Today’s(Topics!Aqueous'Equilibria'III"
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• Brønsted)Lowry.acid)base.equilibria.
• Acid.and.base.strength.
• Hydrolysis.of.salts.
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Related.Problems.from.Spring.2008.Exam.1:.15,.16.&.Spring.2009.Exam.1:.7,.11,.15.
Corresponding.Connect.Problems:.Set.3:.11–16
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Figures.taken.from.Chemistry,.4th.ed..by.J..McMurray.and.R..C..Fay,.Prentice)Hall,.2004..
All,materials,are,©,2012–2013,Lynmarie,A.,Posey,and,Michigan,State,University,unless,otherwise,noted;,
all,rights,reserved.,
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Learning(Objectives((Lecture(8)"
You.should:.
1. be.able.to.apply.the.Brønsted)Lowry.acid.and.base.definitions.to.identify.acids,.bases,.
and.conjugate.acid)base.pairs..
2. know.how.to.write.the.formula.for.the.conjugate.base.of.a.Brønsted)Lowry.acid.or.
conjugate.acid.of.a.Brønsted)Lowry.base..
3. understand.how.strong.acids.and.bases.differ.from.weak.acids.and.bases..
4. know.the.relationship.between.the.relative.strength.of.an.acid.and.the.relative.
strength.of.its.conjugate.base..
5. understand.that.equilibrium.in.acid)base.reactions.favors.transfer.of.a.proton.(H+).
from.the.stronger.acid.to.the.stronger.base.resulting.in.formation.of.the.weaker.acid.
and.the.weaker.base..
6. be.prepared.to.write.balanced.chemical.equations.for.the.reactions.that.occur.when.
acids.and.bases.are.added.to.water.and.to.use.these.equations.to.write.equilibrium.
constant.expressions.for.Ka.and.Kb..
7. be.able.to.use.acid.and.base.ionization.constants.(Ka.and.Kb).to.find.equilibrium.
concentrations.of.[H3O+].or.[OH–]..
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2..
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8. understand.when.and.how.to.use.approximations.in.equilibrium.calculations..
9. know.that.H! O! .is.the.strongest.acid.that.can.exist.in.water..As.a.consequence,.all.
strong.acids.are.leveled.to.H! O! ..Strong.acids.have.conjugate.bases.that.are.
ineffective.as.bases.in.aqueous.solutions.and.have.no.effect.on.pH..
10. know.that.OH ! .is.the.strongest.base.that.can.exist.in.water,.and.as.such,.bases.
stronger.than.OH ! .cannot.exist.in.water.and.will.be.leveled.to.OH ! ..
11. be.able.to.calculate.the.pH.of.solutions.produced.when.weak.acids.or.bases.are.added.
to.water..
12. understand.the.processes.that.occur.when.salts.are.dissolved.in.water..
13. be.able.to.write.balanced.equations.for.dissolution.and.hydrolysis.of.salts.in.water..
14. know.and.be.able.to.apply.the.relationship.between.Ka.for.an.acid.and.Kb.for.its.
conjugate.base..
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3..
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Brønsted!Lowry&Acid!Base%Equilibria"
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The.strength.of.an.acid.is.classified.based.on.its.strength.relative.to.H! O! ..Strong.acids.
are.stronger.than.H! O! ,.and.weak.acids.are.weaker.than.H! O! .(but.stronger.than.H! O)..
Strong"Acids"
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HClO!(!") + H! O(!) ⇌ ClO!
! (!") + H! O ,
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acid! !!!!!!!!!!!!!!base! !!!!!!!!base! !!!!!!!!!!!!!acid! .
! ! > 1.
Equilibrium.favors.the.products,.which.correspond.to.the.weaker.acid.(H! O! ).and.weaker.
base.(ClO!
! )..
Water.serves.as.a.base.in.this.reaction.because.HClO4.is.a.stronger.acid.than.H2O..
ClO!
! .is.the.conjugate.base.of.the.strong.acid.HClO! ..Like.all.conjugate.bases.of.strong.
acids,.it.is.ineffective.as.a.base.in.water..H! O.is.a.stronger.base.than.ClO!
! ..
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4..
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Weak"Acids"
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HCN(!") + H! O(!) ⇌ CN ! (!") + H! O! (!") !!!!!!!!! = 6.2 ⨯ 10 !!" .
. acid! !!!!!!!!!!!base! !!!!!!!!base! !!!!!!!!!!acid! .
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Equilibrium.favors.the.reactants,.which.correspond.to.the.weaker.acid.(HCN).and.weaker.
base.(H2O)..
Water.serves.as.a.base.in.this.reaction.because.HCN.is.a.stronger.acid.than.H2O..
CN ! .is.the.conjugate.base.of.the.weak.acid.HCN..The.conjugate.bases.of.weak.acids.are.
effective.as.bases.in.water..
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5..
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Problem:)Brønsted!Lowry&Acid!Base%Equilibrium)Reaction"
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Consider.the.following.Brønsted)Lowry.equilibrium:.
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HSCN(!") + H! O(!) ⇌ H! O! (!") + ! ______.
Identify.the.missing.product,.and.decide.whether.that.product.is.acting.as.an.acid.or.as.a.
base.in.the.equilibrium.reaction..
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6..
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Acid%Strength%and%Equilibrium"
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(Figure)McMurray.&.Fay).
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7..
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Relative(Strengths(of(Brønsted!Lowry&Acids&and&Bases"
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The,stronger,an,acid,,the,weaker,its,conjugate,base,is;,the,stronger,a,base,,the,weaker,its,conjugate,acid,is.,
(Figure)McMurray.&.Fay).
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8..
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Levelling(Effect(for(Strong(Acids"
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Since.H! O! .is.the.solvent.cation/conjugate.acid.of.H! O,.H! O! .is.the.strongest.acid.that.can.
exist.in.H! O..As.a.consequence,.all.strong.acids.are.leveled.to.H! O! ..No.acid.that.is.
stronger.than.H! O! .can.survive.in.water..
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HClO!(!") + H! O(!) ⇌ ClO!
! (!") + H! O (!") ,!!!!!!!!!!!! ! = 4 ⨯ 10 .
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HCl(!") + H! O(!) ⇌ Cl ! (!") + H! O! (!") ,!!!!!!!!!!!!!!!!!!!! ! = 1 ⨯ 10 ! .
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• Both.of.these.strong.acids.have.the.same.effective.strength.in.water..The.
concentration.of.H! O! .in.strong.acid.solutions.will.be.equal.to.the.mol/L.of.the.
strong.acid.added.to.water..
• The.only.difference.between.the.two.solutions.will.be.the.anions,.which.will.be.
ineffective.as.bases..
• Remember.that.Brønsted)Lowry.acid)base.equilibria.are.governed.by.the.
competition.for.the.! ! .with.the.strongest.base.winning.out..In.the.case.of.equilibria.
involving.strong.acids,.H! O.is.a.much.stronger.base.than.the.conjugate.base.of.the.
strong.acid..
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9..
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Levelling(Effect(for(Strong(Bases"
The.strongest.base.that.can.exist.in.aqueous.solution.is.OH ! ,.which.is.the.solvent.
anion/conjugate.base.of.H! O..Consequently,.all.strong.bases.will.be.levelled.to.OH ! .
because.bases.stronger.than.OH ! .cannot.exist.in.aqueous.solution..
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NH!! (!") + H! O(!) ⇌ NH!(!") + OH ! (!") .
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H ! (!") + H! O(!) ⇌ H!(!") + OH ! (!") .
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Equilibrium.favors.products.(side.of.the.reaction.with.the.weaker.acid.and.weaker.base)..
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10..
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Problem:)Weak)Acids)and)Weak%Bases"
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Write.balanced.chemical.equations.for.the.equilibria.that.result.when.the.substances.
given.below.are.added.to.water..Identify.the.species.acting.as.acids.and.those.acting.as.
bases..Rank.the.participants.in.each.equilibrium.reaction.in.order.of.increasing.acid.and.
base.strength..
1. weak.acid.!!!! .
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11..
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12..
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2. weak.acid.CH3CO2H.(acetic.acid).
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3. weak.base.NH3.
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13..
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Problem:)pH)of)Solutions)Containing)a)Weak)Acid)or)a)Weak)Base"
Find.the.pH.of.the.following.solutions:..
1. 0.20.M.acetic.acid,.!! = 1.8!×!10!! ..
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14..
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2. 0.50.M.ammonia,.!! = 1.8!×!10!! ..
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15..
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3. 0.01.M.NH4+.(ammonium.cation),.!! = 5.56!×!10!!" .
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16..
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Dissolution)of)Salts)in)Water"
Salts.are.ionic.compounds.(excluding.those.containing.OH ! .as.the.anion).consisting.of.
positively.charged.cations.and.negatively.charged.anions,.which.are.held.together.by.
electrostatic.intermolecular.forces..At.room.temperature,.salts.exist.as.solids..
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What.happens.when.salts.(ionic.compounds).are.dissolved.in.water?.
• Dissociation)Salts.separate.into.their.component.cations.and.anions..Dissociation.is.
typically.complete.or.nearly.complete.in.dilute.solutions..
• Solvation/Hydration)Separated.cations.and.anions.are.then.surrounded.by.water.
molecules..
• Hydrolysis.(in.some.cases))When.the.cation.is.a.weak.acid.or.the.anion.is.a.weak.
base,.they.will.react.with.water.in.an.acid)base.reaction.and.establish.an.acid)base.
equilibrium,.which.will.change,the,pH.of.the.solution..
• !! = !! !! .(Stronger.acids.have.weaker.conjugate.bases,.and.weaker.acids.have.
stronger.conjugate.bases.).
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17..
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18..
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Problem:)Dissolution)of)Salts)in)Water)&)pH"
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Which.of.the.following.salts.produces.a.neutral.solution.when.dissolved.in.water?.
(a) NH4ClO4.
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(b) Mg(ClO)2.
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(c) LiCN.
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19..
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20..
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(d) NaF.
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(e) KCN.
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(f) NH4Cl.
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(g) NaClO4.
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(h) Mg(CH3CO2)2.
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21..
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Bonus&Problem:&Determination&of!!! !from%the%pH%of%a%Weak%Acid!
Solution(with(Known(Concentration"
The.pH.of.0.250.M.HF.is.1.890..What.is.the.value.of.Ka.for.hydrofluoric.acid?.
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22..