Page 63 - A Working Method Approach For Introductory Physical Chemistry Calculations

Chapter 5

Equilibrium 11: Aqueous Solution

Equilibria

ACIDS AND BASES
A Brsnsted-Lowry acid is a proton donor (H+). Ionic dissociation is
the breaking up of a reactant to form a cation and an anion. There
are two types of acid. A strong acid is an acid which dissociates
compfetely (100%) in solution, i.e. HA + H20, + H30+ + A-,
where H30 -t represents the hydronium cation (simply water with
an added proton). An example of a strong acid is HN03, since
HN03 + H20 -+ H30+ + NO,. Table 5.1 provides other examples
of strong acids. Since strong acids are dissociated completely in
solution, the reverse reaction does not occur and hence an equili-
brium is not established. Therefore, a forward arrow is used to
illustrate the reaction. A weak acid is an acid which does not
dissociate completely in solution, and hence has an equilibrium
condition, i.e. HA + H20 + H30+ + A-, where Ka =
{[H30'][A-]}/[HA], since the activity, Q, of a pure liquid (water) is
unity, as described in Chapter 4. Ka represents the equilibrium
constant for the dissociation of an acid. Examples of weak acids are
organic acids containing the carboxylic acid functional group
RC02H (R = alkyl group), e.g. CH3CO2H + H20 .P CH~COT +
H3O+, where Ka = ([CH3C02-] [H30+]}/[CH3C02H].
A base is defined as a proton acceptor or a producer of OH- ions.
There are two types of base. Strong bases, such as NaOH, KOH
etc., dissociate completely in solution, according to the reaction
MOH + M+ + OH-. Weak buses, such as ammonia and the amines,
e.g. NH3, CH3NH2, do not dissociate 100% completely and so they
exist in, equilibrium, e.g. CH3NH2 + H20 * CH3NH3+ + OH-,