Chem. 31 – 5/6 Lecture. Announcements Lab Stuff –Reminder that resubmissions must have original...
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Transcript of Chem. 31 – 5/6 Lecture. Announcements Lab Stuff –Reminder that resubmissions must have original...
Chem. 31 – 5/6 Lecture
Announcements• Lab Stuff
– Reminder that resubmissions must have original report attached
– IC and Formal Lab Reports due 5/11• Today’s Lecture
– Chapter 10: Acid – Base Titrations• Strong Acid – Strong Base Titrations• Qualitative Aspects of Acid Base Titrations• Weak Acid – Strong Base Titrations
Chapter 10 – Acid Base Titrations
Strong Acid – Strong Base Titrations• Strong Acid – Strong
Base Titration– How does pH Change as
NaOH is added?– 3 regions to titrations
(different calculations in each region):
• 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 HCl, 25 mL
on this one
Chapter 10 – Acid Base Titrations
Strong Acid – Strong Base Titrations
0.100 M NaOH
0.050 M HCl, 25 mL
Titration Plot
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0 5 10 15 20 25
V(HCl)
pH
Titration Plot
Chapter 10 – Acid Base Titrations
Strong Acid – Strong Base Titrations• What affects shape of
curve?– Concentration of acid
and base– Curves are normally
sharp. Only exception occurs when intial concentrations are very low (e.g. [H+]o, [OH-]o < 10-4 M)
– Note: At low concentrations, one also needs to ensure CO2 doesn’t affect NaOH solutions
Titration Plot
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0 10 20 30
V(HCl)
pH
50 mM acid
1 mM acid
50 uM acid
Note: Base conc. changed by same factor
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations• What affects accuracy and precision?
– Depends on method for determining equivalence point
– End point = “measured” volume of titrant– Titration error (systematic) = difference between
end point and equivalence point– With indicators, systematic errors occur when
pH(end point) ≠ pKa(indicator) – This causes the titration error
– With indicators, random errors occur from difficulty in determining correct color shade which depends on [In-]/[HIn]
– A factor of 2 uncertainty in [In-]/[HIn] (e.g. 0.5 to 2) translates to pHmeas = pHtrue + 0.3
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations• What affects accuracy and precision?
Example 1 – precise + accurate titration
Uncertainty from judging indicator color (e.g. Methyl orange)
sV is small
pH
Vol. Base
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations• What affects accuracy and precision?
Example 2 – mainly imprecise – due to unsharp titration (e.g. low concentrations)
Uncertainty from judging indicator color (e.g. Methyl orange)
sV is large
pH
Vol. Base
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations• What affects accuracy and precision?
Example 3 – mainly inaccurate titration (due to selection of wrong indicator
small unc. in V but significant error
pH
Vol. Base
equiv. pt. pH
indicator color change region (pH < pKa)
Chapter 10 – Acid Base Titrations
General Acid – Base Titrations• Overview:
– What affects accuracy and precision?• Best titrations are sharp• Best titrations measure pH change near actual
equivalence point pH value
Chapter 10 – Acid Base Titrations
Some Questions1. 25.0 mL of 0.200 M NaOH is being titrated
with 0.120 M HNO3. The student wants to estimate the pH after adding 45 mL of HNO3. To do the calculation, should the student calculate [H+] or [OH-] first?
2. For a titration in which HCl is added to a weak base, the pH at the equivalence point is calculated to be 5.21. A student chooses to use Congo Red as an indicator. This indicator has a pKa of 4.0.a) Will selection of this indicator result in a systematic
error?b) Besides, the pKa, what other factor will affect errors
in estimating the equivalence point volume?
Weak Acid – Strong Base Titration
• Weak Acid – Strong Base 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 11 – Acid Base Titrations
Weak Acid – Strong Base Titrations• What affects shape of
curve?– 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)
Titration Plot
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0 5 10 15 20 25
V(HCl)
pH
pKa = 4.76
pKa = 3
pKa = 7
Titration Plot
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0 5 10 15 20 25
V(HCl)
pH
50 mM acid
1 mM acid
50 uM acid
50 mM examplepKa = 4.76 example
