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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 the oxide is left. The resulting oxide belongs to this group of substances.
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), while silicon, phosphorus, sulphur and chlorine oxides are acidic. Some non-metal oxides do not, however, display any acid/base characteristics such as N2O or carbon monoxide.
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.
Example reactions of acidic oxides
- Carbon dioxide, which reacts with water forming 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.
Aluminium oxide (Al2O3) is an amphoteric oxide; it can act as a base or acid. For example with base different aluminate salts will be formed.
Al2O3 + 2 NaOH + 3 H2O -> 2 NaAl(OH)4
Silicon dioxide is a weakly acidic oxide. It will react with strong bases to form silicate salts, but not with water.
SiO2 + 2 NaOH -> Na2SiO3 + H2O
Phosphorus (iii) oxide reacts to form phosphorous acid in water
P4O6 + 6 H2O -> 4 H3PO3
Phosphorus (v) oxide reacts with water to give phosphoric (v) acid
P4O10 + 6 H2O -> 4 H3PO4
Sulfur dioxide reacts with water to form the weak acid, sulphurous acid.
SO2 + H2O -> H2SO3
Sulfur trioxide forms the strong sulphuric acid with water.
SO3 + H2O -> H2SO4
Chlorine (vii) oxide reacts with water to form perchloric acid, a strong acid.
Cl2O7 + H2O -> 2 HClO4
Chlorine (i) oxide reacts with water to form hypochlorous acid, a very weak acid
Cl2O + H2O <-> 2 HOCl
- Organic acid anhydride, similar compounds in organic chemistry
- Base anhydride
- Inorganic anhydride (or anhydride or acid anhydride)
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
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