The negative value of the standard electrode potential
in the second case indicates that hydrogen ions can oxidise zinc (or
zinc can reduce hydrogen ions) In view of this convention, the half reaction for the Daniell cell in
Fig 2 1 can be written as:
Left electrode: Zn(s) ® Zn
2+ (aq, 1 M) + 2 e
–
Right electrode: Cu
2+ (aq, 1 M) + 2 e
– ® Cu(s)
The overall reaction of the cell is the sum of above two reactions
and we obtain the equation:
Zn(s) + Cu
2+ (aq) ® Zn
2+ (aq) + Cu(s)
emf of the cell = Eo
cell = Eo
R – Eo
L
= 0