Oxidation-Reduction Reactions

Oxidation - refers to the loss of electrons by a molecule, atom or ion

Reduction - refers to the gain of electrons by an molecule, atom or ion

Oxidation number - the oxidation number of an atom is the charge that results when the electrons in a covalent bond are assigned to the more electronegative atom; it is the charge an atom would possess if the bonding were ionic

• In oxidation numbers, the charge is written before the number (to distinguish them from actual electronic charges - where the number is placed in front of the charge)

Chemical reactions in which the oxidation state of one or more substances changes are called oxidation-reduction reactions (or redox reactions)

• Keeping track of the oxidation numbers of the elements involved in a reaction will identify whether the reaction is a redox reaction or not

The oxidation numbers for the above elements and ions are:

• Zn = 0, Zn²⁺ = +2
• H⁺ = +1, H₂ = 0
• Thus, the oxidation number of both the Zn(s) and H⁺(aq) change during the course of the reaction, and so, this must be a redox reaction

In this reaction an unambiguous transfer of electrons occurs:

• The elemental form of zinc (i.e. zinc metal) loses two electrons (and becomes soluble zinc ion)
• Two H⁺ ions in solution gain two electrons and form a hydrogen (H₂) molecule

In other types of reactions, we can identify changes in oxidation number, but it is not so clear that a particular atom has gained or lost an electron:

• Hydrogen has been oxidized and oxygen has been reduced in the above reaction involving the production of water from elemental hydrogen and oxygen
• However, the H-O bonds are not ionic, they are polar covalent, so there is not a complete transfer of shared electrons
• Using oxidation numbers is just a convenient bookkeeping method to understand reduction/oxidation types of reactions. It does not necessarily mean that complete electron transfer has occurred.

In a redox reaction, both oxidation and reduction must occur. In other words, something must be oxidized for something else to be reduced

• The compound that contributes the electrons is called the reducing agent. The reducing agent gives up electrons, causing another compound to be reduced (and is therefore oxidized in the process)
• The compound that accepts electrons is called the oxidizing agent. The oxidizing agent accepts electrons from another compound, causing the other compound to be oxidized (in the process, the oxidizing agent is reduced)

In the above reaction of hydrogen and oxygen:

• Hydrogen is the reducing agent (it contributes electrons to oxygen, and is itself oxidized)
• Oxygen is the oxidizing agent (it accepts electrons from hydrogen - oxidizing it, and is itself reduced)