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Catalyst. Lecture 7.9 – Polyprotic Acids and Polyprotic Titrations. Today’s Learning Targets. LT 7.21 – I can explain how to write the equilibrium constants for polyprotic acids and identify its equivalence points. Polyprotic Acids. ⇌. - PowerPoint PPT Presentation
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Catalyst
Lecture 7.9 – Polyprotic Acids and Polyprotic Titrations
Today’s Learning TargetsLT 7.21 – I can explain how to write the
equilibrium constants for polyprotic acids and identify its equivalence points.
Polyprotic AcidsPolyprotic acids are acids have more than one H
that can be considered acidic.H2SO3 H+ + HSO3
- Ka1 = 1.7 x 10-2
HSO3- H+ + SO3
2- Ka2 = 6.4 x 10-8
Each removal of a proton has it’s own Ka value.It is always easier to remove the first proton from a
polyprotic than it is is to remove the second proton.As long as Ka1 > Ka2 by a factor of at least 103, then
we only need to consider first Ka when calculating values like pH.
⇌⇌
Class ExampleThe solubility of CO2 in water at 25 0C is 0.0037
M. the common practice is to assume that all the dissolved CO2 is in the form of carbonic acid (H2CO3), which is produced in the reaction:
CO2 + H2O H2CO3
What is the pH of a 0.0037 M solution of H2CO3. Ka1 = 4.3 x 10-7 and Ka2 = 5.6 x 10-11
⇌
Table TalkCalculate the pH of a solution of oxalic acid
(H2C2O4). Ka1 = 5.9 x 10-2 and Ka2 = 6.5 x 10-5
Polyprotic TitrationsWhen titrating a polyprotic acid with a
strong base, the reaction proceeds in multiple steps.
H3PO4 H+ + H2PO4- Ka1
H2PO4- H+ + HPO4
2- Ka2
HPO42-
H+ + PO43- Ka3
All of the first species must complete react before the second proton is removed.
There will be the same number of equivalence points as there are acidic protons
⇌⇌⇌
Class ExampleHow many acidic protons does the following
polyprotic acid have according to the graph below?
Table TalkHow many acidic protons does the following
polyprotic acid have according to the graph below?
Drill Baby DrillYou have 45 minutes to work
on the drills, integrated practice, and FRQ practice.
Options:Drill – Weak Acid/Base
EquilibriumDrill – BuffersDrill – Weak Acid/Base TitrationsIntegrated Practice – Acids and
BasesFRQ Practice – FRQ #1:
EquilibriumExit Slip in last 10 minutes of
class
Closing TimeClosing TimeRead: 16.6 and 17.3Homework: