[ultrashnotebel]

Hi,

I'm somewhat confused: in which case do you use ke/kb to find the ka of a base's conjuguate acid, and when can you use 1/kb?

I mean, as far as I understand it, and acid reaction can be written like this:

HA + H2O -> A- + H3O+

And the basic reaction from its conjuguate base it:

A- + H2O -> HA + OH-

Therefore if we add the two reactions together, we get:

2H2O -> H3O+ + OH-

Which explains ka.kb=kw

However, in some case I've seen people write the base reaction as:

A- + H3O+ -> HA + H2O

Which is simply 1/ka.

Is this ever correct? Because it actually solves one of my exercises, even though another part of the same exercises uses kw=ka.kb

[chenbeier]

Read this aarticle:

https://www.khanacademy.org/science/chemistry/acids-and-bases-topic/copy-of-acid-base-equilibria/a/relationship-between-ka-and-kb

[ultrashnotebel]

Do you mind taking a look at this? It gives the right answer, but I can't help but feel it isn't correct.

[Borek]

And what is Ke?

[ultrashnotebel]

Kw, in french

[Borek]

However, in some case I've seen people write the base reaction as:

A- + H3O+ -> HA + H2O

Which is simply 1/ka.

I don't see how it is a "base reaction", it is just reversed dissociation.

[Borek]

Do you mind taking a look at this? It gives the right answer, but I can't help but feel it isn't correct.

Checking it is a simple algebra. For each reaction write equilibrium constant, then see if what you did (multiplying the equilibrium constants for the first and second reaction) produces the equilibrium constant for the third reaction.

[ultrashnotebel]

I just kind of just want someone to explain why this isn't correct:

First of all, the question is simply to predict the reaction product of S2- and NH4+, and find its equilibrium constant. We have access to the second ka of H2S (which is the Ka of HS-) and the kb of NH3.

HS- + H2O -> S2- + H3O+        ka = 1.1x10^-12   => kb = kw/ka = 9.1.10^sup]-3[/sup]    which is the constant for the reaction: S2- + H2O -> HS- + OH-

NH3 + H2O -> NH4+ + OH-        kb = 1.774x10^-5   => ka = kw/kb = 5.637x10^-10    which is the equilibrium constant for the reaction: NH4+ + H2O -> NH3 + H3O+

Adding those two reactions together, we find: S2- + NH4+ + 2H2O -> HS- + NH3 + OH- + H3O+

And so by multiplying the constants, we get 51.3x10^-13... Which is not at all the answer I'm given.

[Borek]

As I wrote earlier: write all three reaction equations. For each write formula for the equilibrium constant. Assuming Ka is for NH₄⁺, Kb is for S^2-, and K is the equilibrium constant for the reaction between NH₄⁺ and S^2-, does Ka×Kb=K?

[ultrashnotebel]

Are you messing with me? That's exactly what I just did, and it's still not correct. Have you looked at what I wrote?

I mean no disrespect, but I did see your first reply.

[Borek]

Pleas list all the formulas involved.