Chemistry, Third edition

16 · ACID–BASE EQUILIBRIA

Can we be sure that reaction (16.1) has reached equilibrium at room tempera-

ture? Sophisticated kinetic experimental measurements have shown that reactions in

which protons are lost or gained by molecules in solution are extremely fast. This

means that we may safely assume that all the reactions discussed in this chapter have

already reached equilibrium.

Table 16.1 shows the values of Kwat various temperatures.

286

Table 16.1The ionic product constant of water

at various temperatures

Temperature/°C 1014 Kw/mol^2 dm^6

000 00.114

010 00.293

020 00.681

025 01.008

030 01.471

040 02.916

050 05.476

100 51.3

Variation of Kw

with temperature

The data in Table 16.1

suggest that the

dissociation of water is

endothermic – why?

Exercise 16A

From Table 16.1, at 25 °C

Kw 1.0 10 ^14 mol^2 dm^6

(this is easy to remember) and we can now say that:

When the concentration of hydroxide and hydronium ions in water or in aqueous

solutions are multiplied together, the product is always 1.0  10 ^14 mol^2 dm^6

at 25 °C.

Equation (16.1) shows that two water molecules produce one hydronium and

one hydroxide ion; therefore, the concentration of these ions in water is equal. A

solution containing equal numbers of each ion is said to be neutral. At 25 °C,

1.0 10 ^14 [H 3 O(aq)][OH(aq)]

Therefore,

[H 3 O(aq)][OH(aq)]1.0 10 ^7 mol dm^3

Calculations of pH were introduced in Unit 1 (page 13) and Unit 9 (page 150):

pHlog [H(aq)], or pH  log[H 3 O(aq)]

We can now calculate the pH of pure water (water which contains no dissolved

solids, liquids or gases):

pH of pure water log (1.0  10 ^7 )7.00

(Pure water is rarely encountered. Even distilled or deionized water contains dissolved

carbon dioxide which gives it a pH of about 6. The pH of river water or lake water

may be above 7 if it contains dissolved limestone. Acid rain has a pH as low as 4.)

Definitions of neutral, acidic and basic solutions

Neutral solution

A neutral solution is one where the concentrations of hydroxide and hydronium

ions are equal.At 25 °C, the pH of a neutral solution is 7.

pH measurements require a
meter, a measuring probe
called a glass electrode, and
a reference electrode. These
are often packed together in a
compact case. pH meters are
used to provide rapid and
accurate measurements of
the pH of soils, of waste
waters and of swimming
pools.