DAT destroyer GC question

[Veni vidi vicii]

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NH2OH has a kb of 1.1x10^-8. What represents the dissociation equation? the answer was NH2OH + H20 <---> NH3(+)OH + OH-.

Can an alternative answer be NH2OH + H20 <---> NH(-)OH + H3O+. Since water has kb value of 1x10^-7, it would be the stronger base and deprotonate. Or am I thinking of this wrong? In the answer it says since kb was given you know it's a base so hydroxide ions need to be produced.

 

[whitecoater]

Can an alternative answer be NH2OH + H20 <---> NH(-)OH + H3O+

No, precisely because Kb of H20 is greater (10x) than Kb of NH2OH, so that H20 can only act as conjugate acid against more basic NH2OH.

 

[Veni vidi vicii]

No, precisely because Kb of H20 is greater (10x) than Kb of NH2OH, so that H20 can only act as conjugate acid against more basic NH2OH.

I thought a higher Kb meant more basic, similar to a low pKb, so I would think water would 10x more basic?

 

[whitecoater]

I thought a higher Kb meant more basic, similar to a low pKb, so I would think water would 10x more basic?

Using Bronsted-Lowry definition, acid H20 donates H+, base NH2OH accepts H+.
Using Arrhenius definition, amphoteric H20 produces both H+ and OH-.

 

[Veni vidi vicii]

Using Bronsted-Lowry definition, acid H20 donates H+, base NH2OH accepts H+.
Using Arrhenius definition, amphoteric H20 produces both H+ and OH-.

That's nice and all but that just brings us back to what I originally asked. Why is water acting as an acid when it's 10x more basic than NH2OH?

 

[whitecoater]

That's nice and all but that just brings us back to what I originally asked. Why is water acting as an acid when it's 10x more basic than NH2OH?

Because it just happens that H2O is an amphoteric compound that also produces OH-, not just H+ (equal amount at pH=7). If NH2OH reacts with H2S that carries H+ but not OH-, for example, resulting pH would be lower due to the lack of OH-, though stronger acid H2S still donates its H+ to NH2OH.

NH2OH + H2S <-> NH3 • H2O + S

 

[Veni vidi vicii]

Because it just happens that H2O is an amphoteric compound that also produces OH-, not just H+ (equal amount at pH=7). If NH2OH reacts with H2S that carries H+ but not OH-, for example, resulting pH would be lower due to the lack of OH-, though stronger acid H2S still donates its H+ to NH2OH.

NH2OH + H2S <-> NH3 • H2O + S

Gotcha