Chemistry12Acid-BaseEquilibriumVI

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1. Buffers

Buffers

Anacid-basebufferisasolutionthatresistschangesinpHfollowingtheadditionofrelativelysmallamountsofastrongacidorbase.Example:Considerasolutionof1.0Maceticacid.

CH3COOH(aq)+H2O(l)⇋CH3COO-(aq)+H3O+(aq)

• Aceticacidisaweakacid–onlyasmallpercentoftheweakacidisionized

CH3COOH(aq) + H2O(l) ⇋ CH3COO-(aq) + H3O+(aq)I 1.0M –– 0M 0M

C –x –– +x +x

E 1.0M–x –– x x

Ka=

[CH3COOH] [CH3COO-]Ifastrongbasewasaddedtoasolution,aceticacidwillbetheretoneutralizethebase.Ifastrongacidwasadded,therewouldbenospeciestoneutralizeit.

Inorderforabuffersolutiontobeeffective,equivalentconcentrationsofaweakacidandaconjugatebasemustbeinsolution.

Aregularaceticacidsolution…Anaceticacidbuffersolution…

Considerthefollowingpairsofsolutions:

a) Circlethepairsofchemicalspeciesbelowthatcouldbeusedtoprepareabuffersolution.b) Forthepairsthatyoucircled,writethebufferequation.

HNO3andNaNO3 KFandHF HNO2andHNO3 HCOOHandLiHCOO

NaHSO4andNa2SO4 K2CO3andK2C2O4 HClandNaCl KH2PO4andK2HPO4

(Acid)BufferEquation:

CH3COOH(aq)+H2O(l)⇋CH3COO-(aq)+H3O+(aq)Because… Then…Oringeneral…Thehydroniumionconcentration(andthereforethepH)ofabuffersolutiondependson:

1. theKavalue.2. theratiooftheconcentrationoftheweakacidtoitsconjugatebase.

Usingouraceticacidexample…

CH3COOH(aq)+H2O(l)⇋CH3COO-(aq)+H3O+(aq)

[H3O+]=pH=

Now,let’sshiftoursystem!Wenowadd0.10molHClto1.0Mbuffersolutionwithnovolumechange.

• WouldweexpectthepHtodecreaseorincrease?• Wouldthesystemshiftleftorright?

CH3COOH(aq)+H2O(l)⇋CH3COO-(aq)+H3O+(aq) 1.0M 1.0M

[H3O+]=pH=

Wenowadd0.10molNaOHto1.0Mbuffersolutionwithnovolumechange.

• WouldweexpectthepHtodecreaseorincrease?• Wouldthesystemshiftleftorright?

CH3COOH(aq)+H2O(l)⇋CH3COO-(aq)+H3O+(aq) 1.0M 1.0M

[H3O+]=pH=

Add0.10MHCl Add0.10MNaOH

UnbufferedSystempH=

Add0.10MHCl Add0.10MNaOHBufferSystem

pH=

Wecanalsohaveabasicbuffer:ConsideraNH3/NH4+buffersolution:

NH3(aq)+H2O(l)⇋NH4+(aq)+OH-(aq)Because… Then…Oringeneral…Thehydroxideionconcentration(andthereforethepOHandpH)ofabuffersolutiondependson:

1. theKbvalue.2. theratiooftheconcentrationoftheweakbasetoitsconjugateacid.

UsingourNH3/NH4+example…

NH3(aq)+H2O(l)⇋NH4+(aq)+OH-(aq) 1.0M 1.0M

[OH-]=pH=

Wenowadd0.10molHClto1.0Mbuffersolutionwithnovolumechange.• WouldweexpectthepHtodecreaseorincrease?• Wouldthesystemshiftleftorright?

NH3(aq)+H2O(l)⇋NH4+(aq)+OH-(aq) 1.0M 1.0M

[OH-]=pH=

Wenowadd0.10molNaOHto1.0Mbuffersolutionwithnovolumechange.

• WouldweexpectthepHtodecreaseorincrease?• Wouldthesystemshiftleftorright?

NH3(aq)+H2O(l)⇋NH4+(aq)+OH-(aq) 1.0M 1.0M

[OH-]=pH=

Practice.Considera1.0Mhydrofluoricacidbuffersystem.a. Whatistheequation?

b. CalculatethepHofanundisturbedsystem.

c. CalculatethepHwhen0.050Mhydrobromicacidisaddedtotheoriginalbuffersystem.

d. CalculatethepHwhen0.050MMg(OH)2isaddedtotheoriginalbuffersystem.HebdenWorkbookPg.181#131–138