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1111Acids andAcids and
basesbases
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OH H
+ OH H H O
OH H
H
++
11.1 The Brnsted11.1 The BrnstedLowryLowrydefinition of acids and basesdefinition of acids and bases
An acid is a proton donor
A base is a proton acceptor
An acid-base reaction involves thetransfer of a single proton from one
species to another
A BrnstedLowry acid will donate aproton to a BrnstedLowry base
acid base
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11.1 The Brnsted11.1 The BrnstedLowryLowrydefinition of acids and basesdefinition of acids and bases
For a proton to be measurably acidic, it must
be bound to another atom via an appreciably
acidic bond
Acids tend to contain protons bound to group16 or 17 elements
Basic species require the presence of one or
more lone pairs
Not all species containing lone pairs act as
bases
Bases usually contain group 15 or 16 elements,
the atoms of which are often deprotonated
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Conjugate acid-base pairs For BrnstedLowry acid-base reactions
both the forward and reverse reactions are
acid-base reactions There are always 2 sets of species oneither side of the equation that differ byonly one proton
These are conjugate acid-base pairs
Cl 2 3 Cl
11.1 The Brnsted11.1 The BrnstedLowryLowrydefinition of acids and basesdefinition of acids and bases
acid acid basebase
conjugate pair
conjugate pair
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11.2 Acid11.2 Acid--base reactionsbase reactions
in waterin water The autoprotolysis of water
2 (l) 2 (l) 3 (aq)(aq)
3
is called the hydronium ion is called the hydroxide ion
This is an autoprotolysis reaction
The extent of the autoprotolysis of water
can be determined using the equilibrium
constant for this process
Kw = [ 3 ][] = 1.0 1014 (25.0 C)
Kw is the autoprotolysis constant of water
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11.2 Acid11.2 Acid--base reactionsbase reactions
in waterin water The autoprotolysis of water
Kw very small, autoprotolysis of water
proceeds only to a small extent
Concentrations of 3and in pure
water at 25 C
Kw = [ 3 ][] = 1.0 1014
If [ 3 ] = [
], then14 7 1
3[ ] [ ] 1.0 10 1.0 10 mol L
! ! v ! v
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11.2 Acid11.2 Acid--base reactionsbase reactions
in waterin water The autoprotolysis of water
Pure water is neutral because it contains
equal concentrations of3
and
[ 3 ] > [] in acidic aqueous solutions
[ 3 ] < [] in basic aqueous solutions
There is an inverse relationship between
[ 3 ] and [ ] in aqueous solutions
As the concentration of one increases, the
other must decrease in order to keep Kwconstant
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11.2 Acid11.2 Acid--base reactionsbase reactions
in waterin water The concept of p
To avoid working with inconveniently small
numbers we commonly express [3
] in
terms of the p of the solution
p = log[ 3 ]
p = log[ ]
log means log10, as opposed to naturallogarithms (ln or loge)
[ 3 ] = 10p
[
] = 10
p
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11.2 Acid11.2 Acid--base reactionsbase reactions
in waterin water The concept of p Pure water at 25 C
p = log[ 3 ] = log(1.0 107) = 7.00
p = log[
] = log(1.0 107
) = 7.00 Acidic solutions
[ 3 ] > []
At 25 C p < 7
Basic solutions [ 3 ] < [
]
At 25 C p > 7
At 25 C p p = pKw
p p = 14
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11.2 Acid11.2 Acid--base reactionsbase reactions
in waterin water The strength of acids and bases A strong acid reacts completely with water
to give quantitative formation of 3 A weak acid reacts incompletely with
water to form less than stoichiometricamounts of 3
A strong base reacts completely withwater to give quantitative formation of
A weak base reacts incompletely withwater to form less than stoichiometric
amounts of
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11.2 Acid11.2 Acid--base reactionsbase reactions
in waterin water The strength
of acids andbases
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11.3 Strong acids and bases11.3 Strong acids and bases
The conjugate base of a strong acid is very weak
3 is the strongest acid and is the strongest
base that
can exist inaqueous
solution
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11.3 Strong acids and11.3 Strong acids and
basesbases
p calculations in solutions of strong
acids and bases
Relatively straightforward exercise instoichiometry as strong acids and bases
react with water almost completely
A 0.010 M solution of Cl contains 0.010
mol L1
of 3 .The p of this solution is:
p = log[ 3 ] = log(1.0 102) = 2.00
Cl(aq) 2 (l) H3 (aq) Cl(aq)
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11.3 Strong acids and11.3 Strong acids and
basesbases
contd A 0.010 M solution of Ba( H)2 contains
0.020 M of H.
The p H of this solution is:
p H = log[ H] = log(2.0 102) = 1.70
pH = 14.00 1.70 = 12.30
Ba( H)2(aq) H2 (l) Ba2 (aq) 2 H(aq)
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11.3 Strong acids and11.3 Strong acids and
basesbases
Suppression of the autoprotolysis of
water
Previously we assumed that all of the
H3 or H in solutions of strong acid
or base comes from the acid or base
respectively
There is another potential source of bothH3 and H in aqueous solution,
namely water
[H3 ]total = [H3 ]from acid [H3 ]from H2
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11.3 Strong acids and11.3 Strong acids and
basesbases
Suppression of the autoprotolysis of
water
Except in very dilute solutions of strong
acids, [H3 ]from water is negligible compared
to [H3 ]from acid
In any solution of an acid, the autoprotolysis
of water is suppressed
Add another source ofH3 to water,
instantaneously increase Qw, so that
Qw > Kw, resulting in lower amounts of both
H3and H from the autoprotolysis of
water
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11.3 Strong acids and11.3 Strong acids and
basesbases
Suppression of the autoprotolysis of
water
Calculate the pH of a 1.0
1010 M solution of
HCl(aq)
[H3 ]from HCl = 1.0 1010 M
[H3 ]from water= 1.0 107 M
[H3 ]total = 1.0 107 1.0 1010 M
= 1.0 107 M
pH = log[H3 ]
= log(1.0
107) = 7.00
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11.4 Weak acids and11.4 Weak acids and
basesbases
Weak acids and bases react
incompletely with water
Quantify the extent to which thesereactions occur by looking at the values
of their equilibrium constants
HA(aq) H2
(l) H3
(aq) A(aq)
Ka is called the acidity constant
? A3
a
H O
HK
- - !
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11.4 Weak acids and11.4 Weak acids and
basesbasesB(aq) H2 (l) BH
+(aq) + H(aq)
Kb is called the basicity constant
Ka and Kb values are generally
significantly less than 1 pKa = logKa Ka = 10
pKa
pKb = logKb
Kb = 10pKb
? A
+H OHK
- - !
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11.4 Weak acids and11.4 Weak acids and
basesbases
The strength of a weak acid is
determined by its Ka value
The larger the Ka
the stronger the acid
The smaller the pKa the stronger the acid
The strength of a weak base is
determined by its Kb value The larger the Kb the stronger the base
The smaller the pKb the stronger the
base
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11.4 Weak acids and11.4 Weak acids and
basesbases
KaKb = Kw
pKa
+ pKb
= pKw
These relationships hold for any acid-
base conjugate pair, regardless of the
strength of the acid or base
? A
+ -
3
a
H O HCOO=
HCOOHK
- -
? A -
-
H OOH OH
H OOK
-
-
HCOOH(aq) +H2O(l) H3O+(aq) +HCOO(aq)
HCOO(aq) +H2O(l) HCOOH(aq) +OH(aq)
H2O(l) +H2O(l) H3O+
(aq) +OH
(aq)+ -
3= H O OHwK - -
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11.4 Weak acids and bases11.4 Weak acids and bases
There is an
inverse
relationship
between thestrengths of
the acid and
base
members of aconjugate pair
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11.4 Weak acids and11.4 Weak acids and
basesbases
pH calculations in solutions of weakacids and bases
Must use the value ofKa orKb to
determine [H3O+] or [OH] and hence pH
The pH of a 1.0 M solution of acetic acid, CH3COOHis:
CH3
COOH(aq) +H2
O(l) H3
O+(aq) + CH3
COO(aq)
Cannot solve this equation for [H3O+] as there are
two unknowns: [CH3COO
] and [CH3COOH]
? A
+ -
3 3 5
a
3
H O H OO.8
H OOHK
- - ! v
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11.4 Weak acids and11.4 Weak acids and
basesbases pH calculations in solutions of weak
acids and basesH2O + CH3COOH H3O
+ + CH3COO
Initial concentration 1.0 0 0
Change in concentration x +x +xEquilibrium concentration 1.0x x x
? A
+ -
3 3 5
a
3
H O H OO.8
H OOH .
x xK
x
- - ! ! v
2
51.8 101.0x ! v
2 5 51.0 1. 10 1. 10x ! v ! v5 31. 10 4.2 10 Mx ! v ! v
3pH log 4.2 10 2.3! v !
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11.4 Weak acids and11.4 Weak acids and
basesbases
pH calculations in solutions of salts of
weak acids and bases
What is the pH of a 0.010 M solution of NaOCl?ForHOCl, Ka = 3.0 10
OCl(aq) +H2O(l) HOCl(aq) +OH(aq)
? A -
-
HO l OH
O lK
-
-
147
8
1.0 103.3 10
3.0 10w
b
a
KK
K
v! ! ! v
v
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11.4 Weak acids and11.4 Weak acids and
basesbases pH calculations in solutions of salts of
weak acids and bases
H2O + OCl HOCl + OH
Initial concentration 0.10 0 0Change in concentration x +x +x
Equilibrium concentration 0.10x x x
[HOCl] = [OH] = x
? A
- 2
7b -
HOCl OH= 3.3 10
0.10 0.10OCl
x x xK
x
- ! ! ! v -
2 7 80.10 3.3 10 3.3 10x ! v ! v8 43.3 10 1.8 10 Mx ! v ! v
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pH calculations in solutions of salts ofweak acids and bases
11.4 Weak acids and11.4 Weak acids and
basesbases
-
4
pOH l [OH ]
pOH l 1.8 10
pOH 3.74
!
! v
!
pH = 14.00 - pOH
pH = 14.00 - 3.74
pH = 10.26
The pH f this s luti n is 10.26
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Solutions that contain the salt of a
weak acid and a weak base
When both the cation and anion in a
single salt are able to affect the pH the
net effect is dependent on the relative
strengths of its ions, one functioning as
an acid and the other as a base
NH4HCOO
Ka of NH4+ is 5.6 x 1010
Kb ofHCOO is 5.6 x 1011
Solution of NH4HCOO is slightly acidic
11.4 Weak acids and11.4 Weak acids and
basesbases
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11.4 Weak acids and11.4 Weak acids and
basesbases
Situations where simplifyingassumptions do not work
When a weak acid HA reacts with water
[HA]equilibrium = [HA]initial x
Previously assumed
[HA]equilibrium = [HA]initial
This is only the case if [HA]initial is greaterthan, or equal to, 400 times the value ofKa
If this assumption is not valid, we must
solve a quadratic equation
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11.4 Weak acids and11.4 Weak acids and
basesbases
Situations where simplifying
assumptions do not work
Previously assumed that we could safelyneglect any contribution of [H3O
+] or
[OH] from the autoprotolysis of water
This is only the case if the value of[H3O+] or [OH] from the acid or base is
greater than 1 105
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11.5 The molecular basis11.5 The molecular basis
of acid strengthof acid strength
Binary acids
An acid containing H and only one other
element, generally nonmetal
HF
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11.5 The molecular basis of11.5 The molecular basis of
acid strengthacid strength
Binary acids
Relative acidity increases as we proceeddown any group in the periodic table
Due primarily to the decreasing bonddissociation enthalpy
Across a period the acidity is determinedprimarily by the electronegativity of the
nonmetal X As HX bonds become more polar +
charge on the H atom increases, making iteasier for the proton to separate and bond
to a base
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11.5 The molecular basis of11.5 The molecular basis of
acid strengthacid strength
Oxoacids
Acids composed of hydrogen, oxygenand some other element
A feature common to the structures of alloxoacids is the presence ofOH groupsbonded to some central atom
Reaction of oxoacid with water involves
breaking of an OH bond
O O
O
O
H H SeO O
O
O
H H
sulfuric acid selenic acid
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11.5 The molecular basis of11.5 The molecular basis of
acid strengthacid strength
Oxoacids
The more polar the bond, the stronger
the acid
Two principal factors determine how the
polarity of the OH bond is affected
The electronegativity of the central atom in
the oxoacid
The number of oxygens attached to the
central atom
G-O-H- +
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11.5 The molecular basis of11.5 The molecular basis of
acid strengthacid strength
Oxoacids
As the electronegativity of the centralatom increases, the oxoacid becomes a
better proton donorHIO4 < HBrO4 < HClO4
H3PO4 < H2SO4 < HClO4 As the number of lone oxygens increases,
the oxoacid becomes a better protondonor
O HCl ClO O HO HClO
OCl O HO
O
O
< < 7 as conjugate base formed by addition
ofOH to weak acid
Calculate pH using method for the
determination of pH of a salt of a weak acid
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11.7 Acid11.7 Acid--base titrationsbase titrations
Weak acid strong base titration
The alkaline region
Simply adding excess OH
pH controlled by excess OH
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11.7 Acid11.7 Acid--base titrationsbase titrations
Acid-base indicators
Indicators used in acid-base titration are
weak acids
HIn
Acid form ofHIn must have a different
colour from the conjugate base form In-
HIn(aq) +H2O(l) H3O
+
(aq) +In
-
(aq)acid form base form
colour 1 colour 2
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11.7 Acid11.7 Acid--base titrationsbase titrations
Acid-base indicators
Typically as we pass
the equivalence point
there is a suddenand large change
in pH.
This causes a
sudden shift in theposition of the
equilibrium for
the indicator
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Acid-base indicators
The pKIn of an
indicator should be
equal or close to thepH at the
equivalence point
When performing a
titration we want touse as little indicator
as possible
11.7 Acid11.7 Acid--base titrationsbase titrations
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11.7 Acid11.7 Acid--base titrationsbase titrations
Lewis acids and bases
A Lewis acid is an electron-pair acceptor
A Lewis base is an electron-pair donor
Strength of Lewis acids and bases is not
as readily quantified as BrnstedLowry
counterparts
B
F
F FN
H
H HB N
F H
F HHF
Lewis acid Lewis base adduct
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