Acidic oxide

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 pK_a 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 as seen with 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:

H₂CO₃ → H₂O + CO₂

• Chromium trioxide, which reacts with water forming chromic acid:

CrO₃ + H₂O → H₂CrO₄

• Dinitrogen pentoxide, which reacts with water forming nitric acid:

N₂O₅ + H₂O → 2 HNO₃

• Manganese heptoxide, which reacts with water forming permanganic acid:

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:

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:

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:

[Fe(H₂O)₆]²⁺ → FeO + 2 H⁺ + 5 H₂O

Chromium oxides

Chromium trioxide is the anhydride of chromic acid:

H₂CrO₄ → H₂O + CrO₃

Vanadium oxides

Vanadium trioxide is the anhydride of vanadous acid:

2H₃VO₃ → 3H₂O + V₂O₃

Vanadium pentoxide is the anhydride of vanadic acid:

2H₃VO₄ → 3H₂O + V₂O₅

See also

• Organic acid anhydride, similar compounds in organic chemistry
• Base anhydride