Chemistry, Third edition

290 16 · ACID–BASE EQUILIBRIA

the acidity of a solution. The strength of an acidin water is a property of the acid

molecule, whereas the acidity of a solutionis a qualitative desciption of its pH. A strongly

acidic solution is usually regarded as one with a pH of below 3. A weakly acidic solu-

tion has a pH of 3–7. A low pH might result from a concentrated solution of a weak

acid or from a very dilute solution of a strong acid. But hydrochloric acid is a strong

acid whether its concentration is 0.5 mol dm^3 or 0.0005 mol dm^3.

Weak acids

Ethanoic acid is only partially ionized in aqueous solution:

CH 3 COOH(aq)H 2 O(l)\===\CH 3 COO(aq)H 3 O(aq) (16.3)

ethanoate

(or acetate) ion

and is therefore termed a weak acid:

a weak acid is incompletely ionized in solution

If we apply the equilibrium law to equation (16.3) the equilibrium expression

becomes

Ka(T)

[CH 3 COO(aq)][H 3 O(aq)]

[CH 3 COOH(aq)]

whereKa(T)is the equilibrium constant (‘a’ is for acid) at temperature T. As usual,

the square brackets refer to the equilibrium (not starting) concentrations. Once

again, the concentration of water (a constant) has been incorporated into Ka(T)– this

will be taken for granted in similar expressions below.

The term Ka(T)is commonly known as the acidity constant(or the dissociation

constant) of the acid. For ethanoic acid, Ka(25 °C) 1.8 10 ^5 mol dm^3 , the low

value confirming that very few of the acid molecules have ionized. Hydrogen

fluoride:

HF(aq)H 2 O(l)\===\F(aq)H 3 O(aq)

hydrogen

fluoride hydrofluoric acid

is also a weak acid with Ka(25 °C) 3.5 10 ^4 mol dm^3.

Table 16.2 shows the acidity constants for selected acids at 25 °C. Symbolizing the

acids as AH, the acidity constants are the equilibrium constants for the general reaction

AH(aq)H 2 O(l)\===\A(aq)H 3 O(aq) (16.4)

(If AH is a solid, liquid or gas at room temperature (e.g. C 6 H 5 COOH(s),

CH 3 COOH(l), or HCl(g)) we assume that AH first dissolves in water making

AH(aq), which then reacts according to this equation.)

The higher the Kavalue, the stronger the acid.

The acidity constants of strong acids are difficult to determine experimentally

because the concentration of unionized molecules is minute, but there is no doubt

that the acidity constants of strong acids are much higher than those of the weaker

acids listed in Table 16.2. This reflects the fact that the acid molecules are almost

entirely dissociated in solution.

We may also define pKa, where

pKalogKa

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