Redox Reactions

Chemical reactions are often divided into two categories: oxidation-reduction (or redox reactions) and metathesis reactions.
Metathesis reactions are reactions in which none of the atoms undergoes oxidation or reduction.
Examples of metathesis reactions are:

Acid-base reactions that involve a transfer of H+,

e.g. NaOH + HCl � NaCl + H₂O

Acid-base reactions that involve the sharing of a pair of electrons by a Lewis base and a Lewis acid,

e.g. Cu²⁺ + 4NH₃ � Cu(NH₃)₄²⁺

When at least one atom undergoes a change in oxidation state, the reaction is a redox reaction.

Oxidation is the loss of electrons, while reduction is the gain of electrons.

Consider the reaction I₃^- + S₂O₃^2- � I^- + S₄O₆^2-

This is not balanced. The steps below illustrate how to balance a redox equation.

1. Determine the oxidation number of each atom on both sides of the equation.

   On the left side:
   I: -¹/₃, S: +2, O: -2

   On the right:
   I: -1, S: +5, O: -2

2. Determine which atoms are oxidized and which are reduced.
   S is oxidized and I is reduced.

3. Divide the reaction into oxidation and reduction half-reactions and balance these.

   Oxidation:
   S₂O₃^2- � S₄O₆^2- is balanced to 2S₂O₃^2- � S₄O₆^2- + 2e^-

   Reduction:
   I₃^- � I^- is balanced to I₃^- + 2e^- � 3I^-

4. Combine the half-reactions
   I₃^- + 2S₂O₃^2- � 3I^- + S₄O₆^2-

5. Further balance the equation by inspection if necessary.
   This is unnecessary in this case.

Redox Titration

Supposing we had a solution with an unknown concentration of I₃^- ions. We could determine this concentration by titrating a known volume of the solution against a known concentration of S₂O₃^2-. At the point where the color changes, twice as many S₂O₃^2- ions have been added as there were I₃^- ions, as the above example shows. From these data the concentration of I₃^- ions in the solution can be easily calculated.

This is an example of a redox titration. Redox titrations are similar in principle to acid-base titrations using pH indicators.