What are acid and base dissociation constants?

1 Answer
Sep 7, 2014

They are equilibrium constants for the ionization reactions of acids and bases.

When an acid dissolves in water, it reacts according to the equation

HA + H₂O ⇌ H₃O⁺ + OH⁻

We can write an equilibrium constant expression

#K_"eq" = "[H₃O⁺][OH⁻]"/"[HA][H₂O]"#

Since [H₂O] is a constant, we can write

#K_"eq" × "[H₂O]" = "[H₃O⁺][OH⁻]"/"[HA]"#

We can define a new constant, the acid dissociation constant, #K_a = K_"eq" × "[H₂O]"#. So

#K_a = "[H₃O⁺][OH⁻]"/"[HA]"#

For a base, we can write the equilibrium reaction as

B + H₂O ⇌ BH⁺ + OH⁻

We can define the base dissociation constant, #K_b = K_"eq" × "[H₂O]"#. So

#K_b ="[BH⁺][OH⁻]"/"[B]"#

Thus, the acid and base dissociation constants are the equilibrium constants without the term for the concentration of water.