1. Catalyst

4. Lecture 7.9 – Polyprotic Acids and Polyprotic Titrations

5. Today’s Learning Targets
LT 7.21 – I can explain how to write the equilibrium constants for polyprotic acids and identify its equivalence points.

6. Polyprotic Acids
Polyprotic acids are acids have more than one H that can be considered acidic.
H2SO3 H+ + HSO Ka1 = 1.7 x 10-2
HSO H+ + SO 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. ⇌ ⇌

7. Class Example
The solubility of CO2 in water at 25 0C is 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 M solution of H2CO3. Ka1 = 4.3 x and Ka2 = 5.6 x 10-11 ⇌

8. Table Talk
Calculate the pH of a solution of oxalic acid (H2C2O4). Ka1 = 5.9 x and Ka2 = 6.5 x 10-5

9. Polyprotic Titrations
When titrating a polyprotic acid with a strong base, the reaction proceeds in multiple steps.
H3PO H+ + H2PO Ka1
H2PO H+ + HPO Ka2
HPO H+ + PO 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 ⇌ ⇌ ⇌

11. Class Example
How many acidic protons does the following polyprotic acid have according to the graph below?

12. Table Talk
How many acidic protons does the following polyprotic acid have according to the graph below?

13. Drill Baby Drill
You have 45 minutes to work on the drills, integrated practice, and FRQ practice.
Options:
Drill – Weak Acid/Base Equilibrium
Drill – Buffers
Drill – Weak Acid/Base Titrations
Integrated Practice – Acids and Bases
FRQ Practice – FRQ #1: Equilibrium
Exit Slip in last 10 minutes of class

15. Closing Time
Read: 16.6 and 17.3
Homework: