PHYSICAL CHEMISTRY IN BRIEF

CHAP. 11: ELECTROCHEMISTRY [CONTENTS] 350

trality. Every part of a system also stays electroneutral, which is called thelocal condition
of electroneutrality.

Note: On the molecular level, the local condition of electroneutrality means that in the

near vicinity of a given ion there are primarily ions of the opposite charge which partly

neutralize the given ion charge. In this connection we say that every ion is surrounded by

ionic atmosphere.

The condition of electroneutrality can be expressed mathematically: let us have a compound
Kν+Aν−which dissociates according to the equation

Kν+Aν−=ν+Kz

+

+ν−Az

−

, (11.1)

where Kz

+

is a cation carrying chargez+, and Az

−
is an anion carrying chargez−. We thus
have
ν+z+=ν−z−. (11.2)

This relation can be rewritten to the form

c+z+=c−z−, (11.3)

wherec+is the concentration of cations andc−the concentration of anions. When there are
several kinds of ions present in a system, the generalized condition applies

∑

i

c+izi+=

∑

i

c−iz−i. (11.4)

11.1.5 Degree of dissociation.

Thedegree of dissociationαis a special case of the degree of conversion, see8.2.2, for the
dissociation reactions of the type (11.1). If we divide the numerator and the denominator in
relation (8.5) by the volume, we obtain the degree of dissociation expressed using the initial
and equilibrium concentrations of the dissociating substance

α=

ci 0 −ci

ci 0

. (11.5)