We are now going to look at some reactions that involve a transfer of electrons.
The oxidation number is the charge an atom in a substance would have if the pairs of electrons in each bond belonged to the more electronegative atom. Now this means that in a compound made up of monatomic ions, like NaCl, in which the bonding pairs do belong to the more electronegative atom, the oxidation number equals the ionic charge. The sodium ion has an oxidation number of +1 while the chlorine ion has an oxidation number of -1.
Here are some rules that are helpful in finding the oxidation number of
a specific atom.
You might be heartily confused at this point as to what oxidation numbers are and how to find them. Don't worry, you can usually find the oxidation numbers of elements in ionic compounds by looking at the individual ions in the compound.
In H2SO4, H = +1 and O = -2. Since there are 4 oxygen atoms, the total charge from oxygen is -8. Therefore the charge of S must be +6, since the net charge of the molecule is zero.
When we take an iron metal nail and dip it into a blue solution of copper (II) sulfate, the nail becomes covered with a reddish substance. This reddish susbtance is metallic copper. The equation for this is:
The net ionic equation, that is, the equation in which we have cancelled out the spectator ions (in this case the spectator ion is SO4).
What we have done here is to show you the oxidation numbers above the elements in the equation. This shows that the iron metal loses electrons to form iron(II) ions while copper(II) ions gain electrons to create copper metal. The Cu2+ on the reactancts side of the equation has a 2+ because SO4 has a charge of 2-. Any reaction like this that involves a transfer of electrons is an oxidation-reduction reaction.
We can write the equation above in the form of two half-reactions. A half-reaction is one of two parts of an oxidation-reduction reaction, one of which involves a loss of electrons, and the other a gain of electrons. The half reactions for the iron nail reaction would be:
The first equation shows us that electrons are lost by Fe, (2e-). The second equation shows electrons being gained by Cu2+. You know, we can also express this reaction in terms of what happened to the oxidation number. The oxidation number of iron increases after the reaction, while the oxidation number of copper decreases.
Basically, an oxidation-reduction reaction (redox reaction) is a reaction in which electrons are transferred between species or in which atoms change oxidation numbers. Oxidation is the portion of the redox reaction in which there is a loss of electrons by a species or an increase in the oxidation number of an atom. Reduction is the part of a reaction in which there is a gain of electrons by a species or a decrease in the oxidation number of an atom. Something that is oxidized loses electrons or contains an atom that increases in oxidation number. Something that is reduced gains electrons or has an atom that decreases in oxidation number. An oxidizing agent is a something that oxidizes something, meaning that the oxidation agent is itself reduced. A reducing agent is a something that reduces something else, and is itself oxidized. Notice the example below.
Fe is oxidized and Fe is the reducing agent.
Cu+2 is reduced and Cu+2 is the oxidizing agent.