1. Chem. 1B – 9/20 Lecture

2. Announcements I
Second Mastering Set due today (should cover material to do most problems today)
Exam #1
Next week on Thursday (9/29)
Same format as last year (1 12 point work out problem, 22 multiple choice problems – each worth 4 points plus one bonus multiple choice problem)
Covers material covered through Thurs. (last topic = buffers)

3. Announcements II Today’s Lecture Polyprotic acids
Polyprotic acids – conjugate bases
Dependence of Ka on acid structure
Buffers (Chapter 16)

4. Chem 1B – Aqueous Chemistry Some Practice
Which solution will have a greater fraction of ionization? 0.10 M HClO vs M HF
Ka(HClO) = 2.9 x Ka(HF) = 3.5 x 10-4
(did already)
An unknown base is dissolved in water so that its initial molarity is M. The pH is measured and found to be What is its Kb value?
The Kb for NH3 is 1.76 x What is the pH of a solution initially made to 0.10 M NH4Cl?

5. Chem 1B – Aqueous Chemistry Polyprotic Acids
Generic Example: H2A – has two protons that can be lost through acid reactions (diprotic)
Some Examples:
H2SO4 (sulfuric – first H+ loss is strong acid)
H2SO3 (sulfurous)
H2CO3 (carbonic)
H3PO4 (phosphoric – triprotic)
Reaction of generic diprotic example
H2A(aq) ↔ H+(aq) + HA-(aq) K = Ka1
HA- (aq) ↔ H+(aq) + A2-(aq) K = Ka2

6. Chem 1B – Aqueous Chemistry Polyprotic Acids – in Problems
Solving polyprotic acid problems can be challenging (the concentrations of the products from the first reaction affect the equilibrium in the second reaction)
To simplify the problem, we assume the two reactions occur independently (valid if Ka1 >> Ka2)
Example Problem: calculate [H2CO3], [HCO3-], pH, and [CO32-] for a 1.0 x 10-3 M solution of H2CO3

7. Chem 1B – Aqueous Chemistry Polyprotic Acids – Salts of
While conjugate bases of monoprotic weak acids can only be basic, conjugate bases of polyprotic acids may be acidic or basic
Example: from H2CO3 (carbonic acid), we have HCO3- and CO32- as conjugate bases (from 1st and then 2nd weak acid reactions)
H2CO3 (aq) ↔ H+(aq) + HCO3-(aq) K = Ka1
HCO3-(aq) ↔ H+(aq) + CO32-(aq) K = Ka2
Salts allow us to “start” in the intermediate or basic form

8. Chem 1B – Aqueous Chemistry Polyprotic Acids – Salts of – cont.
The most basic form (CO32-) can only be basic (it has no H+ to lose), while the intermediate form (HCO3-) can react as an acid or as a base
Acid reaction: HCO3-(aq) ↔ H+(aq) + CO32-(aq)
Base reaction: HCO3-(aq) + H2O(l) ↔ H2CO3(aq) + OH-(aq)
To determine if the intermediate form is acidic or basic, we must compare K values for the acid and base reactions
Acid reaction: K = Ka2 = 4.7 x 10-11
Base reaction: K = Kw/Ka1 = 2.2 x 10-8 > Ka2 so basic

9. Chem 1B – Aqueous Chemistry Polyprotic Acids – Salts of – cont.
Rank the following salts from most acidic to most basic:
KHSO4 Na3PO4 KHCO3 KHC2O4
Acid
Ka1
Ka2
Ka3
H2SO4
>> 1
1.2 x 10-2
H3PO4
7.11 x 10-3
6.32 x 10-8
4.5 x 10-13
H2CO3
4.45 x 10-7
4.69 x 10-11
H2C2O4
5.60 x 10-2
5.42 x 10-5

10. Chem 1B – Aqueous Chemistry Molecular Structure – Acidity Relationship
Acid strength depends on ability for H bond to break and on stability of conjugate base formed
More stable conjugate bases means stronger acid
For example, what makes ethanol (C2H5OH) neutral while acetic acid (CH3CO2H) is acidic?
acetate: stabilized by delocalized electrons
ethanol anion (not very stable)

11. Chem 1B – Aqueous Chemistry Molecular Structure – Acidity Relationship
General Rules for Simpler Structures:
Binary Acids: e.g. HCl
more electronegative element makes for stronger acid
longer (and weaker bond) makes for stronger acid (HCl is stronger than HF due to bond strength)
Oxyacids: e.g. HClO2
more oxygens make acid stronger (HClO4 is a strong acid, HClO is a very weak acid)
Other effects
Adding other electronegative elements often makes conjugate base ions more stable
Compound
CH3CO2H
CClH2CO2H
CCl3CO2H
pKa
4.75
2.86
0.66

12. Chem 1B – Aqueous Chemistry Buffers (Chapter 16)
We have discussed some mixtures briefly (e.g. strong acid + weak acid)
One particular type of mixture: weak acid + conjugate base (or weak base + conjugate acid) makes a solution called a buffer
Buffers are desirable because they keep the pH nearly constant even if an acid or base is added
Buffers are very important in biology because many enzymes (protein catalysts) will only work over a narrow pH range

13. Chem 1B – Aqueous Chemistry Buffers (Chapter 16)
Example: Determine pH of a mix of M HC2H3O2 and M Na+C2H3O2- solution (Ka of HC2H3O2 = 1.8 x 10-5)

14. Chem 1B – Aqueous Chemistry Buffers (Chapter 16)
Buffer Solutions:
Question: Was the ICE Problem set up needed?
Answer: No. The assumption of x << [HA], [A-] is valid for all “traditional” buffers
Traditional Buffer
Weak acid (3 < pKa < 11)
Ratio of weak acid to conjugate base in range 0.1 to 10
mM+ concentration range

15. Chem 1B – Aqueous Chemistry Buffers (Chapter 16)
Buffer Solutions:
Since ICE not needed, can just use Ka equation
Ka = [H+][A-]/[HA] = [H+][A-]o/[HA]o
(always valid) (valid for traditional buffer)
But log version more common
pH = pKa + log([A-]/[HA])
Also known as Henderson-Hasselbalch Equation

16. Chem 1B – Aqueous Chemistry Buffers (Chapter 16)
Addition of small amounts of acid to a buffer:
Example: let’s say we have a buffer made to be M NH M NH4Cl in 1.00 L
Calculate the pH
Now lets add moles of HCl. What is the new pH?

17. Chem 1B – Aqueous Chemistry Buffers (Chapter 16)
Change of pH and Buffer Capacity
A buffer is designed to minimize the change in pH from addition of base or acid
What is the most effective pH?
(show next slide)
Can we add too much acid or base to a buffer?
Does the absolute concentration of acid/base affect pH? Why is a higher concentration better?
Buffer Capacity is the ability of the buffer to absorb acid or base without the pH changing significantly

18. Chem 1B – Aqueous Chemistry Buffers (Chapter 16)
Most Effective pH Range
NH3 + NH4Cl in 1.00 L Example [NH3] + [NH4+] = M
Addition of moles HCl
[NH3] (M)
[NH4+] (M) pH
DpH
0.005
0.145
7.78
-2.75
0.030
0.120
8.64
-0.09
0.050
0.100
8.94
-0.07
0.075
9.24
-0.06
9.55
9.84
-0.08
10.71
-0.32
Best region (equal moles of WA and CB