Voltaic/Galvanic Cells

A spontaneous redox reaction generates electricity, which can power electrical devices. But how does it work?

Consider the redox reaction between copper and a silver nitrate solution. When placed in one vessel, the copper is oxidized to cuprous ions, while the silver ions are reduced to a silver precipitate. Here, the transfer of electrons is direct and does not generate an electric current.

However, when these half-reactions are physically separated and connected via an external circuit, an electric current is observed. This set-up is called an electrochemical cell, further defined as a galvanic or voltaic cell for a spontaneous redox reaction.

A galvanic cell consists of two half-cells containing the half-cell reaction mixtures of a solid metal electrode in its corresponding electrolyte solution. Here, it is copper in aqueous copper nitrate and silver in aqueous silver nitrate.

The separated reactants experience a cell potential, but without an external connection, no reaction or flow of electrons occurs. Once externally connected, the electrons are forced to flow indirectly over the external circuit, enabling the reactions to occur at the respective electrodes.

Per convention, oxidation occurs at the anode bearing a negative charge, and reduction at the cathode bearing a positive charge.

As the transfer of electrons occurs, positive ions are formed in the oxidation half-cell and negative ions in the reduction half-cell. This causes a positive and negative charge buildup preventing further electron flow and must be continuously neutralized for the reaction to progress.

Thus, the two half-cells are linked by a salt bridge: an inverted U-tube containing a paste or gel of an inert electrolyte like sodium nitrate. As the redox reaction progresses, the counter-ions flow into the respective half-cells to ensure charge neutrality without mixing of electrolytes.

Special notations, or cell schematics, are used to describe a galvanic cell.

The oxidation is represented on the left side and reduction on the right. A double vertical line indicates a salt bridge connecting the two half-cells, while single vertical lines indicate interfaces between the component phases.

If two or more components exist in the same phase, they are separated using commas.