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Multimedia Chemistry I & II (1996-9-11) [English].img
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chapt11.5c
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à 11.5cèHydrolysis ç Salts
äèPlease predict wheêr ê aqueous solutions ç ê followïg salts will be acidic, basic, or neutral.
(Acid å base dissociation constants are ïclude ï ê Tables)
âèIs an aqueous solution ç AlBr╕ acidic, basic, or neutral?è
AlBr╕ consists ç AlÄó å Brú.èAlÄó is an acidic cation.èThe reaction,
Al(H╖O)╗Äó + H╖O ──¥ Al(H╖O)║(OH)ìó + H╕Oó, shows ê hydrolysis by ê
AlÄó ion.èBrú is ê conjugate base ç HBr, which is a strong acid.
Brú is such a weakly basic anion that it is considered ë be a neutral
anion.èAs a result ç ê acid-base properties ç ê ions, a solution
ç AlBr╕ is acidic.
éSèA salt dissociates completely ïë cations å anions when it
dissolves ï water ë form a dilute solution.èSome ions cause water ë
hydrolyze.èHydrolysis is ê splittïg ç water.èIons may be classified
as eiêr acidic, basic, or neutral.èThe acidic å basic å basic ions
can cause ê hydrolysis.èIn order ë decide wheêr a given solution
will be acidic, basic, or neutral, we just consider ê behavior ç each
ion.
1. Neutral Ions:
èè Neutral ions have no effect on ê acidity or basicity ç ê solu-
tion.èNeutral anions are ê conjugate bases ç strong acids; such as,
Clú, Brú, Iú, NO╕ú,èå ClO╣ú.èNeutral cations are those ç ê Group
1 å Group 2 metals: Lió, Naó, Kó, Caìó, å Srìó.
2. Acidic Ions:
èè The conjugate bases ç strong acids that have anoêr dissociable
proën are acidic anions.èExamples are HSO╣ú, H╖PO╣ú, å HC╖O╣ú.èWith
êse acids ê first proën is ê most easily donated.èSubsequent
proëns are donated less readily å ê anions are weaker acids.
Strictly speakïg ê donation ç a proën by êse ions is not a hydrol-
ysis, because a water molecule is not beïg split.èWe consider êse
reactions here, sïce compounds ç êse ions are considered ë be salts.
èè Transition metal cations are acidic cations; such as, Feìó, FeÄó,
CrÄó, å Znìó.èOêr cations ç relatively small size å high charge
are also acidic, for example, AlÄó.
3. Basic Ions:
èè The conjugate bases ç weak acids are basic.èSome examples ç this
category are: CO╕ìú, SO╕ìú, NO╖ú, PO╣Äú, Sìú, å C╖H╕O╖ú.èBy examïïg
a table ç weak acids, we can predict which anions(conjugate bases) will
be basic.
Will a Na½CO╕ solution be acidic, basic, or neutral?èWe must know what
ions are formed when sodium carbonate dissolves ï water.èWhat ions are
formed?èNa╖CO╕ ─¥ 2Naó + CO╕ìú.èSïce Naó is a Group 1 cation, we know
it is neutral.èThe carbonate ion, on ê oêr hå, is ê conjugate
base ç a very weak base, HCO╕ú, which ï turn is ê conjugate base ç
ê weak acid, H╖CO╕.èThe solution will be basic.èThe hydrolysis
reaction is CO╕ìú + H╖O ──¥ HCO╕ú + OHú.èThis reaction ïcreases ê
hydroxide ion concentration above ê level ï pure water å ê solu-
tion is basic.
Will an AlCl╕ solution be acidic, basic, or neutral?èThis salt dissolves
releasïg AlÄó å Clú ions.èThe Clú ion is neutral.èAlÄó is an acidic
cation.èWe know that six water molecules associate with one AlÄó cation.
The hydrolysis reaction is Al(H╖O)╗Äó + H╖O ──¥ Al(H╖O)║OHìó + H╕Oó.
This reaction ïcreases ê H╕Oó concentration above that ï pure water,
å, thus, ê soltuion is acidic.
It is difficult ë decide wheêr ê acid-base properties ç some ions
simply by lookïg at êir formula.èAn example is HCO╕ú.èThis ion is
amphoteric: capable ç actïg eiêr as an acid or a base.èHow do we
decide?èWe look at ê K╬ å K╧ values.èThe equilibria are:
HCO╕ú + H½O = H╕Oó + CO╕ìú, K╬ = 4.7x10úîî
HCO╕ú + H½O = H╖CO╕ + OHú , K╧ = 2.2x10úô
We see that K╧ > K╬.èConsequently, HCO╕ú is stronger as a base than it
is as an acid.èThe HCO╕ú ion is weakly basic.èA solution ç NaHCO╕ is
weakly basic.
1èWhich one ç ê followïg salts will produce an acidic
èèaqueous solution?
èèA) NaCN B) K╖S
èèC) LiHSO╣ D) Ba(NO╕)╖
üèLiHSO╣ consists ç Lió å HSO╣ú ions.èThe Lió cation is a
neutral ion.èHSO╣ú is ê conjugate base ç H╖SO╣, a very strong acid.
HSO╣ú has an dissociable proën å acts as a weak acid.èIts acid
dissociation constant is 1.2x10úì.èNaCN å K╖S will give basic solu-
tions.èBa(NO╕)╖ is approximately neutral.
Ç C
2èWhich one ç ê followïg salts will yield a basic aqueous
èèsolution?
èèA) KC╖H╕O╖ B) NH╣Cl
èèC) CaCl╖ D) NaH╖PO╣
üèKC╖H╕O╖ is potassium acetate å consists ç Kó å C╖H╕O╖ú ions.
The Kó ion is neutral, but ê acetate ion is basic.èAn aqueous solution
ç KC╖H╕O╖ is basic.èNH╣Cl å NaH╖PO╣ form acidic solutions.èCaCl╖
forms approximately neutral solutions.
Ç A
3èWhich one ç ê followïg aqueous salt solutions will be
èèèèèèbasic?
èèA) CrBr╕ B) NaHC╖O╣
èèC) Mg(HSO╣)╖ D) KNO╖
üèPotassium nitrite consists ç Kó å NO╖ú ions.èThe Kó ion is
neutral.èThe nitrite ion is ê conjugate base ç a weak acid, NO╖ú.
Conjugate bases ç weak acids are slightly basic ï aqueous solution.
The hydrolysis reaction isèNO╖ú + H╖O ──¥ HNO╖ + OHú.èThe oêr three
choices form acidic solutions.
Ç Dè
4èWhich one ç ê followïg aqueous salt solutions will be
èèacidic?
èèA) FeCl╕ B) Na╕PO╣
èèC) Ca(ClO)╖ D) K╖SO╣
üèIron(III) chloride consists ç FeÄó å Clú ions.èThe FeÄó ion
is a transition metal cation.èThese ion have relatively high charge
for êir size.èThey attract ê electrons on oxygen ï ê water mole-
cule, which ên enable one ç ê hydrogens ë be donated as a proën ë
anoêr water molecule.èClú is ê conjugate base ç ê very strong
acid, HCl.èClú is a neutral anion.èThe FeCl╕ solution is acidic å
ê hydrolysis reaction is Fe(H╖O)╗Äó + H½O ──¥ Fe(H╖O)║(OH)ìó + H╕Oó.
Ç A
5èRank ê followïg solutions from ê most acidic ë ê most
èèbasic.è[A] = KNO╕èè [B] = NH╣Clèè [C] = Na╖CO╕
èèA) [A] > [B] > [C] B) [B] > [A] > [C]
èèC) [C] > [A] > [B] D) [C] > [B] > [A]
üèThe ions that compose êse salts å êir acid-base character
is:èSaltèèèèèCationèèèèèè AnionèèèèèèSolution
èè KNO╕èèè Kóè (neutral)èè NO╕úè(neutral)èè Neutral
èè NH╣ClèèèNH╣ó (acidic)èèèClúè (neutral)èè Acidic
èè Na╖CO╕èè Naóè(neutral)èè CO╕ìú (basic)èèè Basic
The order ç ê solutions from most acidic ë most basic is
NH╣Cl > KNO╕ > Na╖CO╕.èIn terms ç ê letters: [B] > [A] > [C].
Ç B
6èWhich ionic reaction shows hydrolysis by sodium carbonate,
èèNa╖CO╕, ë yield a basic aqueous solution?
èèA) CO╕ìú + 2OHú ──¥ H╖O + CO╖ + 2Oìú.
èèB) CO╕ìú + H╖Oè──¥ HCO╕ú + OHú.
èèC) Naó + H½Oèè──¥ NaHìó + OHú.
èèD) Naó + OHúèè──¥ NaOH.
üè We need ë show ê generation ç OHú ions ï order ë show that
ê solution is basic.èThe reaction will not ïvolve Naó, because this
ion is neutral.èThe carbonate ion, CO╕ìú, is ê conjugate base ç HCO╕ú.
Hydrolysis is a reaction that splits water. To act as a base ê carbon-
ate ion must accept a proën.èThe proën comes from ê water.
The hydrolysis reaction is:èCO╕ìú + H╖Oè──¥ HCO╕ú + OHú.
Ç B
7èWhich hydrolysis reaction shows why a solution ç K╖HPO╣ is
èèbasic?
èèA) HPO╣ìú ──¥ PO╕ú + OHú.
èèB) HPO╣ìú + H╖O ──¥ H╖PO╣ú + OHú.
èèC) K╖H + H╖O ──¥ 2Kó + 2OHú.
èèD) PO╣Äú + H╖Oè──¥ HPO╣ìú + OHú.
üèWe need ë identify ê ions that make up ê compound.èThe name
ç ê compound is potassium hydrogen phosphate which helps us ë recog-
nize that it is composed ç Kó å HPO╣ìú ions.èKó is quite content as
a neutral ion å does not participate ï a hydrolysis reaction.èWe need
ë write a reaction showïg HPO╣ìú actïg as a base.èBases accept Hó.
In a hydrolysis reaction, ê Hó comes from ê water.èThe hydrolysis
reaction is: HPO╣ìú + H╖O ──¥ H╖PO╣ú + OHú.
Ç B
8èWill a solution ç NaHC╖O╣ be acidic or basic?è Which
èèèèè hydrolysis reaction is correct?
èèA) basic,è HC╖O╣ú + H╖O ──¥ H╖C╖O╣ + OHú.
èèB) acidic,èHó + H╖O ──¥ H╕Oó.
èèC) basic,è C╖O╣ìú + H╖O ──¥ HC╖O╣ú + OHú.
èèD) acidic,èHC½O╣ú + H╖O ──¥ C╖O╣ìú + H╕Oó.
üèNaHC╖O╣ consists ç Naó å HC╖O╣ú ions.èThe Naó ion is neutral
å does not affect ê pH.èThe HC╖O╣ú ion can acts eiêr as an acid
or a base.èIts K╬ equals 5.4x10úÉ, while its K╧ equals 1.9x10úîÄ.èSïce
K╬ much greater than K╧, ê HC╖O╣ú ion acts as an acid ï water.èThe
solution ç NaHC╖O╣ will be acidic å ê reaction showïg HC╖O╣ú actïg
as an acid isèHC½O╣ú + H╖O ──¥ C╖O╣ìú + H╕Oó.
Ç D
9èWill a solution ç calcium hypochlorite, Ca(ClO)╖ be acidic or
èèèèè basic?è Which hydrolysis reaction is correct?
èèA) basic,è Oìú + H╖O ──¥ 2OHú.
èèB) basic,è ClOú + H╖O ──¥ HClO + OHú.
èèC) acidic,èCaìó + 2H╖O ──¥ Ca(OH)╖ + 2Hó.
èèD) acidic,èCaìó + H╖O ──¥ CaO + 2Hó.
üèCalcium hypochlorite consists ç Caìó å ClOú ions.èThe calcium
ion is essentially neutral.èThe hypochlorite ion is a weak base.èWe
can expect ClOú ë be a weak base, because its conjugate acid is a weak
acid, hypochlorous acid (HClO).èA solution ç Ca(ClO)½ is basic å its
hydrolysis reaction isèClOú + H╖O ──¥ HClO + OHú.
Ç B
10èWill a solution ç sodium hydrogen sulfite, NaHSO╕, be acidic
èèèèèèor basic?è Which hydrolysis reaction is correct?
èèA) basic,è HSO╕ú + H╖O ──¥ H╖SO╕ + OHú.
èèB) basic,è SO╕ìú + H╖O ──¥ HSO╕ú + OHú.
èèC) acidic,èHSO╕ú + H╖O ──¥ SO╕ìú + H╕Oó.
èèD) acidic,èNaó + 2H╖Oè──¥ NaOHè+ H╕Oó.
üèSodium hydrogen sulfite (or sodium bisulfite) contaïs Naó å
HSO╕ú ions.èWe know that ê Group 1 cations are neutral.èThe HSO╕ú ion
can act eiêr as an acid or a base.èWe need ë look at its acid å
base dissociation constant ë decide how it will behave.èThe acid dis-
sociation constant, K╬, is 6.4x10úô.èIts base dissociation constant is
5.9x10úîÄ.èThe Ka value is larger than ê K╧ value.èConsequently, we
know that HSO╕ú acts preferentially as a weak acid.èThe NaHSO╕ solution
will be acidic, å ê hydrolysis reaction is
HSO╕ú + H╖O ──¥ SO╕ìú + H╕Oó.
Ç C