Learning Objectives
Chapter 15
Acid Base Concepts
• Arrhenius Concept of Acids and Base
– a.
Define acid and base according to the
Arrhenius concept.
• Brønsted–Lowry Concept of Acids and
Bases
– a.
Define acid and base according to the
Brønsted–Lowry concept.
Define the term conjugate acid–base pair.
Identify acid and base species.
Define amphiprotic species.
– b.
– c.
– d.
Contents and Concepts
Acid–Base Concepts
1. Arrhenius Concept of Acids and Bases
2. Brønsted–Lowry Concept of Acids and Bases
3. Lewis Concept of Acids and Bases
3. Lewis Concept of Acids and Bases
•
•
Acid and Base Strengths
4. Relative Strengths of Acids and Bases
–
Acid and Base Strengths
4. Relative Strengths of Acids and Bases
5. Molecular Structure and Acid Strength
a. Define Lewis acid and Lewis base.
b. Identify Lewis acid and Lewis base
species.
–
a.
Understand the relationship between the
strength of an acid and that of its conjugate
base.
b. Decide whether reactants or products are
favored in an acid–base reaction.
5. Molecular Structure and Acid Strength
• a.
Self-Ionization of Water and pH
6.Self-Ionization of Water
7.Solutions of a Strong Acid or Base
8.The pH of a Solution
• b.
• c.
• d.
Note the two factors that determine relative
acid strengths.
Understand the periodic trends in the
strengths of the binary acids HX.
Understand the rules for determining the
relative strengths of oxoacids.
Understand the relative acid strengths of a
polyprotic acid and its anions.
1
Self-Ionization of Water and pH
6. Self-Ionization of Water
• a.
• b.
Define self-ionization (or autoionization).
Define the ion-product constant for water.
• When gaseous hydrogen chloride meets
gaseous ammonia, a smoke composed of
ammonium chloride is formed.
7. Solutions of a Strong Acid or Base
– a.
• HCl(g) + NH3(g) NH4Cl(s)
Calculate the concentrations of H3O+ and
OH- in solutions of a strong acid or base
• This is an acid–base reaction.
8. The pH of a Solution
1. Define pH.
2. Calculate the pH from the hydronium-ion
concentration.
3. Calculate the hydronium-ion concentration
from the pH.
4. Describe the determination of pH by a pH
meter and by acid–base indicators.
•
We will examine three ways to explain
acid–base behavior:
H+ and OH• Arrhenius Concept
donor
• Brønsted–Lowry Concept H+ = proton acceptor
• Lewis Concept
electron pair donor
acceptor
Note: H+ in water is H3O+
acid
base
Acid-Base Concepts
• Antoine Lavoisier was
one of the first
chemists to try to
explain what makes a
substance acidic.
– In 1777, he proposed that oxygen was an
essential element in acids.
– The actual cause of acidity and basicity was
ultimately explained in terms of the effect these
compounds have on water by Svante Arrhenius
in 1884.
2
Acid-Base Concepts
Arrhenius Concept of Acids and Bases
• In the first part of this chapter we will look at
several concepts of acid-base theory including:
– The Arrhenius concept
– The Bronsted Lowry concept
– The Lewis concept
This chapter expands on what you learned in
Chapter 3 about acids and bases.
Arrhenius Concept of Acids and Bases
• According to the Arrhenius concept of acids and
bases, an acid is a substance that, when
dissolved in water, increases the concentration
of hydronium ion (H3O+).
• According to the Arrhenius concept of acids and
bases, an acid is a substance that, when
dissolved in water, increases the concentration
of hydronium ion (H3O+).
The H3O+ is
shown here
hydrogen
bonded to three
water molecules.
Arrhenius Concept of Acids and Bases
• A base, in the Arrhenius concept, is a substance
that, when dissolved in water, increases the
concentration of hydroxide ion, OH-(aq).
– Chemists often use the notation H+(aq) for the
H3O+(aq) ion, and call it the hydrogen ion.
– Remember, however, that the aqueous hydrogen
ion is actually chemically bonded to water, that is,
H3O+.
Arrhenius Concept of Acids and Bases
• The Arrhenius concept limits bases to
compounds that contain a hydroxide ion.
• The Brønsted–Lowry concept expands the
compounds that can be considered acids and
bases.
• In the Arrhenius concept, a strong acid is a
substance that ionizes completely in aqueous
solution to give H3O+(aq) and an anion. (See
Animation: Acid Ionization Equilibirum)
– An example is perchloric acid, HClO4.
+
−
HClO4 (aq) + H 2O(l ) → H 3O (aq) + ClO4 (aq )
– Other strong acids include HCl, HBr, HI, HNO3 ,
and H2SO4.
3
Arrhenius Concept of Acids and Bases
• In the Arrhenius concept, a strong base is a
substance that ionizes completely in aqueous
solution to give OH-(aq) and a cation.
• According to the Brønsted-Lowry concept, an
acid is the species donating the proton in a
proton-transfer reaction.
– An example is sodium hydroxide, NaOH.
+
Brønsted-Lowry Concept of Acids and Bases
• A base is the species accepting the proton
in a proton-transfer reaction.
−
NaOH(s) HO→ Na (aq) + OH (aq)
2
– In any reversible acid-base reaction, both forward
and reverse reactions involve proton transfer.
– Other strong bases include LiOH, KOH,
Ca(OH)2, Sr(OH)2, and Ba(OH)2.
Arrhenius Concept of Acids and Bases
• Most other acids and bases that you encounter are
weak. They are not completely ionized and exist
in reversible reaction with the corresponding ions.
• Consider the reaction of NH3 and H20.
NH 3 (aq ) + H 2O(l )
base
– An example is acetic acid, HC2H3O2.
HC2 H 3O 2 (aq ) + H 2O( l )
+
+
−
NH4 (aq) + OH (aq)
−
+
−
NH 4 (aq ) + OH (aq )
H+
−
H 3O (aq) + C2 H 3O 2 (aq)
– Ammonium hydroxide, NH4OH, is a weak base.
NH 4OH(aq )
acid
NH 3 (aq ) + H 2O( l )
+
NH 4 (aq ) + OH (aq )
– In the forward reaction, NH3 accepts a proton
from H2O. Thus, NH3 is a base and H2O is an
acid.
• Brønsted–Lowry Concept of Acids and
Bases
• An acid–base reaction is considered a proton
(H+) transfer reaction.
H+
H+
H+
H+
4
• Consider the reaction of NH3 and H2O.
acid
NH3 (aq) + H2O(l)
+
base
• What is the conjugate acid of H2O?
• What is the conjugate base of H2O?
−
NH4 (aq) + OH (aq)
H+
base
acid
NH 3 (aq ) + H 2O(l )
NH 4 (aq ) + OH (aq )
+
−
The conjugate acid of H2O has gained a proton.
It is H3O+.
– The species NH4+ and NH3 are a conjugate
acid-base pair.
– A conjugate acid-base pair consists of two
species in an acid-base reaction, one acid and
one base, that differ by the loss or gain of a
proton.
The conjugate base of H2O has lost a proton.
It is OH-.
Brønsted-Lowry Concept of Acids and Bases
• Label each species as an acid or base.
Identify the conjugate acid-base pairs.
• Consider the reaction of NH3 and H2O.
base
NH 3 (aq ) + H 2O(l )
acid
+
−
NH 4 (aq ) + OH (aq )
– Here NH4+ is the conjugate acid of NH3
and NH3 is the conjugate base of NH4+.
– The Brønsted-Lowry concept defines a
species as an acid or a base according
to its function in the proton-transfer
reaction.
• Substances in the acid–base reaction that differ
by the gain or loss of a proton, H+, are called a
conjugate acid–base pair. The acid is called the
conjugate acid; the base is called a conjugate
base.
•
Acid
Base
Conjugate Conjugate
base
acid
a.
HCO3-(aq) + HF(aq)
Base
b.
Acid
HCO3-(aq) + OH-(aq)
Acid
Base
H2CO3(aq) + F-(aq)
Conjugate Conjugate
acid
base
CO32-(aq) + H2O(l)
Conjugate Conjugate
base
acid
• A Brønsted–Lowry acid is the species
donating a proton in a proton-transfer reaction;
it is a proton donor.
• A Brønsted–Lowry base is the species
accepting a proton in a proton-transfer
reaction; it is a proton acceptor.
5
• Some species can act as an acid or a base.
• In the Brønsted-Lowry concept:
– An amphoteric species is a species that can act
either as an acid or a base (it can gain or lose a
proton).
1. A base is a species that accepts protons;
OH- is only one example of a base.
2. Acids and bases can be ions as well as molecular
substances.
3. Acid-base reactions are not restricted to aqueous
solution.
4. Some species can act as either acids or bases
depending on what the other reactant is.
– For example, HCO3- acts as a proton donor (an acid) in
the presence of OH−
−2
−
HCO 3 (aq ) + OH (aq ) → CO3 (aq ) + H 2O(l )
–H+
Look at Example 15.1
– An amphoteric species is a species that can act
either as an acid or a base (it can gain or lose a
proton).
– Alternatively, HCO3- can act as a proton acceptor
(a base) in the presence of HF.
−
−
• The Lewis concept defines an acid as an
electron pair acceptor and a base as an
electron pair donor.
– This concept broadened the scope of acidbase theory to include reactions that did not
involve H+.
H+
– The Lewis concept embraces many
reactions that we might not think of as
acid-base reactions.
• The amphoteric characteristic of water is
important in the acid-base properties of
aqueous solutions.
+
• The reaction of boron trifluoride with
ammonia is an example.
:
B
+ :N H
:F:
H
:F
:F:
H
B
N
:F:
H
:
H
:
−
:F:
: :
: :
:F
– Water can also react as a base with the acid HF.
:
−
NH 3 (aq ) + H 2O ( l ) → NH 4 (aq ) +OH (aq )
H+
See Problems 15.35-36
Lewis Concept of Acids and Bases
HCO3 (aq ) + HF(aq ) → H 2CO3 (aq ) + F (aq )
– Water reacts as an acid with the base NH3.
Do Exercise 15.1
H
+
HF(aq ) + H 2O( l ) → F (aq ) + H 3O (aq )
– Boron trifluoride accepts the electron pair, so it is
a Lewis acid. Ammonia donates the electron pair,
so it is the Lewis base.
H+
Do Exercise 15.2
See Example 15.2 and Problems 15.39-42
6
Relative Strength of Acids and Bases
• Consider the equilibrium below.
HC2 H 3O 2 (aq ) + H 2O( l )
acid
base
−
+
H 3O (aq) + C2 H 3O 2 (aq)
acid
base
conjugate acid-base pairs
•
– In this system we have two opposing BrønstedLowry acid-base reactions.
– In this example, H3O+ is the stronger of the two
acids. Consequently, the equilibrium is skewed
toward reactants.
Relative Strength of Acids and Bases
• Consider the equilibrium below.
HC2 H 3O 2 (aq ) + H 2O( l )
• The Brønsted-Lowry concept introduced
the idea of conjugate acid-base pairs
and proton-transfer reactions.
– We consider such acid-base reactions to
be a competition between species for
hydrogen ions.
– From this point of view, we can order
acids by their relative strength as
hydrogen ion donors.
acid
base
−
+
H 3O (aq) + C2 H 3O 2 (aq)
acid
base
conjugate acid-base pairs
– Table 15.2 outlines the relative strength of some
common acids and their conjugate bases.
– This concept of conjugate pairs is fundamental to
understanding why certain salts can act as acids or
bases.
Do Exercise 15.2
See Example 15.3 and Problems 15.45-48
Relative Strength of Acids and Bases
– The stronger acids are those that lose their
hydrogen ions more easily than other acids.
– Similarly, the stronger bases are those that hold
onto hydrogen ions more strongly than other
bases.
– If an acid loses its H+, the resulting anion is now in
a position to reaccept a proton, making it a
Brønsted-Lowry base.
– It is logical to assume that if an acid is considered
strong, its conjugate base (that is, its anion) would be
weak, since it is unlikely to accept a hydrogen ion.
7
Molecular Structure and Acid Strength
• Two factors are important in
determining the relative acid strengths.
– One is the polarity of the bond to which the
hydrogen atom is attached.
– The H atom should have a partial positive charge:
δ+
δ−
H−X
– The more polarized the bond, the more easily the
proton is removed and the greater the acid
strength.
– The second factor is the strength of the bond. Or,
in other words, how tightly the proton is held.
– This depends on the size of atom X.
δ+
δ-
H−X
– The larger atom X, the weaker the bond and the greater
the acid strength.
• For a binary acid, as the size of X in HX
increases, going down a group, acid strength
increases.
• For a binary acid, going across a period, as the
electronegativity increases, acid strength
increases.
Acetate Ion
8
• Which is a stronger acid: HF or HCl?
• Which is a stronger acid: H2O or H2S?
• Which is a stronger acid: HCl or H2S?
HF and HCl
These are binary acids from the same group,
so we compare the size of F and Cl. Because
Cl is larger, HCl is the stronger acid.
• As you go across a row of elements, the
polarity of the H-X bond becomes the
dominant factor.
H 3 N < H 2O < HF
– As electronegativity increases going to the right,
the polarity of the H-X bond increases and the
acid strength increases.
– You can predict the following order of acidic strength.
H 3 N < H 2O < HF
• H2O and H2S
• These are binary acids from the same group, so
we compare the size of O and S. Because S is
larger, H2S is the stronger acid.
• Consider the oxoacids. An oxoacid has
the structure:
H−O−Y−
– The acidic H atom is always attached to an O atom,
which in turn is attached to another atom Y.
• HCl and H2S
• These are binary acids from the same period,
but different groups, so we compare the
electronegativity of O and S. Because Cl is
more electronegative, HCl is the stronger acid.
Molecular Structure and Acid Strength
• Consider a series of binary acids from a
given column of elements.
– As you go down the column of elements, the
radius increases markedly and the H-X bond
strength decreases.
– You can predict the following order of acidic strength.
HF < HCl < HBr < HI
– Bond polarity is the dominant factor in the relative
strength of oxoacids.
– This, in turn, depends on the electronegativity of the
atom Y.
• Consider the oxoacids. An oxoacid has
the structure:
H−O−Y−
– If the electronegativity of Y is large, then the O-H
bond is relatively polar and the acid strength is
greater.
– You can predict the following order of acidic strength.
HOCl > HOBr > HOI
– Other groups, such as O atoms or O-H groups, may
be attached to Y.
– With each additional O atom, Y becomes effectively
more electronegative.
9
• For oxoacids, several factors are relevant: the
number and bonding of oxygens, the central
element, and the charge on the species.
• For a series of oxoacids, (OH)mYOn, acid
strength increases as n increases.
(OH)Cl
n=0
(OH)ClO
n=1
(OH)ClO2
(OH)ClO3
n=2
n=3
Molecular Structure and Acid Strength
• Consider polyprotic acids and their
corresponding anions.
– Each successive H atom becomes more difficult to
remove.
– Therefore the acid strength of a polyprotic acid and
its anions decreases with increasing negative charge.
Weakest
Strongest
– As a result, the H atom becomes more acidic.
– The acid strengths of the oxoacids of chlorine
increase in the following order.
HClO < HClO 2 < HClO 3 < HClO 4
HPO 4
2−
−
< H 2 PO 4 − < H 3 PO 4
• The acid strength of a polyprotic acid and its
anions decreases with increasing negative
charge.
•H2CO3 is a stronger acid than HCO3-.
•H2SO4 is a stronger acid than HSO4-.
•H3PO4 is a stronger acid than H2PO4-.
•H2PO4- is a stronger acid than HPO42-.
• A reaction will always go in the direction from
stronger acid to weaker acid, and from stronger
base to weaker base.
• For a series of oxoacids differing only in the
central atom Y, the acid strength increases
with the electronegativity of Y.
•Decide which species are favored at the
completion of the following reaction:
•HCN(aq) + HSO3-(aq)
•CN-(aq) + H2SO3(aq)
We first identify the acid on each side of the
reaction: HCN and H2SO3.
Next, we compare their acid strength:
H2SO3 is stronger.
Stronger
Weaker
This reaction will go from right to left (), and
the reactants are favored.
10
Self-ionization of Water
• Self-ionization is a reaction in which two like
molecules react to give ions.
Do Exercise 15.4
– The concentration of ions is extremely
small, so the concentration of H2O remains
essentially constant. This gives:
See Problems 15.51-52
+
−
[ H 2O]2 K c = [ H 3O ][OH ]
constant
– We call the equilibrium value for the ion product
[H3O+][OH-] the ion-product constant for water,
which is written Kw.
• Self-Ionization of Water
+
−
K w = [ H 3O ][OH ]
H3O+(aq) + OH-(aq)
• H2O(l) + H2O(l)
Base
Acid
Conjugate
acid
Conjugate
base
– At 25 oC, the value of Kw is 1.0 x 10-14.
– Like any equilibrium constant, Kw varies with
temperature.
– Because we often write H3O+ as H+, the ionproduct constant expression for water can be
written:
+
−
K w = [ H ][OH ]
Self-ionization
of Water
Self-ionization of Water
• Self-ionization is a reaction in which two like
molecules react to give ions.
– In the case of water, the following equilibrium is
established.
+
H 2O (l ) + H 2O (l )
−
H 3 O ( aq ) + OH ( aq )
– The equilibrium-constant expression for this system
is:
+
Kc =
−
[ H 3O ][OH ]
[ H 2O]2
• These ions are produced in equal numbers in pure
water, so if we let x = [H+] = [OH-]
1.0 × 10
−14
= ( x )( x )
x = 1.0 × 10
−14
at 25 oC
= 1.0 × 10 −7
– Thus, the concentrations of H+ and OH- in pure
water are both 1.0 x 10-7 M.
– If you add acid or base to water they are no longer
equal but the Kw expression still holds.
11
Solutions of Strong Acid or Base
H3O+(aq) + OH-(aq)
• H2O(l) + H2O(l)
• We call the equilibrium constant the ionproduct constant, Kw.
Kw = [H3O+][OH-]
At 25°C, Kw = 1.0 × 10-14
• As temperature increases, the value of Kw
increases.
Do Exercise 15.5
See Example 15.4 and Problems 15.53-54
Solutions of Strong Acid or Base
• In a solution of a strong acid you can
normally ignore the self-ionization of water
as a source of H+(aq).
H+(aq)
– The
concentration is usually
determined by the strong acid
concentration.
– However, the self-ionization still exists and
is responsible for a small concentration of
OH- ion.
• As an example, calculate the concentration of OHion in 0.10 M HCl.
Because you started with 0.10 M HCl (a
strong acid) the reaction will produce 0.10
M H+(aq).
+
−
HCl(aq ) → H (aq ) + Cl (aq )
– Substituting [H+]=0.10 into the ion-product
expression, we get:
1.0 × 10
−14
= ( 0.10)[OH − ]
• As an example, calculate the concentration of OHion in 0.10 M HCl.
Because you started with 0.10 M HCl (a strong
acid) the reaction will produce 0.10 M H+(aq).
+
−
HCl(aq ) → H (aq ) + Cl (aq )
– Substituting [H+]=0.10 into the ion-product
expression, we get:
−
[OH ] =
1.0 ×10 -14
= 1.0 × 10-13 M
0.10
• Similarly, in a solution of a strong base you
can normally ignore the self-ionization of
water as a source of OH-(aq).
– The OH-(aq) concentration is usually
determined by the strong base
concentration.
– However, the self-ionization still exists and
is responsible for a small concentration of
H+ ion.
• As an example, calculate the concentration of H+
ion in 0.010 M NaOH.
Because you started with 0.010 M NaOH (a
strong base) the reaction will produce 0.010
M OH-(aq).
H 2O
+
−
→ Na (aq ) + OH (aq )
NaOH(s ) 
– Substituting [OH-]=0.010 into the ionproduct expression, we get:
−
+
1.0 × 10 14 = [ H ](0.010)
12
Because you started with 0.010 M NaOH (a
strong base) the reaction will produce 0.010
M OH-(aq).
H 2O
+
−
→ Na (aq ) + OH (aq )
NaOH(s ) 
– Substituting [OH-]=0.010 into the ionproduct expression, we get:
+
[H ] =
1.0 × 10-14
= 1.0 ×10-12 M
0.010
Solutions of Strong Acid or Base
• By dissolving substances in water, you can
alter the concentrations of H+(aq) and OH(aq).
The pH of a Solution
• Although you can quantitatively describe
the acidity of a solution by its [H+], it is
often more convenient to give acidity in
terms of pH.
– The pH of a solution is defined as the
negative logarithm of the molar hydrogenion concentration.
+
pH = − log[H ]
• For a solution in which the hydrogen-ion
concentration is 1.0 x 10-3, the pH is:
−
pH = − log(1.0 ×10 3 ) = 3.00
– In a neutral solution, the concentrations of H+(aq)
and OH-(aq) are equal, as they are in pure water.
– In an acidic solution, the concentration of H+(aq) is
greater than that of OH-(aq).
– Note that the number of decimal places in
the pH equals the number of significant
figures in the hydrogen-ion concentration.
– In a basic solution, the concentration of OH-(aq) is
greater than that of H+(aq).
• At 25°C, you observe the following
conditions.
– In an acidic solution, [H+] > 1.0 x 10-7 M.
– In a neutral solution, [H+] = 1.0 x 10-7 M.
– In a basic solution, [H+] < 1.0 x 10-7 M.
The pH of a Solution
• In a neutral solution, whose hydrogen-ion concentration
is 1.0 x 10-7, the pH = 7.00.
• For acidic solutions, the hydrogen-ion
concentration is greater than 1.0 x 10-7, so the
pH is less than 7.00
• .
• Similarly, a basic solution has a pH greater
than 7.00.
• Figure 15.8 shows a diagram of the pH scale
and the pH values of some common solutions.
13
A Problem to Consider
Figure 15.8: The pH Scale
• The pH of human arterial blood is 7.40. What
is the hydrogen-ion concentration?
+
[H ] = anti log(−pH )
+
[H ] = anti log(−7.40)
+
[H ] = 10
− 7.40
= 4.0 × 10−8 M
Do Exercise 15.7
See Example 15.5 and Problems 15.67-68
•Calculate the hydronium and hydroxide ion
concentration at 25°C in
•
•
a.
b.
0.10 M HCl
1.4 × 10-4 M Mg(OH)2
a. When HCl ionizes, it gives H+ and Cl-.
So [H+] = [Cl-] = [HCl] = 0.10 M.
a. When Mg(OH)2 ionizes, it gives Mg2+ and 2 OH-.
So [OH-] = 2[Mg2+] = 2[Mg(OH)2] = 2.8 × 10-4 M.
A Problem to Consider
•
•
•
•
A has 5 H3O+ and 5 OH-. It is neutral.
B has 7 H3O+ and 3 OH-. It is acidic.
C has 3 H3O+ and 7 OH-. It is basic.
Listed from most acidic to most basic: B, A, C.
The pOH of a Solution
• A sample of orange juice has a hydrogen-ion
concentration of 2.9 x 10-4 M. What is the pH?
• A measurement of the hydroxide ion
concentration, similar to pH, is the pOH.
+
pH = − log[H ]
−4
pH = − log(2.9 × 10 )
pH = 3.54
– The pOH of a solution is defined as the
negative logarithm of the molar hydroxideion concentration.
−
pOH = − log[OH ]
14
The pOH of a Solution
• A measurement of the hydroxide ion
concentration, similar to pH, is the pOH.
– Then because Kw = [H+][OH-] = 1.0 x 10-14
at 25 oC, you can show that
pH + pOH = 14.00
The pH of a Solution
• A measurement of the hydroxide ion
concentration, similar to pH, is the pOH.
– Then because Kw = [H+][OH-] = 1.0 x 10-14
at 25 oC, you can show that
The pH of a Solution
• The pH of a solution can accurately be
measured using a pH meter (see Figure
15.9).
– Although less precise, acid-base indicators are
often used to measure pH because they usually
change color within a narrow pH range.
– Figure 15.8 shows the color changes of various
acid-base indicators.
Figure
15.9:
A digital
pH meter.
Photo
courtesy of
American
Color.
pH + pOH = 14.00
See Exercise 15.6 and Problems 15.75-76
Do Exercise 15.9-10
http://www.quia.com/rr/4051.html
15
Problem 15.27
Problem 15.28
Figure 15.12: Preparation of Sodium Hydroxide by Hydrolysis
Problem 15.37
16
Problem 15.38
Operational Skills
• Identifying acid and base species
• Identifying Lewis acid and base species
• Deciding whether reactants or products are
favored in an acid-base reaction
• Calculating the concentration of H+ and OHin solutions of strong acid or base
• Calculating the pH from the hydrogen-ion
concentration, and vice versa
17