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: \(H_2S, H_2CO_3, H_2SO_3, and ~ NH_4OH\). However, as discussed here, three of those compounds are actually not gases. They are compounds that immediately decompose to release gases.

For example, \(H_2CO_3\) breaks down to produce carbon dioxide and water according to the reaction here:

\(H_2CO_3 \rightarrow CO_2 + H_2O\)

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

\(H_2CO_3 \rightleftharpoons CO_2 + H_2O\)

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

\(CO_2 + H_2O \rightleftharpoons  H_2CO_3\)

This is tremendously important because it shows that adding \(CO_2\) 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:

\(SO_2 + H_2O \rightleftharpoons H_2SO_3\)

and

\(SO_3 + H_2O \rightleftharpoons H_2SO_4\)

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 \(NO_x\). When \(NO_x\) reacts with water it can create both nitrous and nitric acid:

\(NO_x + H_2O \rightleftharpoons HNO_2 ~ and ~ HNO_3\)

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 (\(CO_2, SO_2, SO_3, and ~ NO_x\) 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:

\(CaO + H_2O \rightleftharpoons Ca(OH)_2\)

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.

\(Na_2O + H_2O \rightleftharpoons 2 NaOH\)

and

\(Al_2O_3 + 3 H_2O \rightleftharpoons 2 Al(OH)_3\)

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

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