If I remember correctly, H+ ions are just used instead of hydronium ion for simplification of the equation, but the actual equation is the one above.

 

[Borek]

Protons can be solvated by several water molecules (so there exists whole series of cations of the general formula H_2n+1O_n⁺). H⁺ is about as good as H₃O⁺ IMHO - works OK as a symbol, doesn't reflect the reality. Add to that fact that OH^- is not isolated in the solution, but solvated as well (yielding H_2n+1O_n+1^- anions), and you should start to see the picture

 

[PWiz]

Protons can be solvated by several water molecules (so there exists whole series of cations of the general formula H_2n+1O_n⁺). H⁺ is about as good as H₃O⁺ IMHO - works OK as a symbol, doesn't reflect the reality. Add to that fact that OH^- is not isolated in the solution, but solvated as well (yielding H_2n+1O_n+1^- anions), and you should start to see the picture

Ahh, I see. But is it safe to say that n is not large? I mean after all, you'll eventually crowd up the entire area around the H+ ion so that the repulsion between the lone pair of electrons on the O atoms of water molecules solvating the ion and the lone pairs on the O atoms of 'unlinked' water molecules is greater than any attractive force the 'unlinked' water molecules might feel toward the H+ ion, preventing these 'unlinked' molecules from solvating the H+ ion as well.
P.S. I can't find a replacement word for 'unlinked' from my Chemistry verbiage memory bank right now :P

 

[Borek]

But is it safe to say that n is not large?

Depends on what counts as "large". Some research suggests structures with n up to 20.

https://en.wikipedia.org/wiki/Hydronium#Solvation

 

[PWiz]

Depends on what counts as "large". Some research suggests structures with n up to 20.

https://en.wikipedia.org/wiki/Hydronium#Solvation

Wow, that's pretty cool. Looks like I've got something to show to my Chem teacher

 

[DrDu]

Protons can be solvated by several water molecules (so there exists whole series of cations of the general formula H_2n+1O_n⁺). H⁺ is about as good as H₃O⁺ IMHO - works OK as a symbol, doesn't reflect the reality. Add to that fact that OH^- is not isolated in the solution, but solvated as well (yielding H_2n+1O_n+1^- anions), and you should start to see the picture

I think the difference is that in H3O+, the proton is covalently bounded to a water molecule while the additional water molecules in the larger clusters are bound via hydrogen bridges.

 

[Borek]

I think the difference is that in H3O+, the proton is covalently bounded to a water molecule while the additional water molecules in the larger clusters are bound via hydrogen bridges.

I would expect the change to be gradual - for small n (like 2 or 3) I would guess the cluster to be quite symmetrical, and all water molecules to be equivalent. Once the structure gets larger the outer layers will be definitely bonded differently.

 

[DrDu]

The only symmetric hydrogen bond I know about is the FHF-. So even in small water clusters, you can uniquely define which one is the hydronium ion.