Chemistry, The Central Science, 11th edition
Theodore L. Brown, H. Eugene LeMay, Jr.,
and Bruce E. Bursten
Chapter 16
Acids and Bases
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Some Definitions
• Arrhenius
– An acid is a substance that, when
dissolved in water, increases the
concentration of hydrogen ions.
– A base is a substance that, when dissolved
in water, increases the concentration
of hydroxide ions.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Some Definitions
• Brønsted-Lowry
– An acid is a proton donor.
– A base is a proton acceptor.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
A Brønsted-Lowry acid…
…must have a removable (acidic) proton.
A Brønsted-Lowry base…
…must have a pair of nonbonding electrons.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
If it can be either…
…it is amphiprotic.
HCO3
HSO4
H2O
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
What Happens When an Acid
Dissolves in Water?
• Water acts as a
Brønsted-Lowry base
and abstracts a proton
(H+) from the acid.
• As a result, the
conjugate base of the
acid and a hydronium
ion are formed.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Conjugate Acids and Bases
• The term conjugate comes from the Latin
word “conjugare,” meaning “to join together.”
• Reactions between acids and bases always
yield their conjugate bases and acids.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.1 (p. 671)
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?
CNSO42H2O
HCO3-
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.1
Write the formula for the conjugate acid
of each of the following:
HSO3FPO43CO
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.2 (p. 671)
The hydrogen sulfite ion (HSO3-) is amphoteric.
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.
In both cases identify the conjugate acid-base pairs.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.2
When lithium oxide (Li2O) is dissolved in
water, the solution turns basic from the
reaction of the oxide ion (O2-) with
water.
Write the reaction that occurs, and
identify the conjugate acid-base pairs.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Acid and Base Strength
• Strong acids are
completely dissociated in
water.
– Their conjugate bases are
quite weak.
• Weak acids only
dissociate partially in
water.
– Their conjugate bases are
weak bases.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Acid and Base Strength
• Substances with
negligible acidity do not
dissociate in water.
– Their conjugate bases are
exceedingly strong.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.3 (p. 673)
For the following proton-transfer
reaction, use the above figure to predict
whether the equilibrium lies
predominantly to the left or to the right
HSO4-(aq) + CO32-(aq)D SO42-(aq) + HCO3-(aq)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.3
For each of the following reactions,
use the above figure to predict
whether the equilibrium lies
predominantly to the left or to the right:
a) HPO42-(aq)+ H2O(l) D H2PO4-(aq)+ OH(aq)
b) NH4+(aq) + OH-(aq) D NH3(aq) + H2O(l)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Acid and Base Strength
• In any acid-base reaction, the equilibrium will
favor the reaction that moves the proton to
the stronger base.
HCl (aq) + H2O (l)  H3O+ (aq) + Cl- (aq)
• H2O is a much stronger base than Cl-, so
the equilibrium lies so far to the right that K
is not measured (K>>1).
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Acid and Base Strength
• In any acid-base reaction, the equilibrium will
favor the reaction that moves the proton to
the stronger base.
CH3CO2H (aq) + H2O (l)
H3O+ (aq) + CH3CO2- (aq)
• Acetate is a stronger base than H2O, so the
equilibrium favors the left side (K<1).
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Autoionization of Water
• As we have seen, water is amphoteric.
• In pure water, a few molecules act as
bases and a few act as acids.
H2O (l) + H2O (l)
H3O+ (aq) + OH- (aq)
• This is referred to as autoionization.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Ion-Product Constant
• The equilibrium expression for this
process is
Kc = [H3O+] [OH-]
• This special equilibrium constant is
referred to as the ion-product constant
for water, Kw.
• At 25C, Kw = 1.0  10-14
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.4 (p. 674)
Calculate the values of [H+] and [OH-] in
a neutral solution at 25oC.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.4
Indicate whether solutions with each of
the following ion concentrations is
neutral, acidic, or basic:
a)
b)
c)
[H+] = 4 x 10-9 M
[OH-] = 1 x 10-7 M
[OH-] = 7 x 10-13 M
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.5 (p. 675)
Calculate the concentration of H+(aq) in
a) a solution in which [OH-] is 0.010 M
(1.0 x 10-12 M)
b) a solution in which [OH-] is 1.8 x 10-9 M (5.0
x 10-6 M)
Assume T = 25oC.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.5
Calculate the concentration of OH-(aq) in
a solution in which
a) [H+] = 2 x 10-6 M
b) [H+] = [OH-]
c) [H+] = 100 x [OH-]
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
pH
pH is defined as the negative base-10
logarithm of the concentration of
hydronium ion.
pH = -log [H3O+]
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
pH
• In pure water,
Kw = [H3O+] [OH-] = 1.0  10-14
• Since in pure water [H3O+] = [OH-],
[H3O+] = 1.0  10-14 = 1.0  10-7
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
pH
• Therefore, in pure water,
pH = -log (1.0  10-7) = 7.00
• An acid has a higher [H3O+] than pure water,
so its pH is <7.
• A base has a lower [H3O+] than pure water,
so its pH is >7.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
pH
These are
the pH
values for
several
common
substances.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.6 (p. 677)
Calculate the pH values for the two
solutions described in Sample Exercise
16.5.
(a) 12.00
b) 5.25)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.6
a) In a sample of lemon juice [H+] is 3.8 x
10-4 M. What is the pH?
(3.42)
b) A commonly available windowcleaning solution has a [OH-] of 1.9 x
10-6 M. What is the pH?
(8.28)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.7 (p. 677)
A sample of freshly pressed apple juice
has a pH of 3.76. Calculate [H+].
(1.7 x 10-4 M)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.7
A solution formed by dissolving an
antacid tablet has a pH of 9.18.
Calculate [H+].
(6.6 x 10-10 M)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Other “p” Scales
• The “p” in pH tells us to take the
negative base-10 logarithm of the
quantity (in this case, hydronium ions).
• Some similar examples are
– pOH: -log [OH-]
– pKw: -log Kw
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Watch This!
Because
[H3O+] [OH-] = Kw = 1.0  10-14,
we know that
-log [H3O+] + -log [OH-] = -log Kw = 14.00
or, in other words,
pH + pOH = pKw = 14.00
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
How Do We Measure pH?
• For less accurate
measurements, one
can use
– Litmus paper
• “Red” paper turns
blue above ~pH = 8
• “Blue” paper turns
red below ~pH = 5
– Or an indicator.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
How Do We Measure pH?
For more accurate
measurements, one
uses a pH meter,
which measures the
voltage in the
solution.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Strong Acids
• You will recall that the seven strong acids are
HCl, HBr, HI, HNO3, H2SO4, HClO3, and
HClO4.
• These are, by definition, strong electrolytes
and exist totally as ions in aqueous solution.
• For the monoprotic strong acids,
[H3O+] = [acid].
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.8 (p. 680)
What is the pH of a 0.040 M solution of
HClO4?
(1.40)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.8
An aqueous solution of HNO3 has a pH
of 2.34. What is the concentration of
the acid?
(0.0046 M)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Strong Bases
• Strong bases are the soluble hydroxides,
which are the alkali metal and heavier
alkaline earth metal hydroxides (Ca2+, Sr2+,
and Ba2+).
• Again, these substances dissociate
completely in aqueous solution.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.9 (p. 680)
What is the pH of
a) a 0.028 M solution of NaOH?
(12.45)
b) a 0.0011 M solution of Ca(OH)2?
(11.34)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.9
What is the concentration of a solution of
a) KOH for which the pH is 11.89?
(7.8 x 10-3 M)
b) Ca(OH)2 for which the pH is 11.68?
(2.4 x 10-3 M)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Overview of Chapter 16
New material:
On your own:
• Arrhenius acids and bases
• Brønsted-Lowry acids and
bases
• Conjugate acid-base pairs
• Relative strengths of acids
and bases
• Autoionization of water
and Kw
• [H+] > pH
• [OH-] > pOH
• pH + pOH = 14.00
• Determine pH and/or pOH
of strong acids and bases
All of above will be
on Thursday’s test
•
•
•
•
•
•
•
•
•
•
•
Application of equilibrium to weak acids
and bases: Ka, Kb
Calculate Ka from pH and vice versa
Calculate percent ionization of weak acids
Polyprotic acids and their successive Ka’s
Types of weak bases
Calculate concentrations of all species of
weak acids and weak bases at equilibrium,
using Ka, Kb, pH or pOH
Relationship between Ka and Kb, derived from
Kw
Application of Ka/Kb: acid-base properties of
salt solutions
Factors that affect acid strength – binary acids
and oxyacids
Carboxylic acids
Acids
Lewis acids and bases
and
Bases
© 2009, Prentice-Hall, Inc.
Application of Equilibrium
to Weak Acids
and Weak Bases
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Dissociation Constants
• For a generalized acid dissociation,
HA (aq) + H2O (l)
A- (aq) + H3O+ (aq)
the equilibrium expression would be
[H3O+] [A-]
Kc =
[HA]
• This equilibrium constant is called the
acid-dissociation constant, Ka.
•
Note that H2O is omitted from Ka because it is a pure liquid
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Dissociation Constants
The larger the Ka, the stronger the acid
If Ka >> 1, then the acid is completely ionized
 strong acid
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Dissociation Constants
The greater the value of Ka, the stronger is the acid.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Ionization Constants and Equations
WS
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Calculating Ka from the pH
• pH  [H+] at equilibrium
• Equilibrium [H+]  RICE table
• Use RICE table to find the [other
species]
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Calculating Ka from the pH
Sample Exercise 16.10
The pH of a 0.10 M solution of formic acid,
HCOOH, at 25C is 2.38. Calculate Ka for
formic acid at this temperature.
We know that
[H3O+] [HCOO-]
Ka =
[HCOOH]
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Calculating Ka from the pH
The pH of a 0.10 M solution of formic acid,
HCOOH, at 25C is 2.38. Calculate Ka for
formic acid at this temperature.
To calculate Ka, we need the equilibrium
concentrations of all three things.
We can find [H3O+], which is the same as
[HCOO-], from the pH.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Calculating Ka from the pH
pH = -log [H3O+]
2.38 = -log [H3O+]
-2.38 = log [H3O+]
10-2.38 = 10log [H3O+] = [H3O+]
4.2  10-3 = [H3O+] = [HCOO-]
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Calculating Ka from pH
Now we can set up a table…
[HCOOH], M
[H3O+], M
[HCOO-], M
Initially
0.10
0
0
Change
- 4.2  10-3
+ 4.2  10-3
+ 4.2  10-3
0.10 - 4.2  10-3
= 0.0958 = 0.10
4.2  10-3
4.2  10-3
At Equilibrium
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Calculating Ka from pH
Note that the [HCOOH] remains at 0.10 M at equilibrium
– sig figs. This will become very important later in this chapter.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Calculating Ka from pH
[4.2  10-3] [4.2  10-3]
Ka =
[0.10]
= 1.8  10-4
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.10
Niacin, one of the B vitamins, has the
following molecular structure:
A 0.020 M solution of niacin has a pH of 3.26.
What is the acid-dissociation constant, Ka, for
niacin?
(1.5 x 10-5)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Calculating Percent Ionization
Sample Exercise 16.11
A 0.10 M solution of formic acid
(HCOOH) contains 4.2 x 10-3 M H+(aq).
Calculate the percentage of the acid
that is ionized.
(4.2%)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Calculating Percent Ionization
Sample Exercise 16.11
[H3O+]eq
• Percent Ionization = [HA]
 100
initial
• In this example
[H3O+]eq = 4.2  10-3 M
[HCOOH]initial = 0.10 M
4.2  10-3
Percent Ionization =
 100
0.10
= 4.2%
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.11
A 0.020 M solution of niacin has a pH of
3.26. Calculate the percent ionization of
the niacin.
(2.8%)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
• Using Ka, we can calculate the concentration
of H+ (and hence the pH).
Example: Calculate the pH of a 0.30 M
solution of acetic acid, HC2H3O2, at 25C.
Ka for acetic acid at 25C is 1.8  10-5.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
1. Write the balanced chemical equation
clearly showing the equilibrium.
CH3COOH(aq) D H+(aq) + CH3COO-(aq)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
2. Write the equilibrium expression.
Look up the value for Ka (in a table).
Ka = [H+][ CH3COO-] = 1.8 x 10-5
[CH3COOH]
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
3. Write down the initial and equilibrium
concentrations for everything except pure
water.
We usually assume that the equilibrium
concentration of H+ is x.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
3. Write down the initial and equilibrium
concentrations for everything except pure
water.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
4. Substitute into the equilibrium constant
expression and solve.
Ka = 1.8 x 10-5 = [H+][C2H3O2-] = (x)(x)
[HC2H3O2]
0.30 – x
Note that Ka is very small (1.8 x 10-5) relative to [HC2H3O2] (0.30 M).
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
4.
Substitute into the equilibrium constant expression and solve.
Keep this x
x2 = 1.8 x 10-5
0.30 – x
Neglect x in the denominator since it is extremely small relative to 0.30.
(Use ballpark figure of 103 X difference for neglecting x in the denominator )
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
4. Substitute into the equilibrium constant
expression and solve.
 x2 = (1.8 x 10-5 )(0.30) = 5.4 x 10-6
  x2 =  (5.4 x 10-6)
x = 2.3 x 10-3 M = [H+]
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
4. Substitute into the equilibrium constant
expression and solve.
Check: Compare x with original [HC2H3O2] of 0.30 M:
2.3 x 10-3 M x 100% = 0.77%, which is < 5%
0.30 M
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Using Ka to Calculate pH
5. Convert x ([H+]) to pH.
pH = - log (2.3 x 10-3) = 2.64
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
What do we do if we are faced with having to solve a
quadratic equation in order to determine the value of x?
Often this cannot be avoided.
•
However, if the Ka value is quite small, we find that we can make a simplifying
assumption.
•
Assume that x is negligible compared to the initial concentration
of the acid.
•
This will simplify the calculation.
•
•
It is always necessary to check the validity of any assumption.
Once we have the value of x, check to see how large it is compared to the
initial concentration.
•
If x is <5% of the initial concentration, the assumption is probably a good
one.
If x>5% of the initial concentration, then it may be best to solve the
quadratic equation or use successive approximations.
•
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.12 (p. 685)
Calculate the pH of a 0.20 M solution of
HCN. Refer to Table 16.2 for Ka.
(5.00)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.12
The Ka for niacin is 1.6 x 10-5. What is
the pH of a 0.010 M solution of niacin?
(3.41)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.13 (p. 687)
Calculate the percentage of HF
molecules ionized in
a)
a 0.10 M HF solution (7.9%)
b)
a 0.010 M HF solution (23%)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.13
In Practice Exercise 16.11, we found that the
percent ionization of niacin (Ka = 1.5 x 10-5) in
a 0.020 M solution is 2.7%. Calculate the
percentage of niacin molecules ionized in a
solution that is
a)0.010 M
(3.9%)
b)a 1.0 x 10-3 M
(12%)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Polyprotic Acids…
…have more than one acidic proton
If the difference between the Ka for the first
dissociation and subsequent Ka values is
103 or more, the pH generally depends only
on the first dissociation.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.14 (p. 689)
HW
The solubility of CO2 in pure water at 25oC
and 0.1 atm pressure is 0.0037 M. the
common practice is to assume that all of the
dissolved CO2 is in the form of carbonic acid
(H2CO3), which is produced by reaction
between the CO2 and H2O:
CO2(aq) + H2O(l) D H2CO3(aq)
What is the pH of a 0.0037 M solution of
H2CO3?
(4.40)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.14 (p. 690)
optional – requires quadratic
a) Calculate the pH of a 0.020 M solution
of oxalic acid (H2C2O4).
(see Table 16.3 for Ka1 and Ka2).
b) Calculate the concentration of oxalate
ion [C2O42-].
(pH = 1.80; [C2O42-] = 6.4 x 10-5 M) Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Weak Bases
Weak bases remove protons from substances.
They react with water to produce hydroxide ion.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Weak Bases
The equilibrium constant expression for
this reaction is
[HB] [OH-]
Kb =
[B-]
where Kb is the base-dissociation constant.
The larger Kb, the stronger the base.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Weak Bases
The equilibrium constant expression for
this reaction is
[NH4+] [OH-]
Kb =
[NH3]
where Kb is the base-dissociation constant.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Weak Bases
Kb can be used to find [OH-] and, through it, pH.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
pH of Basic Solutions
Sample Exercise 16.15 (p. 691)
What is the pH of a 0.15 M solution of NH3?
NH3 (aq) + H2O (l)
NH4+ (aq) + OH- (aq)
[NH4+] [OH-]
Kb =
= 1.8  10-5
[NH3]
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
pH of Basic Solutions
Tabulate the data.
Initially
At Equilibrium
[NH3], M
[NH4+], M
[OH-], M
0.15
0
0
0.15 - x  0.15
x
x
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
pH of Basic Solutions
2
(x)
1.8  10-5 =
(0.15)
(1.8  10-5) (0.15) = x2
2.7  10-6 = x2
1.6  10-3 = x2
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
pH of Basic Solutions
Check the assumption:
1.6  10-3 M x 100% = 1.1%, < 5%
0.15 M
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
pH of Basic Solutions
Therefore,
[OH-] = 1.6  10-3 M
pOH = -log (1.6  10-3)
pOH = 2.80
pH = 14.00 - 2.80
pH = 11.20
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Problem 16.15
Which of the following compounds
should produce the highest pH as a
0.05 M solution: pyridine, methylamine,
or nitrous acid?
(methylamine)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Types of Weak Bases
Weak bases generally fall into one of two
categories.
1. Neutral substances with a lone pair of electrons
that can accept protons.
Most neutral weak bases contain nitrogen.
Amines are related to ammonia and have one or more
N–H bonds replaced with N–C bonds
(e.g., CH3NH2 is methylamine).
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Types of Weak Bases
Like NH3, amines can abstract a proton from a water
molecule by forming an additional N-H bond,
as shown in this figure for methylamine.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Types of Weak Bases
2.
Anions of weak acids are also weak bases.
e.g.: ClO– is the conjugate base of HClO (weak acid):
ClO–(aq) + H2O(l) D HClO(aq) + OH–(aq)
Kb = 3.3 x 10–7
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.16 (p. 692)
A solution made by adding solid sodium
hypochlorite (NaClO) to enough water to
make 2.00 L of solution has a pH of 10.50.
How many moles of NaClO were added to the
water?
(See info immediately above.)
(0.60 mol)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.16
A solution of NH3 in water has a pH of
11.17. What is the molarity of the
solution?
(0.12 M)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Ka and Kb
Ka and Kb are related in this way:
Ka  Kb = Kw
Therefore, if you know one of them, you can calculate the other.
Alternatively, pKa + pKb = pKw = 14.00 (at 25oC)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.17 (p. 695)
Calculate
a) the base-dissociation constant, Kb, for
the fluoride ion (F-);
(1.5 x 10-11)
b) the acid-dissociation constant, Ka, for
the ammonium ion (NH4+).
(5.6 x 10-10)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.17
(Use Appendix D for Ka)
a) Which of the following anions has the largest basedissociation constant: NO2-, PO43-, or N3-?
(PO43-, Kb = 2.4 x 10-2)
b) The base quinoline has the following structure:
Its conjugate acid is listed in handbooks as having a
pKa of 4.90.
What is the base-dissociation constant for quinoline?
Acids
(7.9 x 10-10)
and
Bases
© 2009, Prentice-Hall, Inc.
Acid/Base Properties of Salt
Solutions
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Reactions of Anions with Water
• Anions are bases.
• As such, they can react with water in a
hydrolysis reaction to form OH- and the
conjugate acid:
X- (aq) + H2O (l)
HX (aq) + OH- (aq)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Reactions of Cations with Water
• Cations with acidic protons
(like NH4+) will lower the pH
of a solution.
• Most metal cations that are
hydrated in solution also
lower the pH of the solution.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Reactions of Cations with Water
• Attraction between nonbonding
electrons on oxygen and the
metal causes a shift of the
electron density in water.
• This makes the O-H bond more
polar and the water more acidic.
• Greater charge and smaller size
make a cation more acidic.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Effect of Cations and Anions
1. An anion that is the
conjugate base of a strong
acid will not affect the pH.
2. An anion that is the
conjugate base of a weak
acid will increase the pH.
3. A cation that is the
conjugate acid of a weak
base will decrease the pH.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Effect of Cations and Anions
4. Cations of the strong
Arrhenius bases will not
affect the pH.
5. Other metal ions will
cause a decrease in pH.
6. When a solution contains
both the conjugate base
of a weak acid and the
conjugate acid of a weak
base, the effect on pH
depends on the Ka and Kb
values.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.18 (p. 698)
Determine whether aqueous solutions of each of the following
salts will be acidic, basic, or neutral:
Hint: Analyze each compound – derived from strong or weak acids
and bases?
a)
Ba(CH3COO)2,
b)
NH4Cl
c)
CH3NH3Br
d)
KNO3
e)
Al(ClO4)3
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.18 (p. 698)
a)
b)
c)
d)
e)
Determine whether aqueous solutions of each of the following
salts will be acidic, basic, or neutral:
Ba(CH3COO)2,
anion of weak acid  basic
NH4Cl
cation of weak base  acidic
CH3NH3Br
cation of weak base  acidic
KNO3
neutral – derived from strong acid + strong base
Al(ClO4)3
cation of weak base (Al(OH)3) acidic,
Al3+ - increase the polarity of O-H in H2O
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.18
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
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.19 (p. 698)
Predict whether the salt Na2HPO4 will
form an acidic or a basic solution on
dissolving in water.
(basic)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice Exercise 16.19
Predict whether the dipotassium salt of
citric acid (K2HC6H5O7) will form an
acidic or basic solution in water.
(see Table 16.3 for data)
(acidic)
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Factors Affecting Acid Strength
(Sec. 16.10-16.11)
• The more polar the H-X bond and/or the weaker
the H-X bond, the more acidic the compound.
• So acidity increases from left to right across a row
and from top to bottom down a group.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Factors Affecting Acid Strength
In oxyacids, in which
an -OH is bonded to
another atom, Y, the
more electronegative
Y is, the more acidic
the acid.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Factors Affecting Acid Strength
For a series of oxyacids, acidity increases
with the number of oxygens.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.20 (p. 702)
Arrange the compounds in each of the
following series in order of increasing
acid strength:
a) AsH3, HI, NaH, H2O;
b) H2SO4, H2SeO3, H2SeO4.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Exercise 16.20 (p. 702)
Arrange the compounds in each of the following
series in order of increasing acid strength:
a) AsH3, HI, NaH, H2O
NaH < AsH3 < H2O < HI
b) H2SO4, H2SeO3, H2SeO4
H2SeO3 < H2SeO4 < H2SO4
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice 16.20
In each of the following pairs choose the
compound that leads to the more acidic
(or less basic) solution:
a) HBr, HF;
b) PH3, H2S;
c) HNO2, HNO3;
d) H2SO3, H2SeO3.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Practice 16.20
In each of the following pairs choose the
compound that leads to the more acidic
(or less basic) solution:
a) HBr, HF;
b) PH3, H2S;
c) HNO2, HNO3;
d) H2SO3, H2SeO3.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Factors Affecting Acid Strength
Resonance in the conjugate bases of
carboxylic acids stabilizes the base and
makes the conjugate acid more acidic.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Comparison of Different Types
of Acids and Bases
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Arrhenius (traditional) acids and bases
(C19th)
• Acid: compound containing H that ionizes to yield H+
in solution
• Base: compound containing OH that ionizes to yield
OH- in solution
(Note: does not describe acid/base behavior in
solvents other than water)
Note: Every Arrhenius acid/base is
also a Brønsted-Lowry acid/base.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Bronsted-Lowry Acids and Bases
(1923)
• Acid: H+ (proton) donor
• Base: H+ (proton) acceptor
NH3
ammonia
(B-L base)
+ H 2O
D
water
(B-L acid)
NH4+
ammonium ion
(B-L acid)
+
OHhydroxide ion
(B-L base)
Note: Every Brønsted-Lowry acid/base is
also a Lewis acid/base.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Lewis Acids and Bases (1920’s)
• Acid: accepts pair of e-‘s
• Base: donates pair of e-‘s
..
H+
Lewis acid
+
..
-  H:O:H
[:O:H]
..
..
Lewis base
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Lewis Acids
• Lewis acids are defined as electron-pair
acceptors.
• Atoms with an empty valence orbital can be Lewis
acids.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Lewis Bases
• Lewis bases are defined as electron-pair donors.
• Anything that could be a Brønsted-Lowry base is
a Lewis base.
• Lewis bases can interact with things other than
Acids
protons, however.
and
Bases
© 2009, Prentice-Hall, Inc.
Hydrolysis of Metal Ions
• The Lewis concept may be used to explain
the acid properties of many metal ions.
• Metal ions are positively charged and attract
water molecules (via the lone pairs on the
oxygen atom of water).
• Hydrated metal ions act as acids.
• For example:
Fe(H2O)63+(aq) D Fe(H2O)5(OH)2+(aq) + H+(aq)
Ka = 2 x 10–3
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Hydrolysis of Metal Ions
In general:
• The higher the charge, the stronger the M–OH2
interaction.
•
Ka values generally increase with increasing
charge
• The smaller the metal ion, the more acidic the ion.
•
Ka values generally decrease with decreasing
ionic radius
•
Thus the pH of an aqueous solution increases as
the size of the ion increases (e.g., Ca2+ vs. Zn2+) and
as the charge increases (e.g., Na+ vs. Ca2+ and Zn2+
vs. Al3+).
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Integrative Exercise 16
(p. 706)
Phosphorous acid (H3PO3) has the
following Lewis structure:
Acids
and
Bases
© 2009, Prentice-Hall, Inc.
Sample Integrative Exercise 16
(p. 706)
a) Explain why phosphorous acid is diprotic and not triprotic.
b) A 25.0 mL sample of a solution of H3PO3 is titrated with 0.102 M
NaOH. It requires 23.3 mL of NaOH to neutralize both acidic
protons. What is the molarity of the H3PO3 solution?
c) This solution has a pH of 1.59. Calculate the percent ionization and
Ka1 for H3PO3, assuming that Ka1 >>Ka2.
d) How does the osmotic pressure of a 0.050 M solution of HCl
compare qualitatively with that of a 0.050 M solution of H3PO3?
Explain.
Acids
and
Bases
© 2009, Prentice-Hall, Inc.