Ch19. Electrochemistry

8/18/2019 Ch19. Electrochemistry

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1

Chapter 19

Electrochemistry

Brady and Senese

5th Edition


2/50

2

Index

19.1 Galvanic cells use redox reactions to generate electricity19.2 Cell potentials can be related to reduction potentials

19. Standard reduction potentials can predict spontaneous re

actions

19.! Cell potentials are related to "ree energy changes19.5 Concentrations in a galvanic cell a""ect the cell potential

19.# Electrolysis uses electrical energy to cause che$ical reac

tions

19.% Stoichio$etry o" electroche$ical reactions involves electric current and ti$e

19.& 'ractical applications o" electroche$istry


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19.1 Galvanic cells use redox reactions to generate electricity

Galvanic (eactions

)*re spontaneous redox reactions) (elease energy as heat i" no resistance is $et

) +ay be separated into co$part$ents to harnessenergy as a battery


4/50

19.1 Galvanic cells use redox reactions to generate electricity !

*nato$y o" a Galvanic Cell

) Cells ,co$part$ents containing reactants "or each

hal"-reaction

) Electrodes to conduct current through the solution.

) Salt bridge to o""set ion $ove$ent

) Supporting electrolyte


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19.1 Galvanic cells use redox reactions to generate electricity 5

Electroche$ical Cells

)In all cells/ electrons trans"er bet0een the cathode ,the reduction hal"-cell and the anode ,theoxidation hal"-cell

) Electrical current is conducted through the passage

o" electrons and ions) o prevent charge buildup/

a salt bridge allo0s ionsto $ove bet0een the

cells


6/50

19.1 Galvanic cells use redox reactions to generate electricity #

our urn

o0ards 0hich co$part$ent 0ill electrons "lo0 inan electroche$ical cell3

*. o0ard the cathode

B. o0ard the anodeC. It depends on the reaction


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19.1 Galvanic cells use redox reactions to generate electricity %

our urn

hrough 0hich co$ponents o" the cell 0ill ions not"lo03

*. he electrodes

B. he solutionC. he salt bridge


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19.1 Galvanic cells use redox reactions to generate electricity &

Standard Cell 4otation ,ine Cell 4otation

) Cell reactions separated by 66 that represents the salt bridge

) Electrodes appear at the outsides

) (eaction electrolytes in inner section

) 'hases separated 0ith 6) Species in the sa$e state separated 0ith /

) Concentrations sho0n in ,


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earning Chec7

Balance and identi"y the cathode and anode

Cr8, s +n82, s : +n8!-,aq Cr ,aq ,basic

Cr8, s +n82, s ;28 : +n8!-,aq Cr ,aq

28;-

,aq

Cr8 ;28 e- : Cr #8;-

+n82 !8;- : +n8!

- 2;28 e-

Cathode< Cr8

*node< +n82


10/50

19.1 Galvanic cells use redox reactions to generate electricity 1=

earning Chec7<

Balance and identi"y the cathode and anode

;282,aq C82, g : ;2C28!,aq 82, g ,acidic

;282,aq 2C82, g : ;2C28!,aq 82, g ,acidic

;282 : 82 2; 2e-

2; 2e- 2C82 : ;2C28! ,acidic

Cathodic reaction< 2; 2e- 2C82 : ;2C28!*nodic reaction


11/50


earning Chec7

>here there are no conductive $etals involved in a process/ an inert electrode is used.

C, gr and 't are o"ten used. >rite the standard cell

notation "or the reactions

Cr8, s +n82, s : +n8!-,aq Cr ,aq ,basic

;282,aq C82, g : ;2C28!,aq 82, g ,acidic

C, gr ?+n82, s | +n8!-,aq 66 Cr8, s 6 Cr

,aq 6 C

C, gr 6;282,aq/ ;,a@ 6 82, g 66 C82, g 6 ;2C28!,aq/ ;

,aq 6 C, gr


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our urnA

In the reaction +n82, s 2;,aq : +n2,aq ;282,aq0hich is the correct standard cell notation3

*. 't/ +n82, s6;,aq/ +n2,aq66;286;282,aq/ ;,aq6't

B. 't6;282,aq/ ;,aq6;2866;,aq/ +n2,aq6+n82, s't

C. Both are correct

. 4either is correct


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19.2 Cell potentials can be related to reduction potentials 1

Electrical 'otential

) Every substance has the potential to gainelectrons/ or be reduced in oxidation state

) he relative ease o" gaining electrons is ter$ed the

reduction potential / and is sy$bolied E red

) I" the $atter being observed is in standard state

then E is ter$ed the standard reduction potential

and is sy$bolied as E° red

) Dnits are olt 1 F 1 C-1


14/50

19.2 Cell potentials can be related to reduction potentials 1!

Standard (eduction 'otentials , E Hred

) E Hred are tabulated "or nearlyevery 7no0n substance

) * high value o" E Hred , E Hred =

$eans that the substance is easilyreduced

) E Hred is a relative nu$ber/arbitrarily deter$ined

) *ll substances are co$pared to;/ 0hich has a E Hred o" =.==


15/50

19.2 Cell potentials can be related to reduction potentials 15

Cell 'otentials

) he standard cell potential is calculated as< E Hcell F E Hcathode - E Hanode

) I" the cell is non-standard<

E cell F E cathode - E anode

) In spontaneous redox reactions/ the cathode

portion o" the reaction has a higher

reduction potential than that o" the anode, E cathode E anode


16/50

19.2 Cell potentials can be related to reduction potentials 1#

our urn

Given the t0o hal" reactions/ 0hat 0ould be E Hcell "orthe reaction<

4i, s ;282,aq 2;,aq : 2;28 4i2,aq

*. 1.52

B. -1.52

C. 2.=2

. -2.=2

(eaction E°cell

4i2 2e- : 4i -=.25

;282 2; 2e- : 2;28 1.%%


17/50

19.2 Cell potentials can be related to reduction potentials 1%

(ecogniing the Cathode

) Given the cell reaction/ thecathode is the substance that isreduced ,gains electrons

) he anode is the substance that

creates electrons) E8 the lion says GE(

the loss o" electrons is oxidation and

the gain o" electrons is reduction

) Spontaneously/ the cathode has the$ost + E red o" the choices/ and theanode has the $ost – E red.


18/50

19. Standard reduction potentials can predict spontaneous reactions 1&

'redicting Cells in a Spontaneous (eaction

Cathode ,Gains e-

*node ,oses e-

) Dsually a non-$etal or a cation.

) 8n the E red table/ these

substances have large/ values.) >ater can be a cathode

) *ppears on the left side o" thereduction table

) he cathode reaction occurs asit appears on the reductiontable

) Dsually a $etal or an anion.

) 8n the E red table/ these have

large/ - values.

) >ater can be an anode) *ppears on the right side o" the

reduction table.

) he anode reaction occurs in

the reverse direction "ro$ that0hich appears on the reductionchart


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19. Standard reduction potentials can predict spontaneous reactions 19

earning Chec7

Calculate E Hcell. >hich are spontaneous3

) Cu, s *g,aq : Cu2,aq *g, s

) 'b Cu2 : 'b2 Cu

) Cr 28%2-,aq +n82, s : +n8!-,aq Cr ,aq

Cu2JCu =.! Cr 28%2-JCr 1.

*gJ*g =.&= +n8!-J+n82 1.#95

'b2J'b -=.1

K=.&= - =.!L F =.!#

K=.! - ,-=.1L F =.!%

K1.-1.#95L F -=.#5


20/50

19. Standard reduction potentials can predict spontaneous reactions 2=

our urn

>hich o" the "ollo0ing is not spontaneous3*. 4i2 'b : 'b2 4i

B. 4i 'b2 : 'b 4i2

C. Mn 'b2 : 'b Mn2

. *ll are spontaneous

E Hred

4i2

2e-

: 4i -=.25Mn2 2e- : Mn -=.%#

'b2 2e- : 'b -=.1


21/50

19.! Cell potentials are related to "ree energy changes 21

he her$odyna$ics o" Electroche$istry

) Because reduction potential 0as an arbitrarilyde"ined value/ it has no direct physical $eaning

) It can be related to the "ree energy o" a syste$/

ho0ever as< NG° F -nFΕ°cell

n F the nu$ber o" $oles o" electrons

trans"erred in the process

F F OaradayPs Constant/ 9#/5== CJ$ol e-

) I" the syste$ is not standard/ NG can be de"inedas< NG F -nFΕ cell


22/50


earning Chec7

) Calculate NG° in 7. >hich are spontaneous under standard

conditions3

) Cu, s *g,aq : Cu2,aq *g, s

)'b Cu

2

: 'b2

Cu

) Cr 28%2-,aq +n82, s : +n8!-,aq Cr ,aq

Cu2JCu =.! Cr 28%2-JCr 1.

*gJ*g =.&= +n8!-J+n82 1.#95

'b2J'b -=.1

NGH F -2 $ol Q 9#/5== C $ol-1 Q =.!# C-1 F -&&.& 7

NGH F -2 $ol Q 9#/5== C $ol-1 Q =.!% C-1 F -&=.% 7

NGH F -# $ol Q 9#/5== C $ol-1 Q ,-=.#5 C-1 F 211 7

4ot spontaneous


23/50


our urn

Given that RGH F #5. 7 / 0hat is the value "or E Hcell"or the reaction Cu2,aq ;g,l : ;g2,aq Cu, s3

*. #.5

B. .=.& C. -=.&

. -=.#%#

E. 4one o" these


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19.! Cell potentials are related to "ree energy changes 2!

(elating E H to the E@uilibriu$ Constant

+any redox reactions are reversible/ thus the 0holecell reactions have e@uilibriu$ constants/ K

NGHF nFE H

and/ NGHF T RT ln K

thus/

H

ln

e

÷

=

=nFE

RT

RT K E

nF

K

o


25/50


earning Chec7<

) Calculate K "or the "ollo0ing cells.) Cu, s *g,aq : Cu2,aq *g, s

) 'b, s Cu2,aq : 'b2,aq Cu, s

-1 -1

-1 -1

2 $ol 9#/5== C $ol =.!% CH

&.1! $ol U 29&.15 U 15e e %.& 1=

× × ÷ ÷ ÷× = = = ×nFE

RT K

-1 -1

-1 -1

2 $ol 9#/5== C $ol =.!# CnOEH &.1! $ol U 29&.15 U 15(

e e .5& 1=

× × ÷ ÷ ÷×

= = = × K

E H F =.!%

E H F =.!#


26/50

19.! Cell potentials are related to "ree energy changes 2#

earning Chec7 ,Cont.

Cr 28%2-,aq +n82, s : +n8!-,aq Cr ,aq

-1 -1

-1 -1

# $ol 9#/5== C $ol =.#5 CH

&.1! $ol U 29&.15 U %e e 1.1 1=

× × ÷ ÷ ÷× = = = ×

nFE

RT K

E H F =.#5


27/50

19.! Cell potentials are related to "ree energy changes 2%

our urnA

>hat is the value o" K "or the reaction at 29&.15 U< E Hcell F !. 'b82 !*l : 'b 2*l28

-1 -1

-1 -1

-1 -1

-1 -1

-1 -1

2 $ol 9#/5== C $ol !. G C

&.1! G $ol U 29&.15 U

$ol 9#/5== C $ol !. G C

&.1! G $ol U 29&.15 U

# $ol 9#/5== C $ol !. G C

&.1!

*. e

B. e

C. e

× ×− ÷ ÷×

× ×− ÷ ÷×

× ×

− -1 -1 G $ol U 29&.15 U

. 4one o" the above

÷ ÷×

−−

−−

×××

− K K Jmol

JC Cmol mol e 15.29&1!.&

.!9#5==1211

11


28/50

19.5 Concentrations in a galvanic cell a""ect the cell potential 2&

he 4ernst E@uation

ln−

= + RT

E E QnF

o

) Oor non- standardconditions/

) Substituting "or the "reeenergy/ NG/ ter$s gives us

) ividing by TnO/ 0e getthe 4ernst E@uation

) >hen the te$perature is29& U and ln is convertedto log/ =.=592 log

= −Q

E E n

o

∆G F ∆GH RT ln Q

T nFE F nFE H RT ln Q


29/50

19.5 Concentrations in a galvanic cell a""ect the cell potential 29

earning Chec7< Calculate E cell

) *l6*l,aq ,=.5 M 66Mn2,aq ,=.2 M 6Mn

) *l6*l,aq ,=.5 M 66Mn2,aq ,1 M 6Mn

2

E , =.%# , 1.## =.9= =.=592 K=.5L

E =.9= log =.9=# # K1L

= − − − == − =

o

*lJ*l -1.## Mn2JMn -=.%#

2

E , =.%# , 1.##

=.=592 K=.5L

E =.9= log =.&&5 # K=.2L

= − − −

= − =

o


30/50

19.5 Concentrations in a galvanic cell a""ect the cell potential =

earning Chec7

) *l6*l

,aq ,=.5 M 66*l

,aq ,=.2 M 6*l

) *l6*l,aq ,=.5 M / 25 HC 66*l,aq ,=.5 M / 5= HC 6*l

cell cathode anode

&.1! ,2 U 1,-1.## ln 1.###!

9#/5== K=.5L&.1! ,29& U 1

,-1.## ln 1.##59 9#/5== K=.5L

=.===5

×= − = −

× ×= − = −

×= − =

E

E

E E E

, 1.## , 1.## =

=.=592 K=.5L= log %.&5 1=

K=.2L

−

= − − − =

= − = − ×

E

E

o

*lJ*l -1.##2


31/50


earning Chec7< Oind the Dn7no0n

) Cr6Cr

,33 M 664i2

,=.5 M 64i E cell F =.!9!

) 't6;2,1at$6 ;,3 3p;66;,1 M 6;2,1 at$6't

E cell F =.===5%1

2

=.=592 KxL=.!9! , =.25 , =.%! log

# K=.5L= − − − −

;J;2 =.== Cr JCr -=.%!

4i2J4i -=.25 Cu2JCu =.!

=.9%& M F x? p; F =.==9#5

2

2

=.=592 KxL=.===5%1 ,=.== =.== log

2 K1L= − −

=.222 M F x


32/50


our urn

Consider the reaction sho0n. >hat is the value o" E cell3'b S8!2-,=.1 M 'bS8! 4i2,=.2 M 4i

*. =.99&

B. -=.99&

C. -=.1=1

. 4one o" these

4i2 4i -=.25 'bS8! 'b -=.#

=.=#=

cell

=.=592 1, =.25 , =.# log

2 K=.1LK=.2L= − − − − E


33/50

19.5 Concentrations in a galvanic cell a""ect the cell potential

earning Chec7

Consider the "ollo0ing reaction. Is "avored at high or lo0te$peratures3

't6;2,1 at$6;,p; F !66Cu2,=.1 M 6Cu

2

2

E ,=.! =.==

&.1! K=.===1LE =.! ln

2 9#/5== K=.1L

= −×

= −×

T

o

>hen Q V 1/ reaction is "avored at

higher te$perature


34/50

19.# Electrolysis uses electrical energy to cause che$ical reactions !

Electrolytic (eactions< * (e"ine$ent

) So$e reactions are not "avored as 0ritten, E cath V E anode/ yet 0e 0ould li7e the$ to occur

) o do this/ 0e apply electrical potential to the

syste$ 0hich exceeds the energy need/ and 0e

re$ove any possible co$peting reactions

) Since these reactions are not "avored/ they are

not li7ely to Wshort outX

) hese reactions can o"ten be per"or$ed in one

container/ 0ithout resistance


35/50

19.# Electrolysis uses electrical energy to cause che$ical reactions 5

Electrolysis vs. Electrolytic vs. Galvanic

) 4ote that electrolysis $eans the application o"

electricity

) Electrolytic $eans that the particular reaction

is not spontaneous

Electrolytic Cell Galvanic Cell

Cathode is negative ,reduction

*node is positive ,oxidation 4on-spontaneous

(e@uires a battery

Cathode is positive ,reduction

*node is negative ,oxidationSpontaneous

Is a battery


36/50

19.# Electrolysis uses electrical energy to cause che$ical reactions #

earning Chec7 In the electrolysis o" a@ueous U 2S8! he products o" the

electrolysis are ;2 and 82 gas/ not the WexpectedX products solidU and S28&2-. >hy3

Cathode *node

U e- : U -2.92

;28 2e- : ;2 8;

- -=.&

S8!

2- : S28

&

2- 2e- 2.=1

2;28 : 82 !; 2e- 1.2

>ater co$petes


37/50

19.# Electrolysis uses electrical energy to cause che$ical reactions %

earning Chec7

'redict the li7ely products o" the "ollo0ing/ 0hen an

electrode is dipped into<

* solution o" 4aCl

+olten 4aCl

* solution o" ;Cl

Cl2, g / 4a,l

;2, g / 82, g

;2, g / 82, g


38/50

19.# Electrolysis uses electrical energy to cause che$ical reactions &

our urn

*n electrode is dipped into an a@ueous solutioncontaining U ,aq / 48-,aq/ ;28,l and Cl2,aq.

>hich is the cathode3

*. U ,aq

B. 48-,aq

C. ;28,l

. Cl2,aq

E Hred

!;28 2e- : 2;2 82 -=.&

48-

;

e-

: ;482 ;28 =.9!U e- : U -2.92

Cl2 2e- : 2Cl- 1.#


39/50

19.% Stoichio$etry o" electroche$ical reactions involves electric current and ti$e 9

he Uinetics o" Electrolysis

) OaradayPs E@uation< q F t F nF q F charge ,coulo$bs/ C

F current ,*$peres/ */ or CJs

t F ti$e ,s n F $oles o" electrons trans"erred in the process

F F OaradayPs constant ,9#/5== C $ol-1

) Dsing units tell us ho0 these @uantities are related


40/50

19.% Stoichio$etry o" electroche$ical reactions involves electric current and ti$e !=

Electroplating

) In OaradayPs e@uation/ 0e use the nu$ber o"$oles o" electrons trans"erred/ ne

) Because 0e canPt see electrons/ 0e gauge this by

the a$ount o" $etal deposited or lost

) Dsing the hal"-reaction and stoichio$etry/ 0e can

relate the $etal to the nu$ber o" $oles o"

electrons$etal

$etal$etal $

$oles coe""icient

$ass n++ coe""icient

= ÷ ÷

ee


41/50

19.% Stoichio$etry o" electroche$ical reactions involves electric current and ti$e !1

earning Chec7

) >hat current $ust be supplied to deposit .== g *u "ro$ asolution o" *uCl in 2==.= s3

) ;o0 $uch ti$e ,in s does it ta7e to plate 1=.2 g o" *g

using a =.1 $* po0er source3

1 $ol *u $ol 9#/5== C.== g *u !!1= C F I 2==.= s

19#.9% g 1 $ol *u $ol× × × = ×

!

!

e

e

22.1 *

t F 9 Q 1=% s

1$ol *g 1 $ol 9#/5== C 1= *1=.2 g *g =.1 $* t

1=%.&% g 1 $ol *g $*$ol

−

× × × = × ×

!

!

e

e


42/50

19.% Stoichio$etry o" electroche$ical reactions involves electric current and ti$e !2

our urn

In Chro$e 'lating/ Cr

is plated as Cr. ;o0 long 0ill the process ta7e i" 1.5= g Cr ,++ F 52.== are re@uired and

the current is 5= $*3

*. 1.2 Q 1=# s

B. !.1 Q 1=5

sC. #.5 Q 1=% s

. 1.% Q 1=5 s

E redY

Cr e- : Cr ,s -=.%!!


43/50

19.& 'ractical applications o" electroche$istry !

Batteries - Galvanic Cells

'ri$ary Cell Secondary Cell

4on-rechargeable

*l7aline dry cell

(echargeable

'b storage Battery


44/50

19.& 'ractical applications o" electroche$istry !!

) Shel"-li"e/ rate o" energy output) Energy density ,the ratio o" available energy to

battery volu$e

) Specific energy ,the ratio o" available energy to

0eight

I$portant 'roperties o" Batteries


45/50

19.& 'ractical applications o" electroche$istry !5

)ithiu$ can be used in batteries 0ith highspeci"ic energy ,because o" its lo0 $ass and

high energy density ,because o" its very ,- EHred

) Can be either non-rechargeable ,pri$ary cells or

rechargeable ,secondary cells

ithiu$ Batteries


46/50

19.& 'ractical applications o" electroche$istry !#

ithiu$ Ion Cells ,2H

) onPt actually involve true oxidation and

reduction

) i ions can slip bet0een layers o" ato$s in solids

such as graphite WintercalationX

) he i ion battery is based on the transport o" i ions


47/50

19.& 'ractical applications o" electroche$istry !%

;ydrogen Ouel Cell

) Clean burning) 4o electrode loss

) Easily replenished

);igh operationalte$perature

) ;ighly e""icient


48/50

19.& 'ractical applications o" electroche$istry !&

*pplication< (e"ining

) *l28 is electrolyed in cryolite/ *lO#.


49/50

19.& 'ractical applications o" electroche$istry !9

'uri"ication o" Copper


50/50

*pplication< 'roduction o" 4a8;

Documents

Ch19. Electrochemistry