An Introduction to Environmental Chemistry

The Chemistry of Continental Solids 81

Box 4.5 Dissociation

Dissociation and dissociation constants

(Kdand Ka)

Many chemical components can be described

as ‘ionizable’ meaning they can dissociate

to form charged species (ions). In order to

describe the degree to which compounds

are susceptible to ionization, dissociation

constants (Kd) are derived at equilibrium.

These dissociation constants are a ratio of

dissociated species to undissociated species.

Thus, the larger the value of Kdthe greater

the proportion of dissociated species at

equilibrium. Where protons are generated

the term Kdcan be replaced with the Ka(the

acid dissociation constant, see Box 3.3). Thus,

Kaprovides an indication of a given acid’s

strength. In general:

eqn. 1

where cHA is the concentration of the

undissociated species, cA-is the

concentration of the dissociated base and

cH+is the concentration of dissociated

protons.

For example, phenol dissociates to form

phenate ions and a proton:

eqn. 2

For this equilibrium the dissociation constant

is derived using the expression:

eqn.3

In the case of phenol the value of Ka=1.1¥

10 -^10. This very small value shows that phenol

is a weak acid as there is a far greater

proportion of undissociated phenol than

there is of phenate ions and protons. It is

more common to convert Kavalues to a

KKc c

c

daPhenate H

Phenol

== ◊+

OH O–

+ H+

Phenol Phenate Proton

K cc

a c

AH

HA

= ◊

-+

negative log scale (pKa), similar to that for pH

(see Box 3.5), i.e.:

eqn. 4

Thus, for phenol:

eqn. 5

Dissociation of a hydroxyl functional group is

dependent upon the nature of the rest of the

molecule to which it is attached.

Figure 1 indicates how pKavalues vary

for phenols containing different additional

functional groups (see Section 2.7.1). In the

case of o-nitrophenol the presence of the

nitro functional group (-NO 2 ) increases the

pKavalue, making it more acidic than phenol.

In the case of o-cresol the presence of the

methyl functional group (-CH 3 ) decreases the

pKavalue, making it less acidic than phenol.

These differences are due to the electron-

withdrawing or electron-donating nature of

the functional groups. The electron-

withdrawing/donating nature of a functional

group is governed by the electronegativity

(Box 4.2) of the atoms comprising the

function group, or more specifically by the

polarity of the bonds between these atoms.

The different pKavalues in Fig. 1 are related

to the stability of the phenate ion that forms

as a product of dissociation (Fig. 2). In o-

cresol the negative charge can not be

delocalized (see Section 2.7) onto the methyl

functional group (as it is electron-donating).

Thus, delocalization of the negative charge

is only possible over the aromatic ring. In

o-nitrophenol the negative charge of the

phenate ion can be drawn over a greater

number of atoms as a result of the nitro

functional group being electron-withdrawing.

Thus, the negative charge can be delocalized

over both the aromatic ring and the nitro

functional group. It is this greater

delocalization of the negative charge ino-

nitrophenol that increases the stability of the

phenate ion. As a consequence of this

greater stability the dissociation of the OH-

group on o-nitrophenol is energetically more

favourable then the dissociation of the OH-

group on o-cresol, making o-nitrophenol

more acidic than o-cresol.

pKa=-log. 10 11 10¥ -^10 = 996.

pKKaa=-log 10

(continued on p. 82)