Acidic oxide

Acidic oxides, or acid anhydride, are oxides that react with water to form an acid,[1] 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. For example, sulfurous acid (SO₂), sulfuric acid (SO₃), and carbonic acid (CO₂) are acidic oxides. An inorganic anhydride (a somewhat archaic term) is an 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.[2]

The oxides of period three elements demonstrate periodicity with respect to acidity. As one moves 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.[3] 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

Although it is difficult to classify these oxides as acids, the property is manifest in reactions with bases. For example, carbon dioxide reacts with alkali.

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

Carbon dioxide is the anhydride of carbonic acid:

{\ce {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

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

[4]Silicon dioxide is an amphoteric oxide. It will react with strong bases to form silicate salts, but not with water:

SiO₂ + 2 NaOH → Na₂SiO₃ + H₂O

Silicon dioxide is the anhydride of silicic acid:

{\ce {Si(OH)4 -> 2H2O + 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 (v) acid:

P₄O₁₀ + 6 H₂O → 4 H₃PO₄

Phosphorus trioxide is the anhydride of phosphorous acid:

{\ce {2H3PO3 -> 3H2O + P2O3}}

Phosphorus pentoxide is the anhydride of phosphoric acid:

{\ce {2H3PO4 -> 3H2O + P2O5}}

Sulfur oxides

Sulfur dioxide reacts with water to form the weak acid, sulfurous acid:

SO₂ + H₂O → H₂SO₃

Sulfur trioxide forms the strong sulfuric acid with water (so, sulfur trioxide is the anhydride of sulfuric acid):

SO₃ + H₂O → H₂SO₄

This reaction is important in the manufacture of the acid.

Chlorine oxides

Chlorine(I) oxide reacts with water to form hypochlorous acid, a very weak acid:

{\ce {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:

{\ce {[Fe(H2O)6]^2+ -> FeO + 2H+ + 5H2O}}

Chromium oxides

Chromium trioxide is the anhydride of chromic acid:

{\ce {H2CrO4 -> H2O + CrO3}}

Vanadium oxides

Vanadium trioxide is the anhydride of vanadous acid:

{\ce {2H3VO3 -> 3H2O + V2O3}}

Vanadium pentoxide is the anhydride of vanadic acid:

{\ce {2H3VO4 -> 3H2O + V2O5}}

References

Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

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.
David Oxtoby; H. P. Gillis; Alan Campion. Principles of Modern Chemistry (7th ed.). Cengage Learning. pp. 675–676. ISBN 978-0-8400-4931-5.
Chang, Raymond; Overby, Jason (2011). General chemistry: the essential concepts (6th ed.). New York, NY: McGraw-Hill. ISBN 9780073375632. OCLC 435711011.
Comprehensive Chemistry Volume 1. 113, Golden House, Daryaganj, New Delhi - 110002, India: Laxmi Publications. 2018. p. 6.13. ISBN 978-81-318-0859-7.CS1 maint: location (link)

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[1] 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.

[2] David Oxtoby; H. P. Gillis; Alan Campion. Principles of Modern Chemistry (7th ed.). Cengage Learning. pp. 675–676. ISBN 978-0-8400-4931-5.

[3] Chang, Raymond; Overby, Jason (2011). General chemistry: the essential concepts (6th ed.). New York, NY: McGraw-Hill. ISBN 9780073375632. OCLC 435711011.

[4] Comprehensive Chemistry Volume 1. 113, Golden House, Daryaganj, New Delhi - 110002, India: Laxmi Publications. 2018. p. 6.13. ISBN 978-81-318-0859-7.CS1 maint: location (link)