Redox Reactions

Oxidation reactions are defined as the loss of electrons where the charge increases. Reduction reactions are when there is a gain of electrons, and the charge decreases. When oxidation happens, the oxidation state increases, yet it decreases during reduction.

The oxidation state is a concept very similar to the charge. In fact, it is the same as the charge for pure elements. However, when elements are in a compound, the oxidation state changes. To determine the oxidation state of an element, you simply follow a few rules.

The first rule is that pure elements always have an oxidation state of 0. They also have no charge, so this makes sense. The next rules are all for compounds or ions.

Hydrogen always has an oxidation state of +1, unless it’s with a metal as a hydride (H compound that is not an acid, such as NaH), which turns it into a -1. Oxygen is always a -2, except when it’s in a rare peroxide (O₂⁻²), where it turns into -1. Group (family) 1 metals keep their charge of +1, and group 2 metals keep their charge of +2. Fluorine always has an oxidation state of -1. Chlorine is also -1 unless it is with Oxygen or Fluorine. Finally, the overall sum of the oxidation states must equal the overall charge. The other elements in the compound must change their oxidation state to add/subtract to the overall charge.

For example, H₃PO₄ has an overall charge of 0. Since there are 3 hydrogens, the total hydrogen oxidation state will be +3. The 4 oxygens will have an oxidation state of -8. Since phosphorous has to match the overall charge, it will have an oxidation state of +5.

Synthesis and decomposition reactions can be split into one redox and one reduction reaction called half reactions. In this simple reaction:

MgCl₂ -> Mg + Cl₂

the oxidation states are as follows:

Mg⁺²Cl₂⁻² -> Mg⁰ + Cl₂⁰

which tells us that magnesium is being reduced and chlorine is being oxidized, as their oxidation states change accordingly. The half reactions (with oxidation states) that can be made from this are shown below:

Red: 2e⁻ + Mg⁺² -> Mg⁰

Ox: 2Cl⁻¹ -> Cl₂⁰ + 2e⁻

where e⁻ is an electron. These reactions can create an electron flow from a simple reaction, which is how electricity is created.