- 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- + S2O32- « I- + S4O62-
This is not balanced. The steps below illustrate how to balance a redox equation.
- 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
- Determine which atoms are oxidized and which are reduced.
S is oxidized and I is reduced.
- Divide the reaction into oxidation and reduction half-reactions and balance these.
S2O32- ® S4O62- is balanced to 2S2O32- ® S4O62- + 2e-
I3- ® I- is balanced to I3- + 2e- ® 3I-
- Combine the half-reactions
I3- + 2S2O32- ® 3I- + S4O62-
- Further balance the equation by inspection if necessary.
This is unnecessary in this case.
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 S2O32-. At the point where the color changes, twice as many S2O32- 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.