Oxidation-Reduction Reactions

An easy mnemonic for oxidation and reduction is "OIL RIG"."OIL" = Oxidation Is Losing and "RIG" = Reduction Is Losing.

The atom that oxidizes in a reaction is called the reducing agent. Since sodium oxidizes, it is known as a reducing agent because it lets chlorine reduce.

The atom that reduces in a reaction is called the oxidizing agent. Since chlorine reduces, it is known as an oxidizing agent because it lets sodium oxidize.

Remember how we said that a redox reaction is "kind of" a reaction that involves the transfer of electrons? Well, that can be explained with the reaction between sodium and chlorine gas. It forms NaCl, which is a salt held together by charged ions.

However, what happens if one of the species involved in a reaction is covalent? It can't transfer electrons and gain a net charge, it can only share them. The way we tackle this is through the concept of oxidation number(or oxidation state). The oxidation number is the theoretical charge an atom would have if it were held by ionic bonds. Basically, if you take atoms in a covalent compound, their oxidation numbers are the charges they'd have if they were in an ionic compound. This means that a redox reaction is a chemical reaction that involves a transfer of electrons, both physically(ionic) or formally(covalent).

Below are some important rules for oxidation numbers(ON):

1) The ON of a single element not bonded to any other element is +0.

For example: The ON of O₂ is +0.

2)The ON of a monatomic ion(ion consisting of only one atom) is the charge of that ion.

For example: The ON of Cl is -1 and the ON of Mg²⁺ is +2.

3)The ON of a hydrogen atom is +1 when combined with non-metals. However, the ON of a hydrogen atom is -1 when combined with metals.

Example: in HCl, H has an ON of +1, meaning Cl has an ON of -1(consistent with rule #5) and HCl's ON is 0(consistent with rule #7)

4)The ON of Oxygen is -2 unless in a peroxide. If in a peroxide, Oxygen's ON is then -1.

5) For halogens, Fluorine's ON is always -1. The same goes for every other halogen(Chlorine, Bromine, Iodine, etc.) if they're not combined with oxygen or other halogens.

6)The total sum of ONs in a molecule or polyatomic ion is equal to the charge on that molecule or polyatomic ion.

Example: The charge of water is +0. Both hydrogens have +1 ONs and the single oxygen has an ON of -2, so the sum of the oxidation states is +0.

Using the more formal definition of redox reactions above, it can be concluded that:

Oxidation = increase in ON

Reduction = decrease in ON

These go in tandem with the fact that oxidation is a loss in electrons and reduction is a gain in electrons. Electrons have negative charge so if the ON of an atom decreases(reduction), it is gaining electrons which is consistent with our definitions of oxidation and reduction.

Redox reactions can also occur in acidic and basic solution. Here is the procedure for balancing those:

1)Make an oxidation and reduction half-reaction for the total reaction

Note: These next few steps should be done in both half-reactions

2)Balance all the elements except oxygen and hydrogen

3)Balance the oxygen atoms by adding water(H₂O) to the side lacking oxygen atoms.

4)Balance out the hydrogen(including the atoms brought in with the water) by adding protons(H⁺)

5)Balance the charge on both sides of the reactions by adding electrons the sides that need them

6)If the number of electrons in both half-reactions isn't the same, multiply both half-reactions such that the number of electrons is equal and the stoichiometric coefficients of the species involved are integers.

7)Add up both balanced half-reactions(this'll cancel out electrons if step 6 is done right) and cancel out any extra terms, like extra water molecules.

Follow the next few steps only for Redox Reactions in Basic Solution:

8) If the reactions are in a basic medium, add OH^- ions to both sides of both reactions such that the number of OH^- ions in the reactions equal the number of H⁺ ions. For example, if there's 7 H⁺ ions in a given half-reaction, there should be 7 OH^- ions added.

9)Combine the OH^- ions with H⁺ ions to form water.

10)Cancel out water terms to yield your final balanced reaction.