We know that the neutralization reaction:
It is practically quantitative. In fact, as all the chemical reactions are actually equilibrium reactions, and it will remain a very small amount of H3O+ and OH- unreacted.
The same reaction, roles reversed, takes place in any aqueous solution:
This equilibrium is called water self ionization or water autoionization. Water can either donate or accept a proton. In the reaction of self ionization, a water molecule accepts a proton, while the other donates a proton. Considering that, as we said, the neutralization reaction H3O+/OH- is virtually quantitative, the self-ionization equilibrium is shifted to the left.
Indeed it is so, but there's always a minimum concentration of H3O+ and OH- in solution. This is experimentally demonstrated with electrical conductivity experiments.
The constant auto-ionization of water: Kw
What is really important is that we are also able to calculate the equilibrium constant of this reaction.
Since [H 2 O] can be considered constant and equal to about 55.6 M, you can incorporate in the equilibrium constant.
The value of the equilibrium constant depends only on the temperature, and this is the value for the temperature of 25 ° C.
This expression is very important. It tells us that in any aqueous solution are H 3 O + and OH -. But since H 3 O + and OH - are respectively the acid and the strongest base that can exist in aqueous solutions, we can also deduce that:
When → the solution is acidic
When → the solution is basic
When → the solution is neutral
Moreover, if the solution is neutral you can calculate that semplcemente , For any volume of solution.
I a 0.1 M aqueous solution of hydrochloric acid.
H 3 O +, which is derived from the balance of waterself dissociation will be irrelevant compared to what comes from HCl (being completely dissociated H 3 O + will 0.1M).
Since Kw is a constant and must remain so, the concentration of OH - is calculated as:
then replacing or
It then observes, that when in solution increases the concentration of H 3 O +, that of OH - must necessarily decrease.