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.
Topics Covered in Other Articles
- 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 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 X1-, for example. This can help us remember what reduction is, as the oxidation number gets reduced.
Note: the species getting reduced is called the oxidizing agent. This may be a little confusing at first, but remember the oxidizing agent is the species that causes another to become oxidized—since redox reactions happen in pairs of reduction and oxidation, the species causing oxidation is the one getting reduced.
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)
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 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 X1- 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.
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.
Note: The reducing agent is the species getting oxidized. This is because the reducing agent is the species causing another to become reduced. In a redox reaction, the species getting oxidized causes the other species to get reduced, through the exchange of electrons.
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. The electrons lost in the oxidation half-reaction are then gained in the reduction half-reaction. In this manner, a redox reaction is a chemical reaction wherein electrons are transferred between two species.
Rust is the classic example of oxidation. Iron metal is oxidized to brown iron (III) oxide. Other oxidation examples include zinc metal displacing silver or copper in solution, the zinc is oxidized to the zinc (II) ion. And burning anything in oxygen, for example sugar or magnesium, is an oxidation reaction.
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. These reactions can either come in the form of single replacement, where one element in a compound is replaced by another one, or in the form of a double replacement, where an element in each of the two different reactants gets replaced. 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. In this video of the elephant toothpaste reaction, manganese in potassium permanganate is reduced from the +7 to the +4 and +2 oxidation states, while the oxygen in hydrogen peroxide is oxidized from the -1 to the 0 oxidation state (to elemental oxygen).