Oxidation-reduction, or redox reactions, take place in electrochemical cells. There are two different types of electrochemical cells. Spontaneous reactions occur in galvanic (voltaic) cells and non spontaneous reactions occur in electrolytic cells.
Both types of cells contain electrodes where the oxidation and reduction reactions occur. Oxidation occurs at the electrode called the anode and reduction occurs at the electrode called the cathode.
The anode of an electrolytic cell is positive, where the cathode is negative, because the anode attracts anions from the solution. However, the anode of a galvanic cell is negatively charged, since the spontaneous oxidation at the anode is the source of the cell’s electrons or negative charge. The cathode of a galvanic cell is its positive terminal. In both galvanic and electrolytic cells, oxidation takes place at the anode and the electrons flow from the anode to the cathode.
Redox reaction in a galvanic cell
In a galvanic cell, the redox reaction is a spontaneous reaction. It generally consists of two different metals connected by a salt bridge, or individual half-cells separated by a porous membrane.
Redox reaction in an electrolytic cell
In an electrolytic cell the redox reaction is nonspontaneous. The electrolysis reaction requires electrical energy to induce the reaction. An example of this would be when molten NaCl is electrolyzed to form liquid sodium and chlorine gas.
The sodium ions will migrate toward the cathode, where they are reduced to sodium metal. The chloride ions will migrate to the anode, in a similar fashion, and be oxidized to form chlorine gas. This type of cell is used to produce sodium and chlorine.
The chlorine gas can be collected surrounding the cell. The sodium metal is less dense than the molten salt and is removed as it floats to the top of the reaction container.
A rechargeable battery
A rechargeable battery, as in the case of a AA NiMH cell or a single cell of a lead-acid battery, acts as a galvanic cell when discharging (converting chemical energy to electrical energy), and an electrolytic cell when being charged (converting electrical energy to chemical energy).