[Big-Daddy]

Can Kb be defined in terms of reducing H₃O⁺ or must it be producing OH^-?

For example, for NH₃, it seems we have two choices: NH₃ (aq) + H₃O⁺  NH₄⁺ + H₂O, or NH₃ (aq) + H₂O NH₄⁺ + OH^-. Which is the actual definition? Or are they both ocurring and equivalent in terms of value?

What about the action of NH₄⁺ from a salt? It should just be the reverse reaction of whichever one occurs for NH₃ ... (i.e. so Ka[NH₄⁺]=1/K_b[NH₃])

[AWK]

K_a x K_b = K_w

[Dan]

Can Kb be defined in terms of reducing H₃O⁺ or must it be producing OH^-?

Careful with your terminology, it is a deprotonation not a reduction. If you mean reduction in the sense of "decreasing concentration of", use "consuming" etc. - "reduction" is not a good word to use because it implies redox chemistry in this context.

For example, for NH₃, it seems we have two choices: NH₃ (aq) + H₃O⁺  NH₄⁺ + H₂O, or NH₃ (aq) + H₂O NH₄⁺ + OH^-. Which is the actual definition? Or are they both ocurring and equivalent in terms of value?

The answer to your question is written here: http://en.wikipedia.org/wiki/Base_dissociation_constant#Bases

What about the action of NH₄⁺ from a salt? It should just be the reverse reaction of whichever one occurs for NH₃ ... (i.e. so Ka[NH₄⁺]=1/K_b[NH₃])

No, see AWK's post and again it is written here as well: http://en.wikipedia.org/wiki/Base_dissociation_constant#Bases

[Big-Daddy]

Careful with your terminology, it is a deprotonation not a reduction. If you mean reduction in the sense of "decreasing concentration of", use "consuming" etc. - "reduction" is not a good word to use because it implies redox chemistry in this context.

Yeah, sorry, I meant "decreasing concentration of". I see how this would be a problem (it's just that I was typing at 3 AM!).

No, see AWK's post and again it is written here as well: http://en.wikipedia.org/wiki/Base_dissociation_constant#Bases

OK, so the clear choice for this case is NH₃ (aq) + H₂O  NH₄⁺ + OH^-. Meanwhile, if we describe this equilibrium I just wrote with the constant K_b, K_a for the action of the conjugate base NH₄⁺ is equal to K_w/K_b suggesting the reaction is NH₄⁺ + H₂O  H₃O⁺ + NH₃.

My difficulty with Wikipedia's definition is: say we have an m-protic base B(OH)_m in solution - if it dissociates OH^- ions would that not itself make it a base? e.g. B(OH)_m  B(OH)_m-1⁺ + OH- being Kb1, B(OH)_m-1⁺  B(OH)_m-2²⁺ + OH^- being Kb2, etc. or am I writing the wrong reactions, because in these cases the conjugate acid forms are not being associated to protons? e.g. a concrete example: what are the two dissociations of Ca(OH)2? I would think the reactions were Ca(OH)₂   CaOH⁺ + OH^- and CaOH⁺  Ca²⁺ + OH^- but there does not appear to be proton association here ...

[AWK]

Eg for diprotic:
K_a1 x K_b2 = K_w
K_a2 x Kb₁ = K_w
Number 1 is for the greatest K (a or b)
For triprotic
K_a1 x K_b3 = K_w and so on

[Big-Daddy]

Eg for diprotic:
K_a1 x K_b2 = K_w
K_a2 x Kb₁ = K_w
Number 1 is for the greatest K (a or b)
For triprotic
K_a1 x K_b3 = K_w and so on

Thanks. And to clarify, the Ka values refer to the action of the conjugate acid if it were put into solution (e.g. if you have a salt containing NH₄⁺, it will have a Ka referring to converting to NH₃, and we can get this Ka value from Kw/(Kb of NH₃)) and Kb to the action of the base itself put into solution. Correct?

[AWK]

K_a refers to protolysis of acids (eg NH₄⁺) -proton is moved from acid to water molecule
K_b refers to the protolysis of bases eg. NH₃) - proton is moved from water to base
Numbering of K_a or K_b for polyprotic species depends on value of K. To the highest value number 1 is assigned (for monoprotic acids or bases both numbers at K are equal to 1). Then the sum of numbers n+m at K_an and K_bm in the equation K_an x K_bm = Kw is equal to max. number of exchangeable protons plus 1 (both K_an and K_bm refer to conjugated species)

[Big-Daddy]

K_a refers to protolysis of acids (eg NH₄⁺) -proton is moved from acid to water molecule
K_b refers to the protolysis of bases eg. NH₃) - proton is moved from water to base
Numbering of K_a or K_b for polyprotic species depends on value of K. To the highest value number 1 is assigned (for monoprotic acids or bases both numbers at K are equal to 1). Then the sum of numbers n+m at K_an and K_bm in the equation K_an x K_bm = Kw is equal to max. number of exchangeable protons plus 1 (both K_an and K_bm refer to conjugated species)

Thanks.

What if we have a base B(OH)m which starts dissociating OH^- ions? e.g. B(OH)_m  B(OH)_m-1⁺ + OH^- being Kb1, B(OH)_m-1⁺  B(OH)_m-2²⁺ + OH^- being Kb2, (we can assume Kb1>Kb2) etc. or am I writing the wrong reactions, because in these cases the conjugate acid forms are not being associated to protons? e.g. a concrete example: what are the two dissociations of Ca(OH)2? I would think the reactions were Ca(OH)₂   CaOH⁺ + OH^- and CaOH⁺  Ca²⁺ + OH^- but there does not appear to be proton association here ...

[AWK]

For pH calculations we treat calcium hydroxide as a strong base

[Big-Daddy]

For pH calculations we treat calcium hydroxide as a strong base

That isn't relevant to my question. I was just using calcium hydroxide as an example, my real question relates to the (weak) base B(OH)_m and whether I am writing the Kb expressions for it correctly, since it does not seem to be associating to any protons, only dissociating hydroxide ions.