Redox reactions are about electron transfer. Redox, is a portamento of reduction and oxidation.

Oxidation and Reduction

Reduction is when a substance gains an electron.$^4$

Oxidation is when a substance losses an election.$^4$

The oxidizing agent is the chemical species that caused the oxidization to occur. The reduction agent is the chemical species that causes the reduction to occur.

Oxidation State

While studying redox reactions, you must keep track of the electrons. However for covalent bonds, it is not straight forward as all the atoms within the bond share electrons.$^4$

In order to do so, we have something called as oxidation states. This is what the charge of each atom would be should the bond split.$^4$

Rules

There are a set of rules in order to figure out the oxidation state.$^4$

1. For any single element, whether it be monatomic, diatomic, or polyatomic, is always 0. For example, the oxidation state of $\text{S}_8$ is 0.
2. For a monatomic ion, a charged atom, the oxidation state is equal to the charge. For example the oxidation state of $\text{Fe}^{+2}$ is +2.
3. Oxygen always has an oxidation state of -2. Except, $\text{O}_2$ which has an oxidation state of 0 and $\text{H}_2\text{O}_2$ which has an oxidation state of -1.
4. Hydrogen always has an oxidation state of +1.
5. Fluorine always has an oxidation state of -1.
6. All halogens have an oxidation state of -1. Except when bonded with Fluorine or Oxygen.
7. Neutral compounds all have oxidation states of 0.
8. A polyatomic ion has to work out to have an oxidation number equal to its charge.

Examples

Take the Haber process which is the reaction between nitrogen and hydrogen to make Ammonia.$^4$