Definitions

A reduction-oxidation (redox) reaction is one in which electron transfer occurs between two chemical ’species’. A chemical species can be an atom, ion, radical, or molecule.

One of the species is ‘reduced’ at the same time the other is ‘oxidised’ — the two events are coupled and neither can occur in isolation.

Reduction

The word ‘reduction’ came from observing metal extraction from ore. Ore, on heating, was found to ‘reduce’ (in weight) to metal, with that loss mostly from the release of oxygen gas. Once it was discovered that those metal atoms were also gaining electrons, the term took on this broader definition to cover other species also observed to gain electrons in chemical reactions.

A handy mnemonic is to say that reduction is the receipt of electrons (by a chemical species).

Oxidation

‘Oxidation’ implies oxygen, and this is exactly the origin of this word. Oxygen ‘oxidised’ elements to form oxide compounds — iron was ‘oxidised’ to iron oxides, for example. In the process, oxygen lost electrons, and the term later came to mean ‘loses electrons’ when applied more broadly to other species that also lost electrons in reactions.

A handy mnemonic is to say that oxidation is the loss of electrons (by a chemical species).

Reducing Agent (Reductant, Reducer)

Reducing agent, reductant, and reducer are all synonyms. A reducing agent causes a substance to be reduced, and is itself oxidised.

Oxidising Agent (Oxidant, Oxidiser)

Oxidising agent, oxidant, and oxidiser are all synonyms. An oxidising agent causes a substance to be oxidised, and is itself reduced.

Making Sense of it All

A reducing agent is oxidised and an oxidising agent is reduced, what?!

And oxygen is reduced, not oxidised — how does that work?!

Everything to do with redox is incredibly confusing to grasp at first — it is not you! Let’s break down the words for better understanding.

For example, does it make sense that an agent (of any sort) facilitates something, makes something happen?
“Our sales agent sells our goods for us.”
“Our sales agent makes our goods sell.”

From this, does it then make sense that a reducing agent facilitates reduction, makes reduction happen?
“A reducing agent facilitates the reduction of something else.”
“A reducing agent makes reduction happen.”

Remember at the beginning of this chapter how one of a species is reduced at the same time the other is oxidised, and that neither happens in isolation?

From this, does it now make sense that a reducing agent, by reducing something, must itself be oxidised?

And, after substituting the opposite words in all the above, is it also clearer that an oxidising agent facilitates oxidation, makes oxidation happen?
“An oxidising agent facilitates the oxidation of something else.”
“An oxidising agent makes oxidation happen.”

And that an oxidising agent, by oxidising something, must itself be reduced?

Mnemonics

If you can remember that
reduction is the receipt of electrons, and that
oxidation is the loss of electrons, and that
an agent facilitates, or makes something happen, then you’ve made huge progress in following this stuff!

Also be aware of the synonyms ‘reductant’ and ‘reducer’ for ‘reducing agent’, and ‘oxidant’ and ‘oxidiser’ for ‘oxidising agent’. All are used equally by chemists, but this only adds to the confusion if not aware of them.

And the simplest thing of all to remember, whenever oxygen is involved? Just know that oxygen is reduced, and it automatically follows that anything it is reacting with is oxidised!

Half-Reactions

Half-reactions don’t exist by themselves, but are really useful for describing (and understanding) the reduction-half and the oxidation-half of a complete redox reaction.

Let’s break down a redox reaction by burning charcoal, and let’s further assume that charcoal is pure carbon (‌C‌):
carbon + oxygen gas → carbon dioxide

As a chemical equation:
C + O₂ → CO₂

Here the original meaning of ‘oxidation’ might be clearer, in that oxygen has ‘oxidised’ carbon to carbon dioxide?

Until I write more to explain this further, please, for now, accept that carbon atoms donate four electrons in chemical reactions. Those carbon atoms become ions with a positive charge of 4, written C⁴⁺.

Please also accept, for now, that oxygen atoms acquire two electrons in chemical reactions. Those oxygen atoms become oxygen ions with a negative charge of 2, written O^2-. Oxygen gas (O₂) is two oxygen atoms joined together.

Chemical reactions are written left to right, with the substance(s) undergoing reaction on the left, and the finished product(s) on the right.

Thus the half-reaction for carbon atoms becoming carbon ions is:
C → C⁴⁺ + 4e^-
Here, the starting carbon has lost electrons, and has therefore been oxidised.
[Note how both sides balance (assume C is C⁰ if that helps) to maintain the Law of Conservation of Mass.]

The half-reaction for oxygen atoms becoming oxygen ions is:
O₂ + 4e^- → 2O^2-
Here oxygen has received electrons, and has therefore been reduced.

Putting everything together, we have:
C + O₂ + 4e^- → C⁴⁺ + 4e^- + 2O^2-

As with algebra, remove what’s common to both sides:
C + O₂ + 4e^- → C⁴⁺ + 4e^- + 2O^2-
C + O₂ → C⁴⁺+ 2O^2-
C + O₂ → CO₂

Oxidation State (Oxidation Number)

So far we’ve followed redox reactions in the context of receiving or losing electrons, but this discussion wouldn’t be complete without mentioning oxidation state, also known as the oxidation number.

The oxidation state is the number of electrons removed from or received by an atom during a chemical reaction.

In the half-reaction for carbon above:
C → C⁴⁺ + 4e^-
carbon atoms began with a neutral charge (C⁰ if you like), and ended with a positive charge of +4 (C⁴⁺).

This charge is the oxidation state (or number). Here, carbon atoms not only lost four electrons, but their oxidation number increased from 0 to 4.

Thus oxidation can be defined both as the loss of electrons and/or the increase in oxidation state (oxidation number).

In the half-reaction for oxygen:
O₂ + 4e^- → 2O^2-
this is more confusing because of the diatomic (two-atom) nature of oxygen gas. This may make things a little clearer:
½O₂(g) + 2e^- → O^2- [where (g) signifies the gaseous atomic state]

Here, oxygen atoms also begin with a neutral charge (oxidation number = 0), but end with a negative charge of -2.
Oxygen atoms not only received electrons, but their oxidation number decreased from 0 to -2.

Thus reduction can be defined both as the receipt of electrons and/or the decrease in oxidation state (oxidation number).

Mnemonic

A quick mnemonic regarding oxidation states is to know that in reduction, the oxidation number is reduced too. This makes it a lot easier to remember that oxidation is an increase in oxidation number!

To Summarise

A chemical species is an atom, ion, radical, or molecule.

Reduction

A chemical species is reduced/undergoes reduction when:
1. it receives electrons
2. its oxidation state decreases

Oxidation

A chemical species is oxidised/undergoes oxidation when:
1. it loses electrons
2. its oxidation state increases

Oxygen isn’t oxidised — it does the oxidising. Oxygen is reduced.

Significance

Redox reactions feature prominently in biological systems, as biochemical processes could not proceed at all where it not for an electron flow driving everything.

Every single metabolic mode begins with a redox reaction.

Redox reactions make possible the conversion of atmospheric nitrogen into bioavailable nitrates, and the conversion of soil minerals into plant-available nutrients. Redox reactions are behind the conversion of light and carbon dioxide into carbohydrates during photosynthesis, and for the breakdown of those carbohydrates into energy during cellular respiration.

Technical terms such as ‘oxidation’ and ‘reducing agent’ are well-worth familiarising yourself with should you wish to understand more deeply how biochemical pathways occur at all.