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Acidic oxides, or acid anhydrides, 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. An inorganic anhydride (a somewhat archaic term) is 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 (pH = 5.6) by a factor of 25 compared to pure water (pH = 7). 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 you move across the period, the oxides become more acidic. Sodium and magnesium oxides are alkaline. Aluminium oxides are amphoteric (reacting both as a base or acid). Silicon, phosphorus, sulfur, and chlorine oxides are acidic. Some non-metal oxides, such as nitrous oxide (N2O) and carbon monoxide (CO), do not display any acid/base characteristics.
Acidic oxides can also react with basic oxides to produce salts of oxoanions:
- 2 MgO + SiO2 → Mg2SiO4
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
- CO2 + OH− ⇌ HCO3− + OH− ⇌ CO32− + H2O
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 (Mg,Fe)2SiO4 or as (MgO,FeO)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 acid silicon dioxide.
Examples of reactions
- H2CO3 –> H2O + CO2
- The enzyme carbonic anhydrase, which catalyzes this reaction, is named after this property of carbon dioxide.
- 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
- Al2O3 + 2 NaOH + 3 H2O → 2 NaAl(OH)4
- SiO2 + 2 NaOH → Na2SiO3 + H2O
- P4O6 + 6 H2O → 4 H3PO3
Phosphorus(V) oxide reacts with water to give phosphoric (v) acid:
- P4O10 + 6 H2O → 4 H3PO4
- SO2 + H2O → H2SO3
- SO3 + H2O → H2SO4
This reaction is important in the manufacture of the acid.
- Cl2O + H2O ↔ 2 HOCl
- Cl2O7 + H2O → 2 HClO4
Iron(II) oxide is the anhydride of the aqueous ferrous ion: [Fe(H2O)6]2+ --> FeO + 2H+ + 5H2O