Oxide

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An oxide is a chemical compound containing an oxygen atom and other elements. Most of the earth's crust consists of oxides. Oxides result when elements are oxidized by air. Combustion of hydrocarbons affords the two principal oxides of carbon, carbon monoxide and carbon dioxide. Even materials that are considered to be pure elements often contain a coating of oxides. For example, aluminium foil has a thin skin of Al₂O₃ that protects the foil from further corrosion.

1 Most oxides are insoluble in water
2 Nomenclature
3 Types of oxides
4 Common oxides sorted by oxidation state
5 See also
6 References

[edit] Most oxides are insoluble in water

The oxide anion, O²⁻, is the conjugate base of the hydroxide ion, OH⁻, and is encountered in ionic solid such as calcium oxide. O²⁻ is unstable in aqueous solution − its affinity for H⁺ is so great (pKb ~ -22) that it abstracts a proton from a solvent H₂O molecule:

O²⁻ + H₂O → 2 OH⁻

Although many anions are stable in aqueous solution, ionic oxides are not. For example, sodium chloride dissolve readily in water to give a solution containing the constitutent ions, Na⁺ and Cl^-. Oxides do not behave like this. When an ionic oxide dissolves, the O²⁻ ions become protonated. Although Calcium oxide, CaO, is said to "dissolve" in water, the products include hydroxide:

CaO + H₂O → Ca²⁺ + 2 OH^-

In fact, no monoatomic dianion is known to dissolve in water - all are so basic that they undergo hydrolysis. Concentrations of oxide ion in water are too low to be detectable with current technology.

Authentic soluble oxides do exist of course, but they release oxyanions, not O^2-. Well known soluble salts of oxyanions include sodium sulfate (Na₂SO₄), potassium permanganate (KMnO₄), and sodium nitrate (NaNO₃).

[edit] Nomenclature

In the 18th century, oxides were named calxes or calces after the calcination process used to produce oxides. Calx was later replaced by oxyd.

Oxides can be named after the number of oxygen atoms in the oxide. Oxides containing only one oxygen are called oxide or monoxide, those containing two oxygen atoms dioxide, three trioxide, four tetroxide, and so on following the Greek numerical prefixes.

Two other types of oxide are peroxide, O₂²⁻, and superoxide, O₂⁻. In such species, oxygen is assigned higher oxidation states than oxide.

[edit] Types of oxides

Oxides of more electropositive elements tend to be basic. They are called basic anhydrides; adding water, they may form basic hydroxides. For example, sodium oxide is basic; when hydrated, it forms sodium hydroxide.

Oxides of more electronegative elements tend to be acidic. They are called acid anhydrides; adding water, they form oxoacids. For example, dichlorine heptoxide is acid; perchloric acid is a more hydrated form.

Some oxides can act as both acid and base, at different times. They are amphoteric. An example is aluminium oxide. Some oxides do not show behavior as either acid or base.

The oxides of the chemical elements in their highest oxidation state are predictable and the chemical formula can be derived from the number of valence electrons for that element. Even the chemical formula of ozone is predictable as a group 16 element. One exception is copper for which the highest oxidation state oxide is copper(II) oxide and not copper(I) oxide. Another exception is fluoride that does not exist as expected as F₂O₇ but as OF₂ with the least electronegative element given priority. ^[1]. Phosphorus pentoxide, the third exception is not properly represented by the chemical formula P₂O₅ but by P₄O₁₀