Acid and Base Anhydrides

Acid Anhydrides

In our discussion of double displacement reactions, we said that these reactions occur when a gas is produced.  There were four gases that we look for: $H_{2}S,H_{2}C{O}_3,H_{2}S{O}_3,andN{H}_{4}OH$. However, as discussed here, three of those compounds are actually not gases. They are compounds that immediately decompose to release gases.

For example, $H_{2}C{O}_3$ breaks down to produce carbon dioxide and water according to the reaction here:

$H_{2}C{O}_3\to C{O}_2+H_{2}O$

But, now that we've learned a bit about reversible reactions, we know that this should actually be written as the following:

$H_{2}C{O}_3\rightleftharpoons C{O}_2+H_{2}O$

Of course that means that we could also write the reaction this way:

$C{O}_2+H_{2}O\rightleftharpoons H_{2}C{O}_3$

This is tremendously important because it shows that adding $C{O}_2$ to water will produce carbonic acid and will therefore lower the pH of the solution. This same process can be seen for sulfur dioxide and sulfur trioxide:

$S{O}_2+H_{2}O\rightleftharpoons H_{2}S{O}_3$

and

$S{O}_3+H_{2}O\rightleftharpoons H_{2}S{O}_4$

Nitrogen oxides can do something similar, although nitrogen chemsitry is more varied and more complex. Nitrogen forms a series of oxygen compounds with various ratios. As sch, chemists often write the collection of compounds as $N{O}_x$. When $N{O}_x$ reacts with water it can create both nitrous and nitric acid:

$N{O}_x+H_{2}O\rightleftharpoons HN{O}_{2}andHN{O}_3$

Of course we can't balance that reaction as written, but for our purposes here, what's important is the following:

• ALL of these cases ($C{O}_2,S{O}_2,S{O}_3,andN{O}_x$ are non-metal oxides
• ALL of these compounds form acidic solutions when mixed with water

These compounds (non-metal oxides) are, therefore, called acid anhydrides. A term which simply means that if the compound is put into water it will be an acid. 

This property has significant environmental importance, which is discussed here.

Base Anyhydrides

A related chemistry occurs when a metal oxide is placed in water. For instance, $CaO$ will react with water to produce calcium hydroxide according to the reaction here:

$CaO+H_{2}O\rightleftharpoons Ca(OH{)}_2$

The same reaction will occur with other metal oxides. For example, we can look at the reactions of sodium oxide and aluminum oxide with water.

$N{a}_{2}O+H_{2}O\rightleftharpoons 2NaOH$

and

$A{l}_2{O}_3+3H_{2}O\rightleftharpoons 2Al(OH{)}_3$

As a result of these reactions, metal oxides are called base anhydrides.

This property also has an environmental importance, which is discussed here.