Post on 26-Mar-2015
Acid-Base Equilibrium 1
AP Chemistry
Acid-Base Theory and
Equilibrium Constants
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Models of Acids and Models of Acids and BasesBases Arrhenius ConceptArrhenius Concept: Acids : Acids
produce Hproduce H++ (or H (or H33OO++, hydronium , hydronium ion) in solution; bases produce ion) in solution; bases produce OHOH−− ion. ion.
•HCl + HHCl + H22O ClO Cl−− + H + H33OO++
• acid baseacid baseBrønsted-LowryBrønsted-Lowry: Acids are proton (H: Acids are proton (H++ ) donors bases are proton acceptors.) donors bases are proton acceptors.
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•HA(HA(aqaq) + H) + H22O(O(ll) ) H H33OO++((aqaq) + A) + A−−((aqaq) )
Conjugate Acid-Base Conjugate Acid-Base PairsPairs
conjugate baseconjugate base: everything that remains of the : everything that remains of the acid molecule after a proton is lost.acid molecule after a proton is lost.
conjugate acidconjugate acid: formed when the proton is : formed when the proton is transferred to the base.transferred to the base.
conj conj conj conj conj conj conjconj acid 1 base 2 acid 2 base 1
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•HA(HA(aqaq) + H) + H22O(O(ll) ) H H33OO++((aqaq) + A) + A−−((aqaq))
Acid Dissociation Acid Dissociation Constant, KConstant, Kaa
Ka = [H3O+][A-]
[HA]=
[H+][A -]
[HA]
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Its equilibrium position lies far to the Its equilibrium position lies far to the right. (HNOright. (HNO33))
Large KLarge Kaa value value
Yields a weak conjugate base. (NOYields a weak conjugate base. (NO33−−))
HNOHNO33 + H + H22O HO H33OO++ + NO + NO33−−
Strong Acid:Strong Acid:Acid StrengthAcid Strength
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Its equilibrium lies far to the left. Its equilibrium lies far to the left. (CH(CH33COOH)COOH)
Small KSmall Kaa value value
Yields a much stronger (it is relatively Yields a much stronger (it is relatively strong) conjugate base than water. strong) conjugate base than water. (CH(CH33COOCOO−−
CHCH33COOH + HCOOH + H22O HO H33OO+ + + CH+ CH33COOCOO−−
Acid StrengthAcid StrengthWeakWeak Acid: Acid:
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•B(B(aqaq) + H) + H22O(O(ll) ) OH OH--((aqaq) + HB) + HB++((aqaq))
Base Dissociation Base Dissociation Constant, KConstant, Kbb
Kb = [OH-][HB++]
[B]
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““Strong” and “weak” are used Strong” and “weak” are used in the same sense for bases as in the same sense for bases as for acids.for acids.
strong = complete dissociation strong = complete dissociation (hydroxide ion supplied to (hydroxide ion supplied to solution)solution)
•NaOH(NaOH(ss) ) →→ Na Na++((aqaq) + OH) + OH−−((aqaq))
BasesBases
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weak = very little dissociation (or weak = very little dissociation (or reaction with water)reaction with water)
•HH33CNHCNH22((aqaq) + H) + H22O(O(ll) ) H H33CNHCNH33++((aqaq) + ) +
OHOH−−((aqaq))
Bases Bases (con’t)(con’t)
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•Water can behave either as an acid Water can behave either as an acid or a base.or a base.
•HH22OO + H + H22O O H H33OO++ + + OHOH
conj conjacid 1 base 2 acid 2 base 1
Water is Water is amphotericamphoteric..
KKww = [H = [H++] [OH] [OH−−]]
KKww = 1 = 1 10 10−−1414 at 25°C at 25°C
Acid-Base Equilibrium 2
AP ChemistryThe pH Scale
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pH pH ≈≈ −−log[Hlog[H++] and pOH = ] and pOH = −−log[OHlog[OH--]]
pH in water ranges from 0 to 14.pH in water ranges from 0 to 14.
KKww = 1.00 = 1.00 10 10−−1414 = [H = [H++] [OH] [OH−−]]
ppKKww = 14.00 = pH + pOH = 14.00 = pH + pOH
As pH rises, pOH falls (sum = As pH rises, pOH falls (sum = 14.00)14.00)..
The pH The pH ScaleScale
Example ProblemWhat is the pH, pOH, and [OH-] of a 0.0020M solution of nitric acid?
HNOHNO33 + H + H22O HO H33OO++ + NO + NO33−−
pH pH −−log[Hlog[H++] ] pH = −log(0.0020) pH = −log(0.0020)
pH pH 2.702.70
pH + pOH = 14.00pH + pOH = 14.002.70 + pOH = 2.70 + pOH = 14.0014.00pOH = 11.30pOH = 11.30
[H[H++] [OH] [OH−−] = 1.00 ] = 1.00 10 10−−1414
(0.0020) [OH(0.0020) [OH−−] = 1.00 ] = 1.00 10 10−−1414
[OH[OH−−] = 5.0 ] = 5.0 10 10−−12 12 MM
Acid-Base Equilibrium 3
AP Chemistry
Solving Problems
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List major species in solution.List major species in solution.
Choose species that can produce HChoose species that can produce H++ and write and write reactions.reactions.
Based on K values, decide on dominant Based on K values, decide on dominant equilibrium.equilibrium.
Write equilibrium expression for dominant Write equilibrium expression for dominant equilibrium.equilibrium.
List initial concentrations in dominant List initial concentrations in dominant equilibrium.equilibrium.
Solving Weak Solving Weak Acid Acid Equilibrium ProblemsEquilibrium Problems
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Define change at equilibrium (as “Define change at equilibrium (as “xx”).”).
Write equilibrium concentrations in terms of Write equilibrium concentrations in terms of xx..
Substitute equilibrium concentrations into Substitute equilibrium concentrations into equilibrium expression.equilibrium expression.
Solve for Solve for xx the “easy way.” the “easy way.”
Calculate [HCalculate [H++] and pH.] and pH.
(continued)(continued)
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% Dissociation or % % Dissociation or % IonizationIonization
% dissociation = Amount dissociated (M)
x 100%Initial concentration (M)
Acid-Base Equilibrium 4
AP Chemistry
Ka and Kb Relationships &
Polyprotic Acids
KKaa and K and Kbb RelationshipRelationship
•HA(HA(aqaq) + H) + H22O(O(ll) ) H H33OO++((aqaq) + A) + A−−((aqaq) K) Kaa
AA--((aqaq) + H) + H22O(O(ll) ) OH OH--((aqaq) + HA() + HA(aqaq) ) KKbbHH22O(O(ll) + H) + H22O(O(ll) ) H H33OO++((aqaq) + ) +
OHOH((aqaq))
[H3O+][A-]
[HA]
[OH-][HA]
[A]= [H3O+][OH-]
Ka Kb Kw x =
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•. . . can furnish more than one . . . can furnish more than one proton (Hproton (H++) to the solution.) to the solution.
Polyprotic AcidsPolyprotic Acids
HCO3- H+ + CO3
2- Ka2
H2CO3 H+ + HCO3- Ka1
H2CO3 2H+ + CO32- Ka
Ka1 x Ka2 = Ka(overall)
KKaa and K and Kbb Relationship for Relationship for Polyprotic AcidsPolyprotic Acids
For diprotic acidsKa1 x Kb2 = Kw
Ka2 x Kb1 = Kw
For triprotic acidsKa1 x Kb3 = Kw
Ka2 x Kb2 = Kw
Ka3 x Kb1 = Kw
Acid-Base Equilibrium 5
AP Chemistry
Miscellaneous Acid-Base Concepts
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Acidic and Basic Acidic and Basic SaltsSalts
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•Two factors affecting acidity in binary Two factors affecting acidity in binary compoundscompounds
•Bond PolarityBond Polarity (higher is more acidic) (higher is more acidic)
•Bond StrengthBond Strength (lower is more acidic) (lower is more acidic)
Molecular Structure and Molecular Structure and Acidic PropertiesAcidic Properties
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•1. Acidic Oxides (Acid Anhydrides):1. Acidic Oxides (Acid Anhydrides):
• Nonmetal Oxides (Examples: SONonmetal Oxides (Examples: SO22, CO, CO22, , CrOCrO33))
• OO−−X bond is strong and covalent.X bond is strong and covalent.
•2. Basic Oxides (Basic Anhydrides):2. Basic Oxides (Basic Anhydrides):
• Metal Oxides (Examples: KMetal Oxides (Examples: K22O, CaO)O, CaO)
• OO−−X bond is ionic.X bond is ionic.
Oxides of Metal and Oxides of Metal and NonmetalsNonmetals
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•1. Acidic Oxides (Acid Anhydrides):1. Acidic Oxides (Acid Anhydrides):
• HH22O + COO + CO22 H H22COCO33
• NN22OO55 + H + H22O 2HNO3O 2HNO3
•2. Basic Oxides (Basic Anhydrides):2. Basic Oxides (Basic Anhydrides):
• KK22O + HO + H22O 2KOH O 2KOH
• CaO + HCaO + H22O Ca(OH)O Ca(OH)22
Examples of AnhydridesExamples of Anhydrides
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Lewis AcidLewis Acid: electron pair : electron pair acceptoracceptor
Lewis BaseLewis Base: electron pair donor: electron pair donor
Lewis Acids and BasesLewis Acids and Bases
B B ++ NN F B F B N HN H
FF
FF
FF
FFFF
HH
HH
HH HH
HH
::::
Lewis Lewis LewisLewisAcid Acid BaseBase
Coordinate Coordinate Covalent Covalent
BondBond