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2
Bronsted Acids & Bases
Acid: substance capable of donating a proton (H+).
Base: substance capable of accepting a proton.(definition not dependent on OH-)
3
Conjugate Pairs
Acid & conjugate base pairBase and conjugate acid pair
CH3COOH + H2Oacetic acid water
CH3COO- + H3O+
acetate ion hydronium ion
Acid: H+ donorBase: H+ acceptor
How do you recognize a conjugate pair?
5
Bronsted Base
NH3 H2O
Bronsted bases must have an atom with a lone pair of electrons to accept a proton(“coordinate covalent bond”)
H+
7
Autoionization of Water
or
H2O + H2O H3O+ + OH-
H2O H+ + OH-
What are the conjugate pairs?
shorthand
8
Ion Product of Water
or since [H2O] is ~constant:
Kc[H2O] = Kw = [H+][OH-]
K’c =[H+][OH-]
[H2O]
Remember: H+ is shorthand for H3O+
Kw = [H3O+][OH-]
9
Ion Product of Water
In pure water at 25oC:
[H3O+] = [OH-] = 1.0 x 10-7 M
Thus
Kw = [H3O+][OH-] = 1.0 x 10-14
Kw is temperature dependent, but we will use this value for most calculations.
11
[H+] and [OH-] Calculations
1.In an ammonia solution, what is [H+] if [OH-] is 0.0025 M?
2.What is [OH-] in an HCl solution with [H+] of 1.3 M?
12
pH: DefinitionpH = -log[H+] (no units)
pH increases as [H+] decreases
Acidic: [H+] > 10-7 M & pH < 7Basic: [H+] < 10-7 M & pH > 7Neutral: [H+] = 10-7 M & pH = 7*
*Neutral [H+] = [OH-]Neutral is pH = 7 only at 25oC.
13
pH of Common FluidsStomach fluid 1.5Vinegar 3.0Orange juice 3.5Water + air 5.5Blood 7.4Milk of magnesia 11.5
pH is a log function. A pH increase of 1.0 means [H+] decrease by factor of 10.
15
pOH pX = –log(X)
pOH = -log[OH-]
Kw = [H+][OH-] = 1.0 x 10-14
Taking negative logs:
-log[H+] - log[OH-] = 14.00
pH + pOH = 14.00
16
pH pHun
•If [H+] = 3.2 x 10-4 M, what is the pH?
•If the pH of “acid rain” is 4.82, what is [H+]?
•If a basic solution has [OH-] of 2.9 x 10-4 M, what is pH?
Temp. Dependence of Kw
17
pKw
temp. (oC)
H2O H+ + OH-
Is this reaction endo-or exothermic?
18
Acid Strength
Strong acids: completely ionized in water.
HCl, HBr, HI, HNO3, H2SO4, HClO3
Strong oxyacid if #O - #H > 2This applies to neutral acids, not ions.
H2SO4 + H2O H3O+ + HSO4
-100%
Know!
Strong Acids
19
HCl + H2O H3O+ + Cl-
The anion of a strong acid (Cl-) doesn’t ‘hydrolyze’. Cl- does not react with water to bond with an H+.
20
Weak Acids
Weak acids incompletely ionize in water.
HF, NH4+, organic acids (CH3COOH)
HF + H2O H3O+ + F-
Equilibrium lies far to the left. [HF] >> [F-]
since F- holds onto the H+
21
Strong Bases Completely ionized in water.
NaOH, KOH, Ba(OH)2
KOH K+(aq) + OH-(aq)H2O
Bronsted base
Group 1 & 2 hydroxides. All ionize completely, even though some are only slightly soluble (e.g. Mg(OH)2.)
Strong Bases
22
KOH K+(aq) + OH-(aq)H2O
The cation of a strong base (K+) doesn’t ‘hydrolyze’. K+ does not react with water to bond with an OH-
.Group 1 & 2 ions do not hydrolyze.
23
Weak Bases
Weak electrolytes.
NH3 + H2O NH4+ + OH-
Unlike an acid, a weak base does not ionize. Rather is causes the ionization of water.
Common Weak Bases
24
Ammonia, NH3
Organic amines, e.g. CH3NH2
Pyridines, e.g.
What do these have in common?
25
General Properties(Table 15.2)
HCl + H2O H3O+ + Cl-
1. The stronger the acid
the weaker the conjugate base.
HA + H2O H3O+ + A-
2. H3O+ is strongest acid in water.
Stronger acids yield H3O+.
26
General Properties(Table 15.2)
3.All weak acids have the equilibrium far to the left.
4.OH- is strongest base in water. Stronger bases produce OH-.
O-2 + H2O 2OH-
HF + H2O H3O+ + F-
27
PredictionUse Table 15.2 to predict whether primarily reactants or products will remain when HNO2 is mixed with KCN.
HNO2 + CN- NO2- + HCN
stronger weakeracid acid
28
Prediction
Use Table 15.2 to predict whether Kc is >1 or <1 for the following reaction:
F- + HNO2 HF + NO2-
weaker strongeracid acid
29
Strong AcidCalculate pH of 1.0 x 10-3 M HCl.
HCl H+ + Cl-
Initial: 1.x10-3 0 0Change: –1.x10-3 1.x10-3 1.x10-3
Equilib: 0 1.x10-3 1.x10-3
pH = -log(1.0x10-3) = 3.00
ICE
For strong acids, [H3O+] is same
as starting acid concentration.
One way arrow
30
Strong Base: Try It
What is the pH of a 0.020 M barium hydroxide solution?
31
Weak Acids
HA + H2O H3O+ + A-
or simply:
HA H+ + A-
Acid ionization constant, Ka
Ka = [H+][A-]
[HA]
Two way arrow
32
Acid Ionization Constant
The strength of a weak acid is measured by Ka.
See Table 15.3.
You will need to look up Ka’s!
33
Problems Involving KaWhat is the pH of 0.050 M HF, given Ka = 7.1 x 10-4 ?
or simply:HF H+ + F-
(ignore autoionization of water)
HF + H2O H3O+ + F-
Problems Involving Ka
34
ICE
Initial: 0.050 0 0Change: -x +x +xEqulib.: 0.050-x x x
HF H+ + F-
Ka = x2
(0.050 – x)= 7.1 x 10-4
35
Ka Problem continued
x2
(0.050 – x)= 7.1 x 10-4
There are 2 ways to solve!!!1. quadratic (yuck!)2. successive approximation
36
1. Quadratic
x2
(0.050 – x)= 7.1 x 10-4
x2 + (7.1x10-4)x -3.6x10-5 = 0
x = -b ± b2 – 4ac
2a
a b c
1
37
1. Quadraticx2 + (7.1x10-4)x -3.6x10-5 = 0
You get 2 answers; only one is reasonable.
x = 5.6 x 10-3 M = [H+]
x = -b ± b2 – 4ac
2a
pH = -log(5.6 x 10-3) = 2.25
38
2. Successive Approximation
Ka = x2
(0.050 – x)= 7.1 x 10-4
Assume x is very small compared to 0.050 so that the degree of ionization is small.
This works for Ka ~ 10-4 or less.
HF H+ + F-
39
2. Successive Approximation
x2
0.050 = 7.1 x 10-4
x = 0.0060 M
(ignore ‘x’ here)
Ka = x2
(0.050 – x)= 7.1 x 10-4
40
2. Successive Approximation
Plug this value of ‘x’ into the denominator of the Ka equation, and recalculate another ‘x’.
Repeat until ‘x’ doesn’t change within 2 sig figs.
41
2. Successive Approximation
Put x = 0.0060 M here & calculate another x.
= 7.1 x 10-4Ka = x2
(.050 - x)
x = 0.0056 M
(repeat again and get same value)
42
3. Successive Approximation
x = [H+] = 0.0056 M
pH = -log(0.0056)
= 2.25
Usually 2 cycles will do it!
same as quadratic!
43
Summary (so far)
1. Express Ka in terms of ‘x’, the change in concentration (ICE).
2.Use successive approximation first, then quadratic if needed.
3.Having ‘x’, calculated all else.4. pH depends on both acid strength
(Ka) and acid concentration.
44
Try It !!!
1.Calculate pH of 0.036 M nitrous acid (Ka = 4.5 x 10-4)
2. The pH of 0.10 M methanoicacid is 2.39. What is Ka ?
45
Percent Ionization (PI) Another measure of the
strength of an acid.
HA H+ + A-
=[H+][HA]o
x 100
PI = Ionized acid conc. at eq.Initial acid conc.
X 100
46
For 0.50 M HF
PI = [H+][HF]o
x 100
For 0.050 M HF
PI = 0.00560.050
x 100 = 11%
0.0190.50
x 100 = 3.8%PI =
More dilute greater PI
(See slide 39)
47
Percent Ionization (PI)
[HA]o
PI
0
100 strong acid
weak acid
LeChatelier’s PrincipleHA + H2O H3O
+ + A-
48
Weak Bases (Kb)Treated in the same way.
NH3 + H2O NH4+ + OH-
K = [NH4
+][OH-]
[NH3][H2O]
Name the conjugate pairs.
K[H2O] = Kb = [NH4
+][OH-]
[NH3]
49
Base Ionization Constant, Kb
Problems are solved the same as with acids except calculate [OH-] first. Try it.
What is the pH of 0.40 M NH3solution? Kb = 1.8 x 10-5
See Table 15.4
50
Acid Conjugate Basee.g. acetic acid (HAc) and
sodium acetate (NaAc)
For acetic acid:
HAc + H2O H3O+ + Ac-
Ka =[H3O
+][Ac-][HAc]
51
Acid Conjugate Base
Then: Ac- + H2O HAc + OH-
For sodium acetate:
Nothing happens to Na+ WHY?
First: NaAc Na+ + Ac-H2O
Kb = [HAc][OH-]
[Ac-]
Group 1 & 2 ions do not hydrolyze.
52
Acid Conjugate Base
For a conjugate pair (HA and A-), show that : Ka x Kb = Kwand that the two reactions add to the autoionization of water.
(recall the addition rule for chemical equilibria)
(Also note: pKa + pKb = pKw = 14)
Conjugate Pairs
53
1. Conjugate ion of a weak acid is a weak base.
HF H+ + F-
2. Conjugate ion of a weak base is a weak acid.NH3 + H2O NH4
+ + OH-
3. The stronger the acid/base, the weaker its conjugate.
54
Acid Conjugate Base
Ka Kb = Kw
Try it.
If Ka for acetic acid is 1.8 x 10-5, what is Kb for sodium acetate?
55
Polyprotic Acids
e.g. H2CO3 stepwise ionization
Step 1:
H2CO3 H+ + HCO3-
Ka1 =[H+][HCO3
-][H2CO3]
56
Polyprotic Acids
Step 2: conjugate base of step 1 is the acid of step 2.
HCO3- H+ + CO3
-2
Ka2 =[H+][CO3
-2][HCO3
-]
57
Polyprotic AcidsKa1 is always much greater than Ka2. Why?
See Table 15.5
Ka1 = 4.2E-7Ka2 = 4.8E-11
Overall K
58
What is the product of Ka1 x Ka2?
H2CO3 2H+ + CO3-2
H2CO3 H+ + HCO3-
HCO3- H+ + CO3
-2
Ka1 x Ka2 = 2.0E-17
59
Fun with Oxalic Acid
C2H2O4 is diprotic with
Ka1 = 6.5 x 10-2
Ka2 = 6.1 x 10-5
Calculate conc. of all species in 0.10 M solution of C2H2O4
H-O-C-C-O-H
O O
60
Oxalic AcidStep 1: 1st ionization.
C2H2O4 H+ + C2HO4-
I 0.10 0 0C -x +x +xE 0.10-x x x
Ka1 = x2
0.10 - x= 6.5 x 10-2
61
Oxalic Acid: Step 1
Solve for ‘x’. Approximation method is so far off that you must use quadratic.
x = 0.054 M
X2 + 0.065 x - 0.0065 = 0
62
Oxalic Acid: Step 1 Results
[H+] = 0.054 M
[C2HO4-] = 0.054 M
[C2H2O4] = (0.10 – 0.054) M
= 0.046 M
Now use these as starting values for 2nd ionization.
63
Oxalic Acid: Step 2
I 0.054 0.054 0C -y +y +yE 0.054-y 0.054+y y
C2HO4- H+ + C2O4
-2
2nd ionization
Ka2 = (0.054+y) y
(0.054-y)= 6.1 x 10-5
65
Oxalic Acid: Final Results
[C2H2O4] = 0.046 M
[C2HO4-] = (0.054 – 6.1 x 10-5)M
= 0.054 M
[H+] = (0.054 + 6.1 x 10-5)M
= 0.054 M
[C2O4-2] = 6.1 x 10-5 M
1st only
1st only
1st only
Ka2
66
Oxalic Acid
Note these general results:
2.Second stage conjugate base concentration is equal to Ka2.
1.If Ka1 >> Ka2, [H+] & [first
conjugate base] are result of 1st stage ionization only.
67
Molecular Structure vs. Acid Strength
How is molecular structure of the acid related to acid strength?
HX H+ + X-
Acid strength is measured by degree of ionization (Ka).
68
Acid StrengthDetermine by two factors:
1. Bond strength: the higher the bond dissociation energy, the weaker the acid.
H—A
2. Bond polarity: the more polar the bond, the stronger the acid.
Why?
d+ d-
69
Hydrohalic Acids
HF<<HCl<HBr<HIacid strength
Bond E
nerg
y
Bond P
ola
rity
HF
HCl
HBr
HI
weakstrong
strongstrong
Win
s!
d+ d-H—A
71
Oxyacids
General structure Z—O—H
If Z is highly electronegative or in high oxidation state, it will attract bonding electrons and make the O—H bond more polar, resulting in a stronger acid.
d- d+
72
Oxyacids with Z Atom from same Group
HClO3 vs. HBrO3
Cl more electronegative, therefore HClO3 is stronger.
What is oxidation number of Z ?
H—O—Z—O
O
73
Oxyacids with Same Z
HClO4 >HClO3 >HClO2 >HClO
acid strength
As oxidation state of Cl increases, acid strength increases. Remember the rule for strong acids.
Ox.#+7 +5 +3 +1
74
Oxyacids: Summary
General structure Z—O—H
•High electronegativity•High oxidation number
For element Z:
strongeracid
75
Predict !!!Place in order of decreasing strength and state which are strong/weak:
1.HIO, HBrO, HClO
2.HNO3, HNO2
3.H3PO3, H3PO4
76
Salt “Hydrolysis” Reaction of a cation and/or
anion of a salt with water, changing pH.
Which Ions React with H2O to Change pH?
77
Anions from strong acids do not hydrolyze. E.g. Cl- comes from the strong acid HCl.
Cations from strong bases do not hydrolyze. E.g. Na+ comes from the strong base NaOH.
All other ions hydrolyze.
Know strong acids and bases.
78
“Neutral” SaltsGroup 1 and 2 cations (except Be+2) combined with anions of a strong acid do not hydrolyze and are neutral.
NaNO3 Na+(aq) + NO3-(aq)
pH ~ 7NaOH HNO3
(strong base, (strong acidno hydrolysis) no hydrolysis)
79
“Basic” SaltsSalt of weak acid and strong base is basic.
CH3COONa Na+ + CH3COO-H2O
CH3COO- + H2O CH3COOH + OH-Na+ doesn’t hydrolyze, but:
basic
80
“Basic” Salts
CH3COO- + H2O CH3COOH + OH-
Kb = [CH3COOH ][OH-]
[CH3COO-]
CH3COONa acts like a weak base when added to water.
82
“Acidic” Salts
Salt of strong acid and weak base is acidic.
NH4Cl NH4+(aq) + Cl-(aq)
H2O
Cl- doesn’t hydrolyze, but:
NH4+ + H2O NH3 + H3O
+
acidic
83
“Acidic” Salts
NH4+ + H2O NH3 + H3O
+
or simply: NH4+ NH3 + H+
Ka =[NH3][H
+][NH4
+]
Can look up Ka of NH4+
(or Kb of NH3) to solve problems.
84
“Acidic” SaltsThe most acidic cations are small and highly charged.
(high charge density)
e.g. Al+3, Cr+3, Fe+3, Bi+3, Be+2
Larger, +1 cations do not hydrolyze (K+, Na+).
85
Try It !!Al+3 ion becomes hydrated in water, forming Al(H2O)6
+3, which then hydrolyzes as follows:
What is the pH of 0.020 M AlCl3?
Al(H2O)6+3 Al(OH)(H2O)5
+2 + H+
Ka = 1.3 x 10-5
86
Salts in Which Both Cation & Anion Hydrolyze
The math is involved, so let’s just make some qualitative predictions.
87
Salts in Which Both Cation & Anion Hydrolyze
Kb for anion < Ka for cation, solution will be acidic.
Kb for anion ~ Ka for cation,solution will be ~ neutral.
Kb for anion > Ka for cation, solution will be basic.
88
PredictAre solutions of the following salts acidic, neutral, or basic?
NH4I CaCl2Fe(NO3)3 KCN
LiClO4 NH4CN
Ka~E-10 Kb~E-5
89
HCO3- Acidic or Basic?
HCO3- + H2O H3O
+ + CO3-2
HCO3- + H2O H2CO3 + OH-
Ka = 4.8 x 10-11
Kb = 2.4 x 10-8
Since Kb > Ka, solution is basic.
HSO4-: An Exception!
90
HSO4- is the only common
acidic anion.
Cl-, NO3-, ClO4
- neutral
F-, CO32-, HCO3
-, H2PO4- basic
HSO4- acidic
Remember this guy!
91
Oxides See Table 15.8
basic amphoteric acidic
1 2 13 14 15 16 17
92
Basic (Metallic) Oxides
Na2O + H2O 2NaOH
BaO + H2O Ba(OH)2
Metal oxides are “anhydrous” bases (Bases with the water removed).
93
Acidic (Nonmetal) Oxides
Nonmetal oxides are anhydrous acids.
CO2 + H2O H2CO3
(water exposed to air)
SO3 + H2O H2SO4
(acid rain)
95
Amphoteric Oxides
Al2O3 + 6HCl 2AlCl3 + 3H2O
Al2O3 + 2NaOH + 3H2O
2NaAl(OH)4
Al2O3 is both basic & acidic!
96
Transition Metal Oxides
Some transition metal oxides are acidic if oxidation number is high.
Mn2O7 + H2O 2HMnO4
CrO3 + H2O H2CrO4
(No change in oxidation no.)
+7 +7
+6 +6
97
pH and Solubility
Solubility of “insoluble” basic salts are greatly affected by pH due to common ion effect.
e.g. Mg(OH)2 and BaF2
Since OH- and F- both react with H+, the solubility of Mg(OH)2 and BaF2 are affected by pH.
98
pH and SolubilityMg(OH)2 has Ksp = 1.2 x 10-11
Its pH in water is given by:Mg(OH)2 Mg2+ + 2OH-
Ksp = 1.2 x 10-11 = (Mg+2)(OH-)2
2s = [OH-] = 2.8 x 10-4 M
pH = 10.45
s = 1.4 x 10-4 M
= (s)(2s)2
99
pH and Solubility
Mg(OH)2(s) Mg+2(aq) + 2OH-(aq)
If H+ is added to lower pH below 10.45, the added H+ reacts with OH-, thus lowering the [OH-]. Thus more Mg(OH)2 will dissolve to keep Ksp constant (LeChatelier)
100
Similarly for a basic salt like BaF2:
BaF2(s) Ba+2(s) + 2F-(s)
pH and Solubility
Thus more BaF2 will dissolve at lower pH (LeChat.)
The added H+ to lower pH reacts with F-, forming the weak acid HF.
H+(aq) + F-(aq) HF(aq)