Protonic Acids and Hydroxide Bases
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A compound that increases the concentration of hydronium ions (H3O+) in aqueous solution is an acid. A hydronium ion is a hydrated hydrogen ion, but can be written in a couple of ways;
H+, H+(aq). Don't forget that even though it is written in both of those ways, a hydrogen ion is always associated with at least one water molecule
in an aqueous solution. Hydrogen chloride, HCl, and sulfuric
acid, H2SO4, are both acids; since they have
hydrogen that can be released as protons, they are called protonic
acids. The protonic acids help increase the concentration of the
hydronium ion of water, by the following reactions when they really dissolve:
EQUATION (1)
HCl(g) + H2O(l) ==> H3O+(aq) + Cl-(aq)
EQUATION (2)
H2SO4(l) + H2O(l) ==> H3O+(aq) + HSO4-(aq)
Don't forget that some compounds don't contain any hydrogen but increase the hydronium ion concentration in solution by reacting with water and giving protonic acids as the result. The protonic acids that are given off start to act like electrolytes giving hydrogen ions to
the solution; an example would be Sulfur trioxide. One molecule of
SO3 reacts with one molecule of water to give a mole of sulfuric acid, H2SO4:
EQUATION (3)
SO3(g) + H2O(l) ==> H2SO4(l)
If there is an excess of water present, the hydronium ion concentration
will be increased by the reaction of H2SO4 with
water, just as indicated in Equation (2) above. The net equation below,
Equation (4), can be written as the sum of Equation (3) plus
Equation (2):
EQUATION (4)
SO3(g) + 2H2O(l) ==> H3O+(aq) + HSO4-(aq)
When a covalent molecule separates into ions it is said to ionize. Hydrogen chloride dissolves in water and
ionizes in water; then ionizes into hydronium ions and chloride ions;
nitric acid ionizes into hydronium ions and nitrate ions. A few protonic
acids, such as hydrogen chloride, sulfuric acid, nitric acid, and
perchloric acid, ionize completely. The preceding acids are called
strong acids. Most acids however are weak acids. Only a small amount of the molecules ionize when dissolved in some water. Some examples of weak acids include hydrogen fluoride (HF), acetic acid (CH3CO2H), boric acid (H3BO3), and hydrogen cyanide (HCN). Although the hydronium ion concentration in an aqueous solution of a weak acid is greater that that of pure water, the hydronium ion concentration is a smaller amount than, if the weak acid were to be completely ionized.
A compound that increases the concentration of hydroxide ion (OH-) in a solution is a base. Sodium Hydroxide, NaOH, and calcium hydroxide, Ca(OH)2, are examples of bases;
since they and hydroxide ions, they are also called hydroxide bases. They also increase the hydroxide ion concentration by the
following reactions when they dissolve in water:
EQUATION (5)
H2O (l)
NaOH(s) ==> Na+(aq) + OH-
EQUATION (6)
H2O(l)
Ca(OH)2(aq) ==> Ca2+(aq) + 2OH-(aq)
Many compounds that contain a metal and oxygen react with water to form hydroxides. For example, one mole of sodium oxide, Na2O, reacts with one mole of water to give two moles of sodium hydroxide:
EQUATION (7)
Na2O(s) + H2O(l) ==> 2NaOH(s)
If more that one mole of water is present, the excess basically acts as a solvent; and the sodium hydroxide dissolves with the formation of ions as described by Equation (8) below:
EQUATION (8)
Na2O(s) + H2O(l) ==> 2Na+(aq) + 2OH-(aq)
Ammonia, NH3, is a base because it reacts with water to a limited extent to form ammonium ions, NH4+, and hydroxide ions:
EQUATION (9)
NH3(g) + H2O(l) <==> NH4+(aq) + OH-(aq)
Many hydroxide bases are similar to salts, as these bases are ionic
compounds containing a cation and the hydroxide anion, OH-.
These bases ionize completely in water and give solution that have the
metal cations and hydroxide anions; the preceding are called strong bases. The hydroxides of the alkali metals (Group IA) and of calcium, strontium, barium, and radium (Group IIA, alkaline earth metals) are examples of strong bases. Weak bases are bases, such as beryllium hydroxide, Be(OH)2, which ionize only slightly in water, or bases such as ammonia, which also react with water to some extent. Don't forget that solutions of weak bases contain only a small amount of hydroxide ion and a large amount of undissociated or unreacted molecules of the base. Other weak bases include aluminum
hydroxide, Al(OH)3, and pyriding, C5H5N.
