Acidic oxide
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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.[1] 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. Generally non-metallic oxides are acidic.[2]
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 such as carbon dioxide and phosphorus pentoxide. Some oxides, like aluminium oxides, are amphoteric, while some oxides may be neutral. [3]
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.
Examples
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.
- Carbon dioxide is also the anhydride of carbonic acid:
- Page Module:Chem2/styles.css has no content.H2CO3 → H2O + CO2
- Chromium trioxide, which reacts with water forming chromic acid:
- Page Module:Chem2/styles.css has no content.CrO3 + H2O → H2CrO4
- Dinitrogen pentoxide, which reacts with water forming nitric acid:
- Page Module:Chem2/styles.css has no content.N2O5 + H2O → 2 HNO3
- Manganese heptoxide, which reacts with water forming permanganic acid:
- Page Module:Chem2/styles.css has no content.Mn2O7 + H2O → 2 HMnO4
Further examples
Aluminium oxide
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
Silicon dioxide is an acidic oxide. It will react with strong bases to form silicate salts.[4]
Silicon dioxide is the anhydride of silicic acid:
- Page Module:Chem2/styles.css has no content.H4SiO4 → 2 H2O + SiO2
Phosphorus oxides
Phosphorus(III) oxide reacts to form phosphorous acid in water:
- P4O6 + 6 H2O → 4 H3PO3
Phosphorus(V) oxide reacts with water to give phosphoric acid:
- P4O10 + 6 H2O → 4 H3PO4
Sulfur oxides
Sulfur dioxide reacts with water to form the weak acid, sulfurous acid:
- SO2 + H2O → H2SO3
Sulfur trioxide forms the strong acid sulfuric acid with water:
- SO3 + H2O → H2SO4
This reaction is important in the manufacturing of sulfuric acid.
Chlorine oxides
Chlorine(I) oxide reacts with water to form hypochlorous acid, a very weak acid:
- Page Module:Chem2/styles.css has no content.Cl2O + H2O ⇌ 2 HOCl
Chlorine(VII) oxide reacts with water to form perchloric acid, a strong acid:
- Cl2O7 + H2O → 2 HClO4
Iron oxides
Iron(II) oxide is the anhydride of the aqueous ferrous ion:
- Page Module:Chem2/styles.css has no content.[Fe(H2O)6]2+ → FeO + 2 H+ + 5 H2O
Chromium oxides
Chromium trioxide is the anhydride of chromic acid:
- Page Module:Chem2/styles.css has no content.H2CrO4 → H2O + CrO3
Vanadium oxides
Vanadium trioxide is the anhydride of vanadous acid:
- Page Module:Chem2/styles.css has no content.2H3VO3 → 3H2O + V2O3
Vanadium pentoxide is the anhydride of vanadic acid:
- Page Module:Chem2/styles.css has no content.2H3VO4 → 3H2O + V2O5
See also
- Organic acid anhydride, similar compounds in organic chemistry
- Base anhydride
References
- Page Module:Citation/CS1/styles.css has no content.Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. doi:10.1016/C2009-0-30414-6. ISBN 978-0-08-037941-8.
Page Template:Reflist/styles.css has no content.
- ^ Page Module:Citation/CS1/styles.css has no content.John Daintith (February 2008). "acidic". A Dictionary of Chemistry.
3. Describing a compound that forms an acid when dissolved in water. Carbon dioxide, for example, is an acidic oxide.
- ^ Page Module:Citation/CS1/styles.css has no content.David Oxtoby; H. P. Gillis; Alan Campion. Principles of Modern Chemistry (7th ed.). Cengage Learning. pp. 675–676. ISBN 978-0-8400-4931-5.
- ^ Page Module:Citation/CS1/styles.css has no content.Chang, Raymond; Overby, Jason (2011). General chemistry: the essential concepts (6th ed.). New York, NY: McGraw-Hill. ISBN 9780073375632. OCLC 435711011.
- ^ Page Module:Citation/CS1/styles.css has no content.Comprehensive Chemistry Volume 1. New Delhi, India: Laxmi Publications. 2018. p. 6.13. ISBN 978-81-318-0859-7.
