Advanced acids and bases

Acids, bases and oxides

As stated previously, an acid is a substance that releases hydrogen ions during a reaction and a base is a substance that accepts hydrogen ions in a reaction. This definition includes the behaviour of oxides, which act as acids or bases in different situations.

While a reaction between an acid and metal produces salt and hydrogen, an acid combined with a metal oxide produces salt and water. These are the same products for a reaction between an acid and a base. The equation is:

acid + metal oxide metallic salt + water

This implies that metal oxides are a type of base, if one considers the loose definition of a base as being a 'hydrogen ion acceptor'.

Here is an example of a reaction where sulphuric acid combines with copper oxide to create copper sulphate and water:

H₂SO₄ + CuO CuSO₄ + H₂O

As with acid + base equations, the extent to which the metal oxide reacts to form a salt depends on the strength of the acid.

A non-metal oxide, on the other hand, acts as an acid when dissolved in water because they are hydrogen ion donors. The non-metal oxide needs to be aqueous (dissolved in solution) so that the ions move freely and react with other substances. The equation looks like this:

non-metal oxide + water acid

Sulphurous acid forms after dissolving sulphur dioxide in water:

SO₂ + H₂O H₂SO₃ (aq)

Sometimes acidic oxides are referred to as acid anhydrides, meaning 'removal of water', because the oxide needs the water before it can form the acid. Some naturally occurring oxides such as carbon, nitrogen and sulphur combine with water vapour in the atmosphere to form an acidic substance. Acid rain falls when precipitation carries sulphurous acid to Earth, damaging vegetation, buildings and other structures. See image 1.

Following from the above equations, an aqueous non-metal oxide acting as an acid will form salt and water when combined with a base:

aqueous non-metal oxide + base salt + water

The acid anhydride of sulphur trioxide reacts with sodium hydroxide to produce sodium sulphate and water:

SO₃ + 2NaOH Na₂SO₄ + H₂O

It also follows that a non-metal oxide acting as an acid, reacting with a metal oxide acting as a base will also give salt and water:

non-metal oxide + metal oxide salt + water

The aqueous non-metal oxide sulphurous acid and magnesium oxide reacts to produce magnesium sulphite, salt and water:

H₂SO₃ + MgO MgSO₃ + H₂O

Knowing how acids, bases and oxides react with each other and other substances provides a better foundation for observing and predicting reactions in general.

Acid & carbonates

Carbonates are also hydrogen ion acceptors but because they also contain carbon, the products of a reaction containing an acid and a carbonate is salt, water and carbon dioxide, represented like this:

acid + carbonate salt + water + carbon dioxide

An example of this is when calcium carbonate reacts with hydrochloric acid to form calcium chloride, water and carbon dioxide:

2HCl + CaCO₃ CaCl₂ + CO₂ + H₂O

It is necessary to protect calcium carbonate, better known as limestone or marble, from weather such as acid rain that will react with, and thus erode, buildings made of this material.