Chapter 17 –
Chemistry of Acids
and Bases
Objectives:
1. Know the Bronsted-Lowry and Lewis theories of
acids and bases.
2. Apply chemical equilibrium to acids and bases.
3. Predict reactions between acids and bases.
4. Relate structure and bonding to acid-base
properties.
How do we know ions are present
in aqueous solutions?
The solutions conduct
electricity!
The substances that provide ions
in an aqueous solution are called
ELECTROLYTES.
HCl, CuCl2, and NaCl are
__________electrolytes.
What is a Strong Electrolyte?
They ionize or dissociate completely (or
nearly so) into __________.
HCl, CuCl2, and NaCl are strong
electrolytes.
HCl (aq)
H+(aq)
+
Cl-(aq)
What is a Weak Electrolyte?
Acetic acid ionizes only to a
small extent, so it is a
electrolyte.
CH3CO2H(aq)
weak
CH3CO2-(aq) + H+(aq)
What is an Acid?
An acid - - -> H+ (in water)
PROTON
Because a hydrogen atom is one proton and one
electron, a hydrogen ion (H+) is simply a proton.
Some strong acids
(completely ionized) are:
HCl
hydrochloric
H2SO4
sulfuric
HClO4
perchloric
HNO3
nitric
HNO3
What is an Acid?
An acid ---> H+ in water
HCl(aq)
H+(aq) + Cl-(aq)
What is the Hydronium Ion?
HCl(aq)
H+(aq) + Cl-(aq)
HCl
H 2O
Cl-
hydronium
ion
H3O+
Bronsted – Lowry Theory:
An acid donates a proton
What is a Weak Acid?
CH3CO2H (aq)
H+(aq) + CH3CO2- (aq)
WEAK ACIDS (only partially ionized)
= weak electrolytes
CH3CO2H
acetic acid
H2CO3
carbonic acid
H3PO4
phosphoric acid
HF
hydrofluoric acid
Weak Acid
K = [CH3CO2-][H3O+]
Equilibrium constants =
= 1.8 x 10-5
[CH3CO2H]
____________constants
K > 1 (larger) – _______ Acid
K < 1 (smaller) – ______ Acid
Bronsted-Lowry Acids and Bases
Acid – any substance that can _________ a proton
to any other substance.
Base – any substance that can _________ a proton
from any other substance.
Acids – Monoprotic and Polyprotic
Amphiprotic – can behave as
_____________________.
Polyprotic bases – can accept ____________proton.
Label the following as Acid or Base
HCl
NH3
CO32-
CH3CH2COOH
CN-
H2PO4-
H2SO4
HCO3-
NH4+
H2O
Water is amphiprotic –
self-ionization of water
Write the equation:
Conjugate Acid-Base Pairs
• A pair of compounds or ions that differ
by the presence of one H+ ion is called a
_________________________.
• Every reaction between a Bronsted acid
and Bronsted base involves
___________________and has two
_______________________.
Conjugate Acid-Base
Identify the conjugate acid-base pairs:
1) HCO3- (aq) + H2O (l)
2) HNO3 (aq) + NH3 (aq)
H3O+ (aq) + CO32- (aq)
NH4+ (aq) + NO3– (aq)
Kw
Kw =
Kw and pH
Neutral: [H3O+] = [OH-]
Kw = [H3O+][OH-]
[H3O+] = [OH-] = √1.0 x 10-14
[H3O+] = [OH-] = 1 x 10-7
pH = __________
Neutral = pH = ________
Kw and pH
• Adding acid to pure H2O:
–
–
–
–
Increases the [H3O+] ; some reacts with the [OH-]
[OH-] is ______________
Equilibrium is reached when [H3O+][OH-] = 1 x 10-14 (25oC)
[H3O+] ____ 1 x 10-7 M; [OH-] _____ 1 x 10-7 M
• Adding base to pure H2O:
–
–
–
–
Increases the [OH-] ; some reacts with the [H3O+]
[H3O+] is _______________
Equilibrium is reached when [H3O+][OH-] = 1 x 10-14 (25oC)
[H3O+] ___1 x 10-7 M; [OH-] ____ 1 x 10-7 M
pH
Look at ionization constants to determine
strength of acids and bases.
A solution of HCl has [HCl] = 4.0 x 10-3 M. What
are the concentrations of H3O+ and OH- in this
solution at 25oC?
What is the pH of a 0.0075 M solution of HCl?
What is the hydroxide ion concentration of the
solution?
The pH of a solution of Ba(OH)2 is 9.89 at 25oC. What is
the hydroxide ion concentration in the solution? How
many grams of Ba(OH)2 must have been dissolved?
Other pX Scales
In general
and so
pX = -log X
pOH = - log [OH-]
Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC
Take the log of both sides
-log (10-14) = - log [H3O+] + (-log [OH-])
pKw = _____ = ____ + _____
Ka and
Kb
Strong acids have
_________ ionization
constants (Ka) –
Indicating higher ability
of donating protons –
And a ________ pH.
Strong bases have
_________ ionization
constants (Kb) –
Indicating higher ability
of accepting protons –
And a _________ pH.
Ka and Kb
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
Ka =
B(aq) + H2O(l)
Kb =
BH+(aq) + OH-(aq)
K and Reactions
ACIDS
STRONG
weak
CONJUGATE BASES
weak
STRONG
Reactions always go from the
stronger A-B pair (_______ K) to
the weaker A-B pair (_____ K).
K and Reactions
A strong acid is __________ %dissociated.
Therefore, a STRONG ACID —a good H+ donor—
must have a WEAK CONJUGATE BASE—a poor
H+ acceptor:
HNO3(aq) + H2O(liq)  H3O+(aq) + NO3-(aq)
STRONG A
base
acid
weak B
•Every A-B reaction has two acids and two bases.
•Equilibrium always lies toward the _________ pair.
•Here K is very __________.
K and Reactions
BASE
HNO3 + H2O
STRONG
ACID
ACID
H3O+ + NO3WEAK
BASE
We know from experiment that HNO3 is a strong
acid.
1. It is a stronger acid than H3O+
2. H2O is a stronger base than NO33. K for this reaction is __________.
K and Reactions
Acetic acid is only 0.42% ionized when [HOAc] = 1.0 M. It
is a _____________________.
HOAc
+
H2O
H3O+
+
OAc-
Because [H3O+] is small, this must mean
1. H3O+ is a stronger acid than HOAc
2. OAc- is a stronger base than H2O
3. K for this reaction is ___________.
Ka and Structure
• Inductive effect:
adjacent atoms
____________
electrons.
• O is very
electronegative,
it will attract e• Once a proton is
donated, a
negative charge
is left behind.
• The charge
needs to be
stabilized.
Ka and pKa
pKa = -logKa
pKa = -logKa
• The _____________________ structures that can stabilize
the charge determines the acidity of the substance.
• Because: the negative charge left behind can be distributed
over more atoms (providing stability).
• Exercise: Draw the resonance structures for NO3- and for
NO2-.
Ka and Structure
Exercise: Draw the resonance
structures for NO3- and for NO2-.
Ka and Structure
• The number of resonance structures that can stabilize the
charge determines the acidity of the substance.
• Exercise: Draw the resonance structures for acetate ion.
Ka and Structure
• Acidity is favored by:
– Electronegative atoms attached.
– The possibility of resonance structures for the
anion.
Ka and
Structure
• Methyl (CH3-) group increases the
electronic density on the O-,
making the negative charge more
unstable and then acetic acid is
less acidic than formic acid.
• Propanoate ion is the stronger
base (it is eager to accept a
proton) because the negative
charge is not favored.
Kb and Structure
• Which is the stronger base?
Kb and Structure
Substituent Effects
• The electron cloud in a σ-bond between two unlike atoms is not
uniform and is slightly displaced towards the more electronegative of
the two atoms. This causes a permanent state of bond polarization,
where the more electronegative atom has a slight negative charge(δ-)
and the other atom has a slight positive charge(δ+).
• If the electronegative atom is then joined to a chain of atoms, usually
carbon, the positive charge is relayed to the other atoms in the chain.
This is the electron-withdrawing inductive effect, also known as the -I
effect.
• Some groups, such as the alkyl group are less electron-withdrawing
than hydrogen and are therefore considered as electron-releasing. This
is electron releasing character is indicated by the +I effect.
The inductive effect
Summary:
Substance
Definition
Examples
Strong acids: HCl, HNO3
Arrhenius acid
Provides _____in water
Weak acids: HF, HCN
Cations of weak bases: NH4+
Anions of some polyprotic acids: HSO4 -
Arrhenius base
Provides _____in water
Strong bases: NaOH, Ba(OH)2
Strong acids: HCl, HNO3
Bronsted-Lowry acid
Proton ________
Weak acids: HF, HCN
Cations of weak bases: NH4+
Anions of some polyprotic acids: HSO4 Strong bases: NaOH, Ba(OH)2
Bronsted-Lowry base
Proton _________
Weak bases: NH3
Anions of weak acids: OH -, CN Anions of most polyprotic acids: CO32-, H2PO4 Molecules with an incomplete octet: BF3
Lewis acid
Electron Pair _______
Small highly charged cations: Cu2+, Fe+3
Molecules with polar bonds: CO2
Lewis base
Electron Pair ________
Molecules with a lone pair of electrons: H2O, NH3, CO
Anions: Cl -, OH -, CN -
Identify the acidic protons and the
proton accepting atoms in the following:
Answer using table 17.3:
• Which is the stronger acid?
– Benzoic acid or acetic acid
Which has the stronger conjugate base?
– Acetic acid or benzoic acid
• Which is the stronger base?
– Ammonia or acetate ion
Which has the stronger conjugate acid?
– Ammonia or acetate ion
Aqueous Solutions of Salts
• Some ions are Bronsted bases:
CO32- (aq) + H2O(l)
HCO3- (aq) + OH- (aq)
Kb = 2.1 x 10-4
• Some ions are Bronsted acids:
[Al(H2O)6]3+ (aq) + H2O(l)
[Al(H2O)5 (OH)2+ (aq) + H3O+ (aq)
Ka = 7.9 x 10-6
• Anions that are conjugate bases of strong acids (Cl-, NO3-)
are such weak bases, they ________________ on pH.
• Alkali metal and alkaline earth cations have __________pH.
• Anions of polyprotic acids can be either _______________.
• Transition metal ions M2+ and M3+ can act as ___________.
• ___________anions are conjugate bases of weak acids
(CO32-, F-, CN-) or ions [Al(H2O)6]3+.
Table 17.4 Acid and Base Properties
of Ions in Aqueous Solution
For each of the following salts in water, predict
whether the pH will be acidic, basic, or neutral.
NaNO3
FeCl2
NH4NO3
NH4F
Ka and Kb Relationship
• Ka x Kb = Kw
Reaction between an Acid and a Base
• Write the net ionic equation for the possible reaction
between acetic acid and sodium hydrogen sulfate,
NaHSO4. Does the equilibrium lie to the left or right?
CH3CO2H (aq) + HSO4- (aq)
CH3CO2- (aq)
+ H2SO4 (aq)
Types of acid-base Reactions
•
•
•
•
Strong acid + Strong base (neutralizations)
Weak acid + Strong base
Strong acid + Weak base
Weak acid + Weak base
Strong acid + Strong base
HCl(aq) + NaOH(aq)
NaCl(aq) + H2O(l)
H3O+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)
Na+(aq) + Cl-(aq) + 2 H2O (l)
H3O+(aq) + OH-(aq)
2 H2O (l)
K = 1/Kw = 1 x 1014
• Really large! Reaction goes to completion
• Na+ and Cl- have no effect on pH
• Final solution should be neutral:
Mixing equal molar amounts of a strong acid
with a strong base produces a __________
solution (pH = _____ at 25oC).
Weak acid + Strong base
HCO2H(aq) + NaOH(aq)
HCO2H(aq) + OH-(aq)
HCO2Na(aq) + H2O(l)
H2O (l) + HCO2- (aq)
• OH- is stronger base than HCO2• Reaction should proceed to the right forming
HCO2Na; Na+ is neutral ion and HCO2- is a
basic ion (conjugate base of a weak acid).
• Solution is basic.
Mixing equal amounts of a strong base with a
weak acid produces a ______ whose anion is
the conjugate base of the weak acid. The
solution is __________, pH depending on the
value of _____________________.
Strong acid + Weak base
HCl + NH3
H3O+ (aq) + NH3 (aq)
H2O (l) + NH4+ (aq)
• H3O+ is stronger acid than NH4+
• Equilibrium lies to the right
• NH4Cl; NH4+ is an acidic ion, Cl- will not affect
pH
Mixing equal amounts of a strong acid and a
weak base produces a ________ whose cation
is the conjugate acid of the weak base. The
solution is __________, with the pH
depending on the value of ______________.
Weak acid + Weak base
Acetic acid + ammonia:
CH3CO2H (aq) + NH3 (aq)
•
•
•
•
•
•
NH4+ (aq) + CH3CO2- (aq)
CH3CO2H is stronger acid than NH4+
Equilibrium lies to the right (product favored).
Product is NH4CH3CO2
Ka for NH4+= 5.6 x 10-10
Kb for CH3CO2- = 5.6 x 10-10
Ka = Kb ; solution will be neutral
Mixing equal molar amounts of a weak acid and a
weak base produces a ________ whose cation is
the conjugate acid of the weak base and whose
anion is the conjugate base of the weak acid.
The solution pH depends on the
________________________________.
Determine Ka for butanoic acid. A solution prepared
from 0.055 mol of butanoic acid dissolved in sufficient
water to give 1.0 L of solution has a pH of 2.72 at 25oC.
CH3CH2CH2CO2H (aq) + H2O (l)
H3O+ (aq) + CH3CH2CH2CO2- (aq)
What are the equilibrium concentrations of HF, F- ion,
and H3O+ ion in a 0.015 M solution of HF? What is the
pH of the solution?
HF (aq) + H2O (l)
H3O+(aq) + F-(aq)
What is the pH of the solution that results from mixing
25 mL of 0.016 M NH3 and 25 mL of 0.016 M HCl?
1) Write balanced equation
2) Find amount of product (salt that determines the pH)
3) For pH solve an equilibrium problem
Polyprotic Acids and Bases
• The pH of many inorganic polyprotic acids depends
primarily on the hydronium ion generated in the first
ionization step.
• Same principle is applied for conjugate bases of
polyprotic acids.
• Phosphoric acid:
First ionization step:
Ka1 =
Second ionization step:
Ka2 =
Third ionization step:
Ka3 =
Lewis Acids and Bases
• Gilbert N Lewis (1930s)
• Lewis acid is a substance that can
a
pair of electrons from another atom to form a
new bond.
• Lewis base is a substance that can
a
pair of electrons to another atom to form a
new bond.
A + B:
BA
acid base
acid-base adduct
coordinate covalent bond
Cationic Lewis Acids
• Complex ions or Coordination complexes (due to
coordinate covalent bond).
• [Fe(H2O)6]3+ ; [Co(H2O)6]2+; [Cu(H2O)6]2+;
[Cu(NH3)4]2+
• OH- is a Lewis base
• Amphoteric substance: metal hydroxide that
can behave as an acid or a base: Al(OH)3
Al(OH)3 (s) + OH- (aq)
Al(OH)3 (s) + 3 H3O+ (aq)
[Al(OH)4]- (aq)
Al3+ (aq) + 6 H2O (l)
Molecular Lewis Acids
• Explains why nonmetals behave as _______.
• C in CO2 is slightly positive, a negatively
charged Lewis base OH- can attack this atom
to give bicarbonate ion:
CO2 (g) + OH- (l)
HCO3- (aq)
• Substances which lone pairs of electrons
(central atom) can behave as Lewis _______.
Substances which lack an electron pair behave
as a Lewis _______.
Remember
• Go over all the contents of your
textbook.
• Practice with examples and with
problems at the end of the chapter.
• Practice with OWL.
• Work on your OWL assignment for
Chapter 17.