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ChemistryOnline is another engaging and interactive subject site from the ScholarNET Online Education stable of online learning resources. It covers the major Chemistry topics that are fundamental to every senior Chemistry course - Atomic Structure and Bonding, Aqueous Chemistry, Organic Chemistry, Redox Chemistry as well as Thermochemistry (Energy).
ChemistryOnline has been designed with both the teacher and student in mind, providing a wealth of interesting and interactive materials to help make learning Chemistry a more palatable experience.
In this document:
You should already know:
All about atoms and ions
Introduction
You are used to seeing reactions where cations and anions swap places, like the example below. You could call it an "ion transfer" reaction.
You probably have been introduced to the proton transfer reactions that happen in acid-base chemistry.
This time we are looking at a set of reactions based around something else doing the transfer - electrons. Redox reactions are electron transfer reactions.
Redox
Redox reactions actually have two parts to them, because during the electron transfer, one of the reactants (the one who gives up the electrons) is oxidised and the other reactant (the one who receives the electrons) is reduced. This is reflected in the name "redox", which is a combination of the names of these two reactions - REDuction and OXidation.
You are already familiar with redox reactions. When a metal atom like magnesium loses two electrons to become a magnesium ion, Mg2+, that is an example of oxidation - something losing electrons to become a new species.
When an acid reacts with something and gives off hydrogen gas, the acidic hydrogen ions receive electrons from another reagent so that they can form hydrogen atoms again, and then they pair up to become H2 molecules - bubbles of hydrogen gas. This is an example of reduction, because the hydrogen ions gained an electron each so that they could become hydrogen atoms again.
So a redox reaction needs two reactants - one to donate electrons and the other to accept those electrons at the same time.
Oxidation
Oxidation is the reaction where a species loses an electron.
The early oxidation reactions were ones where a compound reacted and gained oxygen, for example when magnesium metal burns and forms magnesium oxide. Hence the name, oxidation. Then, it was realised that when a substance has hydrogen attached and loses it in the reaction, like an acid reacting and giving off hydrogen gas, that this was an other form of the same oxidation process, even though no oxygen was being added. Then, later it was realised that there were other oxidation reactions that didn't involve either oxygen being gained or hydrogen being lost. Eventually it was realised that the action taking place in all the oxidation reactions was that the reactant was losing one or more electrons to becomes the product.
Reduction
Reduction is the opposite of oxidation, because it is the second half of the redox reaction. The species being oxidised loses electrons to the species being reduced. So therefore the species being reduced is the one that is gaining those electrons.
Originally, the name "reduction" was most likely refering to when a metal is extracted from its ore. An ore is where the metal is in its ion form, and so the metal ion is combined with other cations to form an ionic compound of some sort. Oxygen is a common ingredient of metal ores, and processes to extract metals from their ore often included steps like heating the ore with carbon, to leave a lump of metal behind. We know that the oxygen ions reacted to form carbon dioxide gas and so were removed from the solid, but the chemists' observations originally were that the amount of solid reduced in size as the oxygen was lost, until only a lump of metal remained. Now, the definition of reduction has been identified as the gain of electrons.
Oxidants and reductants
In each pair of reactants, one of them is the reducing agent, also called the reductant, and the other is the oxidising agent, or oxidant. A reducing agent causes the other reagent to be reduced. An oxidising agent causes the other reagent to be oxidised. So a reducing agent reduces the oxidising agent, and the oxidising agent reduces the reducing agent. And so the reducing agent is oxidised itself, and the oxidising agent is itself reduced. Confused?
In this reaction above, the iron II ion (Fe2+) is being oxidised to the iron III ion (Fe3+). This means that it is being oxidised by the other reagent, the dichromate ion (Cr2O72-). So the dichromate ion must be the oxidising agent.
Looking at the other reagent, the dichromate ion is being reduced to the chromic ion (Cr3+). It is being reduced by the iron II ion, so the iron II ion must be the reducing agent.
Some species are better at either oxidising something else or reducing something else (depending on their Eo value - see the standard electrode potential page for more about this), so you tend to find that some species are used more commonly as reductants and others are used more commonly as oxidants.
Checkpoint quiz
Summary of this page
- A redox reaction is one where an electron transfer takes place. At least one electron moves from one reactant to the other.
- In a redox reaction, one of the reactants is oxidised, the other is reduced.
- Oxidation is when a species loses electrons during a reaction.
- Reduction is when a species gains electrons during a reaction.
- A reducing agent is the reactant that gives its electron to the species being reduced.
- An oxidising agent is the reactant that receives the electrons from the species being oxidised.
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