Galvanic Cell Animation and Theory

made by Jelena Simovic Rota for the Physical Chemistry Department, University of Applied Sciences, Munich

A galvanic cell, also known as voltaic cell, is an electrochemical cell that generates electrical energy through a spontaneous reaction taking place in the cell. It consists of two half cells connected via external electrical circuit and via salt bridge.

A half cell contains a metal strip submerged into a solution of the metal's salt. Our example shows zinc-zinc chloride half cell (zinc electrode) and copper-copper sulfate half cell (copper electrode).

Two half cells are connected through a flow of electrons from zinc to copper electrode (external electrical circuit) and through a flow of ions (salt bridge).

The oxidation of Zn_(s) into Zn²⁺ and the reduction of Cu²⁺ into Cu_(s) occur spontaneously due to a difference in the potential energy between Zn and Cu²⁺. The oxidation takes place at the zinc anode and reduction takes place at the copper cathode.

Oxidation: Zn_(s) → Zn²⁺ + 2e^-

Reduction: Cu²⁺ + 2e^- → Cu_(s)

Electrons that are released at the zinc anode flow through the external electrical circuit towards the copper cathode.

A salt bridge is required to maintain electrical neutrality and allow the reaction to continue. Our salt bridge containing KNO₃ is adding NO₃^- into the anode solution and K⁺ into the cathode solution in order to prevent charge build up.

The potential difference between two electrodes is called standard cell potential (E^o_cell) under standard state conditions: concentrations of 1 M, pressure of 1 atmosphere and temperature of 25 ^oC. It can be calculated using standard electrode potential, E^o, in volts, for each of the two half reactions:

E^o_cell = E^o_cathode - E^o_anode

For our example:

E^o_cell = 0.337 V - (-0.763 V) = 1.100 V

In a galvanic cell, where a spontaneous reaction takes place, Gibbs free energy ∆G is negative and cell potential is positive, which is in accordance with the following equation:

∆G = −nFE_cell

where n is the number of moles of electrons and F is the Faraday constant (96485 C/mol).