[FouRRaW]

Hi, i'm supposed to justify this statement

If the following reaction is seen to occur, then HA must be stronger than HB. (Where H is a hydrogen atom and A and B are pretty much any molecule i think)

HA + B^- <--------->   HB + A^-           notice the equlibrium

attempt:   All i know or think is that A^-[/b] can't be very weak.  otherwise it couldn't bring the reaction back eh?  (reverse it) 
Arn't conjugates always the opposite?? like HCl (a strong acid)  isnt' the Conjugate a weak base always?? or no?

Thanks
 

[hmx9123]

The reaction may be at equilibrium, but the equilibrium may lie well toward the products side.  You may be thinking of equilibrium in more mathematical terms, such as 1 = 1.  That is not necessairly the case when talking about chemistry, though, when the equilibrium may lie toward one side or the other.  If HA is a stronger acid than HB, then the equilibrium should lie to the right, as you have stated.  However, there will always be a small percentage of HA that doesn't dissociate, no matter how strong.

[FouRRaW]

K, but why does HA have to be stronger than HB?

[Yggdrasil]

In the equation you wrote, note that HA and HB are acids and A^- and B^- are bases.  Also, HA and A^- are a pair of conjugate acids and bases and HB and B^- are another pair of conjugate acids and bases.  The stronger acid will produce the weaker conjugate base and the stronger conjugate base will produce the weaker conjugate acid.  So, for example, if HA is the stronger acid, A^- will be the weaker base (and B^- will be the stronger base).

Acid/base reactions will generally favor the side which produces the weaker acid and the weaker base.  So if the reaction favors the product side, HB will be the weaker acid and A^- will be the weaker base.  This also means that HA will be the stronger acid and B^- will be the stronger base.

[FouRRaW]

ah ha thanks