Acids and Bases - Derry Area School District
Download
Report
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).
8
Theory of Ionization
Svente Arrhenius
1884
Water
Acid
Base
Neutralization
9
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
11
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,
12
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.
13
Consider…
NH3 (aq) H 2O(l )
NH 4 (aq) OH (aq)
What is the acid?
What is the base?
Production of NH4
14
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,
15
Brønsted-Lowry
Concept of Acids and
Bases
Consider the reaction of NH3 and H2O.
base
acid
H+
16
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+
17
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 )
18
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 )
19
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
20
Conjugate Acids and
Bases
H2O + NH3 NH4+ + OHacid
base
CAN donate H+
conjugate acid
CAN donate OHconjugate base
Conjugate Acid – Base Pairs
21
What’s the Deal With Water?
H2O + NH3 NH4+ + OHacid
base
H2O + HCl H3O+ + Clbase
acid
Amphoteric
Autoionization
Protolysis
22
Brønsted-Lowry
Concept of Acids and
Bases
– HCO3- acts as a proton donor (an acid) in the
presence of OH-
–H+
23
Brønsted-Lowry
Concept of Acids and
Bases
– HCO3 can act as a proton acceptor (a
base) in the presence of HF.
H+
24
Brønsted-Lowry
Concept of Acids and
Bases
In the Brønsted-Lowry concept:
25
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.
26
Lewis Theory
1923
Extended the acid–base theory
electron pair donor =
electron pair receiver =
27
4 ammonia molecules forming a
complex ion with cupric ion
28
Characteristics of
Lewis…
Acids
Bases
Lone e- pair
29
Neutralization = Coordinate
covalent
bond
Both shared e- donated by the
same atom / ion
30
Nesting Theories
Lewis
Bronsted - Lowry
Arrhenius
31
Strength in Water
Depends on degree of ionization
Ease of bond breakage
Stability of resulting ions
Higher the [H+] or [OH-]
32
And the Winner is…
Hydronium H3O+ is the
Hydroxide OH- is the
33
Water is a leveling solvent;.
HCl, HBr, HI
NaOH, Ca(OH)2
34
ACIDS &
H2SO4 v
H2SO2
BASES
NaOH
v S(OH)6
35
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
36
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.
38
Relative Strength of
Acids and Bases
The Brønsted-Lowry concept
introduced the idea of conjugate
acid-base pairs and protontransfer reactions.
39
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
41
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-
HX
42
Molecular Structure and
Acid Strength
Two factors are important in
determining the relative acid
strengths.
d+
d-
HX
43
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.
44
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.
45
Molecular Structure and
Acid Strength
Consider the oxyacids. An oxyacid
has the structure:
HOY
46
Molecular Structure and
Acid Strength
HOY
– You can predict the following order of acidic strength.
47
Molecular Structure and
Acid Strength
Consider the oxyacids. An oxyacid
has the structure:
HOY
48
Molecular Structure and
Acid Strength
Consider the oxyacids. An oxyacid
has the structure:
HOY
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:
51
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
52
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.
53
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.
54
Self-ionization of Water
These ions are produced in equal
numbers in pure water, so if we let x =
[H+] = [OH-]
55
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).
56
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:
57
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:
58
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).
59
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:
60
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:
61
Solutions of Strong Acid
or Base
By dissolving substances in water,
you can alter the concentrations of
H+(aq) and OH-(aq).
62
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+]
63
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.
64
The pH of a Solution
For a solution in which the
hydrogen-ion concentration is 1.0 x
10-3, the pH is:
65
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?
69
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–
72
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
73
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:
75
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).
79
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:
80
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
83
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
86