Electrode potential origin; Zn-Cu cell notation

Origin of electrode potential

A metal rod dipped in a solution of its ions has two tendencies: the metal atoms may pass into solution as ions leaving electrons on the rod (M → Mⁿ⁺ + ne⁻), or ions may take electrons from the rod and deposit (Mⁿ⁺ + ne⁻ → M). An equilibrium is set up and a double layer of charge forms at the interface. The potential difference across it is the electrode potential. For Zn the rod becomes negative; for Cu it becomes positive.

Standard electrode potential

The potential of an electrode in contact with a 1 M solution of its ions at 25°C (1 atm for gases), measured against the standard hydrogen electrode taken as 0 V. Symbol E°.

Zn–Cu cell (Daniell cell)

Cell notation: Zn | Zn²⁺ (1 M) || Cu²⁺ (1 M) | Cu

Anode (oxidation)  : Zn → Zn²⁺ + 2e⁻
Cathode (reduction): Cu²⁺ + 2e⁻ → Cu
Cell reaction      : Zn + Cu²⁺ → Zn²⁺ + Cu

E°cell = E°(Cu²⁺/Cu) − E°(Zn²⁺/Zn) = 0.34 − (−0.76) = 1.10 V.