Half-Cell Reaction

Definition - What does Half-Cell Reaction mean?

A half-cell reaction is either an oxidation reaction in which electrons are lost, or a reduction reaction where electronic are gained. The reactions occur in an electrochemical cell in which the electrons are lost at the anode through oxidation and consumed at the cathode where the reduction occurs.

Half-cell reactions are useful in galvanic and voltaic cells in which the electrons flow from the anode to the cathode through an electrolyte to produce an electromotive force (EMF). Oxidation-reduction half-cell reactions can also occur on metal surfaces and lead to corrosion.

Corrosionpedia explains Half-Cell Reaction

Half-cell reactions occur in an electrochemical cell and results in a deficit or surplus of electrons. Both the donor and acceptor must be present for the transfer of electrons to take place.

Corrosion is considered an electrochemical reaction resulting from the two half-cell reactions of oxidation and reduction (redox). The corrosion process occurs as a result of the formation of voltaic or galvanic cells in which the metal acts as the anode. The metal is oxidized by oxygen in the air in a series of oxidation-reduction reactions similar to those in a battery. Once oxidized, the metal produces free electrons and metal ions. The free electrons reduce the oxygen in the air to form a hydroxide in a complimentary cathodic reaction.

The corrosion electrochemical reaction for iron can be split into the two half-cell reactions of:

Zn = Zn2+ + 2e- (Oxidation)

2H+ + 2e- = H2 (Reduction)

The two half-cell reactions often occur at separate locations on the metal and, because the metal is conductive, the electrons flow through the metal from the anodic to the cathodic region. The presence of water or a thin film of moisture is all what is required to facilitate the movement of the ions.

The four fundamental components required for electrochemical corrosion to occur are:

  • An anode
  • A cathode
  • An ectrolyte to provide a conducting environment for ionic movement
  • An electrical connection

Half-cell reactions that result in corrosion can be stopped by removing or disabling one or more of the above components. Typical methods used include painting, cathodic protection and using sacrificial anodes.

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