Redox Reactions & Oxidation Reduction

Core Concepts

In this tutorial, you will learn what a redox reaction is, the different parts of such a reaction, as well as how to recognize and write redox reactions. You will also learn the difference between oxidation and reduction, and the definition of oxidation. At the end of an article is a video showing a great experiment for budding middle school chemists.

Topics Covered in Other Articles

Vocabulary

Oxidation: a type of chemical reaction where one or more electrons are lost.
Oxidation State / Number: a number assigned to an atom describing its degree of oxidation, meaning how many electrons it has gained or lost.
Reduction: a type of chemical reaction where one or more electrons are gained.
Oxidation Reduction Reaction: a chemical reaction where oxidation and reduction occurs simultaneously

What are redox reactions?

Redox is a shorthand for reduction-oxidation, meaning that a redox reaction is one in which both a reduction reaction and an oxidation reaction takes place at once. It is also shorthand for oxidation reduction reaction. Let’s discuss these two components separately, then circle back to how they combine in a full redox reaction.

Reduction:

Reduction happens when an atom gains one or more electrons during a chemical reaction. That means that its oxidation number decreases. This is because an electron has a negative charge, thus when an atom gains an electron, it gains a negative charge, which decreases the oxidation number. This could look like an atom going from X²⁺ to X¹⁺, or X⁰ to X^1-, for example. This can help us remember what reduction is, as the oxidation number gets reduced.

The oxidizing agent is the species that undergoes reduction. It may be a little confusing initially, but always remember that the oxidizing agent causes another species to become oxidized. Redox reactions occur in pairs of reduction and oxidation. Therefore, the species that facilitates oxidation undergoes reduction. Please note this terminology.

Let’s look at some examples of reduction half-reactions. These are called half-reactions because they make up half of a full redox reaction:

Cu²⁺ (aq) → Cu (s)

F₂ (g) → 2F^– (g)

What is Oxidation?

Oxidation Definition:

There are three different oxidation definitions. “The process or result of oxidizing or being oxidized”, “the gain of oxygen, such as when an element combines with oxygen to form its oxide, like rusting”, and lastly “the loss of electrons during a reaction, which results an atom increasing its oxidation state”. The last definition is the one you should remember for chemistry.

Oxidation is a chemical process in which a substance loses electrons to another substance. This process can result in the formation of a new chemical species, such as an oxide or a salt, and it is usually accompanied by a change in the color, odor, or other properties of the substance. Oxidation is an important part of many chemical reactions, and it is often involved in the formation of new compounds or the release of energy.

Oxidation happens when an atom loses one or more electrons during a chemical reaction, meaning that its oxidation number increases. This is because the atom loses the negative charge of the electron, which is similar to gaining a positive charge, increasing the oxidation number. This could look like an atom going from X^1- to X⁰, or X⁰ to X¹⁺, for example. The phrase “oxidation” is used because historically, the first redox reactions observed were ones involving oxygen. If it helps, however, we can think of it as oxidation because the oxidation number increases/becomes more positive.

Redox examples

When a metal reacts with oxygen to form an oxide, we say that the metal undergoes oxidation while the oxygen undergoes reduction. The transfer of electrons from the metal to the oxygen results in the formation of a new compound and the release of energy. Chemists often use oxidation to explain the behavior of chemicals and the changes they undergo during reactions, making it an important concept in chemistry.

When bleach or hydrogen peroxide turns something white, we say that the substance is oxidized. The hypochlorite ion in the bleach, or the peroxide ion, gains electrons, and the substance that turns white (by default the reducing agent) loses electrons.

We refer to the species undergoing reduction as the oxidizing agent. Initially, it might cause some confusion, but always keep in mind that the oxidizing agent facilitates the oxidation of another species. In redox reactions, the species that causes oxidation is the one that gets reduced, and these reactions occur in pairs of reduction and oxidation.

Let’s look at some examples of oxidation half-reactions:

2I^– (aq) → I₂ (s)

Zn (s) → Zn²⁺ (aq)

Oxidation vs Reduction

Remember OIL RIG: Oxidation Is Loss of electrons – Reduction Is Gain of electrons. Alternatively, you can also remember “LEO GER” (imagine a lion saying “Ger!!!!). LEO means “Loose electrons, oxidation”. GER means “Gain electrons, reduction”.

Redox reactions: involve both

Redox reactions put the two half-reactions (one reduction, one oxidation) together into a complete equation. In a redox reaction, one species undergoes a loss of electrons in the oxidation half-reaction, while another species undergoes a gain of those same electrons in the reduction half-reaction. Thus, a redox reaction is a chemical reaction that involves the transfer of electrons between two species.

Oxidation Examples

Rust is the classic example of oxidation. Iron metal undergoes oxidation to form brown iron (III) oxide. Other examples of oxidation reactions include zinc metal displacing silver or copper in a solution, with zinc getting oxidized to the zinc (II) ion. And burning anything in oxygen, for example sugar or magnesium, is an oxidation reaction. Bleaching something, like hair, blue jeans, or glowing tonic water, is also an oxidation-reduction reaction – the sodium hypochlorite in bleach is the oxidizer.

oxidation reduction redox reaction — Zinc is oxidized to the Zn⁺² ion, and Tin⁺² ions are reduced to beautiful needle-like elemental tin crystals

Examples of Redox Reactions

Let’s take a look at some examples of full redox reactions

CO₂ (g) + H₂ (g) → CO (g) + H₂O (g)

CH₄ (g) + O₂ (g) → CO₂ (g) + H₂O (g)

Zn + CuCl₂ → ZnCl₂ + Zn

Na + Cl → NaCl

How do you recognize redox reactions?

The easiest and primary way to recognize a redox reaction is by look for changes in the oxidation states of atoms from the reactants to the products. One species should have its oxidation number decrease from reactants to products (this, again, is reduction). Another species in the same reaction should have its oxidation number increased from reactants to products (this, again, is oxidation). Just remember – the number of electrons lost should be equal to the number of electrons gained.

Reminder: an atom’s oxidation number (also sometimes called the oxidation state) is a measure of how many electrons it has gained or lost. That is, an oxidation number of zero indicates a neutral atom. Similarly, gaining electrons decreases the oxidation number, since electrons are negative and thus add a negative charge. Losing electrons increases the oxidation number, since the negative charge is being lost. Read more about oxidation states here!

Common types of redox reactions

Aside from using oxidation states to recognize redox reactions, there are other clues to when a redox reaction is happening; namely, there are a few different types of reactions where reduction and oxidation often both take place.

Combination reactions: these reactions combine elements to create a compound, generally taking the form of A + B → AB
Decomposition reactions: these reactions are the reverse of combination reactions; a compound decomposes into its elemental parts. These generally take the form of AB → A + B
Combustion reactions: these reactions involve a fuel (usually organic) and oxygen as reactants, and result in water and carbon dioxide, and occasionally another organic product such as nitrogen. The general form is X + O₂ → CO₂ + H₂O. This equation, of course, needs to balanced according to what X, the organic fuel, is.
Displacement reactions with an oxidation state change. One can classify reactions as either single replacement, where the reaction involves the replacement of one element in a compound by another one, or double replacement, where the reaction involves the replacement of an element in each of the two different reactants. Effectively, swapped. The general form of a single replacement reaction is A + BC → B + AC. The general form of a double replacement reaction is AB + CD → AD + CB.

See our tutorial on balancing redox reactions for more information and further examples!

These are usually NOT redox reactions:

Acid-base reactions
Simple precipitation reactions
Double displacement reactions with no oxidation state change

Video of a simple redox reaction

Let’s look at an oxidation reduction reaction. During the elephant toothpaste reaction in this video, potassium permanganate’s manganese undergoes reduction from the +7 to the +4 and +2 oxidation states, while hydrogen peroxide’s oxygen undergoes oxidation from the -1 to the 0 oxidation state, resulting in the formation of elemental oxygen.