Why Do Bases Release All Hydroxides Simultaneously?

[fire9132]

Homework Statement

Why do bases lose all of their hydroxides at once?

Homework Equations

N/A

The Attempt at a Solution

Polyprotic acids lose their hydrogen atoms in a step-wise fashion, but bases with multiple hydroxides do not do this. I'm not quiet sure why this occurs, but I think that it's because of the structure of the bases compared to the structure to the acids. If I had Sulfuric Acid and added it to water, it would form bisulfate ions (along with protons). If I had Calcium Hydroxide and added water, it would form Calcium ions and hydroxide ions. Is this because Ca(OH) isn't stable? The bisulfate ions are stable, but Ca(OH) isn't because the Ca essentially has just one valence electron and it would just be better for it to lose the other OH and become Ca2+? I'm not sure. Could someone tell me if my explanation is correct or if I'm completely wrong? Thanks!

 

[Borek]

Why do bases lose all of their hydroxides at once?

They don't, they behave exactly as acids do, with separate dissociation steps and separate dissociation constants. No idea why you think different.

 

[fire9132]

My chemistry teacher asked that question... So I don't know...

 

[Borek]

Sorry to say that - your teacher is wrong.

Calcium hydroxide has two dissociation steps, typically we assume the first one to go to the end (very strong base), the second one has a commonly listed pKb value of 1.3 (or 1.4). Actually this is a rather complicated system, with low solubility of the base (so you need to take Ksp into account as well), but it is not that difficult to show experimentally that the solution contains substantial amount of CaOH⁺. That means there exists an equilibrium

CaOH⁺ <-> Ca²⁺ + OH^-

and it is nothing else but base dissociation (technically indistinguishable from the CaOH⁺ complex formation).