The Auto-Ionization of Water

In understanding the Brønsted/Lowry definition of acids and bases, we ran across the idea that water is amphoteric - that is, that water can be both an acid and a base. The question that naturally occurs is “What happens when water (the acid) reacts with water (the base)?” The answer is this:

H2O + H2O ⇄ H3O+1 + OH-1 

Which can, more simply, be written as 

2 H2O ⇄ H3O+1 + OH-1 

The equilibrium expression for this would be

where Kw is the equilibrium constant for this specific reaction with a value of 1.00x10^-14. (Remember that solids and liquids are not included in equilibrium expressions.)

The Two Important Take-Aways

1) [H3O+1] is mathematically tied to the [OH-1]. If you know one of them, you can always find the other using the equilibrium expression.

2) Because [H3O+1]・[OH-1] is always equal to the constant K_w, as one value goes up, the other MUST go down. So a solution with a high concentration of hydronium (an acid) necessarily has a low concentration of hydroxide. A solution with a high concentration of hydroxide (a base) necessarily has a low concentration of hydronium.