oxidation-reduction+reactions

Oxidation-Reduction Half-reactions
In every redox reaction, there is at least one oxidation process, and one reduction process. The electrons that are lost during oxidation will be taken up by reduction.

A chemical that takes electrons from other substances is called an **oxidizing agent**. This chemical itself will undergo reduction.

A chemical that donates electrons is undergoing oxidation, and is called the **reducing agent**.

To find the oxidation and reduction half-reactions in a chemical equation, look for substances that are undergoing changes in oxidation numbers.

For example: copper metal reacts with silver nitrate to form silver metal and a copper (II) nitrate solution.

Cu (s) + 2 AgNO 3 (aq) --> 2 Ag (s) + Cu(NO 3 ) 2 (aq)

First, recognize that copper metal and silver metal are both present in elemental form, and will have oxidation numbers of zero:

Cu o (s) + 2 AgNO 3 (aq) --> 2 Ag o (s) + Cu(NO 3 ) 2 (aq)

Second, chart the compounds and their oxidation numbers: totals on the bottom, individuals on top.


 * 0 ||  || +1 || +5 || -2 ||   || 0 ||   || +2 || +5 || -2 ||
 * Cu || + || 2 Ag || N || O 3 || --> || 2 Ag || + || Cu || (N || O 3 ) 2 ||
 * 0 ||  || +1 || +5 || -6 ||   || 0 ||   || +2 || +10 || -12 ||

The nitrate ion can be kept together or separated. Sometimes it is easier to eliminate spectator ions such as the nitrate ion in this case.


 * 0 ||  || +1 ||   || 0 ||   || +2 ||
 * = Cu ||= + ||= 2 Ag + ||= --> ||= 2 Ag ||= + ||= Cu 2+ ||
 * 0 ||  || +1 ||   || 0 ||   || +2 ||

Also note that the nitrogen and oxygen atoms did not change their individual oxidation numbers, so they are not involved in the redox process.

The oxidation half-reaction is:

Cu o --> Cu 2+

and the reduction half-reaction is:

2 Ag + --> 2 Ag o

Now, include the electrons. Since copper is losing electrons, they appear on the products side. This also balances the net charge of the half-reaction.

Cu o --> Cu 2+ + 2 e-

Silver is gaining two electrons (one per silver ion), so the electrons are reactants, which again balances the net charge of the half-reaction:

2 e- + 2 Ag + --> 2 Ag o

Therefore, copper is the reducing agent, and the silver ion is the oxidizing agent. Two moles of electrons have been transferred in the process.