Half Cell

Definition - What does Half Cell mean?

A galvanic cell or simple battery is made of two electrodes. Each of the electrodes of a galvanic cell is known as a half cell. In a battery, the two half cells form an oxidizing-reducing couple. When two half cells are connected via an electric conductor and salt bridge, an electrochemical reaction is started.

In an electrochemical cell, electrons can flow from one half reaction, or half cell, to another. When a half cell loses electrons from an electrode, this is known as ''oxidation"; when a half cell gains electrons from the electrode, it's known as "reduction." This electron transfer causes corrosion to the electron-losing electrode.

Corrosionpedia explains Half Cell

A half cell is one of the two electrodes of an electrochemical cell. An electrochemical cell comprises two half cells, where every half cell contains an electrode and an electrolyte. A salt bridge or direct contact is needed to connect two half cells. Without a connection between two half cells, no reaction will take place.

In a Zn-Cu battery, when two half cells are connected, the following reaction takes place:

Reduction reaction takes place at the cathode:

Cu2+ + 2 e --> Cu Cu2+ is the oxidizing agent and Cu the reducing agent.

Oxidation reaction takes place at the anode:

Zn --> Zn2+ + 2 e- Zn is the reducing agent, and Zn2+ the oxidizing agent.

The electrons from the Zn will flow from the anode to the surface of the cathode to reduce copper (Cu2+) ion and produce Cu. The oxidation-reduction takes place in a manner that the equilibrium of charges or electrons is established in an overall reaction. The overall reaction therefore involves the reduction of Cu2+ ions by zinc metal, as follows:

Zn(s)

+

Cu2+(aq)

-->

Zn2+(aq)

+

Cu(s)

Since an oxidation reaction takes place at the anode and a reduction reaction takes place at the cathode of an electrochemical cell, the Zn/Zn2+ half cell becomes the anode, and the Cu2+/Cu half cell becomes the cathode. The electrode potential developed by the overall cell reaction can be calculated through adding the potential developed both at the anode and cathode, i.e. at each half cell. Half-cell potential is very important in measuring the corrosion severity of a material.

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