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Equilibria of Weak Acids & Bases Part VI: Polyprotic Acids & Titration Curves. Chap. 17 Sec 6. Dr. C. Yau Spring 2014. 1. Ionization of Polyprotic Acids. Carbonic acid is an example of a polyprotic acid (more specifically, a diprotic acid). What is the formula of carbonic acid? - PowerPoint PPT Presentation
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Equilibria ofWeak Acids & BasesPart VI: Polyprotic Acids
& Titration Curves
Dr. C. Yau
Spring 2014
Chap. 17 Sec 6
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Ionization of Polyprotic AcidsCarbonic acid is an example of a polyprotic acid (more specifically, a diprotic acid).
What is the formula of carbonic acid?
Write the ionization equations of carbonic acid.H2CO3 + H2O H3O
+ + HCO3
HCO3 + H2O H3O
+ + CO32
Note that pH is based on H3O+ and there are 2 sources!
However, Kal > Ka2
so [H3O+] from 1st ioniz >> [H3O
+] from 2nd ioniz
If Kal >> Ka2 then this becomes negligible.2
Ka1 = 4.5x10-7
Ka2 = 4.7 x 10-11
Compare Ka1 & Ka2
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Note HCO3 is produced in one step and
consumed in the 2nd step.
H2CO3 + H2O H3O+ + HCO3
HCO3 + H2O H3O
+ + CO32
Ka1=4.5×10-7; Ka2=4.7×10-11
Example 17.9 p. 807
Give the concentration of all species in 0.040 M H2CO3 as well as the pH.
Do Pract Exer 40, 41 p. 808
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Salts Of Polyprotic Acids…• are amphoteric, and most are basic in water.
• To predict the behavior in water, compare Ka to its Kb.
• Ex. Is an aqueous solution of NaHCO3 likely to be basic or acidic?– NaHCO3 is a salt of neutral Na+ and amphoteric HCO3
-.
– Ka=4.7×10-11; Kb=1.0x10-14/4.5×10-11=2.22 ×10-4.
– Since Kb> Ka, the salt is basic in water.
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Titration of Nitric Acid by NaOHstrong acid by strong base
What species are present at the 10-20 mL range? Why is the pH increasing?What species are present at 25 mL? at 30 mL?
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3
5
7
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0 10 20 30Volume NaOH (mL)
pH Equivalence Point
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Titration of Acetic Acid by NaOH
How is this graph different from the previous one?Why is it starting at pH 3? Why is pH increasing faster at 10-20mL
Why is the equivalent point not at pH = 7?What is the significance of pH at 13 mL?
3456789
101112
0 10 20 30Volume NaOH (mL)
pHEquivalence Pt
26 mL13 mL
pH = 4.74
Significance of this pH?
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Titration of weak vs. strong acid by strong base
Strong acid by strong base: equivalence pt is at pH = 7
Weak acid by strong base: equivalence pt is at pH > 7 (end with basic solution)
Weak acid produces strong conjugate base, which increases the pH.
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How is this graph different from the others? Why?
If the acid is phosphoric acid, what species are present at (a), (b), (c), (d), (e)?
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4
6
8
10
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0 5 10 15 20 25 30
Volume NaOH (mL)
pH
(a) (b)
(c)(d)
(e)
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How is this graph different?Why is the equivalence point at pH lower than 7?
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Reactions Of Acids And Bases
• Produce conjugate acid and bases• May not create neutral products:
– strong acids + strong bases → neutral salts
– weak acids + strong bases → basic salts
– strong acids + weak bases → acid salts
– weak acids + weak bases → amphoteric salts check Ka vs. Kb for dominance (we will not do titration calculations based on this type of reaction)
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Color Indicators• Indicators change color within specific pH
ranges because each indicator is a weak organic acid
H In + H2O H3O+ + In-
• As the concentrations of conjugate base vary, the color varies
• We usually choose an indicator whose pKa=pH at the endpoint of the reaction
• Select an indicator whose color change is light to dark at this pH to optimize visual detection
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Acid-Base Indicators
pH 3.2 to 4.4 methyl orange
pH 4.8 to 6.4 methyl purple
pH 6.0 to 7.6 bromothymol blue
pH 8.2 to 10.0 phenolphthalein
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