Chem. 31 – 5/11 Lecture
Announcements I
• Lab Stuff
– IC and Formal Lab Reports due Today
– Rest of Lab Reports due Thursday
• Final Exam:
–
–
–
–
Monday 5/18 (10:15 to 12:15)
~60% new material; ~40% review material
150 points; so about 50% longer than other exams
Allowed one sheet (8.5” x 11”) of notes
Announcements II
• Thursday
– Finish up Chapter 10 topics
– Review of Material
– Teaching Evaluation (lecture only – separate
evaluations in lab)
• Today’s Lecture
– Chapter 10: Acid – Base Titrations
• Weak Acid – Strong Base Titrations
• Weak Base – Strong Acid Titrations
• Diprotic Compounds and Mixtures (qualitative
understanding)
Weak Acid – Strong Base Titration
• Weak Base – Strong Acid
Titration
– How does pH Change as NaOH
is added?
– Reaction:
HA + OH- ↔ A- + H2O K = 1/Kb
Example – acetic acid:
Ka = 1.74 x 10-5
– 4 regions to titrations (different
calculations in each region):
•
•
•
•
initial pH
before equivalence point
at equivalence point
after equivalence point
– Go to Board to show pH at 5 mL,
12.5 mL, and 15 mL
0.100 M NaOH
0.050 M HA, 25 mL
Chapter 10 – Acid Base Titrations
Weak Acid – Strong Base Titrations
– pKa values (low pKa or
stronger weak acid gives
sharper titration)
– pKa affects position of curve
before and at equivalence
point
– Concentration of acid and
base (higher concentrations
give sharper titrations)
– Concentration affects curve
shape at and after
equivalence point
– 50 mM solution will also have
greater calculation errors due
to poor assumptions (e.g.
buffer calculations)
50 mM
pKexample
= 4.76 example
a
TitrationPlot
Plot
Titration
14.00
14.00
12.00
12.00
10.00
10.00
pH
pH
• What affects shape of curve?
pKa50= mM
4.76acid
8.00
8.00
pKa1=mM
3 acid
6.00
6.00
50 uM acid
pKa = 7
4.00
4.00
2.00
2.00
0.00
0.00
00
55
1010
15 15
V(HCl)
V(HCl)
20 20
25 25
Weak Base – Strong Acid Titration
• Weak Base – Strong Acid
Titration
– How does pH Change as HCl is
added?
– Reaction:
B + H+ ↔ BH+ K = 1/Ka
Example – ammonia:
Ka (BH +) = 5.69 x 10-10
– 4 regions to titrations (different
calculations in each region):
•
•
•
•
initial pH
before equivalence point
at equivalence point
after equivalence point
0.100 M HCl
0.050 M B, 25 mL
Chapter 10 – Acid Base Titrations
Other Titrations
• Weak Base – Strong
Acid
– Has similar 4 regions
as weak acid – strong
base
pH
7
• initial (weak base)
• before equiv. point
(buffer problem)
• equiv. point (weak acid)
• after equiv. point
(excess strong acid)
V(acid)
Chapter 10 – Acid Base Titrations
Other Titrations
• More complex titrations
– polyprotic acid by a strong base (e.g. H2SO3 + OH-)
– This example has pKa1 = 1.81 and pKa1 = 6.97
– Titration involves 2 reactions:
1) H2A + OH- ↔ HA- + H2O
Veq2 = 2Veq1
22) HA + OH ↔ A + H2O
Titration Plot
14.000
12.000
pH
Veq2
Veq1
10.000
8.000
6.000
4.000
2.000
0.000
0
5
10
15
20
V(HCl)
25
30
35
40
45
Chapter 10 – Acid Base Titrations
Example Questions
1. A chemist does a test titration for a weak acid
present at 0.10 M with a strong base and notices
only moderate sharpness.
a)
b)
Will titrations at 0.0050 M be improved or worse?
Will the same indicator be ideal at lower concentrations?
2. Looking at the titration curve below, what type of
titration was performed?
a)
b)
c)
d)
strong base by strong acid
diprotic base by strong acid
base mixture by strong acid
diprotic acid by strong base
pH
V(acid)