Chapter 15 1. Monoprotic Bronsted Acid and Base Bronsted Acids- able to donate protons in the form...
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Transcript of Chapter 15 1. Monoprotic Bronsted Acid and Base Bronsted Acids- able to donate protons in the form...
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Monoprotic Bronsted Acid and BaseBronsted Acids- able to donate protons in the form of hydrogen ions – protons – H+.
AH A- + H+
Bronsted Base- able to accept protons in the form of hydrogen ions or H+.
B- + H+ BH
HCl Cl- + H+
C2H3O2H C2H3O2- + H+
NH3 + H+ NH4+
HO- + H+ H2O
Conjugate Acid-Base Pairs
In the reaction of HF and H2O,• HF/F− is one conjugate acid-base pair.• H2O/H3O+ is the other conjugate acid-base pair.• Each pair is related by a loss and gain of H+.
3
4
Autoionization of Water
Kw is called the ion-product constant of water.
H2O(l) H+(aq) + OH-
(aq)
Kc = [H+][OH-] = Kw
At 250C
Kw = [H+][OH-] = 1.0 x 10-14
[H+] = [OH-][H+] > [OH-][H+] < [OH-]
neutralacidicbasic
pH + pOH = 14
pH = -log [H+]
pOH = -log [OH]
5
Ka and Kb
A-(aq) + H2O(l) AH(aq) + OH-(aq)
Kb =[AH][OH-]
[A-]Ka =
[H+][A-][HA]
HA(aq) H+(aq) + A-
(aq)
HA (aq) H+ (aq) + A- (aq)
A- (aq) + H2O (l) OH- (aq) + HA (aq)
Ka
Kb
H2O (l) H+ (aq) + OH- (aq) Kw
KaKb = Kw
For a acid-base conjugate pair in water:
6
Alternative Notation
Ka =[H+][A-][HA]
HA(aq) H+(aq) + A-
(aq)
acid ionization constant
Ka
acidstrength
[H+] at equilibrium
Initial concentration of [HA]x 100%
percent ionization =
Percent ionization = [H+][HA]0
x 100%
Percent ionization:• Strong acids- % ionization is always 100%
• Weak acids- % ionization decreases as concentration increases (Ka stays the same!).
8
Polyprotic Bronsted Acid and Base• May yield more than one hydrogen ion per molecule.
HS
H
• Ionize in a stepwise manner; they lose one proton at a time.
O O
O
HH
H2CO3 HCO3- + H+
O O
O
H
acid conjugate baseacid
HCO3-
CO32- + H+
acid conjugate base
9
•An ionization constant expression can be written for each ionization stage.
Polyprotic Bronsted Acid and Base• Ionize in a stepwise manner; they lose one proton at a time.
Ka1
Ka2
10
• Conjugate base is the acid for the next equilibrium.
• The second Ka is “always” smaller than the first.
• The acid strength decreases as we go to the species with fewer protons.
• Easier to separate H+ and X- than H+ from X2-.
• Consequently, two or more equilibrium constant expressions must often be used to calculate the concentrations of species in the acid solution.
Polyprotic Bronsted Acid and Base
Example
11
15.11
Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings). Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution.
H2C2O4(aq) H+(aq) + HC2O4-(aq)
HC2O4-(aq) H+(aq) + C2O4
2-(aq)
Ka1
Ka2
0.10 M
Ka1 = 6.5 x 10-2 Ka2 = 6.1 x 10-5
From Table 15.5:
Example
12
15.11
Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings). Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution.
H2C2O4(aq) H+(aq) + (aq)
Initial (M):
Change (M):
Equilibrium (M):
-2 4HC O
0.10 0.00 0.00
-x +x +x
xx0.10 - x
= 6.5 x 10-2
Example
13
15.11
H2C2O4(aq) H+(aq) + (aq)
Initial (M):
Change (M):
Equilibrium (M):
-2 4HC O
0.10 0.00 0.00
-x +x +x
xx0.10 - x
= 6.5 x 10-2
If > 400
Can we neglect x?
[H2C2O4]0
Ka1
we can neglect x
x2 + 6.5 x 10-2x - 6.5 x 10-3 = 0
x = 0.054 M or -0.12 M
Example
14
15.11
Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings). Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution.
H2C2O4(aq) H+(aq) + (aq)
Equilibrium (M):
-2 4HC O
xx0.10 - x
x = 0.054 M [H+] = 0.054 M
[ ] = 0.054 M
[H2C2O4] = 0.046 M
-2 4HC O
the major species are HC2O4-, which acts as the acid in the
second stage of ionization to generate more H+, and C2O42-.
HC2O4-(aq) H+(aq) + C2O4
2-(aq)
Example
15
15.11
(aq) H+(aq) + (aq)
Initial (M):
Change (M):
Equilibrium (M):
-2 4HC O 2-
2 4C O
[H+] = 0.054 M
[ ] = 0.054 M-2 4HC O
For equilibrium 2:
Ka2 = 6.1 x 10-5
0.054 0.054 0.00
-y +y +y
y0.054 - y 0.054 + y
If > 400 [HC2O4
-]0
Ka2
we can neglect y
y = 6.1 x 10-5 M
Example
16
15.11
Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings). Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution.
H2C2O4(aq) H+(aq) + HC2O4-(aq)
HC2O4-(aq) H+(aq) + C2O4
2-(aq)
0.10 M
[H2C2O4] = 0.046 M
[ ] = (0.054 - 6.1 x 10-5) M = 0.054 M
[H+] = (0.054 + 6.1 x 10-5) M = 0.054 M
[ ] = 6.1 x 10-5 M
[OH-] = 1.0 x 10-14/0.054 = 1.9 x 10-13 M
-2 4HC O
2-2 4C O
Ka1 = 6.5 x 10-2
Ka2 = 6.1 x 10-5
17
Last ExampleCalculate the pH of a 0.10 M solution of H3PO4.
H3PO4(aq) ⇄ H+(aq) + H2PO4-(aq) Ka1 = 7.5 x 10-3
H2PO4-(aq) ⇄ H+(aq) + HPO4
2-(aq) Ka2 = 6.2 x 10-8
HPO42-(aq) ⇄ H+(aq) + PO4
3-(aq) Ka3 = 4.8 x 10-13
Acid strength decreases in the order: H3PO4 >> H2PO4
- >> HPO42-
pH of solution is determined mainly by ionization of H3PO4
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Conceptual Question
19
Polyprotic Peptides
PolypeptidesProteins
21
Some factors that influence acid/base strength (Ka/Kb).• Temperature
• Solvent
• Acid Structure– Hydrohalic acids– Oxoacids– Carboxylic acids
Acid/Base Strength
Ka =[H+][A-][HA]
=kfwd
krev
/RTEaAe=k
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Solvent Dependence
AN =
DCE =
H2O =
GP = Gas Phase
HO
H
C NH3C
Cl
Cl
Ka =[H+][A-][HA]
HA (aq) H+ (aq) + A- (aq)
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Hydrohalic Acid Strength The strength of an acid depends on the strength of the X–H
bond that is to be broken.
Relative bond strength:H─F > H─Cl > H─Br > H─I
Relative acid strength:HI > HBr > HCl > HF
Relative acid strength: H2Te > H2Se > H2S > H2O (all very weak acids)
7.2 x 10-4
~109~109
~107
Ka
For H-X, electronegativity does
not play a big role.
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H X H+ + X-
The stronger the bond
The weaker the acid
HF << HCl < HBr < HI
acidityincreases
Hydrohalic Acid Strength
25
Z O H Z O- + H+d- d+
The O-H bond strength/Ka are dependent on:
• electronegativity of Z
• oxidation state of Z
Oxoacid StrengthOxoacids is an acid that contains oxygen and a central atom Z.
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Oxoacids is an acid that contains oxygen and a central atom Z.
If they are from the same group and have the same oxidation number, acid strength increases with increasing electronegativity of Z.
Cl is more electronegative than Br
HClO3 > HBrO3acidityincreases
Oxoacid Strength
27
Oxoacids having the same central atom (Z) but different numbers of attached groups.
Acid strength increases as the oxidation number of Z increases.
HClO4 > HClO3 > HClO2 > HClO
Oxoacid Strength
Example
28
15.12
Predict the relative strengths of the oxoacids in each of the following groups:
(a)ClOH, BrOH, and IOH
(b) HNO3 and HNO2
The O-H bond will be easier to break if:
• Z is very electronegative or
• Z is in a high oxidation state
ClOH > BrOH > IOH
>
29
Carboxylic Acid StrengthGeneral structure for a carboxylic acid:
Where R can be anything: H, CH3, Ph, Cl…The more
electrongative R is the stronger the carboxylic acid.
Proximity of the electrongative atom matters.
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Salts are the ionic product of an acid base neutralization reaction. They are composed of related numbers of cations (positively charged ions) and anions (negative ions) so that the product is electrically neutral (without a net charge).
Acidic and Basic Saltsacid + base salt + water
HCl(aq) + NaOH(aq) NaCl(s) + H2O
XY(s) X+(aq) + Y-(aq)
• Soluble salts dissociate completely when dissolved in water.
• Ions/salts may acidic, basic or neutral.
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Acidic Salts are formed from a strong acid and a weak base.
Neutral salts are formed from a strong acid and strong base.
Basic salts are formed from a strong base and a weak acid.
1. NaCl
2. NaC2H3O2
3. NH4Cl
NaCl + H2OHCl + NaOH
NaC2H3O2 + H2OHC2H3O2 + NaOHw.a. s.b.basic salt
neutral salts.a. s.b.
NH4Cl + H2ONH4OHHCl +s.a. w.b.
acidic salt
Acidic and Basic Salts
33
Neutral Salts• Dissociation and reaction of a neutral salt:
NaCl(aq) Na+(aq) + Cl-(aq)
Na+(aq) + H2O NR
Cl- (aq) + H2O NR
• The concentrations of H+ and OH- in NaCl solution are the same as in pure water solution is neutral.
• Neutral salts are typically formed from a strong acid and strong base (which give weak conjugate acid and bases).
• Salts containing an alkali metal or alkaline earth metal ion (except Be2+) and the conjugate base of a strong acid (e.g. Cl-, Br-, and NO3
-).
34
Basic Salts• Dissociation and reaction of a basic salt:
NaNO2(aq) Na+(aq) + NO2-(aq)
Na+(aq) + H2O NR
NO2-(aq) + H2O HNO2(aq) + OH-(aq)
• The reaction of NO2- with water causes [OH-] > [H+] and
the solution becomes basic.
• Basic salts are typically formed from a weak acid and strong base (which gives a weak conjugate acid and a stronger conjugate base).
Calculate the pH of a 0.15 M solution of sodium acetate (CH3COONa). (Kb of sodium acetate is 5.6 x 10-10)
15.13
CH3COONa (s) Na+ (aq) + CH3COO- (aq)H2O
CH3COO-(aq) + H2O(l) CH3COOH(aq) + OH-(aq)
0.15 M 0.15 M
0.15 M
35
Calculate the pH of a 0.15 M solution of sodium acetate (CH3COONa). (Kb of sodium acetate is 5.6 x 10-10)
15.13
[CH3COO-] = 0.15 M
CH3COO- (aq) + H2O (l) CH3COOH(aq) + OH-(aq)
Initial (M):
Change (M):
Equilibrium (M):
0.15 0.00 0.00
+x+x-x
0.15 - x x x
2 2-10
-6
5.6 × 10 = 0.15 - 0.15
= 9.2 × 10
x x
x
x
If > 400 [CH3COO-]0
Kb
we can neglect x 36
Calculate the pH of a 0.15 M solution of sodium acetate (CH3COONa). (Kb of sodium acetate is 5.6 x 10-10)
15.13
[CH3COO-] = 0.15 M
CH3COO- (aq) + H2O (l) CH3COOH(aq) + OH-(aq)
Initial (M):
Change (M):
Equilibrium (M):
0.15 0.00 0.00
+x+x-x
0.15 9.2 x 10-6 9.2 x 10-6
x = 9.2 x 10-6
[OH-] = 9.2 x 10-6 MpOH = -log (9.2 x 10-6 )pOH = 5.04
pH = 14.00 - 5.04pH = 8.96
37
38
Acidic Salts• Dissociation and reaction of a neutral salt:
NH4NO3 (aq) NH4+(aq) + NO3
-(aq)
NH4+ (aq) + H2O NH3(aq) + H3O+(aq)
Cl- (aq) + H2O NR
• The reaction of NH4+ with water causes [H+] > [OH-], and
the solution becomes acidic.
• Neutral salts are typically formed from a strong acid and weak base (which give weak conjugate acid and bases).
• Salts with the conjugate base of a strong acid and small, highly charged metal cations (e.g. Al3+, Cr3+, and Be2+).
39
Hydrated metal ions polarize coordinated water molecules and, consequently, act as Brønsted-Lowry acids:
Their solutions, therefore, are acidic.
This process is known as hydrolysis.
Hydrated Metal Ions
Al(H2O)6 (aq) Al(OH)(H2O)5 (aq) + H+ (aq)3+ 2+
40
Acidic Salts are formed from a strong acid and a weak base.
Neutral salts are formed from a strong acid and strong base.
Basic salts are formed from a strong base and a weak acid.
1. NaCl
2. NaC2H3O2
3. NH4Cl
NaCl + H2OHCl + NaOH
NaC2H3O2 + H2OHC2H3O2 + NaOHw.a. s.b.basic salt
neutral salt s.a. s.b.
NH4Cl + H2ONH4OHHCl +s.a. w.b.acidic salt
Acidic and Basic Salts
41
• Salts of Strong Acid-Strong Base Reactions: NaCl, NaNO3, KBr, etc.; solutions are neutral
• Salts of Weak Acid-Strong Base Reactions: NaF, NaNO2, NaC2H3O2, etc.; solutions are basic
• Salts of Strong Acid-Weak Base Reactions: NH4Cl, NH4NO3, (CH3)2NH2Cl, C5H5NHCl; Solutions of these salts are acidic
• Salts of Weak Acid-Weak Base Reactions: NH4C2H3O2, NH4CN, NH4NO2, etc..These compounds can be acidic, basic, or neutral, which depends on the relative strength of the acid and the base.
Acidic and Basic Salts
42
Solutions in which both the cation and the anion hydrolyze:
• Kb for the anion > Ka for the cation, solution will be basic
• Kb for the anion < Ka for the cation, solution will be acidic
• Kb for the anion Ka for the cation, solution will be neutral
Acidic and Basic SaltsNaB(s) Na+(aq) + B-(aq)
AHCl(s) AH+(aq) + Cl-(aq)
B-(aq) + H2O BH+(aq) + OH-(aq)
Basic Salt (with B)
Ka =[H+][A]
[HA+]
Kb =[BH+][OH-]
[B-]
Acidic Salt (with AH+)
AH+(aq) A (aq) + H+(aq)
What about AHB (with AH+ and B-)?
AHB(s) AH+(aq) + B-(aq)
Generates H+ Generates OH-
43
Is NH4C2H3O2 acidic, basic or neutral?
NH4C2H3O2(s) NH4+(aq) + C2H3O2
-(aq)
NH4+(aq) + H2O H3O+(aq) + NH3(aq) Ka = 5.6 x 10-10
C2H3O2-(aq) + H2O HC2H3O2(aq) + OH-(aq) Kb = 5.6 x 10-10
Ka = Kb NH4C2H3O2 is neutral
Predicting Acid-Base Property of Salts
44
Predicting Acid-Base Property of Salts
Is (NH4)2SO4 acidic, basic or neutral?
(NH4)2SO4(aq) 2NH4+(aq) + SO4
2-(aq);
NH4+(aq) + H2O H3O+(aq) + NH3(aq) Ka = 5.6 x 10-10
SO42-(aq) + H2O HSO4
-(aq) + OH-(aq) Kb = 8.3 x 10-13
Ka > Kb (NH4)2SO4 is acidic.
45
Acidic and Basic Salts SummaryXY(s) X+(aq) + Y-(aq)
Kb Ka
Example
46
15.14
Predict whether the following solutions will be acidic, basic, or nearly neutral:
(a)NH4I
(b) NaNO2
(c) FeCl3
(d) NH4F
NH4I(s) NH4+(aq) + I-(aq)
acid very weak baseAcidic Salt
NaNO2(s) Na+(aq) + NO2-(aq)
baseneutralBasic Salt
FeCl3(s) Fe+(aq) + 3Cl-(aq)very weak baseacid
Acidic Salt
NH4F(s) NH4+(aq) + F-(aq)
baseacidKa = 5.6 x 10-10 Kb = 1.4 x 10-11
Acidic Salt
48
Oxides• Oxide is a chemical compound that contains at least one
oxygen atom and one other element. Of the general from:
• E can be any element other than O or H.
• Oxides can be acidic, basic amphoteric.
ExOy
49
Oxides
Oxides of highly electropositive metals are basic.
Oxides of electronegative nonmetals are acidic.
Most transition metal oxides are basic.
50
Oxides
Oxides of electronegative nonmetals are acidic.
• Acidity increases left to right SiO2 < P4O10 < SO3 < Cl2O7
•Acidity decreases top-to-bottom N2O5 > P4O10 > As2O5 > Sb2O5
N2O5 + H2O 2HNO3
SO3 + H2O H2SO4
Cl2O7 + H2O 2HClO4
51
Oxides
Oxides of highly electropositive metals are basic.
Na2O(s) + H2O 2NaOH(aq)MgO(aq) + HCl(aq) MgCl2(aq) + H2O
52
Oxides
Some oxides are amphoteric: Can behave as an acid or a base depending on the circumstance.
Al2O3(s) + HCl(aq) 2AlCl3(aq) + H2O
Al2O3(s) + 2NaOH(aq) + 3H2O 2NaAl(OH)4(aq)
54
Lewis Acids and Bases
Gilbert Newton Lewis (1875-1946)
Lewis Dot Structure:
NNeeds 3 electrons
F F F
Need 1 electron each Combine unpaired electrons
N F
F
F
Also proposed a definition for acids and bases. Now known as Lewis acids.
55
Lewis Acids and Bases A base is any species that donates an electron pair. An acid is any species that accepts an electron pair. This definition greatly expands the classes of acids.
The acid-base reaction, in the Lewis sense, is the sharing of an electron pair between an acid and a base resulting in the formation of a bond:
A + :B A–B
N H••
H
H
acid base
F B
F
F
+ F B
F
F
N H
H
H
No H+ or OH- created. No protons donated or accepted!
Example
56
Identify the Lewis acid and Lewis base in each of the following reactions:
(a) C2H5OC2H5 + AlCl3 (C2H5)2OAlCl3
(b) Hg2+(aq) + 4CN-(aq) (aq)
15.15
2-4Hg(CN)
base acid
Here the Hg2+ ion accepts four pairs of electrons from the CN- ions.
baseacid
57
Identify the Lewis acid and Lewis base in each of the following reactions:
Cl– + AlCl3 AlCl4–
H2O + CO2 H2CO3
H+ + OH- H2O
Cu2+ + 4NH3 Cu(NH3)42+
AlCl3 + Cl- AlCl4-
Example
58
Arrhenius acid is a substance that produces H+ in water
base is a substance that produces OH- in water
Acid/Base Definitions
Brønstedacid is a substance that donates H+
base is a substance that accepts H+
Lewisacid is a substance that accepts a pair of electrons
base is a substance that donates a pair of electrons
Venn Diagram
59
60
Acid/Base Venn Diagram
All Brønsted-Lowry acids are Arrhenius acids.
Not all Arrhenius acids are Brønsted-Lowry acids.
etc…
NaHCO3 (aq) + HCl (aq)
NaCl (aq) + H2O (l) + CO2 (g)
Mg(OH)2 (s) + 2HCl (aq)
MgCl2 (aq) + 2H2O (l)
Acid/Base Side Note
61
