Definitions of acids and bases Bronsteds conjugate acidbase pairs concept Determination of H 3 O OH and pH for strong acids and strong bases Determination of H 3 O OH ID: 684297
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Slide1
Acids and Bases
Topics to be covered:
Definitions of acids and bases;
Bronsted’s conjugate acid-base pairs concept;
Determination of [H
3
O
+
], [OH
-
], and pH for strong acids and strong bases;
Determination of [H
3
O
+
], [OH
-
], pH,
K
a
, or
K
b
, and percent ionization for weak acids and weak bases;
Predicting acid/base properties of salts (ionic compounds) and oxides of metals and nonmetals;
The effect of molecular structures and bond energy on the strength of acids and bases.Slide2
Acids and Bases
Arrhenius definition:
Acid
: substance that increases [H
3
O
+
] in solution;
Base
: substance that increases [OH
-
] in solution;
Br
nsted-Lowry definition:
Acid
: reactant that donates proton in a reaction;
Base
: reactant that accepts proton in a reaction;
Lewis definition: -
in the formation of covalent bonds,
Acid
: one that accept electron-pairs
Base
: one that donate electron-pairsSlide3
Strong and Weak Acids
Strong acids ionize completely in aqueous solution
:
HCl
(
aq
)
+ H
2
O
H
3
O
+
(
aq
)
+ Cl
-
(
aq
)
;
H
2
SO
4
(
aq
)
+ H
2
O
H
3
O
+
(
aq
)
+ HSO
4
-
(
aq
)
;
Weak acids ionize only partially in aqueous solution:
HF
(
aq
)
+ H
2
O
⇌
H
3
O
+
(
aq
)
+ F
-
(
aq
)
;
HOCl
(
aq
)
+ H
2
O
⇌
H
3
O
+
(
aq
)
+ ClO
-
(
aq
)
.Slide4
Complete Ionization of Hydrochloric AcidSlide5
Dissociations of Strong and Weak Acids
Slide6
Strong and Weak Bases
Strong bases ionize completely in aqueous solution:
NaOH
(
aq
)
Na
+
(
aq
)
+ OH
-
(
aq
)
;
Ba(OH)
2
(
aq
)
Ba
2+
(
aq
)
+ 2 OH
-
(
aq
);
Weak bases ionize only partially in aqueous solution:
NH
3
(
aq
)
+ H
2
O
NH
4
+
(
aq
)
+ OH
-
(
aq
);
PO
4
3-
(
aq
) + H
2
O
HPO
4
2-
(
aq
)
+ OH
-
(
aq
)Slide7
Strong Acids and Bases
Some of the common strong acids and
bases.Slide8
Brønsted-Lowry: Conjugate Acids & Bases
Consider the following equilibrium:
HA + B
⇌
BH
+
+ A
-
;
Acid
1
Base
2
Conjugate
Conjugate
acid
2
base
1
A
-
is the conjugate base to acid HA;
HA & A
-
are conjugate acid-base pair;
BH
+
is the conjugate acid to base B;
BH
+
& B are also conjugate acid-base pair.Slide9
Brønsted
-Lowry Acid
An acid
HX
donates
proton to
water producing hydronium ion, H
3
O
+
, and the conjugate base X
-
.Slide10
Brønsted
-Lowry Base
A base
NH
3
takes a proton from water to produce hydroxide ion, OH
-
, and the conjugate acid
NH
3
+
.
Slide11
Exercise #1: Conjugate Acids & Bases
Write the formulas of the conjugate bases for the following acids:
(a) H
2
CO
3
(
b) HSO
4
-
(
c) Al(H
2
O)
6
3+
(
d) Cr(OH)
3
(H
2
O)
3
(
Answer: (a) HCO
3
-; (b) SO
4
2-
; (c) Al(H
2
O)
5
(OH)
2+
;
(
d) Cr(OH)
4
(H
2
O)
2
-
)Slide12
Exercise #2: Conjugate Acids and Bases
Write the formulas of the conjugate acids for the following bases:
(a)
CH
3
NH
2
(
b) CO
3
2-
(
c) C
5
H
5
N
(
d)
Al(OH)
3
(H
2
O)
3
(
Answer: (a) NH
4
+
; (b) HCO
3
-
; (c) C
5
H
5
NH
+
;
(
d) Al(OH)
2
(H
2
O)
4
+
)Slide13
Selected Conjugate
Acid-Base Pairs
Acids: strongest on the top, weakest at the bottom;
Conjugate bases; weakest (top), strongest (bottom)
HNO
3
– NO
3
-
H
2
SO
4
– HSO
4
-
H
3
O
+
– H
2
O
HF – F
-
H
3
PO
4
– H
2
PO
4
-
CH
3
COOH – CH
3
COO
-
H
2
PO
4
-
– HPO
4
2-
NH
4
+
– NH
3
H
2
O – OH
-
Slide14
Relative Strength of Acids
and
Their Conjugate Bases
Acids
Conjugate Bases
Very Strong
Very Weak
Strong
Weak
Weak
Strong
Very
Weak
Very
Strong
__________________________________________
Strong
acids lose protons very readily
weak conjugate bases;
Weak acids do not lose protons very readily
strong conjugate bases.Slide15
Relative Strength of Acids and Their Conjugate BasesSlide16
Relative Strength of Acids and their Conjugate Bases
This diagram shows the relative strengths of conjugate acid-base pairs, as indicated by their
ionization constants
in aqueous solution.Slide17
Acid Strength and Ionization Constants
For the ionization or dissociation equilibrium of an acid in aqueous solution, such as:
HA
(
aq
)
+ H
2
O
H
3
O
+
(
aq
)
+
A
-
(
aq
)
;
The
ionization or dissociation constant is expressed as follows:
K
a
=
The
values of
K
a
indicate the relative strength of the acids. Strong acids have very large
K
a
, while weak acids have small Ka’s (Ka << 1)
Slide18Slide19
Acid-Base Properties of Water
Auto-ionization of water:
2H
2
O
H
3
O
+
(
aq
)
+ OH
-
(
aq
)
K
w
= [
H
3
O
+
][OH
-
] = 1.0 x 10
-14
at 25
o
C
Water ionizes to produce both
H
3
O
+
and OH
-
, thus it has both acid and base properties. Kw is called water ionization constant. Pure water at 25oC: [H3O+] = [OH-] = 1.0 x 10-7 M Slide20
Expressing Acidity in pH Scale
pH = -log[H
+
] (
note
: [H
+
] = [H
3
O
+
])
pOH = -log[OH
-
]
p
K
w
= -log(
K
w
); p
K
a
= -log(
K
a
); p
K
b
= -log(
K
b
)
For water,
K
w
= [H
3
O+][OH-] = 1.0 x 10-14 -log(Kw) = -log [H3O+] + (-log[OH-]) pKw = pH + pOH = 14.00At 25oC, pOH = 14 – pHSlide21
Acidity and pH Range
Acidic solutions:
[H
3
O
+
] > 1.0 x 10
-7
M
;
pH < 7;
Basic solutions:
[OH
-
] > 1.0 x 10
-7
M
or [H
3
O
+
] < 1.0 x 10
-7
M
pH > 7;
Neutral solutions:
[H
3
O
+
] = [OH
-
] = 1.0 x 10
-7
M
; pH = 7.00Slide22
Relationship
Between [H
3
O
+
]
and pHSlide23
pH of Common Household ItemsSlide24
Household SubstancesSlide25
[H
3
O
+
] and pH of Strong Acids
Strong acids like
HCl
and
HNO
3
ionize completely in aqueous solution:
HCl
(
aq
)
+ H
2
O
H
3
O
+
(
aq
)
+ Cl
-
(
aq
)
;
HNO
3
(
aq) + H2O H3O+(aq) + ClO4-(aq);In solutions of strong monoprotic acids HA, such as HCl and HNO3
, H3O
+
concentration = initial acid concentration,
[H
3
O
+
] = [HA]
0
For example, in 0.10 M
HCl
, [H
3
O
+
] = 0.10 M, and pH = -log(0.10) = 1.00Slide26
[OH
-
] and pH of Strong Bases
S
trong
bases also ionize
completely:
Examples:
NaOH
(
aq
)
Na
+
(
aq
)
+ OH
-
(
aq
)
;
Ba(OH)
2
(
aq
)
Ba
2+
(
aq
)
+ 2 OH
-
(aq);In base solution such as NaOH, [OH-] = [NaOH]0 For example, in 0.10 M NaOH solution, [OH
-] = 0.10 M
;
In
base
solution such as
Ba(OH)
2
,
[OH
-
] = 2 x [Ba(OH)
2
]
0
For example, in 0.10
M
Ba(OH)
2
solution, [OH
-
] = 0.20
M
;Slide27
[H
3
O
+
] and pH of Weak Acids
In weak acid solutions, [H
3
O
+
] < [HA
]
0
;
[H
3
O
+
] and pH can be calculated from the initial concentration of the acid and its
K
a
value.
For example, in
0.10
M
acetic acid,
CH
3
CO
2
H (
K
a
= 1.8 x
10
-5
)
[H
3
O+] and pH can be calculated using the following “ICE” table. (next slide)Slide28
ICE Table for Acetic Acid
Ionization: CH
3
CO
2
H + H
2
O
H
3
O
+
+ CH
3
CO
2
-
[
I
nitial] 0.10
M
0.00 0.00
[
C
hange] -
x
+
x
+
x
[
E
quil’m
]
(0.10 – x) x x Ka =
=
= 1.8 x 10
-5
By approximation, x
=
= 0.0013
M
;
x
= [H
3
O
+
] = 0.0013
M
; pH = -log(0.0013) =
2.89
Slide29
Calculating [H
3
O
+
] from initial concentration and
K
a
using
approximation method
.
If
K
a
<< [HX]
0
, [H
3
O
+
] can be calculated using approximation method, by assuming that, since
K
a
<<
[HX]
0
, then
x
<< [HX]
0
and ([HX]
0
–
x
)
[HX]
0
.
For example,
=
= 1.8 x 10-5
Since
K
a
(1.8 x 10
-5
)
<< 0.10,
we approximate that
(0.10 –
x
)
0.10;
Then,
= 1.8 x 10
-5
Which leads to:
x
= 1.8 x 10-5)} = 0.0013 M = [H3O+] Slide30
Calculation Percent Ionization
We calculated that, in 0.10
M
CH
3
COOH, the concentration of acid that ionizes is 1.34 x 10
-3
M
.
The percent ionization of 0.10
M
acetic acid:Slide31
Increase in Percent Ionization with Dilution
Percent ionization of weak acid increases as the dilution increases.
For example, if [
CH
3
COOH]
0
= 0.0100
M
[CH
3
COOH]
ionized
4.24 x 10
-4
M
(
calculated using approximation method);
Slide32
Percent Ionization Increases with Dilution
At [CH
3
COOH]
initial
=
0.0010
M
,
[CH
3
COOH]
ionized
=
1.3
x 10
-4
M
(by approximation)
Percent ionization =
x 100 = 13%
The more we dilute the acid solution, the higher the fraction of the acid that will ionize, which increases the degree of ionization.
Is this related to the Le
Chatelier’s
principle? Explain.
Slide33
Exercise #3:
pH and Percent Ionization
Nitrous acid, HNO
2
, has
K
a
= 4.0 x 10
-4
at 25
o
C.
Calculate the pH and percent ionization of HNO
2
in 0.10
M
solution of the acid
.
Chlorous
acid,
HOCl
, has
K
a
= 3.5 x 10
-8
at 25
o
C.
Calculate the pH and percent ionization of
HOCl
in 0.10
M
solution of the acid.
(
Answer:
(1)
pH = 2.20; % ionization = 6.3
%; (2) pH = 4.23; % ionization = 0.059%)Slide34
[OH
-
] and pH of a Weak Base
In a weak base, [OH
-
] < [Base]
0
;
[OH
-
] and pH can be calculated from the initial concentration of the base and its
K
b
value.
For example, in 0.100
M
ammonia, NH
3
, with
K
b
= 1.8 x 10
-5
, [OH
-
] and pH can be calculated using the following “ICE” table.Slide35
ICE Table for the Ionization of Ammonia
Ionization: NH
3
+ H
2
O
NH
4
+
+ OH
-
I
nitial [M]
0.10
0.00 0.00
C
hange [M] -
x
+
x
+
x
E
quilm
. [M] (
0.10
–
x
)
x x
K
b
=
=
= 1.8 x 10
-5
By approximation,
x
=
= 0.0013
M
;
x
= [OH
-
] = 0.0013
M
; pOH = 2.89, and
pH = 11.11
Slide36
Exercise #4: pH of Strong &
Weak Bases
Calculate [OH
-
] and [H
3
O
+
]
in 0.10
M
solution of:
Ethylamine
,
C
2
H
5
NH
2
(
K
b
= 5.6 x
10
-4
);
Pyridine,
C
5
H
5
N (
K
b
= 1.7 x 10-9)What is the pH of each solution?(Answer: [OH-] = 7.5 x 10-3 M; [H3O+] = 1.3 x 10-12 M; pH = 11.89)(Answer: [OH-] = 1.3 x 10-5 M; [H3O+] = 7.7 x 10-10 M; pH = 9.11)Slide37
Polyprotic Acids
Acids with more than one
ionizable
H
+
,
such as
H
2
SO
4
, H
3
PO
4
, H
2
C
2
O
4
,
H
3
C
6
H
5
O
7
, etc.
They ionize
in stages, for
example:
1) H
3
PO
4
(aq) + H2O ⇄ H3O+(aq) + H2PO4-(aq); Ka1 = 7.5 x 10-32) H2PO4-
(aq)
+
H
2
O
⇄
H
3
O
+
(
aq
)
+
HPO
4
2-
(
aq
)
;
Ka2 = 6.2 x 10-83) HPO42-(aq) + H2O ⇄ H3O+(aq) + PO43-(aq); Ka3 = 4.8 x 10
-13
Acid strength decreases in the order:
H
3
PO
4 >> H2PO4- >> HPO42-; pH of solution is determined mainly by ionization of H
3
PO
4
Slide38
Exercise #5:
Concentrations of Species
and pH
of
H
2
SO
4
solution
Calculate the concentrations of H
2
SO
4
, H
3
O
+
, HSO
4
-
, and SO
4
2-
, in 0.10
M
H
2
SO
4
solution. What is the pH of the solution? (H
2
SO
4
is a strong acid and HSO
4
-
has
K
a
= 1.2 x 10-2)(Answer: [H2SO4] = 0.0 M; [H3O+] = 0.11 M; [HSO4-] = 0.090 M; [SO42-] = 0.0098 M; pH = 0.96]Slide39
Salts and Their Corresponding Acids & Bases
What acid
and
base will form
each of the following salts
(ionic compounds)?
Acids
Bases
NaCl
HCl
NaOH
KNO
3
(NH
4
)
2
SO
4
NaC
2
H
3
O
2
BaCl
2
Na
3
PO
4Slide40
Acid-Base Properties of Salt Solutions
Soluble salts dissociate completely when dissolved in
water, producing cations and anions;
Ions produced
by salts may
react with water
to produce
H
3
O
+
, which would make
the solution
acidic
, or
produce
OH
-
, which would make
the solution
basic
.
pH of solution depends on which ion dominates in solution, and on relative values of
K
a
and
K
b
of ions.Slide41
Acid-Base Properties of Salt Solutions
The acid-base property of a salt solution depends on whether the compound is a product of:
Strong acid-strong base reaction
: produces
neutral
salt
Strong acid-weak base reaction
: produces
acidic
salt
Strong base-weak acid reaction
: produces
basic
salt
Weak acid-weak base reaction
: produces salt that is
either
acidic
or
basic
, depending on the relative strength of the acid and the base.Slide42
Types of Salts and Their Solutions
Salts of Strong
Acids-Strong Bases
:
NaCl
, NaNO
3
,
KBr
, etc.; solutions are neutral
Salts of Weak
Acids-Strong Bases
:
NaF
, NaNO
2
, NaC
2
H
3
O
2
, etc.; solutions are basic
Salts of Strong
Acids-Weak Bases
:
NH
4
Cl, NH
4
NO
3
, (CH
3
)
2
NH
2Cl, C5H5NHCl; Solutions of these salts are acidicSalts of Weak Acids-Weak Bases: NH4C2H3O2, NH4CN, NH4NO2, etc.. These compounds can be acidic, basic, or neutral, which depends on the relative strength of the acid and the base.Slide43
Salt of
Strong Acid-Strong Base
Reaction
Dissociation and reaction of a neutral salt:
NaCl
(
aq
)
Na
+
(
aq
)
+ Cl
-
(
aq
)
;
Na
+
(
aq
)
+ H
2
O
“NR”;
(no reaction)
Cl
-
(
aq) + H2O “NR”(Neither Na+ nor Cl- will react with water to produce H+ or OH-, respectively. Concentrations of H
3O
+
and OH
-
in
solution
are the same as in pure
water; therefore,
solution is neutral.)Slide44
Salt
of Weak Acid-Strong Base
Reaction
:
Dissociation and reaction of a basic salt:
NaNO
2
(
aq
)
Na
+
(
aq
)
+ NO
2
-
(
aq
)
;
Na
+
(
aq
)
+ H
2
O
“NR”
NO
2
-
(
aq) + H2O HNO2(aq) + OH-(aq) (NO2- is a stronger conjugate base than H2O; it takes proton from water to produce HNO2 and OH-
, which causes solution to have [OH-
] > [H
3
O
+
]; solution
basic
)Slide45
Salt
of Strong Acid-Weak Base
Reaction
Dissociation and reaction of an acidic salt:
NH
4
NO
3
(
aq
)
NH
4
+
(
aq
)
+ NO
3
-
(
aq
)
;
NH
4
+
(
aq
)
+
H
2
O
NH
3(aq) + H3O+(aq); NO3-(aq) + H2O “NR”;(NH4+
is a stronger conjugate acid than water; it donates proton to H2
O producing H
3
O
+
and NH
3
, which causes
[H
3
O
+
] > [OH
-
], and
solution
i
s
acidic
)Slide46
Salts of Weak Acid-Weak Base Reactions
A salt
produced by
reaction
of
a weak acid
and
a weak base
can be
neutral, acidic, or
basic
; it depends
on the relative
magnitude
of
the
K
a
of
cation
and the K
b
of
anion from the salt.
If
K
a
~
K
b
,
salt is
neutral
;
example
: NH
4
C
2
H3O2If Ka > Kb, salt is acidic; example: NH4NO2If Ka < Kb, salt is basic; example
: (NH4
)
2
CO
3
Slide47
Predicting Acid-Base Property of Salts
Consider a solution containing
NH
4
CH
3
CO
2
NH
4
CH
3
CO
2
(
aq
)
NH
4
+
(
aq
)
+
CH
3
C
O
2
-
(
aq
)
;
NH
4
+(aq) + H2O H3O+(aq) + NH3(aq); Ka = 5.6 x 10-10
CH3
C
O
2
-
(
aq
)
+ H
2
O
CH
3
CO
2
H
(
aq
)
+ OH
-(aq); Kb = 5.6 x 10-10Ka (NH4+) = Kb (CH3CO2-) = 5.6 x
10-10
NH
4
C2H3O2 is neutralSlide48
Predicting Acid-Base Property of Salts
Consider a solution containing
NH
4
NO
2
NH
4
NO
2
(
aq
)
NH
4
+
(
aq
)
+
NO
2
-
(
aq
)
;
NH
4
+
(
aq
)
+ H
2
O
H3O+(aq) + NH3(aq); Ka = 5.6 x 10-10NO2-(
aq)
+ H
2
O
HNO
2
(
aq
)
+ OH
-
(
aq
)
;
K
b
=
2.5
x
10-11Ka (NH4+) > Kb (NO2-) NH4NO2 is acidic;Slide49
Predicting Acid-Base Characteristics of Salts
Now consider a solution containing NH
4
CN.
NH
4
CN
(
aq
)
NH
4
+
(
aq
)
+ CN
-
(
aq
)
;
NH
4
+
(
aq
)
+
H
2
O
H
3
O
+(aq) + NH3(aq); Ka = 5.6 x 10-10CN-(aq) + H
2O
HCN
(
aq
)
+ OH-
(
aq
)
;
K
b
= 1.6 x 10
-5
K
b
(CN
-
)
>
K
a
(NH4+) NH4CN is basicSlide50
Effect of Structure on Acid-Base Properties
Relative bond strength:
H─F >
H─Cl
>
H─Br
> H─I
Relative acid strength:
HI >
HBr
>
HCl
> HF
(only HF
is
classified as a
weak acid in this group)
Relative acid strength:
H
2
Te > H
2
Se > H
2
S > H
2
O (all very weak acids)Slide51
Trends of
Acid Strength in the Periodic Table
As you move from left to right and down the periodic table, the acid strength increases. As you
move from
right to left and up, the base strength increases.Slide52
Effect of Structure on Acid-Base Properties
Electronegativity Effect on Relative Strength of Oxo-Acids:
Same central atom, but different numbers of oxygen
atoms (more O-atoms, stronger acidic):
HClO
4
>
HClO
3
>
HClO
2
>
HClO
(or
HOCl
)
Same number of
oxygen atoms,
but different central
atoms (more electronegative, stronger acid):
HClO
2
>
HBrO
2
>
HIO
2
; (
En
: Cl > Br > I)
CF
3CO2H > CCl3CO2H > CH3CO2H; (En: F > Cl > H)HClO4 > H2SO4 >
H3PO
4
; (
En
: Cl > S > P)Slide53
O
H Bond and Strength of Oxo-Acids
The relative strength of
oxo
-acids, such as HNO
2
and HClO
2
, depends on the strength of O
H bond (bond
b
), which is
influenced by the electronegativity of atom ”E” and other O-atoms attached to it.
M
ore electronegative atom ”E” (and more O-atoms attached to it) leads to weaker
O
H bond and a stronger acid. Stronger
O
H bond produces weaker
acid.
Slide54
Acid-Base Properties of Oxides
Metal oxides are basic or amphoteric
Basic oxides:
Na
2
O
(s)
+ H
2
O
2NaOH
(
aq
)
2Na
+
(
aq
)
+ 2OH
-
(
aq
)
;
MgO
(
aq
)
+
HCl
(
aq
)
MgCl2(aq) + H2OAmphoteric oxides: Al2O3(s) + HCl(aq) 2AlCl3(aq) + H2O Al
2O3
(s)
+ 2NaOH
(
aq
)
+ 3H
2
O
2NaAl(OH)
4
(
aq
)
Slide55
Acid-Base Properties of Oxides
Nonmetal oxides are
acidic:
N
2
O
5
+ H
2
O
2HNO
3
(
aq
)
;
SO
3
+ H
2
O
H
2
SO
4
(
aq
)
;
Cl
2
O
7
+ H2O 2HClO4(aq)Trends: Acidity increases left to right across a period SiO2 < P4O10 < SO3 < Cl2O7 Acidity decreases top-to-bottom down the group
N2O
5
> P
4
O
10
> As
2
O
5
> Sb
2
O
5Slide56
Acid-Base Properties of Oxides
Trend of acid-base properties of oxides in periodic table: from the most basic (on the left) to the most acidic (on the right):
Na
2
O, MgO, Al
2
O
3
, SiO
2
, P
4
O
10
, SO
3
, Cl
2
O
7
Trend of acid-base properties going down a group: most acidic on the left to the most basic on the right:
N
2
O
5
, P
4
O
10
, As
2
O
5
, Sb
2
O
5Slide57
Lewis Acids and Bases
Identify the Lewis acids and Lewis bases in the following reactions:
(a) H
2
O +
CO
2
(
g
)
H
2
CO
3
(
aq
)
;
(b) Cu
2
+
(
aq
)
+
4NH
3
(
aq
)
Cu(NH
3)42+(aq); (c) AlCl3(s) + Cl-(aq) AlCl4-(aq)
(d) BF
3
+ NH
3
F
3
B─NH
3Slide58