Chemistry, Third edition

16 · ACID–BASE EQUILIBRIA

Sulfuric acid – a diprotic acid

Sulfuric acid possesses two acidic hydrogens and is said to be diprotic. There are two

ionization steps, with sulfuric acid itself being the acid in the first step, and the

hydrogensulfate ion being the acid in the second step:

First ionization

H 2 SO 4 (aq)H 2 O(l)\===\H 3 O(aq)HSO 4 (aq) Ka(298 K) large

hydrogensulfate

ion

Second ionization

HSO 4 (aq)H 2 O(l)\===\H 3 O(aq)SO 42 (aq) Ka(298 K) 0.012 mol dm^3

The net reaction is the sum of both steps:

H 2 SO 4 (l)2H 2 O(l)\===\2H 3 O(aq)SO 42 (aq)

TheKavalues show that sulfuric acid is a strong acid and that the hydrogensulfate ion

is a relatively weak acid (although much stronger than most of the acids listed in

Table 16.2).

Dilute sulfuric acid of concentration 0.0100 mol dm^3 contains an H 3 O(aq)

concentration of 0.0145 mol dm^3 , giving a pH of 1.84. Of the total [H 3 O(aq)],

0.0100 mol dm^3 is contributed by the first ionization, and only 0.0045 mol dm^3

of the total [H 3 O(aq)] is provided by the second ionization. If both Kavalues

were large, each reaction would contribute a hydronium ion concentration of

0.0100 mol dm^3 , and the pH of the dilute acid would then be log[0.0200]1.70.

The HSO 4 ion is more than strong enough to be used as an acid in its own right,

and sodium hydrogensulfate crystals (Na,HSO 4 ) are used as a powerful disinfec-

tant. They are also provided in chemistry sets as a safe substitute for sulfuric acid.

Strong and weak bases

The commonest strong bases are the water-soluble hydroxides of sodium, potassium,

calcium, barium and lithium. These are all ionic solids. Since they are strong bases

they are completely ionized in water. For example, NaOH(s) breaks up completely in

water producing Na(aq) and OH(aq):

H 2 O

Na,OH(s) —Na(aq)OH(aq)

sodium hydroxide hydroxide ion

For example, a solution containing 0.5 mol of dissolved NaOH per dm^3 of solution

contains 0.5 mol dm^3 of OH(aq).

A second category of bases are those which produce hydroxide ions in solution by

reactionwith water. An example is ammonia:

NH 3 (aq)H 2 O(l)\===\NH 4 (aq) OH(aq)

Ammonia is a weak basebecause it is incompletely ionized in solution, and

Kb(T)

[NH 4 (aq)][OH(aq)]

[NH 3 (g)]

whereKb(T)is the basicity constant(ordissociation constant) of ammonia at

temperatureT. Generalizing, the basicity constant for a base B is the equilibrium

constant for the reaction:

B(aq)H 2 O(l)\===\BH(aq)OH(aq) (16.8)

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