175Electrochemical Kinetics
For the case when there is a thermodynamic equilibrium, the forward and
reverse reactions must balance each other so that the net current density is zero.
j
i
– j
2
= 0. (5.54)
Thus, for a thermodynamic equilibrium condition,
j
1
= j
2
= j
0
.
(5.55)
j
0
is called the exchange current density for the reaction. At equilibrium, the net
reaction rate is zero, though forward and reverse reactions take place.
The exchange current density is now:
j nFC ke nFC k
FE ERT
00 0
0
−−
Ox,surfRed Red,surf
eq
α ()/
OOx
eq
e
FE ERTβ
/
,
−
0
(5.56)
where E
eq
is the equilibrium potential. The rate constants k
0Red
and k
0Ox
are
dened as
kkThe
GRT
0
1
Ox B
=
−
(/)
/∆
(5.57)
kkThe
GRT
0
2
RedB
=
−
(/
/∆
(5.58)
For equilibrium condition, the rate constants are equal.
k ...