In chemistry, a half reaction (or half-cell reaction) is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction. Often, the concept of half reactions is used to describe what occurs in an electrochemical cell, such as a Galvanic cell battery. Half reactions can be written to describe both the metal undergoing oxidation (known as the anode) and the metal undergoing reduction (known as the cathode). Half reactions are often used as a method of balancing redox reactions. For oxidation-reduction reactions in acidic conditions, after balancing the atoms and oxidation numbers, one will need to add ions to balance the hydrogen ions in the half reaction. For oxidation-reduction reactions in basic conditions, after balancing the atoms and oxidation numbers, first treat it as an acidic solution and then add ions to balance the ions in the half reactions (which would give ). Example: Zn and Cu Galvanic cell Consider the Galvanic cell shown in the adjacent image: it is constructed with a piece of zinc (Zn) submerged in a solution of zinc sulfate () and a piece of copper (Cu) submerged in a solution of copper(II) sulfate (). The overall reaction is: Zn_{(s)}{} + CuSO4_{(aq)} -> ZnSO4_{(aq)}{} + Cu_{(s)} At the Zn anode, oxidation takes place (the metal loses electrons). This is represented in the following oxidation half reaction (note that the electrons are on the products side): Zn_{(s)} -> Zn^2+ + 2e- At the Cu cathode, reduction takes place (electrons are accepted). This is represented in the following reduction half reaction (note that the electrons are on the reactants side): Cu^2+ + 2e- -> Cu_{(s)} Example: oxidation of magnesium Consider the example burning of magnesium ribbon (Mg). When magnesium burns, it combines with oxygen () from the air to form magnesium oxide (MgO) according to the following equation: 2Mg_{(s)}{} + O2_{(g)} -> 2MgO_{(s)} Magnesium oxide is an ionic compound containing and ions whereas and are elements with no charges. The with zero charge gains a +2 charge going from the reactant side to product side, and the with zero charge gains a –2 charge. This is because when becomes , it loses 2 electrons. Since there are 2 Mg on left side, a total of 4 electrons are lost according to the following oxidation half reaction: 2Mg_{(s)} ->