Worked out examples

1. What is the potential for the cell shown below
Zn | Zn²⁺ (1.0 M) || Cu²⁺ (1.0 M) | Cu

Solution: From the table of standard reduction potentials we have the following values

Cu²⁺ + 2 e^- Cu . . . E° = 0.337 – – – (1)
Zn Zn²⁺ + 2 e^- . . . E^* = 0.763 – – – (2)

Add (1) and (2) to yield

Zn + Cu²⁺Zn²⁺ + Cu . . . ΔE°
ΔE° = E° + E^*
= 1.100 V

Note that E^* is the oxidation standard potential, and E° is the reduction standard potential,
E^* = - E°. The standard cell potential is represented by ΔE°.

2. What is the potential for the cell
Ag | Ag⁺(1.0 M) || Li⁺(1.0 M) | Li

Solution: From the table of standard reduction potentials, you find

Li⁺ + e Li . . . E° = - 3.045, – – – (3)
Ag Ag⁺ + e . . . E^* = - 0.799, – – – (4)

According to the convention of the cell, the reduction reaction is on the right. The cell on your left–hand side is an oxidation process. Thus, you add (4) and (3) to obtain

Li⁺ + Ag Ag⁺ + Li. . dE° = - 2.246 V.

How to remember?

OIL RIG: Oxidation Is Losing; Reduction Is Gaining (electrons)
AN OX: (ANode is where OXidation occurs)
RED CAT: (REDuction occurs at the CAThode)