chapter 16. overview: definitions arrhenius bronsted -- conjugate pairs hydronium ion relative...
TRANSCRIPT
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Chapter 16
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Overview:•Definitions Arrhenius Bronsted -- Conjugate Pairs Hydronium Ion
•Relative Strengths Strong/Weak acids and reactions Strong/Weak bases and reactions Ka’s and Kb’s
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•pH and H2O Ionization
•Calculating Equilibrium Concentrations
•Hydrolysis Acidic and Basic Salts
•Polyprotic Acids
•Molecular Structure/Bonding
•Lewis Acids and Bases
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Acids•sour•corrosive•reddens blue vegetable colors•react with bases
Bases•bitter•soapy•restores vegetable colors reddened by acids•react with acids
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Arrhenius Acid:substance that releases or produces H+
HCl(aq) H+(aq) + Cl -
(aq) Arrhenius Base:substance that releases or produces OH - NaOH(aq) Na+
(aq) + OH -(aq) Bronsted Acid:
substance that donates a H+ to anotherHNO3(aq) + H2O(l) H3O+
(aq) + NO3-(aq)
Bronsted Base:substance that accepts a H+ from anotherCO3
2-(aq) + H2O(l) HCO3
-(aq) + OH -
(aq)
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Water Dissociation:Water Dissociation:
2H2O H3O+ + OH -
Hydronium Ion -- because bare protons are unlikely
H+ transfer
HO
H
HO
H
HO
H
HO
H
++
-
- +
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pH and Water Ionization:pH and Water Ionization:
2H2O(l) H3O+(aq) +
OH -(aq)
Kw = [H3O+][OH -] = 1.0 x 10 -14
Reactant Favored
ion-product constant for water
(at 25C)
K = [H3O+][OH -] = K [H2O]2 = [H3O+][OH -] [H2O] 2
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pH = - log [H+] = - log [H3O+]pH = - log [H+] = - log [H3O+]
neutral solution: [H3O+] = [OH -]
2H2O H3O+ + OH - 1.0 x 10 -14 = [H3O+][OH -] =
[H3O+]2
[H3O+] = 1.0 x 10 -
7 M
pH = - log (1.0 x 10 -7) = 7.0 pH = - log (1.0 x 10 -7) = 7.0
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non-neutral solutions:
[H+] > [OH-]
acidic solutions:
pH < 7.0pH < 7.0
basic solutions:[H+] < [OH-]pH > 7.0pH > 7.0
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pH of Strong Acid and Base Solutions: Calculate the pH of a solution containing 0.00100 M of a strong acid such as HCl.
HCl(aq) H+(aq) + OH-
(aq) 0.00100 0.00100 0.00100
pH = - log (0.00100) = 3.00
but. . . .
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What about H3O+ from water?
2 H2O H3O+ + OH -
initial 0.00100 0
change +x +xequil. 0.00100 + x x
1 x 10 -14 = (0.00100 + x)(x)
x = H3O+ and OH- from disso. of water
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[H3O+] (1.0 x 10-11) = 1.0 x 10 -14 [H3O+] = 1.0 x 10 -3 M
pH = 3.0
1 x 10 -14 = (0.00100 + x)(x)x = 1.00 x 10 -11 M =
[OH -]
Kw = [H3O+] [OH -] 0
x is very small compared to 0.00100 & can be neglected
[H+] [H3O+]
the conc. of H+ in a solution of a strong acid is the conc. of the strong acid
pOH = 11.0
Note: pH + pOH = 14.0
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Bottom Line:Bottom Line:
[H+] = [H3O+] = [H5O2+] = [H9O4
+] hydrated
hydrogen ions neutral solution [H3O+] = [OH-] pH = 7
acidic solution [H3O+] > [OH-] pH < 7
basic solution [H3O+] < [OH-] pH > 7
the concentration of [H3O+] in a strong acid
is the concentration of the acid
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Calculate the pH of a solution containing 0.010 M KOH.
KOH K+(aq) +
OH-(aq) 0.010 M 0.010 M
0.010 M
1.0 x 10 -14 = [H+] (0.010 M)[H+] = 1.0 x 10 -12 M
pH = 12.0
strong base, complete rxn, stoichiometric
contribution of [H+] from dissociation of H2O is negligible
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Calculate the [H+] and [OH -] in a solution that has a pH = 8.60
- log [H+] = pH
- log [H+] = 8.60
log [H+] = -8.60
[H+] = anti log (-8.60)
[H+] = 1.8 x 10 -4 M
[OH -] = 5.6 x 10 -11 M
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Basic
Neutral
Acidic
pH [H3O+] [OH-] pOH
7.0 10 -7 10 -7 7.0
14 10 -14 1 0
0 1 10 -14 14
vinegar, cola
milk
ammonia
human blood
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Measuring pH:
pH meter -- electrodes measure pH– most precise method
acid-base indicators– less precise but good when a pH meter is
not available– substances which are differently colored at
different pH values– litmus, phenolphthalein, thymol blue
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Arrhenius Acid:substance that releases or produces H+
HCl(aq) H+(aq) + Cl -
(aq) Arrhenius Base:substance that releases or produces OH - NaOH(aq) Na+
(aq) + OH -(aq) Bronsted Acid:
substance that donates a H+ to anotherHNO3(aq) + H2O(l) H3O+
(aq) + NO3-(aq)
Bronsted Base:substance that accepts a H+ from anotherCO3
2-(aq) + H2O(l) HCO3
-(aq) + OH -
(aq)
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Examples:
NH4+
(aq) + H2O(l) H3O+(aq) +
NH3(aq) acid
base
conjugate acid
conjugate base
PO43-
(aq) + H2O(l) HPO42-
(aq) + OH -
(aq)
H+
base
acid
conjugate acid
conjugate base
H+
conjugate pair
conjugate pair
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HCO3-(aq) + H2O(l) H3O+
(aq) + CO3
2-(aq)
HCO3-(aq) + H2O(l) H2CO3(aq) + OH
-(aq)
Some species can act as an acid or base:
acid
base
conjugate acid
conjugate base
base
acid
conjugate acid
conjugate base
HCO3- is an amphiprotic
substance
H+
H+
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You Must Know:You Must Know:
what an acid and a base is and how to identify both know definitions and properties
the reaction of an acid and a base with water
how to identify acid, base, conjugate acid and conjugate base
what the hydronium ion is and the ionization reaction of water
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Problems: HX + H2O H3O+ + X - A B CA
CB
•What is the conjugate base of:
H2SNH4
+
NH3 H2OOH -
HS -
NH3 NH2
- OH -
O2-
•What is the conjugate acid of:NO3
- HPO4
2-
H2SO4
HNO3 H2PO4
- H3SO4
+
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Relative Strengths:Relative Strengths:
HCl + H2O H3O+ + Cl -
equilibrium is a competition between the bases H2O and Cl
- -- the equilibrium will lie toward the direction of the weaker acid and base
in this case, H2O is a stronger base than Cl - as it competes much more effectively for the H+
stronger B
weaker Bstronger A
weaker A
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HCN + H2O H3O+ + CN -
H2O < CN -
weaker Bweaker A stronger A
stronger B
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Which is the weaker acid:Which is the weaker acid:
H2S + CN - HCN + HS -
HCO3- + SO4
2- HSO4- +
CO32-
HClO4 + H2O H3O+ + ClO4
-
NH4+ + H2O H3O+ +
NH3
HCN
HCO3-
H3O+
NH4+
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The stronger the acid the weaker its conjugate base:
strongest acid
weakest acid strongest conjugate base
weakest conjugate base
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Given the following, which is the weaker conjugate: Given the following, which is the weaker conjugate:
HCl > CH3CO2H
HCN < H3PO4
H2SO4 > H2SO3
NH3 > H2O
HSO4- < CO3
2-
H - > NH3
acids
bases
Cl - CH3CO2-<
CN - H2PO4 -
>
HSO4 -
HSO3-
<
NH4+ H3O+<
H2SO4 HCO3 -
>
H
2 NH4
+ <
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Predicting Direction of Acid/Base Rxns.Predicting Direction of Acid/Base Rxns.
CH3CO2H + CN - HCN + CH3CO2 - stronger
Aweaker A
weaker B
stronger B
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HSO4- + NH3 NH4
+ + SO4
2-
stronger A
weaker A
stronger B
weaker B
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Strong Acids and BasesStrong Acids and Bases•Strong Acids HCl HNO3
HClO4
HClO3 H2SO4 HBr HI
•Strong Bases Grp I hydroxides Grp II hydroxides (except Be)
HX + H2O H3O+ + X -
MOH M+ + OH -
essentially complete rxns.
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Weak Acids and Bases:Weak Acids and Bases:
HX + H2O H3O+ + X -
B + H2O BH+ + OH -
Ka = [H3O+][X -] [HX]
Kb = [BH+][OH -] [B]
< 1< 1
< 1< 1
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Weak Acids Can Be:Weak Acids Can Be:
•cations NH4+ or [Cu(H2O)6]2+
•anions H2PO4- or HCO3
-
•neutral CH3CO2H or HCO2H
Weak Bases Can Be:Weak Bases Can Be:
•anions CO32- or CN -
•neutral NH3 or (CH3)3N
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You Must:You Must:Be able to determine direction of rxn based on acid or base relative strengths
Know the strong acids and strong bases
Be able to recognize weak acids and bases
Know water ionization rxn, constant and expression
Know what pH is and how to calculate it
Know how to calculate equil. conc., pH, pOH for weak acids and bases
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Problem 1: A 0.015 M solution of an unknown base has a pH of 10.09. Is it a strong or weak base? What is the Kb, if it is weak?
Problem 2: What are the equil. conc. of H3O+, acetate ion, acetic acid in a 0.20 M aqueous solution of acetic acid, CH3CO2H?
Use the approximation whenever possible: When the initial weak acid/base conc. > 100 a/b neglect x when it is added to or subtracted from the initial conc.
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If this were a strong base then [OH-] = 0.015 M and pOH = 1.8 and pH = 12.2.
pH = 10.09 and pOH = 3. 91 [OH-] = 1.2 x 10-4
Kb = [HB+][OH-] = (1.2 x 10 -4)2
[B] (0.015)
B + H2O HB+ + OH-
= 9.6 x 10 -7
Answer 1:Answer 1:
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CH3CO2H + H2O H3O+ + CH3CO2-
initial 0.20 0 0change -x +x +xequil. 0.20 -x x x
Ka = 1.8 x 10 -5 = [H3O+][CH3CO2-]
[CH3CO2H]
Ka = 1.8 x 10 -5 = (x)(x) x2 = 3.6 x 10 -6 (0.20 -x) x = 1.9 x 10 -3
[CH3CO2-]=[H3O+] = 1.9 x 10 -
3 M [CH3CO2H] = 0.20 M
[CH3CO2-]=[H3O+] = 1.9 x 10 -
3 M [CH3CO2H] = 0.20 M
pH = 2.7pH = 2.7
Answer 2:Answer 2:
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What is the percent ionization?
% ion = [H3O+] x 100 [CH3CO2H]
= 0.95 %
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Polyprotic Acids and Bases:Polyprotic Acids and Bases:
H3PO4 + H2O H3O+ + H2PO4-
Ka1
H2PO4- + H2O H3O+ + HPO4
2- Ka2
HPO42- + H2O H3O+ + PO4
3- Ka3
H3PO4 + 3H2O 3H3O+ + PO4
3--
(7.5 x 10 -3)(6.2 x 10 -8)(3.6 x 10 -13) = 1.7 x 10 -22
Ka tot = Ka1 Ka2 Ka3Ka tot = Ka1 Ka2 Ka3
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Weak Bases:Weak Bases: Can be anions such as:
– CN-, HSO3-, SO3
2-, HCO3-, CO3
2-, etc.
Can be N-containing compounds, such as:– NH3, (CH3)NH2, (CH3)2NH, (CH3)3N, etc.
React with water
– B- + H2O HB + OH- or
– B + H2O HB+ + OH-
Have base dissociation constants, Kb
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NH3 + H2O NH4+ + OH -
initial 0.20 M 0 0change -x +x +xequil. 0.20 -x x x
Kb = 1.8 x 10 -5 = [NH4+][OH -]
[NH3]Kb = 1.8 x 10 -5 = (x)(x) x2 = 3.6 x 10 -6 (0.20 -x) x = 1.9 x 10 -3
[NH4+]=[OH-] = 1.9 x 10 -3 M
[NH3] = 0.20 M[NH4
+]=[OH-] = 1.9 x 10 -3 M [NH3] = 0.20 M
pOH = 2.7pH = 12.3
pOH = 2.7pH = 12.3
Example:Example:
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CO3 -2 + H2O OH - + HCO3--
Kb1
HCO3 -- + H2O OH - + H2CO3 Kb2
CO3 + 2H2O 2OH -- + H2CO3
2--
Kb tot = Kb1 Kb2
(2.1 x 10 -4)(2.4 x 10 -8) = 5.0 x 10 -12
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Relationship of an acid Ka and
the Kb of its conjugate base:
Relationship of an acid Ka and
the Kb of its conjugate base:
NH3 + H2O NH4+ + OH - Kb
= 1.8 x 10 -5 NH4
+ + H2O NH3 + H3O+ Ka = 5.6 x 10 -10 2H2O H3O+ + OH - Kw = 1 x 10 -14
For Conjugate Acid/Base Pairs:For Conjugate Acid/Base Pairs:Kb Ka = Kw or pKb pKa = pKw
conjugate pair rxns with water
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Find Ka for the conjugate acids of the following bases: Ka = Kw/Kb
Find Ka for the conjugate acids of the following bases: Ka = Kw/Kb
NH3 1.8 x 10 -5
C5H5N 1.7 x 10 -9
HS - 1.8 x 10 -7
CO32- 1.8 x 10 -4
Base Kb Conj. Acid Ka
NH4+ 5.6 x 10 -10
C5H5NH + 5.9 x 10 -6
H2S 5.6 x 10 -8
5.6 x 10 -11 HCO3 -
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Acid/Base HydrolysisAcid/Base HydrolysisMany salts produce solutions that are acidic or basic
Why?Why?
because either (or both) the cation or anion of the salt acted as a weak acid or a weak base
HX + H2O H3O+ + X - or
B + H2O BH+ + OH -
weak acid
weak base
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Which cations will not hydrolyze?Cations of strong bases -- Grp I and II (except Be) Li+ Na+ K+ Rb+ Cs+ Mg2+ Ca2+
Sr2+ Ba2+
Anions of strong acids --
ClO4- ClO3
- SO42- NO3
- Cl - Br - I -
All other cations and anions will hydrolyze
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Which cations will hydrolyze?
Conjugate acids of weak bases:
NH4+ Al3+ Cu2+ etc.
Conjugate bases of weak acids:
CO32 - CH3CO2
- NH3 F - etc.
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What is the pH of the following salt solutions?What is the pH of the following salt solutions?
•NaCl
•NaF
•NH4Cl
•Na2CO3
•Cr(NO3)3
•KNO3
•NaC3H2O4
•NH4CH3CO2
Na+ -- neutral Cl - -- neutralNa+ -- neutral F - -- basicNH4
+ -- acidic Cl - -- neutralNa+ -- neutral CO3
2- -- basicCr3+ -- acidic NO3
- -- neutral
K+ -- neutral NO3+ -- neutral
Na+ -- neutral C3H2O4- --
basicNH4
+ -- acidic CH3CO2- --
basic
N
B
A
B
A
N
B
?
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Is NH4CH3CO2 acidic or basic?
NH4+ + H2O H3O+ +
NH3
Ka = 5.6 x 10 -10
CH3CO2- + H2O CH3CO2H + OH
-
Kb = 5.6 x 10 -
10 weak acid & weak base strength equal -- neutral soln.
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You Must:You Must:Know what hydrolysis is
Know which cations hydrolyze or produce acidic solutions
Know which anions hydrolyze or produce basic solutions
Be able to estimate and calculate the pH of a salt solution
Know how to determine Ka’s & Kb’s of conjugate pairs
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Strength of Acids:Can be increased by anything that facilitates the loss of a Proton (H+)
•Bond Strength
•Polarity or electronegativity
•Central Atom Charge or Oxidation #
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Examples:Examples:
IVA VA VIAVIIA
row 2 CH4 NH3 H2O HF
row 3 SiH4 PH3 H2S HCl
Increasing acid strengthIncreasing acid strength
Incre
asin
g a
cid
stre
ng
thIn
cre
asin
g a
cid
stre
ng
th
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HClO4
HClO3
HClO2
HClO
Incre
asin
g a
cid
str
en
gth
Acid
Cl ox. #
+7
+5
+3
+1
Ka
strong
strong
1.1 x 10 -2 3.0 x 10 -8
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HClO
HBrO
HIO In
cre
asin
g a
cid
str
en
gth
Acid
Ka
3 x 10 -8
2.5 x 10 -9 2.3 x 10 -11
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Organic Acids:Organic Acids:
C C O-
O
H
H
H
C C O
O-
H
H
H
Acetate ion resonance
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C C OH
O
H
H
H
Trifluoroacetic Acid Ka = 5.0 x 10 -1
C C OH
O
F
F
F
Acetic AcidKa = 1.8 x 10 -5
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Lewis Acids and Bases:Lewis Acids and Bases:
Lewis Acid -- accepts a pair of electrons
Lewis Base -- donates a pair of electrons
formation of a coordinate covalent bond
NH
HH
B
FF
F
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NH
HH
H+
NH4+
NH3 + H+
Lewis Base -- donates a pair of electrons
Lewis Acid -- accepts a pair of electrons