AcidBase chemistry amp pH Recognizing acidbase and conjugate baseacid Calculation of pH pOH H 3 O OH Calculating pH for solutions of strong acidsbase Ionization constant ID: 569960
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Slide1
Basic concepts: Acid-Base chemistry & pH
Recognizing acid/base and conjugate base/acid
Calculation of pH,
pOH
, [H
3
O
+
], [OH
-
]
Calculating pH for solutions of strong acids/base
Ionization constant:
K
a
, K
b
Polyprotic
acid (and associated
K
a
values)
pK
a
,
pK
b
Acid-Base properties of salts
Predicting direction of acid-base reaction
Types of acid-base reactions
Calculations with equilibrium constantsSlide2
© 2009, Prentice-Hall, Inc.
Defining Acids & Bases
Arrhenius
An acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions.
A base is a substance that, when dissolved in water, increases the concentration
of
hydroxide ions.Slide3
© 2009, Prentice-Hall, Inc.Some Definitions
Brønsted
-Lowry
An acid is a proton donor.
A base is a proton acceptor
.
A
Brønsted
-Lowry acid…
…must have a removable (acidic) proton.
A
Brønsted
-Lowry base…
…must have a pair of nonbonding electronsSlide4
© 2009, Prentice-Hall, Inc.
If it can be either…
…it is
amphiprotic
.
HCO
3
-
HSO
4
-
H
2
OSlide5
AcidsAcids can be:
monoprotic
: example HC
3
H
3
O
2
Diprotic: example H
2
SO
4
Polyprotic
: example H
3
PO
4
……We’ll talk more about these later.Slide6
© 2009, Prentice-Hall, Inc.
What Happens When an Acid Dissolves in Water?
Water acts as a Brønsted-Lowry base and abstracts a proton (H
+
) from the acid.
As a result, the
conjugate base
of the acid and a
hydronium ion
are formed.Slide7
© 2009, Prentice-Hall, Inc.Conjugate Acids and Bases
The term
conjugate
comes from the Latin word “conjugare,” meaning “to join together.”
Reactions between acids and bases always yield their conjugate bases and acids.Slide8
Strong acids and bases almost completely ionize in water (~100%):
K
strong
>> 1
(product favored)
Weak acids and bases ionize in water to only a small extent (<<100%):
K
weak
<< 1
(Reactant favored)
Equilibrium Constants for Acids & BasesSlide9
The relative strength of an acid or base can also be expressed quantitatively with an equilibrium constant, often called an
ionization constant
. For the general acid HA, we can write:
Conjugate
acid
Conjugate
base
Equilibrium Constants for Acids & BasesSlide10
© 2009, Prentice-Hall, Inc.Dissociation Constants
The greater the value of
K
a
, the stronger is the acid.Slide11
The relative strength of an acid or base can also be expressed quantitatively with an equilibrium constant, often called an
ionization constant
. For the general base B, we can write:
Conjugate
base
Conjugate
Acid
Equilibrium Constants for Acids & BasesSlide12
© 2009, Prentice-Hall, Inc.Weak Bases
Bases react with water to produce hydroxide ion.Slide13
© 2009, Prentice-Hall, Inc.Weak Bases
K
b
can be used to find [OH
-
] and, through it, pH.Slide14
© 2009, Prentice-Hall, Inc.Acid and Base Strength
Strong acids are completely dissociated in water.
Their conjugate bases are quite weak.
Weak acids only dissociate partially in water.
Their conjugate bases are weak bases.Slide15
© 2009, Prentice-Hall, Inc.K
a
and
K
b
K
a
and
K
b
are related in this way:
K
a
K
b
=
K
wTherefore, if you know one of them, you can calculate the other.Slide16
Acids
Conjugate
Bases
Increase strength
Increase strength
Ionization Constants for Acids/BasesSlide17
© 2009, Prentice-Hall, Inc.Polyprotic Acids…
…have more than one acidic proton
If the difference between the
K
a
for the first dissociation and subsequent
K
a
values is 10
3
or more, the pH generally depends
only
on the first dissociation.Slide18
Logarithmic Scale of Relative Acid Strength,
p
K
a
Many chemists use a logarithmic scale to report and compare relative acid strengths.
pK
a
=
log(K
a
)
The lower the
pK
a
, the stronger the acid.
Acid
HCO
3
HClO
HF
pK
a
10.32
7.46
3.14
Equilibrium Constants for Acids & BasesSlide19
© 2009, Prentice-Hall, Inc.Calculating
K
a
from the pH
The pH of a 0.10
M
solution of formic acid, HCOOH, at 25
C is 2.38. Calculate
K
a
for formic acid at this temperature.
We know that
[H
3
O
+
] [COO
-
]
[HCOOH]
K
a
=Slide20
© 2009, Prentice-Hall, Inc.Calculating
K
a
from the pH
The pH of a 0.10
M
solution of formic acid, HCOOH, at 25
C is 2.38. Calculate
K
a
for formic acid at this temperature.
To calculate
K
a
, we need the equilibrium concentrations of all three things.
We can find [H
3
O
+
], which is the same as [HCOO
-
], from the pH.Slide21
© 2009, Prentice-Hall, Inc.Calculating
K
a
from pH
Now we can set up a table…
[HCOOH],
M
[H
3
O
+
],
M
[HCOO
-
],
M
Initially
0.10
0
0
Change
- 4.2
10
-3
+ 4.2
10
-3
+ 4.2
10
-3
At Equilibrium
0.10 - 4.2
10
-3
= 0.0958 = 0.10
4.2
10
-3
4.2
10
-3Slide22
© 2009, Prentice-Hall, Inc.Calculating
K
a
from pH
[4.2
10
-3
] [4.2
10
-3
]
[0.10]
K
a
=
= 1.8
10
-4Slide23
© 2009, Prentice-Hall, Inc.Calculating Percent Ionization
Percent Ionization =
100
In this example
[H
3
O
+
]
eq
= 4.2
10
-3
M
[HCOOH]
initial
= 0.10 M
[H
3
O
+
]
eq
[HA]
initial
Percent Ionization =
100
4.2
10
-3
0.10
= 4.2%Slide24
Calculating Ka
from
experimental
data such as pH:
The pH of a 0.100M solution of
propanioic
acid (CH
3
CH
2
CO
2
H) is 2.94. What is the value of Ka for
propanoic
acid?
Write the ionization equation for this weak acid
Determine the equilibrium expression
Based on pH, determine the [H
3
O
+
]
Develop an ICE chart.Slide25
Use the equilibrium constant to determine the pH of a solution containing a weak acid or base.
What is the pH of 0.050M CH
3
COOH? Ka = 1.8x10-5
Establish equation for ionization of CH3COOH
Set up an ICE chart
Determine equilibrium concentrations of [H3O+]
Calculation pHSlide26
© 2009, Prentice-Hall, Inc.pH of Basic Solutions
What is the pH of a 0.15
M
solution of NH
3
?
[NH
4
+
] [OH
-
]
[NH
3
]
K
b
=
= 1.8
10
-5
NH
3
(
aq
)
+ H
2
O
(
l
)
NH
4
+
(
aq
)
+ OH
-
(
aq
)Slide27
© 2009, Prentice-Hall, Inc.pH of Basic Solutions
Tabulate the data.
[NH
3
],
M
[NH
4
+
],
M
[OH
-
],
M
Initially
0.15
0
0
At Equilibrium
0.15 - x
0.15
x
xSlide28
© 2009, Prentice-Hall, Inc.pH of Basic Solutions
(1.8
10
-5
) (0.15) = x
2
2.7 10
-6
= x
2
1.6 10
-3
= x
2
(x)
2
(0.15)
1.8
10
-5
=Slide29
© 2009, Prentice-Hall, Inc.pH of Basic Solutions
Therefore,
[OH
-
] = 1.6
10
-3
M
pOH = -log (1.6 10
-3
)
pOH
= 2.80
pH = 14.00 - 2.80
pH
= 11.20Slide30
© 2009, Prentice-Hall, Inc.Effect of Cations and Anions
An anion that is the conjugate base of a strong acid will not affect the pH.
An anion that is the conjugate base of a weak acid will increase the pH.
A cation that is the conjugate acid of a weak base will decrease the pH.Slide31
© 2009, Prentice-Hall, Inc.Effect of Cations and Anions
Cations of the strong Arrhenius bases will not affect the pH.
Other metal ions will cause a decrease in pH.
When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the
K
a
and
K
b
values.Slide32
Anions that are conjugate bases of strong acids (for examples,
Cl
or NO
3
.
These species are such weak bases that they have no effect on solution
pH.
Acid–Base Properties of SaltsSlide33
Anions such as
CO
3
2
that are the conjugate bases of weak acids will raise the pH of a solution
.
Hydroxide ions are produced via
“
Hydrolysis
”
.
Acid–Base Properties of SaltsSlide34
Alkali metal and alkaline earth
cations
have no measurable effect on solution
pH.
Since these
cations
are conjugate acids of strong bases, hydrolysis does not occur.
Acid–Base Properties of SaltsSlide35
Basic
cations
are conjugate bases of acidic
cations
such as [Al(H
2
O)
6
]
3+
.
Acidic
cations
fall into two categories: (a) metal
cations
with 2
+
and 3
+
charges and (b) ammonium ions (and their organic derivatives).
All metal
cations
are hydrated in water, forming ions such as [M(H
2
O)
6
]
n+
.
Acid–Base Properties of SaltsSlide36
Acid – Base Properties of Ions in a water Solution
Decide effect of the
cation
on
pH.
Decide effect of the anion on
pH.
Combine the effect of the two
Ka>Kb =acidic, Kb>Ka = basic)
Spectator
Basic
Acidic
Anion
Cl
-
NO
3
-
Br
-
ClO
4
-
I
-
(anion of strong acids)
C
2
H
3
O
2
-
F
-
CO
3
2-
PO
4
3-
(CB of weak acid)
Cation
Li
+
Ca
+
Na
+
Sr
2+
K
+
Ba
2+
(
cation
of strong base)
NH
4
+
Mg
2+
Al
3+
Transition metal ions
(CA of weak base)Slide37
Salt
pH of (
aq
) solution
CaCl
2
NH
4
Br
NH
4
F
KNO
3
KHCO
3
Acid–Base Properties of Salts: PracticeSlide38
Acid–Base Properties of Salts: Practice
Salt
pH of (
aq
) solution
CaCl
2
Neutral
NH
4
Br
Acidic
NH
4
F
Basic
KNO
3
Neutral
KHCO
3
BasicSlide39
3.
Acid-base properties of salts
of a weak acid or weak base:
Identify if the following ions contribute toward an aqueous solution being acid, basic or neutral. If acidic or basic, write a net ionic equation to explain the behavior:
NO
3
-
2. PO
4
3-
3. HCO
3
-Slide40
pH calculation of a salt solution
What is the pH of a 0.10M solution of NH
4
Cl?
The K
b
of NH
3
= 1.8x10
5-
Identify equilibrium equation.
Identify dissociation constant
Develop an ICE chart
Complete calculations
Determine pH from [H
3
O+]Slide41
According to the
Brønsted
–Lowry theory, all acid–base reactions can be written as
equilibria
involving the acid and base and their conjugates.
All proton transfer reactions proceed from the stronger acid and base to the weaker acid and base.
Predicting the Direction of Acid–Base ReactionsSlide42
When a weak acid is in solution, the products are a stronger conjugate acid and base. Therefore equilibrium lies to the left.
All proton transfer reactions proceed from the stronger acid and base to the weaker acid and base.
Predicting the Direction of Acid–Base ReactionsSlide43
Strong acid
(
HCl
) +
Strong base
(
NaOH
)
Net ionic equation
Mixing equal molar quantities of a strong acid and strong base produces a neutral solution.
Types Acids–Base ReactionsSlide44
Weak acid
(HCN) +
Strong base
(
NaOH
)
Mixing equal amounts (moles) of a
strong base
and a
weak acid
produces a salt whose anion is the conjugate base of the weak acid. The solution is basic, with the pH depending on
K
b
for the anion.
Types Acids–Base ReactionsSlide45
Weak acid: Strong BaseCalculate the pH of solution prepared by mixing 100mL of O.1M HC
2
H
3
O
2
+ 0.1M
NaOH
.
(step one)
HC
2
H
3
O
2
+
NaOH
Na+(aq
) +
C
2
H3O2
- (aq)+ H2O(l)(step two) C2H
3O2- (aq)+ H2O(l) ↔ HC
2
H
3
O
2
(
aq
)
+
OH
-
(
aq
)
Slide46
Strong acid
(
HCl
) +
Weak base
(NH
3
)
Mixing equal amounts (
moles
) of a
weak base
and a
strong acid
produces a conjugate acid of the weak base. The solution is basic, with the pH depending on
K
a
for the acid.
Types Acids–Base ReactionsSlide47
Weak acid
(CH
3
CO
2
H) +
Weak base
(NH
3
)
Mixing equal amounts (moles) of a
weak acid
and a
weak base
produces a salt 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 relative K
a
and K
b
values.
Types Acids–Base ReactionsSlide48
pH Stoichiometry Problem:Calculate the pH for the following reaction:
50 ml of 0.10M Nitric acid is combined with 50 ml of 0.15M Barium Hydroxide.
50mL of 1.25M acetic acid is mixed with 50 ml of 2.0M sodium hydroxide.Slide49
© 2009, Prentice-Hall, Inc.Lewis Acids
Lewis acids are defined as electron-pair acceptors.
Atoms with an empty valence orbital can be Lewis acids
.
Lewis structure-central atom is missing a pair of electrons
Example: metal
cations
, non-metal oxides-CO
2Slide50
© 2009, Prentice-Hall, Inc.Lewis Bases
Lewis bases are defined as electron-pair donors.
Anything that could be a
Brønsted
-Lowry base is a Lewis base.
Lewis bases can interact with things other than protons, however
.
Lewis dot structure exhibits lone pair electrons on central atom. Example: NH
3
, H
2
OSlide51
Lewis Acid/BaseGeneral equation for a Lewis acid-base reaction:
A
(acid)
+ B
:
(base)
B
A
(adduct)
The A-B adduct is called a
coordinate covalent bond
.
ex: formation of the hydronium ion (H
3
O
+
),
H
+
= Lewis Acid
H2O = Lewis Base
H3O+ = AdductSlide52
p. 794Slide53
Properties of Acid-Base behaviorTrends for Binary Acids (H-X)
Atomic radius
Electronegativity
Both factors impact bond strength
(remember strong acids give up H+ so acid strength increases when bond strength decreases)
Trends for Oxy-Acids (H-O-X)
Electronegativity
(Increase X-O bond increases acid strength)
Number of oxygen atoms (resonance)Slide54
p. 792Slide55
Fig. 17-11, p. 792
Unique adduct compounds involving transition metals.
Generally characterized by their colors.Slide56
Example: Coordinate complexesUnique adduct compounds involving transition metals.
Generally characterized by their colors.