Page 269 - Physical chemistry understanding our chemical world

236 ACIDS AND BASES

Dissolving a solute generally shifts the reaction in Equation (6.2) from left to
right, thereby increasing the concentration of ionic species in solution. This increased
number of ions causes the conductivity κ of water to increase, thereby making it a
fatally efficient conductor of charge.

Why is bottled water ‘neutral’?

Autoprotolysis

The labels of many cosmetic products, as well as those on most
The word ‘criterion’ is bottles of drinking water, emphasize how the product is ‘neutral’,
used of a principle or implying how it is neither acidic nor alkaline. This stipulation is
thing wechoosetouse deemed to show how healthy the water is. But how do they know?
as a standard when
And, furthermore, what is their criterion for testing?
judging a situation. A better way of defining ‘neutral’ is to say equal numbers of
The plural or crite-
protons and hydroxide ions reside in solution (both types of ion
rion is ‘criteria’, not
‘criterions’. being solvated). How does this situation arise? Autoprotolysis,as
mentioned above, represents the self-production of protons, which
is achieved by the splitting of water according to Equation (6.2).
‘Autoprotolysis’ comes It is clear from Equation (6.2) how the consequence of such auto-
from proto–indicating protolytic splitting is a solution with equal numbers of protons and
the proton, and lysis, hydroxide ions.
which is a Greek root When water contains no dissolved solutes, the concentrations of
meaning ‘to cleave the solvated protons and the hydroxide ions are equal. Accord-
or split’. The prefix ingly, from our definition of ‘neutral’ above, we see why pure
auto means ‘by self’ + −
water should always be neutral, since [H 3 O (aq) ] = [OH (aq) ].
or ‘without external As with all physicochemical processes, the extent of Equation
assistance’.
(6.2) may be quantified by an equilibrium constant K.We callit
the autoprotolysis constant, as defined by

−
+
[H 3 O (aq) ][OH (aq) ]
K = (6.3)
[H 2 O] 2
The water term in the denominator of Equation (6.3) is always large when com-
pared with the other two concentrations on the top, so we say it remains constant.
This assumption explains why it is rare to see the autoprotolysis constant written as
Equation (6.3). Rather, we usually rewrite it as

+
−
K w = [H 3 O (aq) ][OH (aq) ] (6.4)
2
where K in Equation (6.3) = K w × [H 2 O] in Equation (6.4). We will only employ
Equation (6.4) from now on. We call K w the autoprotolysis constant or ionic product
of water.