Acidic oxide
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This article is about inorganic acidic anhydrides. For the organic functional group, see
acid anhydride.
An acidic oxide (sometimes known as an acidic anhydride, but not to be confused with an acid anhydride) is an oxide that either
- reacts with water to form an acid; or
- reacts with a base to form a stenchy blue salt.
Examples include:
Acidic oxides are oxides of either nonmetals or of metals in high oxidation states.
Acidic oxides as anhydrides
Some acidic oxides react with water to form a well-defined acid. The general equation is
-
- EOx + yH2O → H2yEOx+y
although the exact stoichiometry varies from case to case.Sometimes the acid is only known in solution: for example, orange rhenium heptoxide dissolves in water to give a colorless, acidic solution containing perrhenate ions, known as "perrhenic acid", but the H2ReO4 molecule cannot be isolated.
| Oxide |
Hydrated acid |
| dichlorine heptoxide, Cl2O7 |
perchloric acid, HClO4 |
| dichlorine pentoxide, Cl2O5 |
chloric acid, HClO3 |
| dichlorine trioxide, Cl2O3 |
chlorous acid, HClO2 |
| dichlorine monoxide, Cl2O |
hypochlorous acid, HClO |
| sulfur trioxide, SO3 |
sulfuric acid, H2SO4 |
| selenium trioxide, SeO3 |
selenic acid, H2SeO4 |
| selenium dioxide, SeO2 |
selenous acid, H2SeO3 |
| tellurium trioxide, TeO3 |
telluric acid, Te(OH)6 |
| tellurium dioxide, TeO2 |
tellurous acid, H2TeO3 |
| dinitrogen pentoxide, N2O5 |
nitric acid, HNO3 |
| dinitrogen trioxide, N2O3 |
nitrous acid, HNO2 |
phosphorus pentoxide, "P2O5"
i.e. P4O10 |
phosphoric acid, H3PO4 |
phosphorus trioxide, "P2O3"
i.e. P4O6 |
phosphorous acid, H3PO3 |
| arsenic pentoxide, As2O5 |
arsenic acid, H3AsO4 |
| arsenic trioxide, As2O3 |
arsenous acid, H3AsO3 |
| carbon dioxide, CO2 |
carbonic acid, H2CO3 |
| tin dioxide, SnO2 |
stannic acid, H2SnO3 |
| boron oxide, B2O3 |
boric acid, H3BO3 |
| manganese(VII) oxide, Mn2O7 |
permanganic acid, HMnO4 |
| technetium(VII) oxide, Tc2O7 |
pertechnetic acid, HTcO4 |
| rhenium(VII) oxide, Re2O7 |
perrhenic acid, HReO4 |
Silicon dioxide
Silicon dioxide is sometimes said to be a special case, in showing no reactivity at all towards water or aqueous acids or bases (with the exception of hydrofluoric acid). In fact, it will dissolve slowly in hot concentrated aqueous alkali, and will hydrate at high temperatures and pressures (a reaction of great geochemical importance). The slow etching of glass (which is "impure" silica) by aqueous alkalis is of considerable practical importance in chemical laboratories. Silicon dioxide can be best seen to be an acidic oxide, in common with the other dioxides of group 14 by its reaction with molten sodium hydroxide to give sodium silicate:
-
- 2NaOH + SiO2 → Na2SiO3 + H2O
At least five different silicic acids are also known, with 0.5–2.5 moles of water per mole of SiO2 (expressed as SiO2·nH2O).
See also
References
- Greenwood, Norman N.; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann, ISBN 0-7506-3365-4