Transcript Acids and Bases - Derry Area School District

Acids and Bases
1
Acid-Base Concepts

Antoine Lavoisier was one of the
first chemists to try to explain what
makes a substance acidic.
2
Acid-Base Concepts

In the first part of this chapter we
will look at several concepts of
acid-base theory including:
3
Common
Characteristics
Acids
Bases
SOLUTION
NO Current
Nonelectrolytic solution
Molecular Solution
Molecule = no net charge
EXCEPTIONS:
Covalent / Hydrohalic Acids
Conducts Current
Electrolytic Solution
Acid, Base or Salt Solution
Ion = charged particle
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+).
– Remember, however, that the aqueous hydrogen
ion is actually chemically bonded to water, that is,
H3O+.
6
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+).
The H3O+ is shown
here hydrogen
bonded to three
water molecules.
7
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).
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Theory of Ionization



Svente Arrhenius
1884
Water
 Acid


Base
Neutralization
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Arrhenius Concept of
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.
– Other strong acids include
10
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.
See Base Ionization animation
– An example is sodium hydroxide,.
– Other strong bases include
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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.
– An example is acetic acid,
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Arrhenius Concept of
Acids and Bases

The Arrhenius concept is limited in
that it looks at acids and bases in
aqueous solutions only.
– Broader definitions of acids and bases are
discussed in the next sections.
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Consider…
NH3 (aq)  H 2O(l )


NH 4 (aq)  OH  (aq)
What is the acid?
What is the base?
Production of NH4
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Brønsted-Lowry
Concept of Acids and
Bases
• According to the Brønsted-Lowry concept, an
acid is

A base is
– In any reversible acid-base reaction,
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Brønsted-Lowry
Concept of Acids and
Bases

Consider the reaction of NH3 and H2O.
base
acid
H+
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Brønsted-Lowry
Concept of Acids and
Bases

Consider the reaction of NH3 and H2O.
NH 3 (aq )  H 2O(l )
acid
base

NH 4 (aq )  OH  (aq )
H+
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Brønsted-Lowry
Concept of Acids and
Bases

Consider the reaction of NH3 and H2O.
base
NH 3 (aq )  H 2O(l )
acid

NH 4 (aq )  OH  (aq )
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Brønsted-Lowry
Concept of Acids and
Bases

Consider the reaction of NH3 and H2O.
base
NH 3 (aq )  H 2O(l )
acid

NH 4 (aq )  OH  (aq )
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Bronsted – Lowry
Theory

Acid

Base

When an acid loses a proton, its
conjugate base is formed.

When a base accepts a proton, its
conjugate acid is formed
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Conjugate Acids and
Bases
H2O + NH3  NH4+ + OHacid
base
CAN donate H+
conjugate acid
CAN donate OHconjugate base
Conjugate Acid – Base Pairs
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What’s the Deal With Water?
H2O + NH3  NH4+ + OHacid
base



H2O + HCl  H3O+ + Clbase
acid
Amphoteric
Autoionization
Protolysis
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Brønsted-Lowry
Concept of Acids and
Bases
– HCO3- acts as a proton donor (an acid) in the
presence of OH-
–H+
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Brønsted-Lowry
Concept of Acids and
Bases
– HCO3 can act as a proton acceptor (a
base) in the presence of HF.
H+
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Brønsted-Lowry
Concept of Acids and
Bases

In the Brønsted-Lowry concept:
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Lewis Concept of Acids
and Bases

The Lewis concept defines an acid
as an electron pair acceptor and
a base as an electron pair donor.
– The Lewis concept embraces many reactions that
we might not think of as acid-base reactions.
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Lewis Theory


1923
Extended the acid–base theory
electron pair donor =
electron pair receiver =
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4 ammonia molecules forming a
complex ion with cupric ion
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Characteristics of
Lewis…

Acids

Bases
Lone e- pair
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
Neutralization = Coordinate
covalent
bond

Both shared e- donated by the
same atom / ion
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Nesting Theories
Lewis
Bronsted - Lowry
Arrhenius
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Strength in Water


Depends on degree of ionization

Ease of bond breakage

Stability of resulting ions
Higher the [H+] or [OH-]
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And the Winner is…

Hydronium H3O+ is the

Hydroxide OH- is the
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
Water is a leveling solvent;.

HCl, HBr, HI

NaOH, Ca(OH)2
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ACIDS &
H2SO4 v
H2SO2
BASES
NaOH
v S(OH)6
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H3PO4
weaker than HNO3
+5
+5
HNO2
weaker than H3PO3
+3
+3
H
H
H
N
O
O
P
O
H
O
O
Trigonal planar
Tetrahedral
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Relative Strength of
Acids and Bases

The Brønsted-Lowry concept
introduced the idea of conjugate
acid-base pairs and protontransfer reactions.
– We consider such acid-base reactions to be a
competition between species for hydrogen ions.
37
Relative Strength of
Acids and Bases

The Brønsted-Lowry concept
introduced the idea of conjugate
acid-base pairs and protontransfer reactions.
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Relative Strength of
Acids and Bases

The Brønsted-Lowry concept
introduced the idea of conjugate
acid-base pairs and protontransfer reactions.
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Relative Strength of
Acids and Bases
Consider the equilibrium below.


HC2 H 3O 2 (aq)  H 2O(l )
H 3O (aq)  C2 H 3O 2 (aq)

acid
base
acid
base
conjugate acid-base pairs
40
Relative Strength of
Acids and Bases

Consider the equilibrium below.


HC2 H 3O 2 (aq)  H 2O(l )
H 3O (aq)  C2 H 3O 2 (aq)
acid
base
acid
base
conjugate acid-base pairs
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Molecular Structure and
Acid Strength

Two factors are important in
determining the relative acid
strengths.
– The H atom should have a partial positive charge:
d+
d-
HX
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Molecular Structure and
Acid Strength

Two factors are important in
determining the relative acid
strengths.
d+
d-
HX
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Molecular Structure and
Acid Strength

Consider a series of binary acids
from a given column of elements.
– You can predict the following order of acidic strength.
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Molecular Structure and
Acid Strength

As you go across a row of
elements, the polarity of the H-X
bond becomes the dominant
factor.
– You can predict the following order of acidic strength.
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Molecular Structure and
Acid Strength

Consider the oxyacids. An oxyacid
has the structure:
HOY
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Molecular Structure and
Acid Strength
HOY
– You can predict the following order of acidic strength.
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Molecular Structure and
Acid Strength

Consider the oxyacids. An oxyacid
has the structure:
HOY
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Molecular Structure and
Acid Strength

Consider the oxyacids. An oxyacid
has the structure:
HOY
49
Molecular Structure and
Acid Strength

Consider polyprotic acids and their
corresponding anions.
– Therefore the acid strength of a polyprotic acid and
its anions decreases with increasing negative charge.
50
Self-ionization of Water
Self-ionization is a reaction in which two
like molecules react to give ions. (See
Animation: Self-ionization of Water to
Form H+ and OH- in Equilibrium)
– In the case of water, the following equilibrium is
established.

– The equilibrium-constant expression for this
system is:
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Self-ionization of Water

Self-ionization is a reaction in which two
like molecules react to give ions. [i.e.
H2O]
– The concentration of ions is extremely
small, so the concentration of H2O remains
essentially constant. This gives:
constant
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Self-ionization of Water

Self-ionization is a reaction in which two
like molecules react to give ions.
–.
– At 25 oC, the value of Kw is 1.0 x 10-14.
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Self-ionization of Water

Self-ionization is a reaction in which two
like molecules react to give ions.
– Because we often write H3O+ as H+, the ionproduct constant expression for water can be
written:
– Using Kw you can calculate the concentrations of
H+ and OH- ions in pure water.
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Self-ionization of Water

These ions are produced in equal
numbers in pure water, so if we let x =
[H+] = [OH-]
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Solutions of Strong Acid
or Base

In a solution of a strong acid you
can normally ignore the selfionization of water as a source of
H+(aq).
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Solutions of Strong Acid
or Base
As an example, calculate the
concentration of OH- ion 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).

– Substituting [H+]=0.10 into the ion-product
expression, we get:
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Solutions of Strong Acid
or Base
As an example, calculate the
concentration of OH- ion 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).

– Substituting [H+]=0.10 into the ion-product
expression, we get:
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Solutions of Strong Acid
or Base

Similarly, in a solution of a strong
base you can normally ignore the
self-ionization of water as a source
of OH-(aq).
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Solutions of Strong Acid
or Base
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).

– Substituting [OH-]=0.010 into the ion-product
expression, we get:
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Solutions of Strong Acid
or Base
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).

– Substituting [OH-]=0.010 into the ion-product
expression, we get:
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Solutions of Strong Acid
or Base

By dissolving substances in water,
you can alter the concentrations of
H+(aq) and OH-(aq).
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Solutions of Strong Acid
or Base

At 25°C, you observe the following
conditions.
– In an acidic solution, [H+]
– In a neutral solution, [H+]
– In a basic solution, [H+]
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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.
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The pH of a Solution

For a solution in which the
hydrogen-ion concentration is 1.0 x
10-3, the pH is:
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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,
• Similarly, a basic
• Figure 16.6 shows a diagram of the pH scale
and the pH values of some common solutions.
66
Figure 16.8: The pH
Scale
A Problem to Consider

A sample of orange juice has a hydrogen-ion
concentration of 2.9 x 10-4 M. What is the pH?
68
A Problem to Consider

The pH of human arterial blood is 7.40. What is
the hydrogen-ion concentration?
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The pH of a Solution

A measurement of the hydroxide
ion concentration, similar to pH, is
the pOH.
70
The pH of a Solution

A measurement of the hydroxide
ion concentration, similar to pH, is
the pOH.
71
What is the [H3O+] of a .050 M Sr(OH)2 solution?
Sr(OH)2 ↔ Sr2+ + 2OH.050 M
0
0
(strong base)
0
.050 M .100 M
[OH-] = .100 M
2H2O ↔ H3O+ + OH–
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Negligibility

Applies only to addition and
subtraction calculations

Variable very small
10-3, 10-4, 10-5 and
 Less than 5% of the number to
which it is added / subtracted

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pH and pOH
p = -log
pKw = -logKw
pH
pOH
74
A Problem to Consider

An ammonia solution has a
hydroxide-ion concentration of 1.9
x 10-3 M. What is the pH of the
solution?
You first calculate the pOH:
Then the pH is:
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If and acid solution has a [H3O+] of .05
M, what are its pH and pOH ?
pH = -log [H3O+]
=
=
Kw =
=
=
=
pOH =
pOH =
76
pH + pOH =
77
Calculate the pH and pOH of a household ammonia
solution that contains 2.5 mol of NH3 per liter of
solution. Assume 10% ionization.
NH3 + H2O ↔ NH4+ + OHInitial
Change
@equilib
78
The pH of a Solution

The pH of a solution can
accurately be measured using a
pH meter (see Figure 16.9).
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Net Ionic Equation

Real physical state of every component of the reaction
 Strong acids and strong bases in ionic form
 Soluble salts in ionic form
 Pure substances, oxides, gases and solids in
molecular form
CaCO3(s) + HCl ?
Molecular Equation:
Total
Ionic Equation:
Net Ionic Equation:
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Salts

Product of acid – base rxn.

Types
1.
Normal
81
2.
Acidic
3. Basic
82
Indicators


Organic dyes
Change color over pH range
Acidic
pH<7
Red
Yellow
Colorless
Red
Neutral
7
[Litmus]
[Bromothymol Blue]
[Phenolphthalein]
[Methyl Orange]
Basic
pH>7
Blue
Blue
Deep Pink
Yellow
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Animation: Acid Ionization
Equilibrium
(Click here to open QuickTime animation)
Return to Slide 7
84
Animation: Self-Ionization
of Water to Form H+ and
OH- in Equilibrium
(Click here to open QuickTime animation)
Return to Slide 38
85
Figure
16.9:
A digital
pH
meter.
Photo
courtesy of
American
Color.
Return to
Slide 60
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