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Acid-Base Equilibria1. Application of equilibrium concepts. 2. Not much else new in the way of theory
is presented. 3. Specific focus on aqueous (H2O is
solvent) systems.4. Assume we are at equilibrium unless
otherwise specified.
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I. Acids & Bases
A. General ideas:
1. Some H nuclei are quite stable in theirbonding arrangements. Ex.: H nuclei inmethane (CH4) have almost no tendencyto leave their bonding e! pair and goelsewhere.
CH4 + H2O þ no reaction
Therefor, the H atoms in CH4 are not acidic!
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B. Brønsted-Lowry Theory
1. An acid is a substance that can donate a H+.
2. A base is a substance that can accept a H+. Requires non-bonding e! pair!!!
3. A general expression of an acid-base rxn.:
HA + B W BH+ + A!
acid base acid base
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4. Specific expression:
HCl + H2O W H3O+ + Cl! acid base acid base
C. Conjugates:
1. Conjugate base is what is “leftover” after theacid has donated its H+.
2. Conjugate acid is what is “leftover” after thebase has accepted its H+.
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HNO3 + NH3 W NH4+ + NO3
!
You fill in the blanks above.
Problems:
a) Rxn. of CH3COOH (acetic acid) with water:
b) Rxn. of NH3 (weak base) with water:
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D. Acid & Base Strength (context dependent!!!)
1. 1st, a qualitative approach. Then, quantitative.
2. Think about H2O functioning as a base:
HA(aq) + H2O(l) W H3O+(aq) + A!
(aq) acid base acid base
The 2 bases are competing for the H+ ion. Willthe H+ ion spend most of its time associatedwith the weaker of the 2 bases or the stronger?
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A base does what?
Does a strong base do this better than a weakbase?
3. A relative strength picture: When you dissolve1 mole of HCl in 1 L of water you will get ahigher [H3O+] than when you do the same withCH3COOH. Which is a stronger acid?
Closure question: If HA is a strong acid, is itsconjugate base strong or weak?
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A brief aside: When a H+ ion is released in water,it does not exist as a free H+ ion. It binds to water:
1. H3O+ is called the hydronium ion.
2. H5O2+, H7O3
+, etc., likely exist as well.
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4. A quantitative approach (by far the best): Ka
a) Can you write an equilibrium constantexpression for:
HCl + H2O W H3O+ + Cl!
Keq = [H2O] is so large, it is essentially constant. Therefore wecan simplify Keq into Ka.
Ka = By definition:
Keq × [H2O] = Ka
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b) Can you write a Ka expression for:
CH3COOH + H2O W H3O+ + CH3COO!
Ka =
c) The Ka value for HCl ~ 1 x 106, the value forCH3COOH = 1.8 x 10!5
Which is a stronger acid? HCl or CH3COOH
Logic!? Counting page?
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E. The Dissociation of Water
1. Water is interesting. It can be both an acid anda base:
H2O + H2O W H3O+ + OH! Lewis dot? acid base acid base
On your own, write this reaction using Lewisstructures instead of molecular formulas:
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2. Can we write a Keq expression for the above?
“Keq” =
3. Can we simplify this? Go to steps #4-6 first,then come back here.
Kw =
4. In “pure” water @ 25°C, [H3O+] = 1.0 x 10!7 M.
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5. In “pure” water at 25°C, [OH!] = ?
6. What is [H2O] = ?
7. Now solve for Kw: Kw = [H3O+] [OH!]
Problem: If [H3O+] for a solution = 3.84 x10!9, what is [OH!]?
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II. The pH Scale (power of Hydrogen)There are many reasons for using the pH, as opposed to [H3O+].
Convenience in expressing the numbers is probably most importantto you.
A. By definition: pH = !log [H3O+], so:
[H3O+] = 10!pH
1. Let’s start with an example of pH. Calculate the pH of a 1.0 x 10!2 M solution ofHBr (a strong acid).
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2. Make sure you can also find [H3O+] & [OH!] ifgiven pH .
Calculate [H3O+] & [OH!] for a gastric juicesample with a pH = 2.09.
B. Note that as [H3O+] increases, pH decreases. Logic: pH = negative log of [H3O+].
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C. What about rounding? Essentially the x /÷rule plus one.
!log (1.0 x 10!9) = 9.00
!log (3.28 x 10!5) = 4.484 126 156.
!log (3.28 x 10!9) = ?.??? ??????.
Can you see the pattern yet?
D. Why do we care about pH? (Apollo 13 clip?)
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III. Reactions Between Acids and Bases A. Neutralization
1. HA(aq) + NaOH(aq) W H2O(l) + NaA(aq) acid base water salt
2. I like the following approach better:
HA(aq) + OH!(aq) W H2O(l) + A!
(aq)
What would we call Na, in 1 above?
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3. pH indicators have acid & conjugate baseform. The forms are different colors:
HInd(aq) + OH!(aq) W H2O(l) + Ind!
(aq) acid base acid base
Color 1 Color 2
4. pH paper has indicator linked to the paper
5. In a titration, the indicator tells you when youhave just overshot (reached endpoint). Referto Titration lab exercise for more details.
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What volume of a 0.2000 m [NaOH] solutionwould be required to neutralize 40.00 mL of a0.1207 M solution of HCl?
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IV. Acid-Base Buffers
A. Buffers function to limit change in pH when anacid or base is added to a system.Do you think the blood has buffers?
B. A buffer is prepared by combining a weak acidwith its conjugate base (in what ratio?)
For acetic acid/acetate, CH3COOH combined withCH3COONa.
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What happens to CH3COONa when itdissolves it in water?
H2O
CH3COONa ö
C. Acid part of the buffer neutralizes added base,while the base neutralizes added acid.
Added strong base
CH3COOH + OH! ö
Added strong acid
CH3COO! + H3O+ ö
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D. The Henderson-Hasselbalch Transformation
1. Can we rearrange the Ka expression to get anequation with pH = ?
[H3O+] [A!]Start with Ka = Finish with pH = pKa + [HA] log([A!]'[HA])
If pH = !log [H3O+], what do you think pKa is?
pKa =
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2. Why is the Henderson-HasselbalchTransformation useful? (Buffer issues.)
a) Do you think it would be “nice” to have both HAand A! present in a buffer? yes or no?
b) Why?
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c) What happens to pH in the Henderson-Hasselbalch when [HA] = [A!]?
i) Substitute [HA] = [A!] and simplify. What do youobtain?
ii) What does the result in 2 c) i) mean in terms of mostappropriate choice of weak acid to buffer at a givenpH?
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3. Medical Perspective: Which of dissociation ofH2CO3 is physiologically relevant?
CO2(aq) + H2O(l) W H2CO3(aq)
H2CO3(aq) + H2O(l) W HCO3!
(aq) + H3O+(aq)
pKa1 = 6.38
HCO3!
(aq) + H2O(l) W CO32!
(aq) + H3O+(aq)
pKa2 =10.32
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4. Let’s look at the Normal Values section of: http://www.nlm.nih.gov/medlineplus/ency/article/003855.htm
a) Can you relate the terms in pH'PaCO2'PaO2'SaO2'HCO3
! to variables in the Henderson-Hasselbalchtransformation?Which values relate directly to acid-base chemistry?
pH = pKa + log([A!]'[HA])
pH PaCO2 PaO2 SaO2 HCO3
!
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b) How do you know if HCO3! is acting like an acid or a
base? See pKa values above and think of CO2 eventuallyleaving the body.
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Counting H3O+ formed for strong acid & weak acid.To get more comfortable with quantitative aspects of acid strength, we are
going to try counting the number of acid molecules that would dissociate toform H3O+. While we would normally use concentration values (and countingmolecules is not quite proper), most students understand the concept better bythinking about numbers of molecules/ions, as opposed to concentrations ofthese.
We will assume we are placing 1 x 106 molecules of HCl into water andallowing the dissociation reaction to proceed until equilibrium is reached. (Could we apply Ka if we had not reached equilibrium?) .
(Reaction quotient tells you if you have reached equilibrium.)
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For HCl, Ka =
HCl + H2O W H3O+ + Cl!
Start
After 1 reacts
After 10 react
@ equilibrium
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Now do the same for acetic acid. Ka =
CH3COOH + H2O W H3O+ + CH3COO!
Start After 1 reacts After 10 react @ equilibrium
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Change in acid-conjugate base ratios as a function of pH
pH 65432 7 8 9 10
H2O H3O+
OH-11
101
1001
1,0001
104
1105
11
101
1001
1000
R
CO
O
H
+ H2O
Aspirin,HA form
R
CO-
O
Aspirin,A- form
H3O++pKa = 3.0
Aspirin HAA-
11
101
110
1100
11000 104
11051
H2CO3
pKa: 3
HAA-
pKa: 6.37
+ H2OCarbonic acid,HA form
Carbonic acid,A- form
H3O++pKa = 6.37
C
O
OOHH C
O
O-OH
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5. Can you fill in the blanks for carbonic acid(H2CO3), above? Two approaches:
a) Fill in ratios for pH values 2.37, 3.37, 4.37,etc., remember log term in H-H equation.
b) Use the H-H equation to calculate the ratioof H3O+'OH! for pH values 2, 3, 4, etc.
6. Does this approach give different way to lookat acid strength? How dos this relate to loss offunctions/in CO2 poisoning (re. Apollo 13)?
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Change in protein activity as a function of pH
pH 65432 7 8 9 10
+ H2OHProtein H3O++pKa = 6.0
HProtein 10001
1001
101
11 10
1100
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Most proteins have acid/base groups. Assume here that a protein has no activity in its HProtein form & is 100% active in the Protein- form.Would the activity be higher at pH 7.4 or 6.0???What would a graph of activity vs. pH look like?
Protein-
Protein-
inactive active
10001
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activ
ity (%
)
20
40
60
80
100
pH642 8 10
pH Activity Relationship
