Reduction/oxidation (redox) reactions are an important class of chemical reactions since they are the driving force behind a vast range of process, both desirable (for example breathing in mammals) and undesirable (for example rusting of iron). A redox reaction is characterized by the fact that electrons are produced (in an oxidation reaction) or are used by the reaction (in a reduction reaction). An oxidation reaction must always be paired with a reduction reaction, as the oxidation reaction produces the electrons required by the reduction reaction.
The electrons transferred in a redox reaction arise from the change of the valence state of materials in the redox reaction. If a material gives up or loses an electron, then its valance state becomes more positive (since an electron has a negative charge) and the reaction is called an oxidation reaction. Since an oxidation reaction gives up electrons, it will always have electrons as one of its products. By definition, the oxidation reaction occurs at the anode. The chemical reaction shown below is an oxidation reaction where zinc metal (with a neutral valance state or valance charge = 0) is oxidized to give a zinc ion, which has a 2+ valence charge. The two electrons lost by the zinc metal are products of the oxidation reaction. The zinc ion does not exist as separate entity, and therefore must for either a solid salt (in which case its mobility and availability is not useful for redox reactions) as a dissolved salt in a solution. The (aq) after the zinc ion indicates that it is aqueous. Note that since the overall aqueous solution must be electrically neutral, there must also be ions with positive charge in the solution. In examining only the behavior of the battery reaction, these may not be specified. However, they will play a role in the solubility of the Zn water (or an alternate solvent).
If a material gains an electron then its valance state decreases or reduces due to the negative charge of the electrons and the reaction is a reduction reaction. The reaction below is a reduction reaction in which a copper ion with a valance state of 2+ is reduced to copper metal, with a valence state of zero. Since a reduction reaction requires electrons, it will always have electrons as one of the reactants. The reduction reaction occurs at the cathode.
The total redox reaction consists of both of the two reactions together. For the example of copper and zinc above, the total reaction is shown below. Since the reaction with zinc metal (ie the reactant of the oxidation reaction) is providing the electron required to reduce the copper, the zinc is the reducing agent and the zinc itself is oxidized. Copper ions in this case are the oxidizing agent - they oxidize the zinc and are themselves reduced. Note that since the electrons appear on both sides of the chemical equation, they may be omitted when writing the redox reaction. Further note that for redox reaction, it is important to balance not only the elements in the chemical reactions, but also the electrons.