AcidBase Equilibria

1
Chapter 16

• Acid-Base Equilibria

2
Dissociation of water
Autoionization or autoprotolysis
Ion-product constant Autoprotolysis constant
3
Kw HOH- 1.0x10-14 When H OH-
neutral. Doesnt usually happen. As one
increases, the other decreases the product must
equal 1.0x10-14. When H gt OH- acidic
OH- gt H basic
4
H is a proton with no electrons. In water

ù
é
H
O
H
ú
ê
H
û
ë
Hydronium ion
5
Bronstead-Lowry Acid-Base Acid - Can donate a
proton Base - Can accept a proton Doesnt have
to be in H2O. Can be in other solvents.
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Conjugate Acid-Base Pairs
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The stronger an acid, the weaker its conjugate
base. The weaker an acid, the stronger its
conjugate base.
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pH scale pH -log H Remember Kw
(1x10-7)(1x10-7) 1.0x10-14 pH -log H
-log (1x10-7) pH 7 (neutral) H pH acidi
c gt 1.0x10-7 lt 7.00 basic lt 1.0x10-7 gt 7.00
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You can also speak in terms of OH- pOH -log
OH- 14 - pH Because pH pOH
-log Kw 14
10
Measure pH by pH meter Acid-base indicators
Litmus red pH lt 5 blue pH gt
8 Figure 16.7 shows several acid-base indicators
and their ranges
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Strong Acids and Bases Strong electrolytes Compl
etely ionize HA H2O ? A- H3O Bases form
hydroxides in solvent
12
In H2O, Alkali metal hydroxides Alkaline earth
metal Hydroxides (except Be) Many are
insoluble Also, substances that will abstract a
H from H2O. O2- H2O ? 2OH- Na2O or CaO would
do this. O2-, H-, N3- bases that would do this.
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Weak acids
Only partially ionize
Acid dissociation constant
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Larger Ka means stronger acid. ex.
0.020M solution pH 3.26 ? Ka
pH -log H 3.26 H 5.50x10-4
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?
H
A-
H
HA
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16
Can calculate pH in same manner if you have Ka
and concentration of solution. Lets use niacin
again.
?
H
A-
H
HA
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Simplifying Assumption x is very very small
compared to 0.010M sooooooooo, ignore x in
denominator
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pH -log H x H 3.9x10-4 pH 3.41 What
percent of niacin molecules ionized?
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Polyprotic Acids ex. H2SO4
H3PO4 H2SeO4 H2SO4 ? H HSO4- Ka1
1.7x10-2 HSO4- ? H SO42- Ka2 6.4x10-8 Ka1
always larger than Ka2 If Ka1/ Ka2 ? 103, can
estimate pH by Ka1 only.
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Weak Bases ex. Amines an organic
substituted ammonia
methyl amine
ammonia NH3
CH3
N
H
H
N
H
H
H
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H
CH3 H2O ?
N
H
N
CH3 OH-
H
H
H
Anions of weak acids
ClO- H2O ? HClO OH- Kb 3.3x10-7
Can use this in the same manner in which you used
Ka.
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Ka and Kb How are they related?
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1) 2) 3)
When two reactions are added together, the
equilibrium constant for the third reaction is
given by the product of equilibrium constants of
equations 1 and 2.
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K1 x K2 K3 rxn 1 rxn 2 rxn 3
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Special Case Ka x Kb Kw For conjugate
acid-base pairs.
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Bond polarity and Bond strength effect on
Acid-base behavior In binary acids ?
polarity(across a row) ? acidity ? bond
strength(in a group)? acidity ? stability of
conj. base ? acidity
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Metal hydrides are basic or show no acid/base
properties in H2O. Nonmetal hydrides are acidic
or show no acid/base properties in H2O (except
NH3) Acidity increases moving down a group.
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Oxyacids
O
Have unprotonated and protonated oxygens.
H
O
S
O
H
O
H3PO4
Y
O
H

• As electronegativity of Y increases, acidity
  increases.
• As number of unprotonated oxygens increases,
  acidity increases (effect of formal charge and
  oxidation number)
• Ex. HClO, HClO2, HClO3, HClO4

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Carboxylic Acids
O
COOH Carboxyl group
C
R
OH
R H or an organic group. The more electron
withdrawing R is, the greater the acidity (this
stabilizes anion and weakens O-H bond) ex.
H
F
O
O
C
H
C
C
F
C
O
O
H
H
F
H
Acetic acid Ka 1.8x10-5
Trifluoroacetic acid Ka 5.0x10-1
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Lewis Acids and Bases This is a completely
different definition for acid/base chemistry than
what you have seen thus far!!! Lewis acid
electron pair acceptor Lewis base electron pair
donor Not giving them away, just has them
available to share.
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H Bronstead-Lowry acid also a Lewis
acid H electron pair acceptor
OH-
Electron pair donor Lewis base also
Bronstead-Lowry base
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H
H
BH3 not a Bronstead-Lowry acid, but its a Lewis
acid
B
H
Incomplete Octet
H
Lewis Base has an electron pair available to
attack an area that is e- deficient
N
H
H
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Transition metal ions are often Lewis Acids.
They have vacant d orbitals. (s and p also)
H
H
O
O C O Can be a Lewis Acid because e-
density around the C is bound in just 2
directions.
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H
O
O
H
C

O
C

O
O
Carbonic acid
Hydrolysis of metal ions Metal ions have positive
charge so they attract the lone e- pair on H2O
molecules
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6 of these
Because the metal is (), e- density of H2O moves
toward the metal. When this happens, there is
less e- density in waters O-H bonds, so H can
come off easier ? pH will drop.
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The higher the charge density of the metal ion,
the greater the acidity of its aqua complex.