An acidic oxide is an oxide that either produces an acidic solution upon addition to water, or acts as an acceptor of hydroxide ions effectively functioning as a Lewis acid. Acidic oxides will typically have a low pKa and may be inorganic or organic. A commonly encountered acidic oxide, carbon dioxide produces an acidic solution (and the generation of carbonic acid) when dissolved.
The acidity of an oxide can be reasonably assumed by its accompanying constituents. Less electronegative elements tend to form basic oxides such as sodium oxide and magnesium oxide, whereas more electronegative elements tend to produce acidic oxides as seen with carbon dioxide and phosphorus pentoxide. Some oxides like aluminium oxides are amphoteric.
Acidic oxides are of environmental concern. Sulfur and nitrogen oxides are considered air pollutants as they react with atmospheric water vapour to produce acid rain.
Carbonic acid is an illustrative example of the lewis acidity of an acidic oxide.
- CO2 + 2OH− ⇌ HCO3− + OH− ⇌ CO32− + H2O
This property is a key reason for keeping alkali chemicals well sealed from the atmosphere, as long-term exposure to carbon dioxide in the air can degrade the material.
- 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
- 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 manufacturing of sulfuric acid.
- Cl2O7 + H2O → 2 HClO4
Iron(II) oxide is the anhydride of the aqueous ferrous ion:
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3. Describing a compound that forms an acid when dissolved in water. Carbon dioxide, for example, is an acidic oxide.
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