Today’s(Topics!Electrochemistry-II"
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• Electrochemical,cells,
• Half0cell,potentials,
• Nernst,equation,
• Equilibrium,in,electrochemical,cells,
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Related,Problems,from,Spring,2008,Exam,2:,26,,27,&,Spring,2009,Exam,2:,20,,21,
Corresponding,Connect,Problems:,Set,8:,1–11
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Figure,taken,from,Chemistry,,4th,ed.,by,J.,McMurray,and,R.,C.,Fay,,Prentice0Hall,,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(18)"
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You,should:,
1. understand,the,difference,between,voltaic/galvanic,and,electrolytic,electrochemical,
cells.,
2. understand,why,standard,reduction,potentials,are,used,and,how,they,are,
determined.,
3. know,how,the,standard,states,used,for,E°,and,ΔG°,are,defined,for,gases,,solids,,
liquids,,and,solutes.,
4. be,able,to,write,balanced,half0reactions,,determine,overall,cell,reactions,,calculate,
the,standard,reduction,potential,,and,predict,the,direction,of,electron,,anion,,and,
cation,flow,based,on,a,sketch,of,an,electrochemical,cell,or,the,description,of,an,
electrochemical,cell,given,in,shorthand,notation.,
5. understand,the,relationship,between,chemical,energy,(Gibbs,free,energy,change,for,
a,redox,reaction),and,electrical,energy,(electromotive,force,or,cell,potential),in,an,
electrochemical,cell.,
6. be,prepared,to,use,standard,reduction,potentials,to,calculate,the,standard,cell,
potential,E°,for,an,electrochemical,cell.,
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2,,
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7. be,able,to,use,standard,reduction,potentials,to,predict,the,strength,of,oxidizing,and,
reducing,agents.,
8. be,able,to,calculate,cell,potentials,for,electrochemical,cells,of,arbitrary,composition,
using,the,Nernst,equation,and,to,predict,if,the,cell,will,operate,as,a,voltaic/galvanic,
cell,(i.e.,,is,the,reaction,spontaneous?),
9. understand,the,relationship,between,standard,cell,potentials,E°,and,equilibrium,
constants,K,for,redox,reactions,and,be,able,to,use,this,relationship,to,determine,K,
based,on,a,measured,E°,or,vice,versa.,
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3,,
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Electrochemical,Cells"
Electrochemical,cells,cause,interconversion,between,chemical,and,electrical,energy.,
1. Voltaic,(Galvanic),
• Spontaneous,chemical,reaction,(ΔG,<,0),generates,a,current.,
2. Electrolytic,
• Electric,current,drives,a,nonspontaneous,(ΔG,>,0),chemical,reaction.,
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4,,
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Half!Cells,&Half!Cell$Reactions,$&$Half!Cell$Potentials"
• In,order,to,make,it,straightforward,to,predict,the,properties,of,any,electrochemical,
cell,,we,define,a,halfHcell,as,the,compartment,in,which,either,the,oxidation,or,
reduction,half0reaction,occurs.,,
• If,the,potentials,of,half0cells,are,measured,with,respect,to,a,reference,halfHcell,,then,
the,potential,of,an,electrochemical,cell,created,by,combining,any,two,half0cells,can,
be,predicted.,Note:,The,potential,of,a,halfHcell,cannot,be,measured,in,isolation.,It,can,
only,be,measured,when,it,is,combined,with,another,half0cell,,just,as,oxidation,cannot,
occur,separately,from,reduction.,
• In,practice,,the,potentials,of,half0cells,are,measured,relative,to,the,standard,
hydrogen,electrode,(SHE),,and,by,convention,,the,SHE,is,assigned,a,half0cell,potential,
of,0.0,V.,
• By,convention,,half0cell,potentials,are,tabulated,as,reduction,potentials.,
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5,,
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The,Daniell,Cell,introduced,in,last,lecture,has,a,standard,potential,,E°,=,1.10,V,
[Standard,conditions,correspond,to,1,M,concentration,for,solutes,,1,atm,for,gases,,and,25°C],
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How,much,of,the,
potential,difference,
comes,from,the,Zn/Zn2+,
half0cell,,and,how,much,
from,the,Cu/Cu2+,half0
cell?,
(Figure-McMurray & Fay)
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6,,
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Cell$Notation"
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In,the,shorthand,notation,used,to,describe,electrochemical,cells,,the,following,
conventions,are,used:,
|,(single,vertical,line),represents,a,phase,change,
||,(double,vertical,line),represents,a,salt,bridge,or,membrane,separating,two,half0
cells,
,,(comma),appears,between,ions,found,in,the,same,solution,
(,),States,and,concentrations,for,solutions,are,indicated,between,the,parentheses,
The,anode,appears,on,the,left,and,cathode,on,the,right.,
Example:,Daniell,cell,
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Zn!(!)|Zn!! (!")||Cu!! (!")|Cu!(!),
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Zn(!) + Cu!! (!") ⟶ Zn!! (!") + Cu(!) ,
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Oxidation,occurs,at,the,Zn(s),electrode,(anode),,and,reduction,occurs,at,the,Cu(s),
electrode.,Consequently,,the,reaction,for,the,cell,as,written,is,
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7,,
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Half!Cell$Reduction$Potentials"
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The,standard,hydrogen,electrode,(SHE),,which,is,assigned,E°,=,0.0,V,by,convention,,is,
represented,in,cell,notation,as,
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Pt!(!)|H! (!, 1!atm)|H ! (!", 1!!),
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Zn!(!)|Zn!! (!", 1!!)||H ! (!", 1!!)|H! (!, 1!atm)|Pt!(!),
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The,electrochemical,cell,given,below,has,a,measured,standard,cell,potential,E°,=,0.76,V,,
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• E°,>,0,,ΔG°,<,0.,This,means,that,the,cell,operate,spontaneously,as,written,,with,
oxidation,occurring,at,the,Zn,anode,in,the,Zn(!)|Zn!! (!", 1!!),half0cell,(left),and,
reduction,occurring,at,the,Pt,cathode,in,the,H ! (!", 1!!)|H! (!, 1!atm)|Pt!(!),half0cell,
(right).,
• Zn,is,oxidized.,Consequently,,it,is,a,better,reducing,agent,than,H2.,
• Since,Zn,is,being,oxidized,in,this,cell,,the,corresponding,standard,reduction,potential,
for,the,reaction,
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Zn!! (!") + 2e! ⟶ Zn(!) ,
is,E°red,=,–0.76,V,
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Daniell!Cell:%Half!Cell$Potentials$Relative$to$the$SHE"
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Example:)Cell)Potential)I"
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Consider,the,following,electrochemical,cell:,
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Ni(!)|Ni!! (!", 1!!)||H ! (!", 1!!)|H! (!, 1!atm)| Pt ! !!!!° = 0.25!!,
1. What,species,are,oxidized,and,reduced,in,this,cell?,
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2. What,are,the,standard,reduction,potentials,for,the,half0cells?,
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3. How,does,Ni,compare,to,Zn,and,H2,as,a,reducing,agent?,
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Now,consider,the,following,electrochemical,cell:,
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Pt(!)|H! (!, 1!atm)|H ! (!", 1!)| Cu!! !", 1!! Cu ! !!!!!° = 0.34!!,
1. What,species,are,oxidized,and,reduced,in,each,cell?,
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2. What,are,the,standard,reduction,potentials,for,the,half0cells?,
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3. How,does,Cu,compare,to,Zn,and,H2,as,a,reducing,agent?,
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Example:)Cell)Potential)II"
Consider,the,cell,
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Zn(!)|Zn!! (!", 1!!)||Ni!! (!", 1!!)|Ni(!),
1. What,is,the,cell,potential,E°?,
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2. What,is,!" ! ?,
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Nonstandard)Conditions)&)the)Nernst)Equation"
Recall,that,
, !" = !"° + !"ln!,
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Since,
, !" = −!"#!!and!!!"° = −!"#°,
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substitution,yields,
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−!"# = −!"#° + !"ln!,
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! = !° −
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where,
!"
ln!!!!!(Nernst!equation),
!"
R,=,8.3145,J,mol–1,K–1,,
F,=,96485,C,mol–1,
n,=,#,of,moles,of,electrons,transferred,(sometimes,represented,by,ν),
T,is,the,absolute,temperature,in,K,
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Example:)Application)of)the)Nernst)Equation"
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Consider,a,Daniell,Cell,
, Zn ! Zn!! !" Cu!! !" Cu ! !!!!!° = 1.1!!,
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that,has,been,operating,for,a,while,at,25°C.,During,this,time,,the,concentration,of,Cu!! ,
has,been,reduced,from,1,M,to,1!×!10!! !M,(99.99%,of,the,Cu!! ,has,been,consumed).,The,
Zn!! ,concentration,will,have,increased,to,2,M.,What,is,the,cell,potential?,
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Equilibrium)&)Electrochemical)Cells"
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Eventually,,the,potential,of,an,electrochemical,cell,will,go,to,zero,,which,corresponds,to,
the,system,reaching,equilibrium.,
At,equilibrium,Q,=,K,and,E,=,0,(ΔG,=,0),
,
! = !° −
!"
ln!,
!"
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0 = !° −
!"
ln!,
!"
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!° =
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!"
ln!,
!"
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Example:)Equilibrium)&)Electrochemical)Cells"
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What,is,the,equilibrium,constant,for,the,reaction,occurring,in,the,Daniell,cell,at,25°C?,
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Some%Typical%Questions!Pertaining)to)Electrochemical)Cells"
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Consider,the,reaction,
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–
!!
16H ! (!") + 2MnO!
! (!") + 10I (!") → 5I!(!") + 2Mn (!") + 8H! O(!) ,
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The,standard,half0cell,reduction,potentials,are:,
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!!
! ! MnO!
= ! +1.51!!,
! Mn
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! ! I! I ! = ! +0.54!!,
1. What,is,the,value,of,! ! ,for,a,cell,based,on,the,reaction,given,above?,
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–
!!
16H ! (!") + 2MnO!
! (!") + 10I (!") → 5I!(!") + 2Mn (!") + 8H! O(!) ,
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2. What,is,the,value,of,n,in,the,expression,!"° = −!"#°,for,the,reaction,given,above?,
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3. Is,this,reaction,spontaneous,or,not,spontaneous?,
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4. What,is,the,value,of,!"°,for,the,reaction,given,above?,
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5. If,the,coefficients,in,the,balanced,chemical,equation,are,changed,(doubled,,for,
example),,what,happens,to,the,values,of,∆!°,and,E°?,How,can,one,change,and,not,the,
other?,
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