Page 102 - Basic physical chemistry for the atmospheric sciences

88 Basic physical chemistry

HA(aq) + H20(l +z ) H 3 0 + ( aq) + A - ( aq) (5.8)
we can measure the strength of HA by the equilibrium constant for the
forward reaction of (5.8)

(5.9)

Ka is called the acid-dissociation (or ionization) constant for HA.
The dissociation constant for the conjugate base A - of HA is given
by the equilibrium constant for the forward reaction of
A - ( aq) + H20(1) +z HA(aq) + OH - (aq) (5 . 1 0)

which is
[HA(aq)][OH -(aq)]
Kb (5 . 1 1 )
[A -(aq)]
5
From Eq . (5 .9) and ( . 1 1 )
s
KaKb = [Hp + (aq)][OH - (aq)] (5 . 1 2)
+
From Eqs. (5. 1 2) and (4.22), remembering that [H30 + ( aq)] = [H ( aq)],
we see that
(5 . 1 3 )

where, Kw is the ion-product constant for water. Thus, the product o f
the acid-dissociation constant f o r an acid and the base-dissociation
f
f
constant o r its conjugate base is equal to the ion-product constant o r
water. Some values of acid-dissociation constants are given in Appen­
dix IV . Values of the corresponding conjugate base-dissociation con­
stants can be derived from the acid-dissociation constant u s ing Eq.
(5. 1 3 ).
The strongest acid that can exist in water is H +(aq), because any
material that releases a proton more readily than water will completely
tt
ionize in water to form + (aq); for example, HCl, H 2 S04, and HN03
l
are 1 0 0% ionized in water. Similar y , OH - (aq) is the strongest base
that can exist in water, because any base that is a stronger proton
acceptor will remove protons from the water to become what is called
2
2
)
completely protonated [e . g . , s - ( aq , 0 - ( aq) and H - (aq) are 1 0 0%
protonated in water] .