Oxidation/Reduction Reactions

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).

Figure 2: Oxidation reaction (the valence state of the reactant increases) of zinc metal to a zinc ion. The (s) after the zinc indicates that it is in solid form. The zinc ion has (aq) after to indicate that it is aqueous, (ie in solution).

Insert move of solid zinc, with electron transfer and then zinc ion going into solution.

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.

Reduction reaction of Cu ions to form copper metal. The valence state of copper is reduced from 2+ to 0.

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.

Pop-up window: Mnemonic Devices: Remembering which reaction occurs where and does what

There are several mnemonic devices to assist in remembering the terms in a battery. To remember the difference between a reduction and an oxidation reaction, you can use any of the below methods. The first method it just based on the words describing reduction. In a reduction reaction, the valance state is reduced, meaning that it gains a negative charge, ie an electron, and therefore the electron must be a reactant in a reduction reaction. Thus the word itself – reduction – identifies the process that occurs there. An imaginative mnemonic to remember which reaction is the oxidation and the reduction is "LEO (the Lion goes) GER (grrr)" in which LEO is short for "Loss of Electrons - Oxidation" and GER is short for "Gain of Electrons - Reduction". Another way is to remember the difference between oxidation and reduction reactions, (which is not quite is the spirit of renewable energy), is "OIL RIG": Oxidation Involves Loss of electrons, Reduction Involves Gain of electrons."

There are also a few mnemonics to help remember at which reaction occurs at the anode (the oxidation) and which at the cathode (the reduction)."The RED CAT and AN OX", groups the abbreviations for REDuction and CAThode together and also the abbreviations for ANode and OXidation. Another way to remember which reactions occur at the anode and cathode is to note that the letter "O" (for oxidation) occurs before the letter "R" (for reduction) and similarly that the letter "A" (for anode) occurs before the letter "C" (for cathode). Thus, the oxidation occurs at the anode and the reduction occurs at the cathode. Finally, you may also remember that both "anode" and "oxidation" begin with vowels and both "cathode" and "reduction" begins with consonants.

A final piece of information to remember is that the anode is negative and the cathode positive. The mnemonic for this is similar to that of the anode and cathode. The anode is negative, the cathode is positive, "A" comes before "C" and "N" comes before "P". In a battery the situation is slightly more complex, as the physical location of the oxidation and reduction reaction changes between when the battery is charging and discharging. By convention, the terms anode and cathode are defined according to conventional rules when the battery is discharging and retain the same names when the battery is being charged.