From UCDavic Chemwiki
Because hydronium ions can be formed by transferring protons to water molecules, it is convenient when dealing with aqueous solutions to define an acid as a proton donor. This definition was first proposed in 1923 by the Danish chemist Johannes Brönsted (1879 to 1947) and the English chemist Thomas Martin Lowry (1874 to 1936). It is called the Brönsted-Lowry definition of an acid, and we will use it for the majority of this text. The Brönsted-Lowry definition has certain advantages over Arrhenius’ idea of an acid as a producer of H3O+(aq). This is especially true when acid strengths are compared, a subject we shall come to a bit later. Consequently, when we speak of an acid, we will mean a proton donor, unless some qualification, such as Arrhenius acid, is used.
Again, a base can be considered to be the opposite of an acid. Therefore it is logical, in the Brönsted-Lowry scheme, to define a base as a proton acceptor, that is, a species which can incorporate an extra proton into its molecular or ionic structure. For example, when ammonia (NH3) dissolves in water, the ammonia accept protons from water molecules according to the equation
`NH_3 + H_2O -> NH_4^(+) + OH^(-)`
The added proton transforms the ammonia, into a ammonium ion. Removal of a proton from a water molecule leaves behind a hydroxide ion also, accounting for the OH– on the right side of the equation. Since it can accept protons, ammonia serves as a base. When a base is added to water, its molecules or ions accept protons from water molecules, producing hydroxide ions. Thus the general properties of solutions of bases are due to the presence of hydroxide ions [OH–(aq)]. Any aqueous solution which contains a concentration of hydroxide ions greater than the 1.00 × 10–7 mol dm–3 characteristic of pure water is said to be basic. Unlike the hydronium ion, which forms very few solid compounds, hydroxide ions are often present in solid crystal lattices. Therefore it is possible to raise the hydroxide-ion concentration above 1.00 × 10–7 mol dm–3 by dissolving compounds such as NaOH, KOH, or Ba(OH)2. Hydroxide ions can accept protons from water molecules, but of course such a proton transfer has no net effect because the hydroxide ion itself becomes a water molecule through the following reaction:
`HOH + OH^(-) -> HO^(-) + HOH`
Nevertheless, the hydroxide ion fits the Brönsted-Lowry definition of a base as a proton acceptor.
Example 2
Write a balanced equation to describe the proton transfer which occurs when the base sodium hydride, NaH, is added to water.
Subpages (1): Example 2