Chapter 17 Principles of Reactivity: Chemistry of Acids and Bases

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Chapter 17 Principles of Reactivity Chemistry of
Acids and Bases
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• Acids and Bases
• Arrhenius definition
• Brønsted definition
• Relative strengths of conjugate acid-base pairs
• Effect of structure on acid-base strength
• Acid-base equilibria
• Definitions
• pH, pOH
• Ka, Kb, pKa, pKb
• Kw autodissociation of water
• Ka and Kb for conjugate acid-base pairs
• pH Calculations
• Strong acids and bases
• Weak acids and bases
• Polyprotic acids
• Salts
• Lewis Acids and Bases

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I. Acids and Bases
A. Arrhenius definition
acid base
produces hydronium ion (H3O) in aqueous
solution produces hydroxide ion (OH) in aqueous
solution
donates a proton (hydrogen ion, H) accepts a
proton
A conjugate acid is formed by adding a proton to
something. A conjugate base is formed by
removing a proton from something.
acid
conjugate acid
base
conjugate base
base
conjugate acid
acid
conjugate base
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I. Acids and Bases
B. Brønsted definition
Brønsted definition fits even when not in aqueous
solution
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I. Acids and Bases
C. Relative strengths of conjugate acid-base pairs
???? If HA is a stronger acid then A is a
weaker base. If HA is a weaker acid then A is a
stronger base.
???? If B is a stronger base then BH is a
weaker acid. If B is a weaker base then BH is a
stronger acid.
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I. Acids and Bases
D. Effect of structure on acid-base strength
General
The strength of HA depends on the stability of A.
A is more stable when A is more
electronegative the negative charge is more
delocalized
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I. Acids and Bases
D. Effect of structure on acid-base strength
1. binary acids hydrogen one other element
(a) As A becomes more electronegative, A is more
stable (weaker base), and HA is a stronger acid.
Thus HF gt H2O gt NH3 gt
CH4 (dissociates 1 107 1016
1023) Similarly HCl gt H2S gt PH3 gt SiH4
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I. Acids and Bases
D. Effect of structure on acid-base strength
1. binary acids hydrogen one other element
(b) As A becomes larger, the negative charge is
more delocalized, A is more stable (weaker
base), and HA is a stronger acid. (overcomes
opposite trend in electronegativity)
Thus HI gt HBr gt HCl gt HF
Similarly H2Te gt H2Se gt H2S gt H2O
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I. Acids and Bases
D. Effect of structure on acid-base strength
2. oxoacids HnXOm (HNO3, H2SO4, H3PO4, etc.)
a. electronegativity of central atom (with same
number of oxygens) e.g., HClO4 is a stronger
acid than HBrO4
Thus HClO4 gt HBrO4 gt HIO4 HClO3 gt HBrO3 gt
HIO3 HClO2 gt HBrO2 gt HIO2 HClO gt HBrO gt
HIO And HClO4 gt H2SO4 gt H3PO4 HBrO4 gt H2SeO4 gt
H3AsO4 etc.
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I. Acids and Bases
D. Effect of structure on acid-base strength
2. oxoacids HnXOm (HNO3, H2SO4, H3PO4, etc.)
b. number of lone oxygens (with same central atom)
negative charge more delocalized, anion more
stable ? stronger acid
Thus HClO4 gt HClO3 gt HClO2 gt HClO HBrO4 gt
HBrO3 gt HBrO2 gt HBrO etc. And H2SO4 gt H2SO3
H2SeO4 gt H2SeO3 HNO3 gt HNO2 H3PO4 gt H3PO3
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I. Acids and Bases
D. Effect of structure on acid-base strength
3. carboxylic acids
(10-6 dissociated)
(1 dissociated)
negative charge delocalized, anion more stable,
stronger acid
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I. Acids and Bases
D. Effect of structure on acid-base strength
Which in each pair would be the stronger
acid? H2SO3 or HClO3 CH3OH or CH3SH HOBr or
HBrO3 HBrO2 or HClO3 F2CHCO2H or
FCH2CO2H ClCH2CH2CO2H or CH3CHClCO2H HNO3 or
HNO2 PH3 or NH3 HClO4 or HBrO4 H2O or
HF H2S or H2Se FCH2CO2H or ClCH2CO2H H2Se or
HBr HF or HCl
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I. Acids and Bases
Summary acid and base strength
Absolute strengths (in water)
HA
H3O A
strong acid nonbasic K ? (100
dissociated) weak acid weak base K 103 -
1010 neutral strong base K 1015 nonacid v.
strong base K 1020
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I. Acids and Bases
Summary acid and base strength
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I. Acids and Bases
E. Acid-base equilibria
On which side does the equilibrium lie in the
following reactions? OCl HCl NH4
ClO4 F H2O NH2 H2O
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II. Definitions
A. pH, pOH
Definition pX -logX pH -logH3O pOH
-logOH H3O 10pH OH 10pOH
H3O pH 0.01 M 1 x 107 M 3 x 1010 M
pH H3O 3.0 8.4
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II. Definitions
A. pH, pOH
sig figs in pH calculations e.g., H3O 2.3
x 103 M log H3O log 2.3 - 3
log H3O -2.64
pH -log H3O 2.64
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II. Definitions
B. Ka, Kb, pKa, pKb
HA H2O
H3O A
Kc
pKa -log Ka if pKa 5 then Ka 105 if
pKa 8 then Ka 108
stronger acid weaker acid
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II. Definitions
B. Ka, Kb, pKa, pKb
B H2O
BH OH
Kc
pKb -log Kb if pKb 4 then Kb 104 if
pKb 9 then Kb 109
stronger base weaker base
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II. Definitions
C. Kw autodissociation of water
H2O H2O
H3O OH
pH 14 - pOH
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II. Definitions
D. Ka and Kb for conjugate acid-base pairs
HA
H3O A
Ka
H3OOH Kw
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III. pH Calculations
A. Strong acids and bases
100 dissociated ? for strong acid H3Oeq
HAI base OHeq Bi
e.g., 1.0 x 103 M HCl H3O pH OH pOH

e.g., 2.5 x 102 M NaOH OH pOH H3O
pH
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III. pH Calculations
B. Weak acids and bases
H3OA HA
HA
H3O A
Ka
Solve equilibrium expressions
BHOH B
B
BH OH
Kb
e.g., What is the pH of 0.10 M HC2H3O2? (Ka
1.8 x 105)
x H3O 1.3 x 103 M (assumption valid) pH
2.87
assume x ltlt 0.10
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III. pH Calculations
B. Weak acids and bases
e.g., What is the pH of 0.25 M N2H4? (Kb 1.7 x
106)
N2H4
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III. pH Calculations
B. Weak acids and bases
e.g., If a 0.10 M solution of a weak acid has a
pH of 4.28, what is the Ka for the acid?
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III. pH Calculations
B. Weak acids and bases
e.g., What concentration of C5H5N (Kb 1.7 x
109) will give a solution with pH 9.80?
C5H5N
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III. pH Calculations
C. Polyprotic acids
H2SO4, H2SO3, H2CO3, etc.
Lose their protons in separate steps
(Usually, Ka1 gtgt Ka2)
Assume 1) H2A, H3O, and HA can be
determined from the 1st step. (i.e., HA
dissociates only very little.) 2) A2 can be
determined from the 2nd step.
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III. pH Calculations
C. Polyprotic acids
e.g., What are the equilibrium concentrations of
all species in 0.10 M Vitamin C (ascorbic acid,
H2C6H6O6)?
x 2.8 x 103 M
y 1.6 x 1012 M
? H2A 0.10 M H3O HA 2.8 x 103
M A2 1.6 x 1012 M
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III. pH Calculations
C. Polyprotic acids
e.g., What are the equilibrium concentrations of
all species in a 0.10 M solution of oxalic acid,
H2C2O4? (Ka1 5.9 x 10-2, Ka2 6.4 x 10-5)
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III. pH Calculations
D. Salts
HA B ?? BHA (salt)
Salt may be acidic or basic depending on the
nature of BH and A.
conjugate acid BH H2O
B H3O
conjugate base A H2O
HA OH
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III. pH Calculations
D. Salts
1. acidic salts
e.g., What is the pH of a 0.10 M solution of
NH4Cl?
NH4 Cl
conjugate acid of a weak base ? weak
acid conjugate base of a strong acid ? nonbasic
(spectator)

x H3O 7.5 x 106 M pH 5.13
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III. pH Calculations
D. Salts
2. basic salts
e.g., What is the pH of a 0.10 M solution of
NaCN? Ka(HCN) 4.9 x 1010
Na CN
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IV. Lewis Acids and Bases
acid accepts a pair of electrons base donates a
pair of electrons
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IV. Lewis Acids and Bases
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IV. Lewis Acids and Bases