Acid-Base Equilibria Acid-Base Properties of Water Water is an
amphoteric substance. It is a weak electrolyte and is a poor
conductor of electricity. However, it undergoes dissociation to a
small extent: H2O(l) H+(aq) + OH-(aq) 2 H2O(l) H3O+(aq) + OH-(aq)
Which are the conjugate acid-base pairs? Autoionization of water
The equilibrium constant expression for the autoionization of water
is given by
Kc =
[H+][OH-] [H2O]
The concentration of water in aqueous solutions is considered
constant.
Kc [H2O] = [H+][OH-] Kw = [H+][OH-] Kw is the ion-product
constant of water at a specific temperature. At 25C, Kw has a value
of 1.0 x 10-14.
Acid-Base EquilibriaCalculate the concentration of H+ in (a) a
solution in which [OH-] is 0.010 M; (b) a solution in which [OH-]
is 1.8 x 10-9 M. (Unless otherwise stated, the temperature is 25C.)
(a) Kw = [H+][OH-] (b) Kw = [H+][OH-] Kw 1.0 x 10-14 Kw 1.0 x 10-14
H+ = = H+ = = -] [OH 0.010 [OH-] 1.8 x 10-9 H+ = 1.0 x 10-12 M Is
this solution acidic or basic? H+ = 5.6 x 10-6 M Is this solution
acidic or basic?
What is the pH and pOH of each of the solutions given above?
Recall: pH = -log [H+], pOH = -log [OH-]
Acid-Base Equilibria Relative strengths of acids and bases A
strong acid completely transfers its protons while its conjugate
base has negligible tendency to be protonated. A weak acid only
partially dissociates and its conjugate base is a weak base. A
substance with negligible acidity (e.g. CH4) has a conjugate base
that is very strong.
Acid-Base Equilibria Strong acids and strong basesHCl(aq) H+(aq)
+ Cl-(aq) Kc = [H+][Cl-] [HCl] Since HCl is a strong acid and
dissociates almost completely in the solution, [HCl] is almost
zero. Therefore, HCl(aq) H+(aq) + Cl-(aq) Kc = [H+][Cl-] [HCl] [H+]
= [Cl-] = initial concentration of HCl before dissociation took
place] The above trend goes for strong bases.
(a) Calculate the pH of 0.10 M HNO3 (b) Calculate the pH of 1.0
x 10-10 M HCl (c) Calculate the pH of 0.0011 M Ca(OH)2
Acid-Base Equilibria Weak acids Consider the ionization of a
weak acid in water:
Ka is the acid dissociation constant at a specific temperature.
Just like in molecular equilibrium, an ICE table is used in solving
acid-base equilibrium problems.
Acid-Base Equilibria Weak acids
The following approximation can be used since Ka is very
small:
Acid-Base Equilibria Weak acids
When can we do the approximation? The approximation is valid if
the result is 5% of the initial concentration.
The pH of a 0.10 M solution of formic acid (HCOOH) is 2.39. What
is the Ka of the acid? (1.8 x 10-4)
Acid-Base Equilibria Percent ionization Another measure of acid
strength is percent ionization which has the formula:
The extent of dissociation of a weak acid depends on the initial
concentration of the acid.
Acid-Base Equilibria Weak bases and Base dissociation
constant
What is the pH of a 0.26 M methylamine solution (Kb = 4.4 x
10-4)? What is the value of Ka for the conjugate acid of
methylamine? Acid-base properties of Salts A salt is one of the
products of an acid-base reaction (aside from water). The cation of
a salt is from a base and its anion is from an acid. Salt
hydrolysis refers to the reaction of a cation or an anion of a
salt, or both, with water. Salts can be neutral, acidic, or basic
depending on the strength of the acid and base that reacted to form
the salt.
The conjugate base has a Kb value at a specific temperature.
What is the value of Kb for acetate anion?
Acid-Base Equilibria Acid-base properties of Salts If a strong
acid and a strong base react, the resulting salt is neutral. If a
strong acid and a weak base react, the resulting salt is acidic. If
a weak acid and a strong base react, the resulting salt is basic.
If a weak acid and a weak base react, the pH of the resulting
solution depends on the Kb of the conjugate base and Ka of the
conjugate acid.
Acid-Base Equilibria Acid-base properties of Salts Tell whether
the following salts are neutral, acidic or basic: (a) LiClO4 (b)
Na3PO4 (c) NH4CN Calculate the pH of a 0.24 M sodium formate
solution (HCOONa). Ka for HCOOH = 1.7 x 10-4 Calculate the pH of a
0.10 M NH4Cl solution. The Kb value for NH3 = 1.8 x 10-5.
Buffers A buffer solution is a solution consisting of (1) a weak
acid or base and (2) its salt; both must be present. A buffer
solution has the ability to resist changes in pH upon addition of
small amounts of either acid or base. An example of a buffer
solution is made up acetic acid (CH3COOH) and acetate anion
(CH3COO-). If an acid is added to the system, the acetate ion will
serve as a base to neutralize the added acid: If a base is added to
the system, the acetic acid will serve as the acid to neutralize
the added acid:
Which of the following are buffer systems? (a) KF/HF; (b)
KBr/HBr; (c) Na2CO3/NaHCO3