Acid-Base Equilibria
Chapter 16
1. Arrhenius Acids and Bases
 Arrhenius
Acid –produce H+
 HCl(aq)  H+ + Cl Arrhenius Base- produce OH+
 NaOH(aq)  Na + OH
2. Bronsted-Lowry Acids and Bases
 B-L
acid-base rxns involve a
+
transfer of a proton (H )
 B-L Acid = donates H+
 B-L Base = accepts H+
2. Bronsted-Lowry Acids and Bases
 HCl
+ NH3  Cl- + NH4+
 HCl + H2O  H3O+ + Cl H2O + NH3  NH4+ +OH Amphiprotic – a substance
that can be an acid or base
2. Bronsted-Lowry Acids and Bases
conjugate acid-base pairs differ only by a
proton
Sample Exercise 16.1 Identifying Conjugate Acids and Bases
(a) What is the conjugate base of each of
the following acids: HClO4, H2S, PH4+,
HCO3–?
(b) What is the conjugate acid of each of
the following bases: CN-, SO42–, H2O,
HCO3– ?
Sample Exercise 16.2 Writing Equations for Proton-Transfer
Reactions
The hydrogen sulfite ion (HSO3–) is
amphiprotic.
(a) Write an equation for the reaction of
HSO3– with water, in which the ion
acts as an acid
(b) Write an equation for the reaction of
HSO3– with water, in which the ion
acts as a base.
The Autoionization of Water

acid
 The
base
ion product of water,
Kw=[H3O+][OH-] = 1x10-14
The Autoionization of Water



[H+] =[OH-] is neutral
[H+] > [OH-] is acidic
[H+] < [OH-] is basic
Sample Exercise 16.5 Calculating [H+] from [OH-]
 Calculate the concentration of H+(aq)
in
 (a) a solution in which [OH–] is 0.010
M,
 (b) a solution in which [OH–] is 1.8
×10–9 M .
3. Lewis Acids and Bases
A
lewis acid is an electron
pair acceptor
 A lewis base is an electron
pair donor
 Doesn’t pertain to just H+
3. Lewis Acids and Bases
The pH Scale --- key equations
 pH
= -log[H+]
 [H+]=10-pH
 The pH decreases as [H+]
increases
 pOH = -log[OH-]
 [OH-]=10-pOH
 pH + pOH = 14
The pH Scale
The pH Scale
 Measuring
pH
 1. pH meters
 2. Titration and Acid-Base
Indicators-colored substance
that can exist as an acid or
base, each having its own color
pH meter simulation
http://group.chem.iastate.edu/Greenbo
we/sections/projectfolder/flashfiles/acid
basepH/ph_meter.html
 Acid-Base Solutions
http://phet.colorado.edu/en/simulation/ac
id-base-solutions
pH Scale
http://phet.colorado.edu/en/simulation/p

Sample Exercise 16.7 Calculating [H+] from pH
A sample of freshly pressed apple juice
has a pH of 3.76. Calculate [H+], [OH-],
and pOH.
Strong Acids and Bases
7
strong acids ???
 Strong acids are strong
electrolytes and completely
ionize in aqueous soln
Strong Acids and Bases
 In
aqueous solutions of
+
strong acids, the [H ] equals
the original concentration of
acid, calculate the pH from
[H+]
Sample Exercise 16.8 Calculating the pH of a Strong Acid
What is the pH of a 0.040 M solution of
HClO4?
Strong Acids and Bases
 Strong
bases ???
 Strong bases are strong
electrolytes and completely
ionize in aqueous soln
Strong Acids and Bases
 In
aqueous solns of strong
bases, the [OH-] depends on
the coefficient of the
equation
 Calculate pH from pOH
Sample Exercise 16.9 Calculating the pH of a Strong Base
What is the pH of
(a) a 0.028 M solution of NaOH
(b) a 0.0011 M solution of Ca(OH)2?
Weak Acids
 weak
acids are partially
ionized which is represented
as an equilibrium reaction
 HA + H2O  H3O+ + A or HA H+ + A-
Weak Acids
 Ka
is the acid-dissociation
constant and the magnitude
of it indicates the tendency
of the acid to ionize.
 The larger the Ka, the
stronger the acid.
Weak Acids
 It
is possible to calculate Ka
from pH
Sample Exercise 16.10 Calculating Ka from Measured pH
A student prepared a 0.10 M solution of
formic acid (HCOOH) and measured its
pH. The pH at 25 ºC was found to be
2.38. Calculate Ka for formic acid at this
temperature.
Weak Acids
 Or
you can calculate pH from
Ka
 What is the pH of 0.30M
-5
HC2H3O2 ? (Ka = 1.8x10 )
Sample Exercise 16.11 Calculating Percent Ionization
A 0.10 M solution of formic acid (HCOOH)
contains 4.2 × 10–3 M H+(aq) .
Calculate the percentage of the acid
that is ionized.
Warm up:
Sample Exercise 16.12 Using Ka to Calculate pH
(1) Calculate the pH of a 0.20 M solution
of HCN. (Ka = 4.9x10-10.)
(2) Calculate the pH of 0.20M HNO3.
Sample Exercise 16.13 Using Ka to Calculate Percent Ionization

Calculate the percentage of HF
molecules ionized in (a) a 0.10 M HF
solution, (b) a 0.010 M HF solution
Weak Acids
 Polyprotic
Acids have more than
one ionizable H atom
 It is always easier to remove
the first proton from a
polyprotic acid than the second
 Ex. H2CO3
Weak Bases
 Weak
bases react with water
by taking away a proton
forming the conjugate acid
of the base and OH- ions
+
 B + H2O  BH + OH
Weak Bases
 Types
of Weak Bases
 1. Neutral substances that have an
atom with a nonbonding pair of
electrons that can serve as a
proton acceptor (substances w/N)
2. Anions of weak acids ; conjugate
base of weak acids
Weak Bases
Weak Bases
Sample Exercise 16.15 Using Kb to Calculate OH¯
Calculate the pH of a 0.15 M solution of NH3.
Kb = 1.8x 10-5
Relationship between Ka and Kb
 Ka
x Kb=Kw = 1.0 x 10-14
 pKa = -logKa
 pKb=-log Kb
 pKa + pKb = pKw = 14.00 at
25oC
Relationship between Ka and Kb
 As
the strength of an acid
increases (larger Ka), the
strength of its conjugate
base decreases (smaller Kb)
 Or vice versa
Sample Exercise 16.17 Calculating Ka or Kb for a Conjugate
Acid-Base Pair
Calculate (a) the base-dissociation
constant, Kb, for the fluoride ion (F–);
(b) the acid dissociation constant, Ka,
for the ammonium ion (NH4+).
Acid-Base Properties of Salt Solutions
 salt
solutions can be acidic,
neutral, or basic
 the pH of a salt solution can
be predicted by the ions that
make up the salt
Acid-Base Properties of Salt Solutions
 General
Rules
 1. An anion that is the
conjugate base of a SA will
not affect the pH of a
solution (Cl-)
Acid-Base Properties of Salt Solutions
 2.
An anion that is the
conjugate base of a WA will
increase the pH (CN )
Acid-Base Properties of Salt Solutions
 3.
A cation of a SB will not
+
affect pH (Na )
Acid-Base Properties of Salt Solutions
 4.
A cation that is the
conjugate acid of a WB will
cause a decrease in pH
(NH4+)
Acid-Base Properties of Salt Solutions
 5.
With the exception of ions
of group 1A and heavier
members of 2A, metal ions
will cause a decrease in pH
Acid-Base Properties of Salt Solutions
 6.
When a soln contains both
the conj. base of a WA and
the conj. acid of a WB, the
ion with the largest Keq will
have the greatest influence
on pH
Sample Exercise 16.18 Determining Whether Salt Solutions Are
Acidic, Basic, or Neutral
Determine whether aqueous solutions of
each of the following salts will be acidic,
basic, or neutral:
(a) Ba(CH3COO)2
(b) NH4Cl
(c) CH3NH3Br
(d) KNO3
(e) Al(ClO4)3.
Practice Exercise 16.18 Determining Whether Salt Solutions Are
Acidic, Basic, or Neutral
In each of the following, indicate which
salt in each of the following pairs will
form the more acidic (or less basic)
0.010 M solution:
(a) NaNO3 or Fe(NO3)3
(b) KBr or KBrO;
(c) CH3NH3Cl or BaCl2
(d) NH4NO2 or NH4NO3.
Titration simulation
 http://group.chem.iastate.edu/Greenbo
we/sections/projectfolder/flashfiles/stoic
hiometry/acid_base.html
 http://chemilp.net/labTechniques/AcidBaseIdicatorS
imulation.htm
