[OrganicH2O]

A question states "rank the following conjugate acids in the order you would predict, from most stable to least stable". The question is referring to the specific molecules, and not the conjugate acids of those molecules, so that part is awkwardly worded.

The two molecules are H₂O and NH₃. The correct answer from the homework is definitely NH₃ is more stable than H₂O.

I am a little confused by this question because it seems to be implying "Rank the following acids in stability relative to the stability of their conjugate bases"  Given how the question is phrased, does this seem like a valid question or is this just mind reading? I understand that NH₃ is a weaker acid than H₂O because it's anionic conjugate base is unstable, but does that mean NH₃ is "more stable" than H₂O.

After all, if we phrased the question: rank the relative stability of the following conjugate bases, wouldn't we then predict the opposite order? H₂O would be more stable than NH₃ because there is a bigger energy change for H₂O to form the conjugate acid.

By the logic of this question, we would be saying that the relative stability of these two molecules changes depending on how a question is phrased. But the relative stability of two molecules can't suddenly change based on how somebody phrases a question, can it?

And as a side note, isn't basicity the better judge of stability? Water's lone pairs are in lower energy orbitals because oxygen is more electronegative. So in terms of raw stability, isn't water more stable than NH₃?

[hollytara]

The unspoken truth of any argument from stability is "relative to...."

The conventional discussion of carbocations looks at stability of 1o vs 2o vs 3o.  But the starting material for a 3o cation has to have a crowded Carbon with 3 alkyl groups and a leaving group.  Changing to a carbocation will relieve some steric strain at that C.  So... is it that 3o cations are more stable or 3o halides are less stable ... or a little of both. 

These are not easy questions with absolute answers.

But H2O is a stronger acid than NH3; and NH3 is a stronger base than H2O.  How you assess relative energies won't change that.

[OrganicH2O]

Not to distract from my original question, but I think with carbocation stability it actually is possible to use thermodynamic data to prove that increasing substitution is stabilizing a carbocation. Specifically, thermodynamic data shows that more substituted alkanes are more stable than their linear counterparts. E.g. the rearrangement of tert-butyl bromide to isobutyl bromide is actually up hill in energy, and the rearrangement of n-butyl bromide to sec-butyl bromide is downhill in energy. So I think if branching in the starting material is actually increasing stability, then the ease of forming more substituted carbocations must come from the inherent stability of the carbocation, and not destabilization of the starting material.

I was not able to find data on the relative stability of tert-butyl bromide vs sec-butyl bromide, but I'd guess it would show the same trend.

[OrganicH2O]

Unfortunately none of those examples of thermodynamic data actually change the carbon skeleton. But thermodynamic data does show  that isobutane is more stable than n-butane.