Acidic oxide
794849
163956889
2007-10-12T02:16:49Z
Shalom Yechiel
608951
italics
:''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 (chemistry)|base]] to form a [[salt (chemistry)|salt]].
Examples include:
*[[Carbon dioxide]] which reacts with water to produce [[carbonic acid]].
*[[Sulfur dioxide]], which does not form the non-existent [[sulfurous acid]] but does react with bases to form [[sulfite]]s.
*[[Silicon dioxide]], which does not react with water but will react with bases to form [[silicate]]s
*[[Chromium trioxide]], which reacts with water to form [[chromic acid]].
Acidic oxides are oxides of either [[nonmetal]]s or of [[metal]]s in high [[oxidation state]]s.
== Acidic oxides as anhydrides ==
Some acidic oxides react with water to form a well-defined acid. The general equation is
::EO<sub>''x''</sub> + ''y''H<sub>2</sub>O → H<sub>2''y''</sub>EO<sub>''x''+''y''</sub>
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 H<sub>2</sub>ReO<sub>4</sub> molecule cannot be isolated.
{|class="wikitable"
|-
! Oxide !! Hydrated acid
|-
| [[dichlorine heptoxide]], Cl<sub>2</sub>O<sub>7</sub>
| [[perchloric acid]], HClO<sub>4</sub>
|-
| [[dichlorine pentoxide]], Cl<sub>2</sub>O<sub>5</sub>
| [[chloric acid]], HClO<sub>3</sub>
|-
| [[dichlorine trioxide]], Cl<sub>2</sub>O<sub>3</sub>
| [[chlorous acid]], HClO<sub>2</sub>
|-
| [[dichlorine monoxide]], Cl<sub>2</sub>O
| [[hypochlorous acid]], HClO
|-
| [[sulfur trioxide]], SO<sub>3</sub>
| [[sulfuric acid]], H<sub>2</sub>SO<sub>4</sub>
|-
| [[selenium trioxide]], SeO<sub>3</sub>
| [[selenic acid]], H<sub>2</sub>SeO<sub>4</sub>
|-
| [[selenium dioxide]], SeO<sub>2</sub>
| [[selenous acid]], H<sub>2</sub>SeO<sub>3</sub>
|-
| [[tellurium trioxide]], TeO<sub>3</sub>
| [[telluric acid]], Te(OH)<sub>6</sub>
|-
| [[tellurium dioxide]], TeO<sub>2</sub>
| [[tellurous acid]], H<sub>2</sub>TeO<sub>3</sub>
|-
| [[dinitrogen pentoxide]], N<sub>2</sub>O<sub>5</sub>
| [[nitric acid]], HNO<sub>3</sub>
|-
| [[dinitrogen trioxide]], N<sub>2</sub>O<sub>3</sub>
| [[nitrous acid]], HNO<sub>2</sub>
|-
| [[phosphorus pentoxide]], "P<sub>2</sub>O<sub>5</sub>"<br/>i.e. P<sub>4</sub>O<sub>10</sub>
| [[phosphoric acid]], H<sub>3</sub>PO<sub>4</sub>
|-
| [[phosphorus trioxide]], "P<sub>2</sub>O<sub>3</sub>"<br/>i.e. P<sub>4</sub>O<sub>6</sub>
| [[phosphorous acid]], H<sub>3</sub>PO<sub>3</sub>
|-
| [[arsenic pentoxide]], As<sub>2</sub>O<sub>5</sub>
| [[arsenic acid]], H<sub>3</sub>AsO<sub>4</sub>
|-
| [[arsenic trioxide]], As<sub>2</sub>O<sub>3</sub>
| [[arsenous acid]], H<sub>3</sub>AsO<sub>3</sub>
|-
| [[carbon dioxide]], CO<sub>2</sub>
| [[carbonic acid]], H<sub>2</sub>CO<sub>3</sub>
|-
| [[tin dioxide]], SnO<sub>2</sub>
| [[stannic acid]], H<sub>2</sub>SnO<sub>3</sub>
|-
| [[boron oxide]], B<sub>2</sub>O<sub>3</sub>
| [[boric acid]], H<sub>3</sub>BO<sub>3</sub>
|-
| [[manganese(VII) oxide]], Mn<sub>2</sub>O<sub>7</sub>
| [[permanganic acid]], HMnO<sub>4</sub>
|-
| [[technetium(VII) oxide]], Tc<sub>2</sub>O<sub>7</sub>
| [[pertechnetic acid]], HTcO<sub>4</sub>
|-
| [[rhenium(VII) oxide]], Re<sub>2</sub>O<sub>7</sub>
| [[perrhenic acid]], HReO<sub>4</sub>
|-
|}
== 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 [[geochemistry|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 + SiO<sub>2</sub> → Na<sub>2</sub>SiO<sub>3</sub> + H<sub>2</sub>O
At least five different [[silicic acid]]s are also known, with 0.5–2.5 moles of water per mole of SiO<sub>2</sub> (expressed as SiO<sub>2</sub>·''n''H<sub>2</sub>O).
== See also ==
*[[Basic oxide]]
*[[Amphoteric oxide]]
*[[Neutral oxide]]
== References ==
*{{Greenwood&Earnshaw}}
[[Category:Acidic oxides| ]]
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