Upload
cili
View
35
Download
1
Embed Size (px)
DESCRIPTION
Aqueous Equilibria: Acids and Bases. Chemistry 4th Edition McMurry/Fay. Acid–Base Concepts01. Arrhenius Acid: A substance which dissociates to form hydrogen ions (H + ) in solution. HA( aq ) H + ( aq ) + A – ( aq ) - PowerPoint PPT Presentation
Citation preview
Dr. Paul CharlesworthMichigan Technological UniversityDr. Paul Charlesworth
Michigan Technological University
C h a p t e rC h a p t e r 1515Aqueous Equilibria:
Acids and BasesAqueous Equilibria:
Acids and Bases
Chemistry 4th EditionMcMurry/Fay
Chemistry 4th EditionMcMurry/Fay
Chapter 15 Slide 2Prentice Hall ©2004
Acid–Base Concepts 01Acid–Base Concepts 01
Arrhenius Acid: A substance which dissociates to form hydrogen ions (H+) in solution.
HA(aq) H+(aq) + A–(aq)
Arrhenius Base: A substance that dissociates in, or reacts with water to form hydroxide ions (OH–).
MOH(aq) M+(aq) + OH–(aq)
Chapter 15 Slide 3Prentice Hall ©2004
Acid–Base Concepts 02Acid–Base Concepts 02
• Brønsted–Lowry Acid: Substance that can donate H+
• Brønsted–Lowry Base: Substance that can accept H+
• Chemical species whose formulas differ only by one proton are said to be conjugate acid–base pairs.
Chapter 15 Slide 4Prentice Hall ©2004
Acid–Base Concepts 03Acid–Base Concepts 03
Chapter 15 Slide 5Prentice Hall ©2004
Acid–Base Concepts 04Acid–Base Concepts 04
Chapter 15 Slide 6Prentice Hall ©2004
• A Lewis Acid is an electron-pair acceptor. These are generally cations and neutral molecules with vacant valence orbitals, such as Al3+, Cu2+, H+, BF3.
• A Lewis Base is an electron-pair donor. These are generally anions and neutral molecules with available pairs of electrons, such as H2O, NH3, O2–.
• The bond formed is called a coordinate bond.
Acid–Base Concepts 05Acid–Base Concepts 05
Chapter 15 Slide 7Prentice Hall ©2004
Acid–Base Concepts 06Acid–Base Concepts 06
Chapter 15 Slide 8Prentice Hall ©2004
Acid–Base Concepts 07Acid–Base Concepts 07
• Write balanced equations for the dissociation of each of the following Brønsted–Lowry acids.(a) H2SO4 (b) HSO4
– (c) H3O+
• Identify the Lewis acid and Lewis base in each of the following reactions:
(a) SnCl4(s) + 2 Cl–(aq) æ SnCl62–(aq)
(b) Hg2+(aq) + 4 CN–(aq) æ Hg(CN)42–(aq)
(c) Co3+(aq) + 6 NH3(aq) æ Co(NH3)63+(aq)
Chapter 15 Slide 9Prentice Hall ©2004
Dissociation of Water 01Dissociation of Water 01
• Water can act as an acid or as a base.
H2O(l) æ H+(aq) + OH–(aq)
• This is called the autoionization of water.
H2O(l) + H2O(l) æ H3O+(aq) + OH–(aq)
Chapter 15 Slide 10Prentice Hall ©2004
Dissociation of Water 02Dissociation of Water 02
• This equilibrium gives us the ion product constant for water.
Kw = Kc = [H+][OH–] = 1.0 x 10–14
• If we know either [H+] or [OH–] then we can
determine the other quantity.
Chapter 15 Slide 11Prentice Hall ©2004
Dissociation of Water 03Dissociation of Water 03
• The concentration of OH– ions in a certain household
ammonia cleaning solution is 0.0025 M. Calculate the
concentration of H+ ions.
• Calculate the concentration of OH– ions in a HCl
solution whose hydrogen ion concentration is 1.3 M.
Chapter 15 Slide 12Prentice Hall ©2004
pH – A Measure of Acidity 01pH – A Measure of Acidity 01
• The pH of a solution is the negative logarithm of the
hydrogen ion concentration (in mol/L).
pH = –log [H+]
pH + pOH = 14
Acidic solutions: [H+] > 1.0 x 10–7 M, pH < 7.00
Basic solutions: [H+] < 1.0 x 10–7 M, pH > 7.00
Neutral solutions: [H+] = 1.0 x 10–7 M, pH = 7.00
Chapter 15 Slide 13Prentice Hall ©2004
pH – A Measure of Acidity 02pH – A Measure of Acidity 02
• Nitric acid (HNO3) is used in the production of fertilizer, dyes, drugs, and explosives. Calculate the pH of a HNO3 solution having a hydrogen ion concentration of 0.76 M.
• The pH of a certain orange juice is 3.33. Calculate the H+ ion concentration.
• The OH– ion concentration of a blood sample is 2.5 x 10–
7 M. What is the pH of the blood?
Chapter 15 Slide 14Prentice Hall ©2004
pH – A Measure of Acidity 03pH – A Measure of Acidity 03
Chapter 15 Slide 15Prentice Hall ©2004
Strength of Acids and Bases01Strength of Acids and Bases01
• Strong acids and bases: are strong electrolytes
that are assumed to ionize completely in water.
• Weak acids and bases: are weak electrolytes that
ionize only to a limited extent in water.
• Solutions of weak acids and bases contain ionized
and non-ionized species.
Chapter 15 Slide 16Prentice Hall ©2004
Strength of Acids and Bases02Strength of Acids and Bases02
HClO4
HI
HBr
HCl
H2SO4
HNO3
H3O+
HSO4–
HSO4–
HF
HNO2
HCOOH
NH4+
HCN
H2O
NH3
ClO4–
I–
Br –
Cl –
HSO4 –
NO3 –
H2O
SO42–
SO42–
F –
NO2 –
HCOO –
NH3
CN –
OH –
NH2 –
ACID CONJ. BASE ACID CONJ. BASE
Incr
easi
ng A
cid
Str
engt
h
Incr
easi
ng A
cid
Str
engt
h
Chapter 15 Slide 17Prentice Hall ©2004
Strength of Acids and Bases03Strength of Acids and Bases03
• Stronger acid + stronger base
weaker acid + weaker base
• Predict the direction of the following:
HNO2(aq) + CN–(aq) æ HCN(aq) + NO2–(aq)
HF(aq) + NH3(aq) æ F–(aq) + NH4+(aq)
Chapter 15 Slide 18Prentice Hall ©2004
Acid Ionization Constants 01Acid Ionization Constants 01
• Acid Ionization Constant: the equilibrium constant for the ionization of an acid.
HA(aq) + H2O(l) æ H3O+(aq) + A–(aq)
• Or simply: HA(aq) æ H+(aq) + A–(aq)
[HA]]][A[H
aK
Chapter 15 Slide 19Prentice Hall ©2004
Acid Ionization Constants 02Acid Ionization Constants 02
7.1 x 10 –4
4.5 x 10 –4
3.0 x 10 –4
1.7 x 10 –4
8.0 x 10 –5
6.5 x 10 –5
1.8 x 10 –5
4.9 x 10 –10
1.3 x 10 –10
HF
HNO2
C9H8O4 (aspirin)
HCO2H (formic)
C6H8O6 (ascorbic)
C6H5CO2H (benzoic)
CH3CO2H (acetic)
HCN
C6H5OH (phenol)
F–
NO2 –
C9H7O4 –
HCO2 –
C6H7O6 –
C6H5CO2 –
CH3CO2 –
CN –
C6H5O –
ACID Ka CONJ. BASE Kb
1.4 x 10 –11
2.2 x 10 –11
3.3 x 10 –11
5.9 x 10 –11
1.3 x 10 –10
1.5 x 10 –10
5.6 x 10 –10
2.0 x 10 –5
7.7 x 10 –5
Chapter 15 Slide 20Prentice Hall ©2004
Strength of Acids and Bases03Strength of Acids and Bases03
(a) Arrange the three acids in order of increasing value of Ka.
(b) Which acid, if any, is a strong acid?(c) Which solution has the highest pH, and which has the
lowest?
Chapter 15 Slide 21Prentice Hall ©2004
HA æ H+ + A
(M): 0.50 0.00 0.00 (M): –x +x +x
Equilib (M): 0.50 –x x x
Acid Ionization Constants 04Acid Ionization Constants 04
• Initial Change Equilibrium Table: Determine the pH
of 0.50 M HA solution at 25°C. Ka = 7.1 x 10–4.
InitialChange
(aq) (aq)-(aq)
Chapter 15 Slide 22Prentice Hall ©2004
Acid Ionization Constants 05Acid Ionization Constants 05
• pH of a Weak Acid (Cont’d):
1. Substitute new values into equilibrium expression.
2. If Ka is significantly (>1000 x) smaller than [HA] the expression
(0.50 – x) approximates to (0.50).
3. The equation can now be solved for x and pH.
4. If Ka is not significantly smaller than [HA] the quadratic
equation must be used to solve for x and pH.
Chapter 15 Slide 23Prentice Hall ©2004
Acid Ionization Constants 06Acid Ionization Constants 06
• The Quadratic Equation:
• The expression must first be rearranged to:
• The values are substituted into the quadratic and
solved for a positive solution to x and pH.
aacbb
x2
42
02 cbxax
Chapter 15 Slide 24Prentice Hall ©2004
Acid Ionization Constants 07Acid Ionization Constants 07
• Calculate the pH of a 0.036 M nitrous acid (HNO2)
solution.
• What is the pH of a 0.122 M monoprotic acid whose
Ka is 5.7 x 10–4?
• The pH of a 0.060 M weak monoprotic acid is 3.44.
Calculate the Ka of the acid.
Chapter 15 Slide 25Prentice Hall ©2004
Acid Ionization Constants 08Acid Ionization Constants 08
• Percent Dissociation: A measure of the strength of an acid.
• Stronger acids have higher percent dissociation.
• Percent dissociation of a weak acid decreases as
its concentration increases.
100%[HA]
][HonDissociati %
Chapter 15 Slide 26Prentice Hall ©2004
Base Ionization Constants 01Base Ionization Constants 01
• Base Ionization Constant: The equilibrium constant for the ionization of a base.
• The ionization of weak bases is treated in the same
way as the ionization of weak acids.
B(aq) + H2O(l) æ BH+(aq) + OH–(aq)
• Calculations follow the same procedure as used for
a weak acid but [OH–] is calculated, not [H+].
Chapter 15 Slide 27Prentice Hall ©2004
Base Ionization Constants 02Base Ionization Constants 02
5.6 x 10 –4
4.4 x 10 –4
4.1 x 10 –4
1.8 x 10 –5
1.7 x 10 –9
3.8 x 10 –10
1.5 x 10 –14
C2H5NH2 (ethylamine)
CH3NH2 (methylamine)
C8H10N4O2 (caffeine)
NH3 (ammonia)
C5H5N (pyridine)
C6H5NH2 (aniline)
NH2CONH2 (urea)
C2H5NH3+
CH3NH3+
C8H11N4O2+
NH4+
C5H6N+
C6H5NH3+
NH2CONH3+
BASE Kb CONJ. ACID Ka
1.8 x 10 –11
2.3 x 10 –11
2.4 x 10 –11
5.6 x 10 –10
5.9 x 10 –6
2.6 x 10 –5
0.67
Note that the positive charge sits on the nitrogen.
Chapter 15 Slide 28Prentice Hall ©2004
Diprotic & Polyprotic Acids 01Diprotic & Polyprotic Acids 01
• Diprotic and polyprotic acids yield more than one
hydrogen ion per molecule.
• One proton is lost at a time. Conjugate base of first
step is acid of second step.
• Ionization constants decrease as protons are
removed.
Chapter 15 Slide 29Prentice Hall ©2004
Diprotic & Polyprotic Acids 02Diprotic & Polyprotic Acids 02
Very Large1.3 x 10 –2
6.5 x 10 –2
6.1 x 10 –5
1.3 x 10 –2
6.3 x 10 –8
4.2 x 10 –7
4.8 x 10 –11
9.5 x 10 –8
1 x 10 –19
7.5 x 10 –3
6.2 x 10 –8
4.8 x 10 –13
H2SO4
HSO4–
C2H2O4
C2HO4–
H2SO3
HSO3–
H2CO3
HCO3–
H2SHS–
H3PO4
H2PO4–
HPO42–
ACID Ka CONJ. BASE Kb
HSO4 –
SO4 2–
C2HO4–
C2O42–
HSO3 –
SO3 2–
HCO3–
CO3 2–
HS–
S 2–
H2PO4–
HPO42–
PO43–
Very Small7.7 x 10 –13
1.5 x 10 –13
1.6 x 10 –10
7.7 x 10 –13
1.6 x 10 –7
2.4 x 10 –8
2.1 x 10 –4
1.1 x 10 –7
1 x 10 –5
1.3 x 10 –12
1.6 x 10 –7
2.1 x 10 –2
Chapter 15 Slide 30Prentice Hall ©2004
Molecular Structure and Acid Strength 01Molecular Structure and Acid Strength 01
• The strength of an acid depends on its tendency to
ionize.
• For general acids of the type H–X:
1. The stronger the bond, the weaker the acid.
2. The more polar the bond, the stronger the acid.
• For the hydrohalic acids, bond strength plays the
key role giving: HF < HCl < HBr < HI
Chapter 15 Slide 31Prentice Hall ©2004
Molecular Structure and Acid Strength 02Molecular Structure and Acid Strength 02
• The electrostatic potential maps show all the hydrohalic
acids are polar. The variation in polarity is less
significant than the bond strength which decreases
from 567 kJ/mol for HF to 299 kJ/mol for HI.
Chapter 15 Slide 32Prentice Hall ©2004
Molecular Structure and Acid Strength 03Molecular Structure and Acid Strength 03
• For binary acids in the same group, H–A bond strength decreases with increasing size of A, so acidity increases.
• For binary acids in the same row, H–A polarity increases with increasing electronegativity of A, so acidity increases.
Chapter 15 Slide 33Prentice Hall ©2004
Molecular Structure and Acid Strength 04Molecular Structure and Acid Strength 04
• For oxoacids bond polarity is more important. If we consider the main element (Y):
Y–O–H
• If Y is an electronegative element, or in a high
oxidation state, the Y–O bond will be more covalent
and the O–H bond more polar and the acid stronger.
Chapter 15 Slide 34Prentice Hall ©2004
Molecular Structure and Acid Strength 05Molecular Structure and Acid Strength 05
• For oxoacids with different central atoms that are from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity.
Chapter 15 Slide 35Prentice Hall ©2004
Molecular Structure and Acid Strength 06Molecular Structure and Acid Strength 06
• For oxoacids having the same central atom but different numbers of attached groups, acid strength increases with increasing central atom oxidation number.
• As shown on the next slide, the number of oxygen atoms increases the positive charge on the chlorine which weakens the O–H bond and increases its polarity.
Chapter 15 Slide 36Prentice Hall ©2004
Molecular Structure and Acid Strength 07Molecular Structure and Acid Strength 07
• Oxoacids of Chlorine:
Chapter 15 Slide 37Prentice Hall ©2004
Molecular Structure and Acid Strength 08Molecular Structure and Acid Strength 08
• Predict the relative strengths of the following groups of oxoacids:
a) HClO, HBrO, and HIO.
b) HNO3 and HNO2.
c) H3PO3 and H3PO4.
Chapter 15 Slide 38Prentice Hall ©2004
Acid–Base Properties of Salts 01Acid–Base Properties of Salts 01
• Salts that produce neutral solutions are those
formed from strong acids and strong bases.
• Salts that produce basic solutions are those formed
from weak acids and strong bases.
• Salts that produce acidic solutions are those
formed from strong acids and weak bases.
Chapter 15 Slide 39Prentice Hall ©2004
Acid–Base Properties of Salts 02Acid–Base Properties of Salts 02
• Calculate the pH of a 0.15 M solution of sodium
acetate (CH3COONa). What is the percent
hydrolysis?
• Calculate the pH of a 0.24 M sodium formate
solution (HCOONa).