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Dangerous goods label for oxidizing agents

An oxidizing agent (also called an oxidant, oxidizer or oxidiser) can be defined as either:

  1. a chemical compound that readily transfers oxygen atoms, or
  2. a substance that gains electrons in a redox chemical reaction

In both cases, the oxidizing agent becomes reduced in the process.

In simple terms:

  • The oxidizing agent is reduced.
  • The reducing agent 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.

Contents

Definition

Oxidising, literally, means converting to oxide. This process can apply to metals (iron converts to iron oxide), nonmetals (sulfur converts to sulfur oxide), and organic matter (mainly carbon and hydrogen converts to carbon oxide and hydrogen oxide). An obvious oxidizer is oxygen, which forms about 21% of air.

Later, the use of the term expanded to include any time where formal charge is increased (losing electrons), and applies to substances which contain no oxygen (typically halogens and substances rich in these elements, and less commonly sulfur). Ironically, the action of fluorine upon many oxides is an oxidation even if the reaction results in the removal of the oxygen from a substance. It is thus the generic opposite of reduction, where formal charge is decreased (gaining electrons). With this definition, the "oxide" part of the term is misleading, since oxygen might have no presence in the reaction, yet no alternative term has gained favor.

Many common oxidizers contain oxygen (KClO4 is KCl "plus" 2 O2) and can be considered compact storage of oxygen; a given volume of potassium perchlorate contains much more oxygen than the same volume of air. The fuel and oxidizer can be comminuted, which allows the reaction to proceed much faster than if oxygen is brought in from air. Gunpowder is one historically important application.

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

Iron (Fe) has been oxidized because the oxidation number increased and is the reducing agent because it gave electrons to the oxygen (O). Oxygen (O) has been reduced because the oxidation number has decreased and is the oxidizing agent because it took electrons from iron (Fe)

Electron acceptor

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

In one definition, an oxidizing agent receives - or accepts - electrons from a reagent. In this context, the oxidizing agent is called an electron acceptor. A classic oxidizing 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 oxidizing agent transfers oxygen atoms to the substrate. In this context, the oxidizing 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 oxidizing agent is a substance that is not necessarily combustible, but may, generally by yielding oxygen, cause or contribute to the combustion of other material (Australian Dangerous Goods Code, 6th Edition). By this definition some materials that are classified as oxidizing agents by analytical chemists are not classified as oxidizing agents in a dangerous materials sense. An example is potassium dichromate, which does not pass the dangerous goods test of an oxidizing agent.

Common oxidizing agents

Common oxidizing 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

References

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