Oxide
An oxide is a compound in which one or more oxygen atoms are bonded to another type of atom, often a metal. In oxidative reactions, oxygen combines chemically with another substance.
In nonmetal oxides, the bonding is primarily covalent (characterized by the sharing of electrons)rather than ionic (accomplished as a result of opposite charges). Many familiar nonmetallic oxides, including carbon monoxide, carbon dioxide, and sulfur dioxide, exist as molecules. Most nonmetals can form more than one oxide. Normally the more highly oxidized oxide forms when the nonmetal burns at ordinary temperatures in an abundance of oxygen, allowing the maximum number of oxygens to bond, e.g., CO2. High temperatures and/or little oxygen favor formation of the less oxidized oxide, e.g., CO. Many nonmetal oxides react with water to form acids; these substances are called acid anhydrides.
When oxygen reacts with metal, the reaction product is usually an ionic solid containing an oxide ion. Many transition metals form more than one oxide. In these cases, it is usually the oxide containing the positive ion of highest charge that is formed at ordinary temperatures. Many ionic oxides react with water to form the corresponding metal hydroxide in water. Compounds that react with water to form hydroxide ions are called basic anhydrides.
A few transition metals form oxides in which the bonding is more covalent than ionic.Titanium oxide and manganese oxide have macromolecular structures more like that of SiO2. Neither of these compounds react with water to form hydroxide ions.
Most metal oxides can be reduced to the pure metal by heating them in the presence of an element that has a strong affinity for oxygen. Carbon, in the form of coke, is most frequently used for this purpose. In this reaction, the decomposition products of coke react with the metal oxide to produce carbon dioxide and the pure metal.
In the case of hematite ore (Fe2O3), a mixture of the ore, coke, and limestone is first placed in a blast furnace, then pure or compressed air is introduced at the base of the furnace to burn the coke, whose decomposition products remove the oxygen atoms from the hematite.
Oxides of highly active metals cannot be reduced in the presence of coke, but their pure metals can sometimes be obtained by electrolysis. An example is the reduction of bauxite to yield pure aluminum by means of electrolysis.
In the case of certain ores containing relatively inactive metals such as mercury, separation can be achieved by heating the ore in air, i.e., by oxidative calcination (also known as roasting). In the case of more chemically active metals, e.g., zinc, roasting produces the metal oxide instead of the free metal.
Although the most common anion formed by oxygen is monatomic, oxygen can also form diatomic ions known as peroxides (bivalent) and superoxides (monovalent). Metal peroxides undergo a violent reaction with water to form a solution containing hydrogen peroxide. Superoxides react with water to produce molecular oxygen and hydrogen peroxide.
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