Oxidizing agent
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An oxidizing agent (also called an oxidant, oxidizer or oxidiser) can be defined as a substance that removes electrons from another reactant in a redox chemical reaction. The oxidizing agent is "reduced" by taking electrons onto itself and the reactant is "oxidized" by having its electrons taken away. Oxygen is the prime example of an oxidizing agent, but it is only one among many.
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.
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:
- Oxidation half reaction: Fe0 → Fe3+ + 3e−
- Reduction half reaction: O2 + 4e− → 2 O2−
Iron (Fe) has become oxidized 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 oxidizing 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 oxidizing agent has acquired multiple meanings.
In one definition, an oxidizing agent accepts - or gains - electrons. In this context, the reducing agent is called an electron donor. 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
- 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)
- Persulfuric acids (H2SO5 and H2SO8)
- 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
- Sodium perborate
- Nitrous oxide (N2O)
- Silver oxide (Ag2O)
- Osmium tetroxide (OsO4)
- Tollens' reagent
- 2,2'-Dipyridyldisulfide (DPS)
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 |