Bases have characteristics opposite those of acids, and bases can be neutralized by acids. Therefore it is logical, in the Brönsted-Lowry scheme, to define a base as a protonThe positively charged particle in an atomic nucleus; its mass is similar to the mass of a hydrogen atom. acceptor, that is, a species which can incorporate an extra proton into its molecular or ionic structure. For example, when barium oxide, BaO, dissolves in water, oxide ions accept protons from water molecules according to the equation
BaO + H2O → Ba2+ + 2OH–
i.e., O–2 + H2O → OH– + OH–
The added proton transforms the oxide ion, into a hydroxide 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, barium oxide (or, more specifically, oxide ion) 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 concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of hydroxide ions greater than the 1.00 × 10–7 mol dm–3 characteristic of pure water is said to be basic. [[Image:|thumb|]]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:
HOH + OH– → HO– + HOH (1)
Nevertheless, the hydroxide ion fits the Brönsted-Lowry definition of a base as a proton acceptor.
EXAMPLE 1 Write a balanced equationA representation of a chemical reaction that has values of the stoichiometric coefficients of reactants and products such that the number of atoms of each element is the same before and after the reaction. to describe the proton transfer which occurs when the base sodium hydride, NaH, is added to water.
Solution NaH consists of Na+ and H– ions. Since positive ions repel protons, the H– ion is the only likely base. Again it may be useful to use two hypothetical steps: (l) donation of a proton by an H2O molecule, and (2) acceptance of a proton by the base. As in Example 11.6, we can then sum the steps
H+ + OH–
H+ → H2
H– + H2O → H2 + OH– overall
Note that adding a proton to H– balances the excess electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. of that ion, producing a neutral H2 molecule. Note also that charges balance in all equations.