Acid-base reactions  (Page 23/5)

Introduction

There are many different types of chemical reactions that can take place. In this chapter, we will be looking at a few of the more common reaction types: acid-base and acid-carbonate reactions, redox reactions and addition, elimination and substitution reactions.

Acid-base reactions

What are acids and bases?

In our daily lives, we encounter many examples of acids and bases. In the home, vinegar (acetic acid), lemon juice (citric acid) and tartaric acid (the main acid found in wine) are common, while hydrochloric acid, sulfuric acid and nitric acid are examples of acids that are more likely to be found in laboratories and industry. Hydrochloric acid is also found in the gastric juices in the stomach. Even fizzy drinks contain acid (carbonic acid), as do tea and wine (tannic acid)! Bases that you may have heard of include sodium hydroxide (caustic soda), ammonium hydroxide and ammonia. Some of these are found in household cleaning products. Acids and bases are also important commercial products in the fertiliser, plastics and petroleum refining industries. Some common acids and bases, and their chemical formulae, are shown in [link] .

Most acids share certain characteristics, and most bases also share similar characteristics. It is important to be able to have a definition for acids and bases so that they can be correctly identified in reactions.

Defining acids and bases

A number of definitions for acids and bases have developed over the years. One of the earliest was the Arrhenius definition. Arrhenius (1887) noticed that water dissociates (splits up) into hydronium (H ${}_3$ O ${}^{+}$ ) and hydroxide (OH ${}^{-}$ ) ions according to the following equation:

${\mathrm{H}}_2\mathrm{O}\leftrightharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{OH}}^{-}$

Arrhenius described an acid as a compound that increases the concentration of H ${}_3$ O ${}^{+}$ ions in solution, and a base as a compound that increases the concentration of OH ${}^{-}$ ions in a solution. Look at the following examples showing the dissociation of hydrochloric acid and sodium hydroxide (a base) respectively:

1. $\mathrm{HCl}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{Cl}}^{-}$ Hydrochloric acid in water increases the concentration of H ${}_3$ O ${}^{+}$ ions and is therefore an acid .
2. $\mathrm{NaOH}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Na}}^{+}+{\mathrm{OH}}^{-}$ Sodium hydroxide in water increases the concentration of OH ${}^{-}$ ions and is therefore a base .

However, this definition could only be used for acids and bases in water . It was important to come up with a much broader definition for acids and bases.

It was Lowry and Bronsted (1923) who took the work of Arrhenius further to develop a broader definition for acids and bases. The Bronsted-Lowry model defines acids and bases in terms of their ability to donate or accept protons.

Acids and bases

According to the Bronsted-Lowry theory of acids and bases, an acid is a substance that gives away protons (H ${}^{+}$ ), and is therefore called a proton donor . A base is a substance that takes up protons, and is therefore called a proton acceptor .