Page 93 - Basic physical chemistry for the atmospheric sciences

Solution chemistry and aqueous equilibria

Weak electrolytes ionize incompletely in water. For example, accl i c
l
acid dissolves only partial y in water (to form vinegar)

but only a small fraction of the acetic acid forms ions in this way, so
the solution is only a weak electrical conductor. The ionization of
CH3COOH(I) represented by Reaction (4 . 1 9 ) differs from that of
.
HCl(g) in Reaction (4 1 8 ) in a significant way; namel , it is a truly
y
reversible reaction [indicated by the two-way arrows in Reaction
(4. 1 9 )] and can therefore be represented by an equilibrium-constant
expression.
Water itself is a very weak electrolyte

H 2 0(1) � H + (aq) + OH -(aq) (4.20)

where H + (aq) and OH -(aq) are the hydrogen and hydroxide ions,
respectivel , in solution. The longer arrow from right to left in Reac­
y
tion (4.20) can, if desired, be used to emphasize that there are many
more H 0 molecul e s than there are H + (aq) and OH - ( aq) ions. We
2
could define an equilibrium constant for Reaction (4.20) by
[H + (aq)][OH - ( aq)]
K = (4. 2 1 )
[HzO(I)]

However, the concentration of H 0 molecules in water is so large
2
(55 . 5 M), and so few ions are formed that [H20(1)] is virtually constant.
Therefore, [H20(1)] is usually combined with K and called the ion­
product constant o r water (Kw)
f
(4.22)

where,

.
Kw = [H 0(1)]K = 55.5K (4 2 3)
2
At 25°C, Kw = 1 . 00 x 1 0 - 1 4 (when concentrations are expressed in
moles per liter).
Exercise 4.8. What are the concentrations of H + ( aq) and OH -(aq)
in pure water at 25°C?
Solution. In pure water

[H + (aq)] = [OH - ( aq)]