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Chapter 16-Acids and Bases Slide 1
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Chapter 16Chapter 16AcidAcid--BaseBase Equilibria Equilibria
AP CHEMISTRY
Content from The Central Science, 9th ed David P. White,
revised 2004 by CMB
Chapter 16-Acids and Bases Slide 2
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• Acids: taste sour and cause dyes to change color.• Bases: taste bitter and feel soapy.• Arrhenius Definition:
• acids increase [H+]• bases increase [OH-] in solution.
• Arrhenius: acid + base salt + water.• Arrhenius limits us to aqueous solution.
Acids and Bases: A Brief Acids and Bases: A Brief ReviewReview
Aqueous Acids (strong and weak).mov Aqueous Bases (strong and weak).mov
Chapter 16-Acids and Bases Slide 3
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Hydronium: The H+ Ion in Water
• The H+(aq) ion is simply a proton with no electrons.
• Generally we use H+(aq) and H3O+
(aq) interchangeably.
• Brønsted-Lowry: acid donates H+ and base accepts H+.• Brønsted-Lowry base does not need to contain OH-.
• Consider HCl(aq) + H2O(l) H3O+(aq) + Cl-
(aq):
• HCl donates a proton to water. Therefore, HCl is an acid.
• H2O accepts a proton from HCl. Therefore, H2O is a base.
• Water can behave as either an acid or a base-its amphoteric.• Amphoteric substances can behave as acids and bases.
BrBrønsted-Lowry Acids and ønsted-Lowry Acids and Bases Bases
Example on next page
HCl(aq) + H2O(l) H3O+(aq) + Cl-
(aq):
Chapter 16-Acids and Bases Slide 5
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The Ion Product of Water• In pure water the following equilibrium is established
• at 25 C
• The above is called the autoionization of water.
• In neutral, pure water, [H3O+] = 1 x 10-7 M
• In neutral, pure water, [OH-] = 1 x 10-7 M
The The AutoionizationAutoionization of Water of Water
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
14-3
-3
22
22
-3
100.1]OH][OH[
]OH][OH[]OH[
]OH[
]OH][OH[
w
eq
eq
K
K
K
Chapter 16-Acids and Bases Slide 6
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“p” = -log• In most solutions [H+(aq)] is quite small.• We define
• In neutral water at 25 C, pH = pOH = 7.00.• In acidic solutions, [H+] > 1.0 10-7, so pH < 7.00.• In basic solutions, [H+] < 1.0 10-7, so pH > 7.00.• The higher the pH, the lower the pOH, the more basic the
solution.• There are no theoretical limits on the values of pH or pOH.
(e.g. pH of 2.0 M HCl is -0.301.)
The pH ScaleThe pH Scale
]OHlog[pOH ]Hlog[]OHlog[pH -3
Logarithm algebra is needed here
Chapter 16-Acids and Bases Slide 7
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• pH meter – measures conductivity as a function of [H3O+].
• Precise, but calibration is often needed for accuracy
• Indicators – dyes that change their bonding & color as a function of [H3O+].
• Less precise than pH meters, but quick and cheap
• Most indicators tend to be red in more acidic solutions.
The pH Scale, & The pH Scale, & Measuring pHMeasuring pH
Natural Indicators.mov
Indicators & the pH ScaleIndicators & the pH ScaleCarbonDioxide as an acid.MOV
Transition IntervalTransition Interval: : the range of pH that the range of pH that they are effective.they are effective.
EndpointEndpoint: the pH : the pH that produces a that produces a noticeable color noticeable color changechange
Chapter 16-Acids and Bases Slide 9
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Definition• Conjugate Base = Whatever is left of the acid after the
proton is donated.• Conjugate Acid = Whatever remains of the base after it
accepts a proton.• Consider
– After HA (acid) loses its proton it is converted into A- (base). Therefore HA and A- are conjugate acid-base pairs.
– After H2O (base) gains a proton it is converted into H3O+ (acid). Therefore, H2O and H3O+ are conjugate acid-base pairs.
• Conjugate acid-base pairs differ by only one proton.
Conjugate Acid-Base PairsConjugate Acid-Base Pairs
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Chapter 16-Acids and Bases Slide 10
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Identify Conjugate pairs:Identify Conjugate pairs:
KClOHKOHHCl 2
0342432 HPOHPOHOHAcid
Acid
Base
Base Conjugate Acid
Conjugate Base
Conjugate Base
Conjugate Acid
Chapter 16-Acids and Bases Slide 11
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Relative Relative Strengths Strengths of Acids of Acids
and Basesand Bases
Strengths of Acids or Bases is determined by the extent to which a species dissociates, or ionizes in water.
Chapter 16-Acids and Bases Slide 12
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RulesRules: Relative Strengths of Acids and Bases• The stronger the acid, the weaker the conjugate base. • The stronger the base, the weaker the conjugate acid.• A “strong” acid’s conjugate base (e.g. Cl-) has negligible
acid-base properties.• A “strong” base’s conjugate acid has negligible acid-base
properties.• Conjugates of weak acids are weak bases.• Conjugates of weak bases are weak acids.
• Acid strength increases across a period and down a group.
• Base strength decreases across a period and down a group.
ConjugateConjugate Acids and Acids and Base Strength Base Strength
Chapter 16-Acids and Bases Slide 13
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Strong Acids and BasesStrong Acids and Bases*MEMORIZE**MEMORIZE*
• Six Strong Acids*:
• HClO4 HI HBr HCl H2SO4 HNO3HNO3(aq) H+
(aq) + NO3-(aq)
• Strong Bases:
• NaOH KOH Ba(OH)2 etc-OH (those soluble)
• Strong bases need not have OH- (e.g.: H-(aq) + H2O(l) H2(g) + OH-
(aq) )
• All others, consider to be “weak” (not completely dissociated)
• Strong acids & bases are strong electrolytes.
• “Weak” acids and bases = Ions are in equilibrium with unionized compounds.
• *Your book: HClO3 is also strong, but not on AP test.
Strong and weak acids-electrolytes.mov
]HA[]A][H[ -
aK
Chapter 16-Acids and Bases Slide 14
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The of the acid-base on the AP• Weak acids are only partially ionized in
solution.• Therefore, weak acids are in equilibrium:
• Ka is the acid dissociation constant.
• The larger the Ka the stronger the acid (i.e. the more ions are present at equilibrium relative to unionized molecules).
• If Ka >> 1, then the acid is completely ionized and the acid is a strong acid.
Weak AcidsWeak Acids
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
]HA[]A][OH[ -
3
aK
Chapter 16-Acids and Bases Slide 15
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Some Weak Acids:Some Weak Acids:
Chapter 16-Acids and Bases Slide 16
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• Weak acids are simply equilibrium calculations.• The pH gives the equilibrium concentration of H+.
• Using Ka, the concentration of H+ (and hence the pH) can be calculated:1. Write the balanced chemical equation clearly showing the
equilibrium.
2. Write the equilibrium expression. Find the value for Ka.
3. Write down the initial and equilibrium concentrations for everything except pure water. We usually assume that the change in concentration of H+ is x.
4. Substitute into the equilibrium constant expression and solve. Remember to turn x (at equilibrium) into pH if necessary.
We can also solve for Ka from pH or [H3O+] in this manner.
Calculating pH or [HCalculating pH or [H33OO++] from K] from Kaa
Chapter 16-Acids and Bases Slide 17
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100]HA[
]OH[ionization %
0
3
eqm
Using %Ionization to Calculate pH or Ka• Percent ionization is another method to assess acid strength.• For the reaction
• The higher percent ionization, the stronger the acid.• Percent ionization of a weak acid decreases as the molarity
of the solution increases.• For acetic acid, 0.05 M solution is
2.0 % ionized whereas a 0.15 M solution is 1.0 % ionized.
% Ionization% Ionization
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Chapter 16-Acids and Bases Slide 18
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Polyprotic Acids have more than one ionizable H+
• The protons are removed in steps not all at once:
• It is always easier to remove the first proton in a polyprotic acid than the second.
• Therefore
• Ka1 > Ka2 > Ka3 etc.
Polyprotic AcidsPolyprotic Acids
H2SO3(aq) H+(aq) + HSO3-(aq) Ka1 = 1.7 x 10-2
HSO3-(aq) H+(aq) + SO3
2-(aq) Ka2 = 6.4 x 10-8
Chapter 16-Acids and Bases Slide 19
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• Weak bases remove protons from substances.
• There is an equilibrium between the base and the resulting ions:
• Example:
• The base dissociation constant, Kb, is defined as
Weak BasesWeak Bases
Weak base + H2O conjugate acid + OH-
NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)
]NH[]OH][NH[
3
-4
bK
Chapter 16-Acids and Bases Slide 20
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Trends in Weak Bases• Bases generally have lone pairs or negative charges in order
to attack protons.• Most neutral weak bases contain nitrogen.• Amines are related to ammonia and have one or more N-H
bonds replaced with N-C bonds (e.g., CH3NH2 is methylamine).
• Anions of weak acids are also weak bases. Example: OCl- is the conjugate base of HOCl (weak acid):
Weak BasesWeak Bases
ClO-(aq) + H2O(l) HClO(aq) + OH-(aq) Kb = 3.3 x 10-7
Chapter 16-Acids and Bases Slide 21
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Some Weak BasesSome Weak Bases
Chapter 16-Acids and Bases Slide 22
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• When two reactions are added to give a third, the equilibrium constant for the third reaction is the product of the equilibrium constants for the first two:
• For a conjugate acid-base pair:
• The larger the Ka, the smaller the Kb. That is, the stronger the acid, the weaker the conjugate base.
• Taking negative logarithms:
Conjugate Acid/Base StrengthsConjugate Acid/Base Strengths
213 KKK
baw KKK
baw pKpKpK
Chapter 16-Acids and Bases Slide 23
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Ionic salts dissociate completely• Acid-base properties of salts are a consequence of the
reaction of their ions in solution.• The reaction in which ions produce H+ or OH- in water is
called hydrolysis.• Anions from strong acids are neutral.• Anions from weak acids are basic:
• Polyatomic cations with ionizable protons can be considered conjugate acids of weak bases:
Acid-Base Properties of Acid-Base Properties of SaltSalt Solutions Solutions
X-(aq) + H2O(l) HX(aq) + OH-(aq)
NH4+(aq) + H2O(l) NH3(aq) + H3O+(aq)
Chapter 16-Acids and Bases Slide 24
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Consider H-X:
For this substance to be an acid we need:• H-X bond to be polar• H-X bond must be weak enough to be broken• The conjugate base, X-, must be stable• HF is a weak acid because the bond energy is high and not
easily broken. H-I is a strong acid, easy to break H-I bond.• Acid strength increases across a period and down a group.• Base strength decreases across a period and down a group.
Binary Acid StrengthsBinary Acid Strengths
Chapter 16-Acids and Bases Slide 25
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Binary Acids
Acid-Base Behavior and Acid-Base Behavior and Chemical StructureChemical Structure
Chapter 16-Acids and Bases Slide 26
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Oxyacids contain O-H bonds• All oxyacids have the general structure Y-O-H.• The strength of the acid depends on Y and the relative
location of the electron density near Y:– If Y is a metal (low electronegativity), then the substance is a base.
– If Y has intermediate electronegativity (e.g. I, EN = 2.5), the substance is a weak oxyacid.
– If Y has a large electronegativity (e.g. Cl, EN = 3.0), the electrons are located closer to Y than O and the O-H bond is polarized to lose H+.
– More O atoms attached to Y increase the O-H bond polarity and the strength of the acid increases (e.g. HOCl is a weaker acid than HClO2 which is weaker than HClO3 which is weaker than HClO4 which is a strong acid).
Oxy-acidsOxy-acids
Chapter 16-Acids and Bases Slide 27
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Oxyacids
Acid-Base Behavior and Acid-Base Behavior and Chemical StructureChemical Structure
Chapter 16-Acids and Bases Slide 28
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Carboxylic Acids• Carboxylic acids all contain the COOH group.• All carboxylic acids are weak acids.• When the carboxylic acid loses a proton, it generate the
carboxylate anion, COO-.
Acid-Base Behavior and Acid-Base Behavior and Chemical StructureChemical Structure
RC
OH
O
Chapter 16-Acids and Bases Slide 29
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• Recall: Brønsted-Lowry acid is a proton donor.• Focusing on electrons: a Brønsted-Lowry acid can be
considered as an electron pair acceptor.• Lewis acid: electron pair acceptor.• Lewis base: electron pair donor.• Note: Lewis acids and bases do not need to contain protons.
• Lewis acids generally have an incomplete octet (e.g. BF3).
• Transition metal ions are generally Lewis acids.• Lewis acids must have a vacant orbital
(into which the electron pairs can be donated).
Lewis Acids and BasesLewis Acids and Bases
Lewis Acids and Bases.mov
Chapter 16-Acids and Bases Slide 30
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Hydrolysis of Metal Ions (can be tricky)• Alkali and Alkaline-earth metals form bases in solution.• Transition metal ions are positively charged and attract
water molecules (via the lone pairs on O).• The higher the charge, the smaller the metal ion and the
stronger the M-OH2 interaction.
• Hydrated metal ions (water-complexes) act as weak acids:
• [H3O+] increases as the size of the ion decreases (Ca2+ / Zn2+) and as the charge increases (Na+ / Ca2+ and Zn2+ / Al3+).
• Na+ = no acidic behavior, Al3+ = very acidic cation
Metals and pHMetals and pH
Fe(H2O)63+(aq) Fe(H2O)5(OH)2+(aq) + H+(aq)
Ka = 2 x 10-3
Chapter 16-Acids and Bases Slide 31
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Hydrolysis of Metal Ions
Lewis Acids and BasesLewis Acids and Bases
End of chapter 16 (phew)