Page 270 - Physical chemistry understanding our chemical world

PROPERTIES OF LOWRY–BRØNSTED ACIDS AND BASES 237

Table 6.2 Values of the autoprotolysis constant K w as a function of
temperature
◦
Temperature T / C 0 18 25 34 50
K w × 10 14 0.12 0.61 1.04 2.05 5.66
Source: Physical Chemistry, W. J. Moore (4th Edn), Longmans, London, 1962,
p. 365.

The value of K w is 1.04 × 10 −14 at 298 K when expressed in
−3
concentration units of mol dm . Like all equilibrium constants, its Note how K w has units.
value depends on the temperature. Table 6.2 lists a few values of
K w as a function of temperature. Note how K w increases slightly K w is often re-ex-
as the temperature increases. pressed as pK w ,where
It should now be clear from Equation (6.4) how water splits the ‘p’ is a mathe-
(dissociates) to form equal number of protons and hydroxide ions, matical operator (see
hence its neutrality, allowing us to calculate the numbers of each p. 246). pK w has a
from the value of K w . value of 14 at 298 K.

Worked Example 6.1 What is the concentration of the solvated proton in super-pure
water?

+
−
Since [H 3 O (aq) ] = [OH (aq) ], we could rewrite Equation (6.4) as
+
K w = [H 3 O (aq) ] 2
Taking the square root of both sides of this expression, we obtain

[H 3 O (aq) ]/mol dm −3 = K w = [10 −14 1/2
+
]
so
+
[H 3 O (aq) ] = 10 −7 mol dm −3
The concentration of solvated protons in super-pure water is clearly very small.

What is ‘acid rain’?

Hydrolysis

Acid rain is one of the worst manifestations of the damage we, as humans, inflict on
our planet. Chemicals combine with elemental oxygen during the burning of fossil
fuels, trees and rubbish to generate large amounts of ‘acidic oxides’ such as nitrogen
monoxide (NO), carbon dioxide (CO 2 ) and sulphur dioxide (SO 2 ).
Natural coal and oil contain many compounds of nitrogen. One of the worst products
of their combustion is the acidic oxide of nitrogen, NO. At once, we are startled by
this terminology, because the Lowry–Brønsted definition of an acid involves the
release of a proton, yet nitrogen monoxide NO has no proton to give.