[maccha]

When a weak acid is dissociated in water, I know that this equilibrium occurs:

A- + H20 <--> HA + OH-

But does this equilibrium also occur?

HA <--> A- + H+

Is this basically the same thing? I'm so confused. My prof wrote out that both will occur..

[Borek]

When you start with a weak acid second equation describes better what is happening in the solution. First equation better fits the situation when you start with a salt of a weak acid.

Are they the same thing? Yes, once you realize that there is another equilibrium present:

H₂O <-> H⁺ + OH^-

When you remember about it A^- + H₂O is in a way equivalent to A^- + H⁺ + OH^-.