ACIDS AND BASES II
Topic 18
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Chem2_Dr. Dura
IB CHEMISTRY GR.12
TOPICS
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Chem2_Dr. Dura
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18.1 Calculations Involving Acids and Bases
18.2 Buffer Solutions
18.3 Salt Hydrolysis
18.4 Acid-Base Titrations
18.5 Indicators
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IB STANDARDS
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18.1.1 State the expression for the ionic product
constant of water (Kw).
18.1.2 Deduce [H+(aq)] and [OH-(aq)], for water at
different temperatures given Kw values.
18.1.3 Solve problems involving [H+(aq)], [OH-(aq)],
pH and pOH.
18.1.4 State the equation for the reaction for any
weak acid or weak base with water, and hence deduce
the expressions for Ka and Kb.
Chem
2_Dr.
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Dura
IB STANDARDS
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Chem2_Dr. Dura
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18.1.5 Solve problems involving solutions of
weak acids and bases using the expressions:
Ka x Kb = Kw
pKa + pKb = pKw
pH + pOH = pKw
18.1.6 Identify the relative strengths of acids
and bases using values of Ka, Kb, pKa and pKb.
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The Ion Product of Water
H2O (l)
H+ (aq) + OH- (aq)
[H+][OH-]
Kc =
[H2O]
[H2O] = constant
Chem2_Dr. Dura
Kc[H2O] = Kw = [H+][OH-]
The ion-product constant (Kw) is the product of the molar
concentrations of H+ and OH- ions at a particular temperature.
At 250C
Kw = [H+][OH-] = 1.0 x 10-14
[H+] = [OH-]
Solution Is
neutral
[H+] > [OH-]
acidic
[H+] < [OH-]
basic
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KW IS TEMPERATURE DEPENDENT
Temp oC
1.5 x 10-15
6.8 x 10-15
1.0 x 10-14
[H+] in pure
water
3.9 x 10-6
8.2 x10-6
1.0 x10-7
pH
Chem2_Dr. Dura
0
20
25
Kw
7.47
7.08
7.00
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pH = - log [H+],
[H+] = 10-pH
pOH = -log [OH-], [OH-] = 10-pOH
pH = 7
acidic
[H+] > [OH-]
[H+] > 1 x 10-7
pH < 7
basic
[H+] < [OH-]
[H+] < 1 x 10-7
pH > 7
pH
Chem2_Dr. Dura
[H+] = [OH-]
At 250C
[H+] = 1 x 10-7
Solution is
neutral
[H+]
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At 250C
[H+][OH-] = Kw = 1.0 x 10-14
Chem2_Dr. Dura
-log [H+] – log [OH-] = 14.00
pH + pOH = 14.00
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Example 1: The pH of rainwater collected in a certain region
of the northeastern United States on a particular day was
4.82. What is the H+ ion concentration of the rainwater?
pH = -log [H+]
Chem2_Dr. Dura
[H+] = 10-pH = 10-4.82 = 1.5 x 10-5 M
Example 2: The OH- ion concentration of a blood sample is
2.5 x 10-7 M. What is the pH of the blood?
pH + pOH = 14.00
pOH = -log [OH-] = -log (2.5 x 10-7) = 6.60
pH = 14.00 – pOH = 14.00 – 6.60 = 7.40
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Weak Acids (HA) and Acid Ionization Constants
HA (aq) + H2O (l)
HA (aq)
H3O+ (aq) + A- (aq)
H+ (aq) + A- (aq)
Chem2_Dr. Dura
[H+][A-]
Ka =
[HA]
Ka is the acid ionization constant
Ka
weak acid
strength
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Weak Bases and Base Ionization Constants
NH3 (aq) + H2O (l)
NH4+ (aq) + OH- (aq)
Kb is the base ionization constant
Kb
Chem2_Dr. Dura
[NH4+][OH-]
Kb =
[NH3]
weak base
strength
Solve weak base problems like weak acids
except solve for [OH-] instead of [H+].
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IONIZATION CONSTANTS OF CONJUGATE ACIDBASE PAIRS
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Consider a weak acid (HA) and its conjugate base (A-) in
water:
HA (aq)  H+(aq) + A-(aq)
Ka = [H+][A-] / [HA]
A-(aq) + H2O(l)  HA(aq) + OH-(aq)
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Kb = [HA][OH-] /[A-]
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Ka x Kb = [H+][OH-] = Kw
Chem
2_Dr.
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Dura
IB STANDARDS
Chem2_Dr. Dura
18.2.1 Describe the composition of a buffer
solution and explain its action.
18.2.2 Solve problems involving the composition
and pH of a specified buffer system.
18.3 Deduce whether salts form acidic, alkaline or
neutral aqueous solutions.
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BUFFER SOLUTIONS
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A buffer solution is a solution of:
acid or a weak base and
2.The salt of the weak acid or weak base
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Both must be present!
Chem2_Dr. Dura
1.A weak
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The common ion effect is the shift in equilibrium caused by the
addition of a compound having an ion in common with the
dissolved substance.
Chem2_Dr. Dura
The presence of a common ion suppresses the
ionization of a weak acid or a weak base.
Example: A buffer consisting of CH3COONa (salt) and
CH3COOH (weak acid).
CH3COONa (s)
Na+ (aq) + CH3COO- (aq)
CH3COOH (aq)
H+ (aq) + CH3COO- (aq)
common
ion
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• A buffer solution has the ability to resist changes in pH upon
the addition of small amounts of either acid or base.
Chem2_Dr. Dura
How buffers work
Consider an equal molar mixture of CH3COOH and
CH3COONa
Add strong acid
H+ (aq) + CH3COO- (aq)
Add strong base
OH- (aq) + CH3COOH (aq)
CH3COOH (aq)
CH3COO- (aq) + H2O (l)
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Consider mixture of salt NaA and weak acid HA.
NaA (s)
Na+ (aq) + A- (aq)
HA (aq)
H+ (aq) + A- (aq)
Ka [HA]
=
[A-]
-log [H+] = -log Ka - log
Chem2_Dr. Dura
[H+]
[H+][A-]
Ka =
[HA]
[HA]
[A-]
-]
[A
-log [H+] = -log Ka + log
[HA]
[A-]
pH = pKa + log
[HA]
pKa = -log Ka
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HENDERSON-HASSELBALCH EQUATION
pH of buffers can be determined from HendersonHasselbalch Equation.
Chem2_Dr. Dura
[conjugate base]
pH = pKa + log
[acid]
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Exercise: Which of the following are buffer systems? (a)
KF/HF (b) KBr/HBr, (c) Na2CO3/NaHCO3
(b) HBr is a strong acid
not a buffer solution
Chem2_Dr. Dura
(a) KF is a weak acid and F- is its conjugate base
buffer solution
(c) CO32- is a weak base and HCO3- is its conjugate acid
buffer solution
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Problem: Calculate the pH of the 0.30 M NH3/0.36 M NH4Cl
buffer system. What is the pH after the addition of 20.0 mL of
0.050 M NaOH to 80.0 mL of the buffer solution?
NH4+ (aq)
start (moles)
end (moles)
pKa = 9.25
0.029
0.001
NH4+ (aq) + OH- (aq)
0.028
0.0
[0.30]
pH = 9.25 + log
= 9.17
[0.36]
Chem2_Dr. Dura
[NH3]
pH = pKa + log
[NH4+]
H+ (aq) + NH3 (aq)
0.024
H2O (l) + NH3 (aq)
0.025
final volume = 80.0 mL + 20.0 mL = 100 mL
[NH4
+]
0.028
0.025
=
[NH3] =
0.10
0.10
[0.25]
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pH = 9.25 + log
= 9.20
[0.28]
Acid-Base Properties of Salts
Neutral Solutions:
Salts containing an alkali metal or alkaline earth metal ion
(except Be2+) and the conjugate base of a strong acid
(e.g. Cl-, Br-, and NO3-).
Na+ (aq) + Cl- (aq)
Basic Solutions:
Chem2_Dr. Dura
NaCl (s)
H2O
Salts derived from a strong base and a weak acid.
NaCH3COOH (s)
H 2O
CH3COO- (aq) + H2O (l)
Na+ (aq) + CH3COO- (aq)
CH3COOH (aq) + OH- (aq)
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Acid-Base Properties of Salts
Acid Solutions:
Salts derived from a strong acid and a weak base.
NH4+ (aq)
NH4+ (aq) + Cl- (aq)
NH3 (aq) + H+ (aq)
Chem2_Dr. Dura
NH4Cl (s)
H2O
Salts with small, highly charged metal cations (e.g. Al3+,
Cr3+, and Be2+) and the conjugate base of a strong acid.
Al(H2O)3+
6 (aq)
Al(OH)(H2O)52+(aq) + H+ (aq)
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Acid-Base Properties of Salts
Solutions in which both the cation and the anion hydrolyze:
Kb for the anion > Ka for the cation, solution will be basic
•
Kb for the anion < Ka for the cation, solution will be acidic
•
Kb for the anion  Ka for the cation, solution will be neutral
Chem2_Dr. Dura
•
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IB STANDARDS
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Chem2_Dr. Dura
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18.4.1Sketch the general shapes of graphs of pH
and volume for titrations of strong and weak
acids and bases and explain their important
features.
18.5.1 Describe qualitatively the action of acidbase indicator.
18.5.2 State and explain how the pH range of an
acid-base indicator relates to its pKa value.
18.5.3 Identify an appropriate indicator for a
titration given the equivalence point of titration
and the pH range of the indicator.
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Titrations
In a titration a solution of accurately known concentration is added
gradually to another solution of unknown concentration until the
chemical reaction between the two solutions is complete.
Indicator – substance that changes color at (or near) the
equivalence point
Chem2_Dr. Dura
Equivalence point – the point at which the reaction is complete
Slowly add base
to unknown acid
UNTIL
The indicator
changes color
(pink)
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Strong Acid-Strong Base Titrations
NaOH (aq) + HCl (aq)
OH- (aq) + H+ (aq)
H2O (l) + NaCl (aq)
H2O (l)
Chem2_Dr. Dura
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Weak Acid-Strong Base Titrations
CH3COOH (aq) + NaOH (aq)
CH3COONa (aq) + H2O (l)
CH3COOH (aq) + OH- (aq)
CH3COO- (aq) + H2O (l)
At equivalence point (pH > 7):
OH- (aq) + CH3COOH (aq)
Chem2_Dr. Dura
CH3COO- (aq) + H2O (l)
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Strong Acid-Weak Base Titrations
HCl (aq) + NH3 (aq)
H+ (aq) + NH3 (aq)
NH4Cl (aq)
NH4Cl (aq)
At equivalence point (pH < 7):
NH3 (aq) + H+ (aq)
Chem2_Dr. Dura
NH4+ (aq) + H2O (l)
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Acid-Base Indicators
HIn (aq)
H+ (aq) + In- (aq)
Color of acid (HIn) predominates
[HIn]
[In-]  10
Color of conjugate base (In-) predominates
Chem2_Dr. Dura
[HIn]
[In-]  10
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The titration curve of a strong acid with a strong base.
Chem2_Dr. Dura
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Which indicator(s) would you use for a titration of HNO2
with KOH ?
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Chem2_Dr. Dura
Weak acid titrated with strong base.
At equivalence point, will have conjugate base of weak acid.
At equivalence point, pH > 7
Use cresol red or phenolphthalein
Acid-Base Properties of Salts
Acid Solutions:
Salts derived from a strong acid and a weak base.
NH4+ (aq)
NH4+ (aq) + Cl- (aq)
NH3 (aq) + H+ (aq)
Chem2_Dr. Dura
NH4Cl (s)
H2O
Salts with small, highly charged metal cations (e.g. Al3+,
Cr3+, and Be2+) and the conjugate base of a strong acid.
Al(H2O)3+
6 (aq)
Al(OH)(H2O)52+(aq) + H+ (aq)
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Acid-Base Properties of Salts
Solutions in which both the cation and the anion hydrolyze:
Kb for the anion > Ka for the cation, solution will be basic
•
Kb for the anion < Ka for the cation, solution will be acidic
•
Kb for the anion  Ka for the cation, solution will be neutral
Chem2_Dr. Dura
•
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Chemistry In Action: Antacids and the Stomach pH Balance
NaHCO3 (aq) + HCl (aq)
NaCl (aq) + H2O (l) + CO2 (g)
Chem2_Dr. Dura
Mg(OH)2 (s) + 2HCl (aq)
MgCl2 (aq) + 2H2O (l)
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