1.1 | Reduction and Oxidation

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Electrochemistry is the study of the the interactions between chemicals (molecules and materials) and electricity (exchange of charge as ions or electrons). A redox reaction is when electrons are exchanged between two chemical species. The species that loses electrons is call the reducing agent (reductant) and the species that gains electrons is called the oxidizing agent (oxidant). One example of a redox reaction is shown in Fig. 1.

Fig. 1 Redox reaction between copper(II) and iron(III) ions. Copper is reduced and iron is oxidized. Copper is the oxidant and iron is the reductant.

Reduction is the addition of electrons to a chemical species. Oxidation is the removal of electrons from a chemical species. A reaction in which a species is reduced is not possible without another species in the balance reaction being oxidized. This is because just like mass, charge is conserved in all chemical reactions.

A Periodic Trend: The Strength of Oxidants and Reductants

For the elements and elemental ions the strength of an oxidant typically increases with the elements electronegativity, Fig. 2. The most electronegative elements (e.g. Fluorine) are very strong oxidants and the most electropositive elements (e.g. Cesium) are very strong reductants. Strong oxidants easily accept electrons and strong reductants easily donate electrons. Strong oxidants are very poor reductants and strong reductants are very poor oxidants.

Fig. 2 The trend of electronegativity across the periodic table. Eletronegativity increases from left to right and decreases from top to bottom. The most electronegative elements are strong oxidants and the most electropositive elements are strong reductants.

Ionic Charge and Redox Strength

The oxidizing or reducing strength of an elemental ion is also influenced by the ions charge. For example, Fe²⁺ is a strong reductant than Fe³⁺ because Fe³⁺ has a a more negative charge and therefore more can easily donates electrons. Intuitively, the electrons in the more electron rich ion are less attracted to the nucleus of Fe²⁺ because more electrons better screen the nuclear charge than in the case of the Fe³⁺ ion. Therefore, the electrons are more weakly bound to the Fe²⁺ ion and are more easily donated.

Likewise, Cu²⁺ is a stronger oxidant compared to Cu⁺ because Cu²⁺ has a more positive charge that can more strongly attract electron from other chemical species.

Practice: Identifying oxidation and reduction in chemical reactions

1. For each of the following, identify the strongest oxidant.

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2. For each of the following, identify the strongest reductant.

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