Oxidizing agent

European Union Chemical hazard symbol for oxidising agents

Dangerous goods label for oxidising agents

An oxidizing agent (also oxidant, oxidizer or oxidiser) is the element or compound in an oxidation-reduction (redox) reaction that accepts an electron from another species. Because the oxidizing agent is gaining electrons, we say it has been reduced.

The oxidizing agent itself is reduced, as it is taking electrons onto itself, but the reactant is oxidized by having its electrons taken away by the oxidizing agent. Oxygen is the prime (and eponymous) example among the varied types of oxidizing agents.

Overview

• The oxidizing agent takes electrons from another species, and thus itself is reduced.
• The reducing agent gives electrons to another species, and thus itself is oxidized.
• All atoms in a molecule can be assigned an oxidation number. This number changes when an oxidant acts on a substrate.
• Redox reactions occur when oxidation states of the reactants change.

Example of oxidation

The formation of iron(III) oxide;

4Fe + 3O₂ → 2Fe₂O₃

In the above equation, the iron (Fe) has an oxidation number of 0 before and 3+ after the reaction. For oxygen (O) the oxidation number began as 0 and decreased to 2−. These changes can be viewed as two "half-reactions" that occur concurrently:

1. Oxidation half reaction: Fe⁰ → Fe³⁺ + 3e⁻
2. Reduction half reaction: O₂ + 4e⁻ → 2 O²⁻

Iron (Fe) has become oxidised because its oxidation number increased and was the reducing agent because it gave electrons to the oxygen (O). Oxygen (O) has been reduced because the oxidation number has decreased and was the oxidising agent because it took electrons from iron (Fe).

Electron acceptor

Because the process of oxidation is so widespread (fire, explosives, chemical synthesis, corrosion), the term oxidising agent has acquired multiple meanings.

In one definition, an oxidising agent accepts - or gains - electrons. In this context, the reducing agent is called an electron donor. A classic oxidising agent is the ferrocenium ion [Fe(C₅H₅)₂]⁺, which accepts an electron to form Fe(C₅H₅)₂. Of great interest to chemists are the details of the electron transfer event, which can be described as inner sphere or outer sphere.

In more colloquial usage, an oxidising agent transfers oxygen atoms to the substrate. In this context, the oxidising agent can be called an oxygenation reagent or oxygen-atom transfer agent. Examples include [MnO₄]⁻ (permanganate), [CrO₄]²⁻ (chromate), OsO₄ (osmium tetroxide), and especially [ClO₄]⁻ (perchlorate). Notice that these species are all oxides, and are in fact polyoxides. In some cases, these oxides can also serve as electron acceptors, as illustrated by the conversion of [MnO₄]⁻ to [MnO₄]²⁻, manganate.

Dangerous materials definition

The dangerous materials definition of an oxidising agent is a substance that is not necessarily combustible, but may, generally by yielding oxygen, cause or contribute to the combustion of other material. By this definition some materials that are classified as oxidising agents by analytical chemists are not classified as oxidising agents in a dangerous materials sense. An example is potassium dichromate, which does not pass the dangerous goods test of an oxidising agent.

Common oxidising agents

• Oxygen (O₂)
• Ozone (O₃)
• Hydrogen peroxide (H₂O₂) and other inorganic peroxides
• Fluorine (F₂), chlorine (Cl₂), and other halogens
• Nitric acid (HNO₃) and nitrate compounds
• Sulfuric acid (H₂SO₄)
• Peroxydisulfuric acid (H₂S₂O₈)
• Peroxymonosulfuric acid (H₂SO₅)
• Chlorite, chlorate, perchlorate, and other analogous halogen compounds
• Hypochlorite and other hypohalite compounds, including household bleach (NaClO)
• Hexavalent chromium compounds such as chromic and dichromic acids and chromium trioxide, pyridinium chlorochromate (PCC), and chromate/dichromate compounds
• Permanganate compounds such as potassium permanganate
• Sodium perborate
• Nitrous oxide (N₂O)
• Silver oxide (Ag₂O)
• Osmium tetroxide (OsO₄)
• Tollens' reagent
• 2,2'-Dipyridyldisulfide (DPS)

Common oxidising agents and their products

Agent	Product(s)
O2 oxygen	Various, including the oxides H2O and CO2
O3 ozone	Various, including ketones, aldehydes, and H2O; see ozonolysis
F2 fluorine	F−
Cl2 chlorine	Cl−
Br2 bromine	Br−
I2 iodine	I−, I3−
OCl− hypochlorite	Cl−, H2O
ClO3− chlorate	Cl−, H2O
HNO3 nitric acid	NO nitric oxide NO2 nitrogen dioxide
Hexavalent chromium CrO3 chromium trioxide CrO42− chromate Cr2O72− dichromate	Cr3+, H2O
MnO4− permanganate MnO42− manganate	Mn2+ (acidic) or MnO2 (basic)
H2O2, other peroxides	Various, including oxides and H2O

See also

• Dye
• Organic oxidation
• Reducing agent