Equilibrium ❮ 241● Strong acids completely dissociate in water, whereas weak acids only partially
dissociate.
● Weak acids and bases establish an equilibrium system.
● Under the Brønsted–Lowry acid–base theory, acids are proton (H+) donors and bases are
proton acceptors.
● Conjugate acid–base pairs differ only in a single H+; the one that has the extra H+ is the acid.
● The equilibrium for a weak acid is described by Ka, the acid dissociation constant.
It has the form: =+−
K[H ][A]
a [HA]. Know how to apply this equation.
● Most times the equilibrium concentration of the weak acid, [HA], can be approximated
by the initial molarity of the weak acid.
● Knowing Ka and the initial concentration of the weak acid allows the calculation of
the [H+].
● Water is an amphoteric substance, acting either as an acid or a base.
● The product of the [H+] and [OH-] in a solution or in pure water is a constant, Kw,
called the water dissociation constant, 1.0 × 10 -^14. Kw = [H+] [OH-] = 1.0 × 10 -^14 at
25 °C. Know how to apply this equation.
● The pH is a measure of the acidity of a solution. pH = - log[H+]. Know how to apply
this equation and estimate the pH from the [H+].
● On the pH scale 7 is neutral; pH > 7 is basic; and pH < 7 is acidic.
● pH + pOH = pKw = 14.00. Know how to apply this equation.
● (^) Kb is the ionization constant for a weak base. (^) =
+−
K
[HB][OH]
b [HB]. Know how to
apply this equation.
● Ka × Kb = Kw for conjugate acid–base pairs. Know how to apply this equation.
● Buffers are solutions that resist a change in pH by neutralizing either an added acid or
an added base.
● The Henderson–Hasselbalch equation allows the calculation of the pH of a buffer
solution:
=+
−
pHpK log[A ]
a [HA]. Know how to apply this equation.
● The buffer capacity is a quantitative measure of the ability of a buffer to resist a change
in pH. The more concentrated the acid–base components of the buffer, the higher its
buffer capacity.
● A titration is a laboratory technique to determine the concentration of an acid or base
solution.
● An acid–base indicator is used in a titration and changes color in the presence of an
acid or base.