PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 377

Note:The dissociation of a weak acid, equation (11.52), may be also written as

HA + H 2 O = H 3 O++ A−,

see the Note in section11.1.2. Then we have for the dissociation constant

K=

cH 3 O+cA−γ^2 ±

aH 2 OcHAγHA

1

cst

ForaH 2 O= 1, which applies practically every time, this relation is identical with equation

(11.53), sincecH 3 O+=cH+.

11.5.4 Dissociation of a weak monoacidic base

A weak monoacidic base BOH dissociates according to the equation

BOH = B++ OH−. (11.55)

The equilibrium constant of this reaction is called thebase dissociation constant, for which
we have

K=

cB+cOH−γ^2 ±

cBOHγBOH

1

cst

. (11.56)

If we neglect the contribution of the ions formed in consequence of water dissociation to the
concentration of the hydrogen ions, we may rewrite equation (11.56) to

K=

α^2 c γ±^2

(1−α)γBOH

1

cst

, (11.57)

wherecis the initial concentration of the base andαis the degree of dissociation.

11.5.5 Dissociation of weak polybasic acids and polyacidic bases

A weak dibasic acid H 2 A dissociates in the first stage according to the equation

H 2 A = H++ HA− (11.58)