What are acid and base dissociation constants?

Question

5466 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2014-09-07T14:54:58

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.