Acidic oxides, or acid anhydride, are oxides that react with water to form an acid, or with a base to form a salt. They are oxides of either nonmetals or of metals in high oxidation states. Their chemistry can be systematically understood by taking an oxoacid and removing water from it, until only an oxide remains. The resulting oxide belongs to this group of substances. For example, sulfurous acid, sulfuric acid, and carbonic acid are acidic oxides. An inorganic anhydride is an acid anhydride without an organic moiety. Acidic oxides are not Brønsted–Lowry acids because they do not donate protons; however, they are Arrhenius acids because they increase the hydrogen ion concentration of water. For instance, carbon dioxide increases the hydrogen ion concentration of rain water by a factor of 25 compared to pure water. They are also Lewis acids, because they accept electron pairs from some Lewis bases, most notably base anhydrides. The oxides of period three elements demonstrate periodicity with respect to acidity. As one moves across the period, the oxides become more acidic. Sodium and magnesium oxides are alkaline. Aluminium oxides are amphoteric. Silicon, phosphorus, sulfur, and chlorine oxides are acidic. Some non-metal oxides, such as nitrous oxide and carbon monoxide, do not display any acid/base characteristics. Acidic oxides can also react with basic oxides to produce salts of oxoanions: Acidic oxides are environmentally relevant. Sulfur and nitrogen oxides are considered air pollutants as they react with atmospheric water vapour to produce acid rain.
Reactions of acidic oxides
Although it is difficult to classify these oxides as acids, the property is manifest in reactions with bases. For example, carbon dioxide reacts with alkali. For this reason, alkali is kept in stoppered vessels to inhibit reaction with atmospheric carbon dioxide. In geochemistry complex silicates are often written as though they are the products of an acid-base reaction. For example, the chemical formula of the mineral olivine can be written either as 2SiO4 or as 2SiO2. This mineral is said to be ultramafic, meaning that it has a very high nominal content of the bases magnesium oxide and iron oxide and hence, a low content of the acidsilicon dioxide.
Examples of reactions
Carbon dioxide is the anhydride of carbonic acid:
Chromium trioxide, which reacts with water forming chromic acid
Dinitrogen pentoxide, which reacts with water forming nitric acid
Manganese heptoxide, which reacts with water forming permanganic acid
Examples
Aluminium oxide
is an amphoteric oxide; it can act as a base or acid. For example, with base different aluminate salts will be formed:
reacts with water to form the weak acid, sulfurous acid: Sulfur trioxide forms the strong sulfuric acid with water : This reaction is important in the manufacture of the acid.
