Chapter 14. Acids and Bases
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Transcript of Chapter 14. Acids and Bases
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Chapter 14.Acids and Bases
Early attributes of acids and bases (1600's)
Acids BasesTaste sour Taste bitterTurn litmus red Turn litmus blueReact with metals Feel slipperyReact with carbonates React with fats
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Arrhenius Acids and Bases
Definitions of Svante Arrhenius, 1884 First working theory about acids and bases
Acids contain hydrogen and produce H1+ ions in water.
Bases contain hydroxide ions (OH1-) and are soluble in water.
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Acids and Bases
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Arrhenius Acids and Bases
Acids are molecular compounds; a covalent bond attaches the hydrogen ion to the ad-jacent atom.
Ionization, the separation of the molecule into ions, occurs when the molecule dis-solves in water.
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Arrhenius Acids and Bases
Bases are ionic compounds; the hydroxide ion exists in the crystal structure of the solid compound.
Dissociation occurs when the ionic solid dissolves in water, releasing the ions to move about.
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Arrhenius Acids and Bases
Common Acids:
HCl(aq), H2SO4, H3PO4, HNO3
HC2H3O2 = CH3COOH = acetic acid
Common Bases:NaOH, KOH
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Bronsted-Lowry Acids and Bases
The Arrhenius definition has some problems:
It's restricted to water.
It doesn't explain why solutions of some molecular compounds (NH3) and salts (Na2CO3) are basic.
It doesn't explain why some salt solutions are acidic (aqueous Al3+, Fe3+ solutions).
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Bronsted-Lowry Acids and Bases
Definitions of Brønsted and Lowry, 1923 Most widely used theory of acids and bases
Acids are proton donors. Bases are proton acceptors.
Reactions:
HCl(aq) + H2O(l) H3O1+(aq) + Cl1-(aq)
H3O1+(aq) + NH3(aq) NH41+(aq) + H2O(l)
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Bronsted-Lowry Acids and Bases
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Bronsted-Lowry Acids and Bases
Formation of water by the transfer of protons from H3O1+ ions to OH1 ions.
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Bronsted-Lowry Acids and Bases
Works in solvents other than water
Solves the base problem:
NH3(aq) + H3O1+(aq) NH41+(aq) + H2O(l)
CO32-(aq) + H3O1+(aq) HCO3
1-(aq) + H2O(l)
Doesn't solve the acid problem;What is it with Al3+(aq) and Fe3+(aq)?
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Lewis Acids and Bases
Definitions of Gilbert Lewis, 1923 Most general theory of acids and bases
Acids are electron pair acceptors.
Bases are electron pair donors.
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Bronsted-Lowry Acids and Bases
Conjugate acid-base pairs:A reaction between and acid and a base
produces a conjugate acid and a conjugate baseHCl(aq) + H2O(l) H3O1+(aq) + Cl1(aq)
Acid Base Conjugate Conjugate Acid
Base
H3O1+(aq) + NH3(aq) H2O(l) + NH41+(aq)
Acid Base Conj. Conj. Base Acid
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Bronsted-Lowry Acids and Bases
Choose the acid, base, conjugate acid, and conjugate base:
HCOOH(aq) + NH3(aq) HCOO1(aq) + NH41+ (aq)
H2PO41(aq) + H2O(l) HPO4
2(aq) + H3O1+ (aq)
H2O(l) + HPO42(aq) + H3O1+ (aq) + PO4
3(aq)
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Bronsted-Lowry Acids and Bases
Amphoteric substances can act as both acids and bases:HCOOH(aq) + H2O(l) HCOO1(aq) + H3O1+ (aq)
NH3 (aq) + H2O(l) NH41+ (aq) + OH1(aq)
HPO42(aq) + OH1(aq) PO4
3(aq) + H2O(l)
HPO42(aq) + H3O1+ (aq) H2PO4
2(aq) + H2O(l)
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Mono-, Di- and Triprotic Acids
Monoprotic acids can transfer one protonCH3COOH + H2O CH3COO1 + H3O1+
Diprotic acids can transfer two protonsH2CO3 + H2O HCO3
1 + H3O1+
HCO31 + H2O CO3
2 + H3O1+
The first proton transfer is complete before the second one starts.
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Mono-, Di- and Triprotic Acids
Triprotic acids can transfer three protonsH3PO4 + H2O H2PO4
1 + H3O1+
H2PO41 + H2O HPO4
2 + H3O1+
HPO42 + H2O PO4
3 + H3O1+
The first proton transfer is complete before the second one starts. The second proton transfer is complete before the third one starts.
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Strengths of Acids and Bases
Acids differ in the extent of ionization when they are put in solution
Strong acids ionize completely. There are only a few strong acids.
Weak acids do not ionize completely. Most acids are weak acids.
The equilibrium constant, Ka, is a measure of the strength of an acid.
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The Strong Acids
Formula NameHCl(aq) Hydrochloric
acidHBr(aq) Hydrobromic
acidHI(aq) Hydriodic acidHNO3 Nitric acidHClO4 Perchloric acidHClO3 Chloric acidH2SO4 Sulfuric acid*
*first proton
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Some Weak Acids
Formula Name Ka
HSO41 Hydrogen sulfate 1.2 x 102
C9H8O4 Acetylsalicylic acid 3.0 x 104
HCOOH Formic acid 1.8 x 104
HC3H5O3 Lactic acid 1.4 x 104
CH3COOH Acetic acid 1.8 x 105
H2CO3 Carbonic acid 4.3 x 107
H2S(aq) Hydrosulfuric acid 1.0 x 107
HCN(aq) Hydrocyanic acid 4.9 x 1010
C6H5OH Phenol 1.3 x 1010
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A comparison of the number of H3O1+ ions present in strong acid and weak acid solu-tions of equal concentration.
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The Strong BasesSoluble compounds that contain OH1-
Group 1A Hydroxides Group 2A Hydroxides
LiOHNaOHKOH Ca(OH)2
RbOH Sr(OH)2
CsOH Ba(OH)2
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The Weak Bases
Ammonia:
NH3 + H2O NH41+ + OH1
Kb = 1.8 x 105
NH41+ + H2O NH3 + H3O1+
Ka = 5.6 x 1010
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The Weak Bases
Anions from weak acids:
CH3COO1 + H2O CH3COOH + OH1
CO32- + H2O HCO3
1- + OH1
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SaltsA salt is a compound containing a metal or
polyatomic cation, and a nonmetal or polyatomic anion (except OH1).NaCl, NH4Cl, BaSO4, CaCO3, Al2(SO4)3
Neutralization reactions between an acid and a base produce a salt and water.HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)
2 Al(OH)3(s) + 3 H2SO4(aq) 6 H2O + Al2(SO4)3 (aq)
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Hydrolysis of Salts
Hydrolysis is a reaction of a substance with water. Salts may hydrolyze to form H3O1+ or OH1 along with other products.
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Hydrolysis of Salts
The salt of a weak acid and a strong base gives a weakly basic aqueous solution.
NaOH + HC2H3O2 NaC2H3O2 + H2O
NaC2H3O2 + H2O HC2H3O2 + OH1 + Na1+
Reestablishes equilibrium between acetate anion and acetic acid.
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Hydrolysis of Salts
The salt of a weak base and a strong acid gives a weakly acidic aqueous solution.
NH3 + HCl NH4Cl
NH4Cl + H2O NH3 + H3O1+ + Cl1
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Hydrolysis of Salts
The salt of a weak base and a weak acid can give a weakly acidic, neutral, or weakly ba-sic aqueous solution, depending on acid strengths.
NH4C2H3O2 + H2O HC2H3O2 + NH3
The salt of a strong acid and a strong base give a neutral solution.
NaCl + H2O Na1+ + Cl1 + H2O
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Hydrolysis of SaltsSome metal ions, if they're small and have a
high charge, give acidic solutions.
Al3+ + 2 H2O AlOH2+ + H3O1+
Keq = 1.4 x 105
Fe3+ + 2 H2O FeOH2+ + H3O1+
Keq = 6.3 x 103
Cr3+ + 2 H2O CrOH2+ + H3O1+
Keq = 1.6 x 104
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Net Ionic Equations2 Al(OH)3(s) + 3 H2SO4(aq)
6 H2O(l) + Al2(SO4)3(aq)
2 HCl(aq) + CaCO3(s)
2 CaCl2(aq) + CO2(g) + H2O(l)
Ionic Equations show dissolved ionic sub-stances as ions rather than as compounds.
Net Ionic Equations show only the participat-ing species. "Spectator" ions are not shown.
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Net Ionic EquationsIonic Equations2 Al(OH)3(s) + 6 H1+(aq) + 3 SO4
2(aq) 6 H2O(l) + 2 Al3+(aq) + 3 SO4
2(aq)
2 H1+(aq) + 2 Cl2(aq) + CaCO3(s)
Ca2+(aq) + 2 Cl2(aq) + CO2(g) + H2O(l)
Net Ionic Equations (NIE's)2 Al(OH)3(s) + 6 H1+ 6 H2O(l) + 2 Al3+
2 H1+ + CaCO3(s) Ca2+ + CO2(g) + H2O(l)
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Self-Ionization of WaterThe self-ionization of water is an acid-base
reaction in which one water molecule trans-fers a proton to another.
2 H2O H3O1+ + OH1 Kw = 1.0 x 1014
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Self-Ionization of Water
2 H2O H3O1+ + OH1 Kw = 1.0 x 1014
Kw = ion product constant for water
Kw = 1.0 x 1014 = [H3O1+] [OH1]
In pure water, [H3O1+] = [OH1] = 1.0 x 107M
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The relationship between [H3O1+] and [OH1] in aqueous solution is an inverse proportion; when [H3O1+] is increased, [OH1] decreases, and vice versa.
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Self-Ionization of WaterAn acidic solution has [H3O1+] > 1.0 x 107 M
[OH1] < 1.0 x 107 M
A basic solution has [OH1] > 1.0 x 107 M[H3O1+] < 1.0 x 107 M
A neutral solution has [H3O1+] = [OH1] = 1.0 x 107 M
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Self-Ionization of Water
Examples:
In a 0.015 M solution of HCl, what is the concentration of OH1?Is the solution acidic or basic?
[OH1] is 4.0 x 105. What is [H3O1+]?
Is the solution acidic or basic?
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The pH Scale
[H3O1+] can vary over a wide range, and is often low. Often, you need scientific notation to express it. This isn't always convenient.
A simpler way to write [H3O1+] is pH
pH = log [H3O1+]
[H3O1+] = 10pH
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Common (base 10) Logarigthms
A logarithm is the power to which a base, such as 10, must be raised to produce a given number.
Number Logarithm
0.010 = 1.0 x 10-2 -2.00 1.0 = 1.0 x 100 0.00
10 = 1.0 x 101 1.00
Coefficient Exponent Characteristic Mantissa
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Powers of 10 and their logarithms
0
1
2
3
4
5
6
7
8
0.E+001.E+052.E+053.E+054.E+055.E+056.E+057.E+058.E+059.E+051.E+06
Power of 10
Base 10 logarithm
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Common (base 10) Logarigthms
What happens if the coefficient of the number isn’t 1.0?
Number Logarithm
0.050 = 5.0 x 10-2 -1.30 5.0 = 5.0 x 100 0.70
50 = 5.0 x 101 1.70
Coefficient Exponent Characteristic Mantissa
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Integers and their logarithms
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4 5 6 7 8 9 10
Integer
Base 10 logarithm
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Common (base 10) Logarigthms
How logarithms simplify mathematics:
2.594 x 103 x 6.022 x 1023 = 1.562 x 1027
log(2.594e3) + log(6.022 e23) = log(1.562e27)
3.4140 + 23.7797 = 27.1937
antilog(27.1937) = 1027.1937 = 1.562 x 1027
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Common (base 10) Logarigthms
How logarithms simplify mathematics:
Slide rules use logarithmic scales for multiplication and division.
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The pH ScaleAn acidic solution has [H3O1+] > 1.0 x 107 M
pH < 7.0
A basic solution has [OH1] > 1.0 x 107 MpH > 7.0
A neutral solution has [H3O1+] = [OH1] = 1.0 x 107 M
pH = 7.0
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A pH meter is used to measure pH values. The pH of vinegar is 2.32 (left). The pH of milk of magnesia in water is 9.39 (right).
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The pH Scale
Give the pH for
[H3O1+] = 0.010 M
= 4.2 x 103 M = 1.0 x 107 M = 6.8 x 1010 M = 1.0 x 1012 M
Are the solutions acidic or basic?
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The pH Scale
Give [H3O1+] for
pH = 3.00 = 4.50 = 6.85
= 7.00 = 10.75
Are the solutions acidic or basic?
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Buffers
A buffer is a solution that resists major changes in pH when acids or bases are added to it.
A buffer containsA weak acid to react with added baseA weak base to react with added acidMost often, the acid and base are conjugate pairs
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Buffers
A buffer made of equimolar amounts of a weak acid and its conjugate base will have a pH equal to log Ka.
log Ka = pKa
Adding a acid will shift the pH of the buffer down, adding base will shift the pH of the buffer up.
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BuffersExamples:
What is the pH of a buffer made of 0.10 mole of CH3COOH and 0.10 mole of NaCH3COO?
What is the pH of a buffer made with 1.0 mole of ammonia and 1.0 mole of ammonium chloride?
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BuffersA buffer is made with 0.10 mole of CH3COOH
and 0.10 mole of NaCH3COO.
What component of the buffer reacts with added H3O1+?
What component of the buffer reacts with added OH1?
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Buffers
A buffer is made with 1.0 mole of NH3 and 1.0 mole of NH4Cl.
What component of the buffer reacts with added H3O1+?
What component of the buffer reacts with added OH1?
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Acid-Base TitrationsIn an acid-base titration, a measured volume
of an acid or base of known concentration is reacted with a measured volume of a base or acid of unknown concentration.
The reaction is conducted in a way that exactly equimolar amounts of H3O1+ and OH1 are combined.
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Acid-Base TitrationsA student titrates 2.00 mL of vinegar (acetic
acid in water) with 15.85 mL of 0.1048 M NaOH. What is the concentration of acetic acid in the vinegar? Calculate both molarity and mass percent. The density of vinegar is 1.05 g/mL.
CH3COOH(aq) + NaOH(aq)
CH3COONa(aq) + H2O(l)