Given : `Zn^(2+)(aq)+2e rarr Zn(s), E^(@)=-0.76V` and
`Fe^(2+)(aq)+2e rarr Fe(s), E^(@)=-0.44V`.
At standard conditions, what cause minimum external potential be required to cause the following reaction : `Fe(s)+Zn^(2+)(aq)rarr Fe^(2+)(aq)+Zn(s)`?

Explanation

For the given reaction,
Anode reaction : `Fe(s)rarr Fe^(2+)(aq)+2e`
Cathode reaction : `Zn^(2+)(aq)+2e rarr Zn(s)`
and `E_("cell")^(@)=E_("cathode")^(@)-E_("anode")^(@)`
`[-0.76V-(-0.44)]V=-0.32V`
The negative value of `E_("cell")^(@)` implies that the given reaction is non - spontaneous under standard conditions.
Hence, for the given non - spontaneous reaction to occur, an external potential above 0.32V is required. So, the minimum external potential required is just above 0.32V.