Today’s(Topics!Aqueous'Equilibria'II"
"
• Acid%&%Base%Definitions%
• Aqueous%solutions%(Arrhenius,%Brønsted8Lowry)%
• Nonprotonic%solvents%
• Donor8acceptor%models%(Lewis,%Usanovich)%
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Related%Problems%from%Spring%2008%Exam%1:%14%&%Spring%2009%Exam%1:%9,%19%
Corresponding%Connect%Problems:%Set%3:%2–5
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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(7)"
You%should:%
1. know%the%Arrhenius%acid%and%base%definitions%for%aqueous%solutions%and%be%able%to%
apply%them%to%identify%acids%and%bases.%
2. know%the%Brønsted8Lowry%definitions%for%acids%and%bases.%
3. be%able%to%identify%Brønsted8Lowry%acids%and%bases%as%well%as%conjugate%acid8base%
pairs.%
4. understand%that%in%the%context%of%the%Brønsted8Lowry%definitions,%acid8base%reactions%
involve%competition%for%H+%with%the%strongest%acid%giving%up%H+%and%the%strongest%base%
receiving%H+.%
5. understand%how%the%acid8base%definitions%for%aqueous%solutions%can%be%expanded%to%
treat%other%solvents%(protonic%and%nonprotonic).%
6. know%the%Lewis%acid%and%base%definitions%and%be%able%to%identify%Lewis%acids%and%
bases%using%these%definitions.%
7. know%the%Usanovich%acid%and%base%definitions.%
8. understand%the%underlying%premises%of%Donor8Acceptor%Theory.%
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2%%
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Acid!Base%Reactions"
"
What"is"an"acid"or"a"base?"The"answer"depends"on"the"definition"acid"and"base."
Acid8Base%Definitions%
• In%Protonic%Solvents%
• Aqueous%
• Arrhenius%
• Brønsted8Lowry%
• Nonaqueous%
• In%Nonprotonic%Solvents%
• Donor8Acceptor%Models%
• Lewis%
• Usanovich%
• Donor8Acceptor%Theory%
%
%
3%%
%
Arrhenius"
An%Arrhenius#acid%is%a%substance%that%increases%the%hydrogen%(H+)/hydronium%ion%(H3O+)%
concentration%in%aqueous%solution.%
An%Arrhenius#base%increases%the%hydroxide%ion%(OH–)%concentration%in%aqueous%solution.%
Based%on%the%Arrhenius%definitions,%a%neutralization#reaction%occurs%when%an%acid%and%
base%are%combined%and%results%in%formation%of%water%
%
H! O! + OH – ⟶ 2H! O%
%
In%more%general%terms,%the%cation%of%the%solvent%(H3O+)%combines%with%the%anion%of%the%
solvent%(OH–)%to%form%the%neutral%solvent%(H2O),%hence%the%designation%of%this%reaction%as%a%
neutralization%reaction.%%
%
%
4%%
%
Other&Protonic&Solvents"
"
Extension%of%the%concept%of%the%neutralization%reaction%based%on%the%Arrhenius%definitions%
leads%to%acid%and%base%definitions%that%are%applicable%to%solvents%other%than%water%%
• An%acid%is%a%substance%that%produces%the%solvent#cation%by%transferring%H+%to%the%
solvent.%
• A%base%produces%the%solvent#anion%by%removing%H+%from%the%neutral%solvent.%
Examples:%
• In%liquid%NH3,%an%acid%produces%NH4+%and%a%base%produces%NH2–.%
• In%glacial%acetic%acid%(CH3CO2H),%an%acid%produces%CH3CO2H2+%and%a%base%produces%
CH3CO2–%
The%corresponding%neutralization#reaction%corresponds%to%combining%the%solvent#cation%
and%solvent#anion%to%form%the%neutral%solvent:%
%
%
%
NH!! + NH!! ⟶ 2NH ! %
%
CH ! CO ! H!! + CH ! CO!
! ⟶ 2CH ! CO! H%
%
%
5%%
%
Nonprotonic"
"
Now%consider%application%of%the%following%definitions%to%nonprotonic%solvents%
• An%acid%is%a%substance%that%produces%the%solvent#cation.%
• A%base%produces%the%solvent#anion.%
Example:%
• In%the%solvent%BrF3,%an%acid%will%produce%BrF!! %and%a%base%will%produce%BrF!! %
%
SbF! + BrF! ⟶ BrF!! + SbF!! %
%
SbF! %acts%as%an%acid%and%! ! %is%transferred%from%the%base%to%the%acid.%
• The%corresponding%neutralization%reaction%is%
%
BrF!! + BrF!! ⟶ 2BrF ! %
%
CaO(!"#$) + SiO! (!"#$) ⟶ CaSiO! %
%
!
!!
!!
NO!
! (!"#$) + S! O! (!"#$) ⟶ NO! + 2SO! %
%
A%second%related%example%involves%the%transfer%of%! !! %from%the%base%to%the%acid%(Lux8
Flood)%
%
%
%
%
6%%
%
***Brønsted!Lowry***"
"
The%Brønsted8Lowry%definitions%for%acids%and%bases%are%the%most%important%definitions%for%
aqueous%acid8base%equilibria.%
• An%acid%is%a%proton%(H+)%donor.%
• A%base%is%a%proton%(H+)%acceptor.%
In%the%context%of%the%Brønsted8Lowry%definitions,%acid8base%reactions%involve%transfer#of#a#
proton#(H+)#from#an#acid#to#a#base.%
%
%
%
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7%%
%
Example:%Addition%of%the%weak%base%NH3%to%water%
%
%
%
NH! (!") + H! O(!) ⇌ NH! ! (!") + OH ! (!") !!!!!!!!!!K ! = 1.8 ⨯ 10 !! %
%
base! !!!!!!!!!!acid! !!!!!!!!acid! !!!!!!!!!!!!!base! %
%
• Each%acid8base%reaction%consists%of%two%conjugate#acid/base#pairs.%Conjugate%acid8
base%pairs%are%species%that%differ%by%one%H+.%The%conjugate%base%of%the%stronger%acid%is%
the%weaker%base%in%the%reaction.%%
• The%bases#in#the#reaction#compete#for#H+%with%the%stronger%base%getting%the%proton.%
Equilibrium%favors%the%side%of%the%reaction%in%which%the%stronger%base%has%the%proton.%
In%the%reaction%above,%OH ! %is%a%stronger%base%than%NH! ;%therefore%equilibrium%favors%
reactants.%%
%
%
%
%
8%%
%
Example:%Ionization%of%the%weak%acid%HCN%in%aqueous%solution%
%
%
%
HCN(!") + H! O(!) ⇌ CN ! (!") + H! O! (!") !!!!!!K ! = 6.2 ⨯ 10 !!" %
%
acid! !!!!!!!!!!!base! !!!!!!!!base! !!!!!!!!!!acid! %
%
• CN ! %is%a%stronger%base%than%H! O;%consequently,%equilibrium%favors%reactants.%
• HCN%is%a%weaker%acid%than%H! O! .%Equilibrium%favors%the%side%of%the%equation%with%the%
weaker%base%and%weaker%acid.%
%
%
%
%
9%%
%
Brønsted!Lowry"definitions"applied"to"nonprotonic"systems–transfer"of"! ! !or!! !! "
Example:%A%nonprotonic%system%where%! ! %is%transferred:%
%
SbF! + BrF! ⇌ BrF!! + SbF!! %
%
acid! !!!!!base! !!!!!acid! !!!!!!!base! %
In%general,%
%
%
Acid
!
!
!
⇌
!!
,!!
Base%
• Bases%tend%to%be%negatively%charged%or%neutral,%while%acids%tend%to%be%positively%
charged%or%neutral.%%
• Based%on%electrostatic%considerations,%
• bases%are%likely%to%give%up%an%anion%or%react%with%a%cation%%
• acids%are%likely%to%give%up%a%cation%or%react%with%an%anion.%
• When%acid%gives%up%a%cation,%it%becomes%a%base.%When%a%base%accepts%a%cation,%it%is%
transformed%to%an%acid.%%
• If%the%ion%being%transferred%in%the%acid8base%reaction%is%an%anion,%the%base%is%the%anion#
donor%and%the%acid%is%the%anion#acceptor.%
%
%
10%%
%
***Lewis***"
"
The%Lewis%definitions%for%acids%and%bases%expand%the%definitions%to%consider%what%
happens%to%the%electrons.%
• A%Lewis%acid%accepts%a%pair%of%electrons.%
• A%Lewis%base%donates%a%pair%of%electrons.%
%
%
11%%
%
Usanovich"
The%Usanovich%definitions%further%expand%the%definitions%of%acids%and%bases.%
• An%acid%will%give%up%a%cation%or%produce%a%solvent%cation,%accept%an%anion,%or%accept%an%
electron.%(In%the%acid8base%reaction%the%charge%on%the%acid%will%become%more%
negative.)%
• A%base%will%give%up%an%anion%or%produce%a%solvent%anion,%accept%a%cation,%or%donate%an%
electron.%(In%the%acid8base%reaction%the%charge%of%the%base%will%become%more%positive.)%
Note%that%the%Usanovich%definitions%include%transfer%of%electrons,%which%means%that%these%
definitions%are%applicable%to%oxidation8reduction%(redox)%reactions.%
%
%
12%%
%
Donor%Acceptor%Theory"
"
Donor8acceptor%theory%builds%on%the%idea%introduced%by%Lewis%that%acid8base%reactions%
involve%pairs%of%electrons.%
In%donor8acceptor%theory,%the%lowest%unoccupied%molecular%orbital%(LUMO)%of%the%acid%
combines%with%the%highest%occupied%molecular%orbital%(HOMO)%of%the%base%to%form%a%
bonding%molecular%orbital%(MO).%With%formation%of%the%bonding%molecular%orbital,%the%
energies%of%the%electrons%from%the%HOMO%of%the%base%are%reduced.%
• The%acid%provides%its%LUMO%in%formation%of%the%new%bonding%MO%and%effectively%
behaves%as%an%electron%acceptor.%
• The%base%provides%its%HOMO%in%formation%of%the%new%bonding%MO%as%well%as%the%
electrons%from%its%HOMO%to%the%new%bonding%MO%and%effectively%behaves%as%an%
electron%donor.%
%
%
%
%
13%%
%
14%%
%
%
%
%
Typical(Questions"
"
1. Which%statement%is%applicable%to%the%Lewis%theory%of%acids%and%bases?%
(a) Acids%produce%hydronium%ions%in%aqueous%solution.%
(b) The%hydroxide%ion%is%the%strongest%base%that%can%exist%in%water.%
(c) Bases%donate%electron%pairs.%
(d) Every%acid%has%a%conjugate%base.%
(e) All%acid8base%reactions%involve%transfer%of%a%hydrogen%ion.%
%
%
%
%
15%%
%
2. Which%statement%distinguishes%the%Lewis%theory%of%acid8base%behavior%from%the%
theories%of%Arrhenius%and%Brønsted8Lowry?%
(a) Acids%produce%hydronium%ions%in%aqueous%solution.%
(b) Strong%acids%are%those%acids%stronger%than%the%hydronium%ion%in%aqueous%solution.%
(c) The%hydroxide%ion%is%the%strongest%base%that%can%exist%in%water.%
(d) Acids%are%electron%pair%acceptors.%
(e) Every%base%in%aqueous%solution%has%a%conjugate%acid.%
(f) All%acid8base%reactions%involve%the%transfer%of%a%hydrogen%ion.%
%
%
%
%
16%%
%
Bonus&Problem:)Gas!phase&Equilibrium&and&!! !
Ammonium%chloride%is%placed%in%a%vessel%and%equilibrium%is%established%
K ! = 0.64!at!400°C%
% NH! Cl(!) ⇌ NH!(!) + HCl(!) ,
%
What%is%the%partial%pressure%of%NH3%at%equilibrium?%
%
%
17%%
%
Bonus&Problem:)Gas!phase&Equilibrium&and&!! "
"
Quantities%of%0.180%moles%of%Br2%and%0.180%moles%of%Cl2%are%introduced%to%a%3.0%L%reaction%
vessel,%and%equilibrium%is%established%for%the%following%reaction:%
% Br!(!) + Cl!(!) ⇌ 2BrCl(!) , K ! = 36.0%
%
How%much%BrCl%is%present%at%equilibrium?%What%is%the%value%of%KP?%
%
%
%
%
18%%