Chem 106
Thursday, March 10, 2011
Chapter 17 Acids and Bases
Ka and acid strength
Acid + base reactions: Four types (s +s, s + w, w + s, and w + w)
Determining K from concentrations and pH
pH of aqueous solutions of weak acids or bases, or salts
Polyprotic acids
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OWL 17
OWL 18
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Which solution has a lower pH?...
1. 0.050 M sulfuric acid
2. 0.050 M nitric acid
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M
0.
05
0
0.
05
0
M
su
l
ni
tri
c..
.
fu
r..
.
5
3
Which solution has the lower pH?
1. 0.050 M sulfuric acid
2. 0.050 M nitric acid
H2SO4 has two ionizable H’s,
therefore [H3O+] = 0.10 M, and
pH = -log(0.10) = 1.00
HNO3 has only one ionizable H,
therefore [H3O+] = 0.050 M, and
pH = -log(0.050) = 1.30
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17.4 Ka vs. Acid/Base strength
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Put in same
order as
table 17.3
Acid
Ka
Conjugate base Kb
HF
7.20 x 10-4
F1.39 x 10-11
Aspirin 3.00 x 10-4
Asp3.33 x 10-11
HCN
4.00 x 10-10
CN2.50 x 10-5
Think of it this way:
HF is a stronger acid. That is , F- more readily GIVES UP a H+ ion.
So, F- is correspondingly is less able to ACCEPT a proton.
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Relative Acid/Base Strength from K for Overall Reaction
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How is Kc for a H+ transfer reaction related to the Ka’s of the two acids?
When an acid and base react, the conjugate base of the
acid and the conjugate acid of the base are formed.
HF
pKa = 3.2
+ NH3
F-
+ NH4+
Kc > 1
pKa = 9.2
The proton transfer equilibrium tends to form the conjugate base of
the stronger acid.
If the stronger acid is on the left, KC >1
If the stronger acid is on the right, KC < 1
If the two acids are of equal strength (equal Ka’s) KC = 1
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HF
+ NH3

F-
+ NH4+
You can easily prove for yourself that
F-
+ NH4+
HF + NH3
Kc < 1
If the stronger acid is on the left, KC >1
If the stronger acid is on the right, KC < 1
If the two acids are of equal strength (equal Ka’s) KC = 1
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(that is, favors the products)
HC9H7O4 > C6H5COOH
HC9H7O4 > HClO
2
C6H5COOH > HClO
1
3
HC9H7O4 > C6H5COOH
C6H5COOH > HClO
HC9H7O4 > C6H5COOH > HClO
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You can use the same reasoning to compare BASE strengths.
I.e., the acid-base equilibrium tends away from the stronger base.
FKb = 1x10-11
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+ NH4+
HF + NH3
Kb = 2x10-5
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CONJUGATE
BASE
1
3
Conjugate
ACID
BASE
ACID
2
4
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CONJUGATE
BASE
1
3
Conjugate
ACID
BASE
ACID
2
4
Lets look these up – what are the structures and Kas
Benzoic acid
Tris acid form
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6.5 x 10-5
8.7 x 10-9
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K < 1 for the following reaction. Which are the
stronger acid, weaker base?
HOAc +
Br-

OAc-
+ HBr
17
9
6
cc,
O
A
H
H
B
O
A
r,
Br
-
Br
c,
O
A
H
r,
O
A
c-
2
B
HBr, OAcHOAc, BrHBr, BrHOAc, OAc-
H
1.
2.
3.
4.
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K < 1 for the following reaction. Which are the stronger
acid and weaker base?
HOAc +
1.
2.
3.
4.
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Br-

OAc-
+ HBr
HBr, OAcHOAc, BrHBr, BrHOAc, OAc-
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17.6 Types of Acid-Base Reactions
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Types of Acid/Base Reactions
1.
Strong Acid + Strong Base ----> Water + salt
Example:
HCl
+
H 3 O+
Net Ionic Equation:
NaOH
+
---->
OH-
.. pH = Neutral
H2O + NaCl
----> 2 H2O
-------------------------------------------------------------------------------2.
Weak Acid + Strong Base ----> Water + weak base
Example:
HF
+
NaOH
Net Ionic Equation: HF
+
OH-
... pH = Basic
----> H2O + NaF
----> H2O + F-
(Conjugate base of a weak acid = Basic.)
-------------------------------------------------------------------------------3.
Strong Acid + Weak Base ----> Water + Weak Acid
HCl
Example:
Net Ionic Equation:
F-
+
---->
H3O+ + F- ---->
... pH = Acidic
HF
HF + H2O
(Conjugate acid of a weak base = Acidic.)
-------------------------------------------------------------------------------4.
Weak Acid + Weak Base ---->
Net Ionic Equ:
HCN
+
F-
---->
Weak Acid + Weak Base ..
HF +
CN-
( pH depends on Ka’s )
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Strong acid
A
Strong base
HClO4 + H2O -> H3O+ + ClO4NaOH -> OH- + Na+
D
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H3O+ + OH- --> 2 H2O
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Strong base
Weak acid
C
HNO2(aq) + OH-(aq)
Stronger acid
Ka = 1 x 10-4


NO2-(aq)
+
H2O(l)
Weaker acid
Ka = 1 x 10-14
Strong bases react completely “100%” with any acid.
Equal moles of acid base make essentially sodium nitrite
(NaNO2).
So the real question is: what does the NO2- ion do? React
slightly with water (here) to make OH-.
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Strong acid
Weak base
A

HCl + H2O  H3O+ + ClH3O+(aq) +
Stronger acid
OAc-(aq) 
Weak base
H2O(l) + HOAc(aq)
Weaker acid
Strong acids react completely “100%” with any base.
Equal moles of HCl and acetate make a solution of the
weak acid (acetic acid).
This solution is acidic.
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Determining K from concentrations and pH
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HA(aq) + H2O(l)

H3O+(aq)
+
A-(aq)
Strong acids react completely with any base,
even water. So [H3O+] = MHA
Weak acids react partially with water as determined by
the dissociation constant Ka. In this case, [H3O+] < MHA
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A 0.015 M solution of hydrogen cyanate HOCN has a pH = 2.67.
What is Ka for HOCN?
[H3O+] = 10-2.67 = 0.00214 M
I
C
E
HOCN + H2O  H3O+ + OCN0.015
0
0
-0.00214 M
+0.00214 M
+0.00214 M
0.01286 M
0.00214 M
0.00214 M
= 3.56 x 10-4
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Predicting the pH of a weak acid or weak base
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What is the pH of a
0.0030 M solution of
potassium cyanide
(Kb = 2.5 x 10-5)?
For dilute acids or bases, you
may have to use the quadratic
equation or successive
approximations.
For more concentrated
solutions, one approximation is
generally sufficient.
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This is the last part of
the problem, which I
did not finish in class
(we got to [OH-] =
2.611 x 10-4 M.
Use the quadratic equation or successive approximations if the
concentration of the weak acid < 100 x Ka.
One approximation is sufficient if the
concentration of the weak acid > 100 x K a.
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Polyprotic acids
Strong (H2SO4): Ka’s large
Weak (H3PO4): Ka1 > Ka2 > Ka3
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H3PO4  H2PO4- + H3O+
Ka1 = 7 x 10-3
H2PO4-  HPO42- + H3O+
Ka2 = 6 x 10-8
HPO42-  PO43- + H3O+
Ka3 = 4 x 10-13
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