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Transcript of Ch19. Electrochemistry
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8/18/2019 Ch19. Electrochemistry
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1
Chapter 19
Electrochemistry
Brady and Senese
5th Edition
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8/18/2019 Ch19. Electrochemistry
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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$ents to harnessenergy as a battery
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19.1 Galvanic cells use redox reactions to generate electricity !
*nato$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 $ove bet0een the
cells
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19.1 Galvanic cells use redox reactions to generate electricity #
our urn
o0ards 0hich co$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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19.1 Galvanic cells use redox reactions to generate electricity 9
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
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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
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19.1 Galvanic cells use redox reactions to generate electricity 11
earning Chec7
>here there are no conductive $etals involved in a process/ an inert electrode is used.
C, gr and 't are o"ten used. >rite the standard cell
notation "or the reactions
Cr8, s +n82, s : +n8!-,aq Cr ,aq ,basic
;282,aq C82, g : ;2C28!,aq 82, g ,acidic
C, gr ?+n82, s | +n8!-,aq 66 Cr8, s 6 Cr
,aq 6 C
C, gr 6;282,aq/ ;,a@ 6 82, g 66 C82, g 6 ;2C28!,aq/ ;
,aq 6 C, gr
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19.1 Galvanic cells use redox reactions to generate electricity 12
our urnA
In the reaction +n82, s 2;,aq : +n2,aq ;282,aq0hich is the correct standard cell notation3
*. 't/ +n82, s6;,aq/ +n2,aq66;286;282,aq/ ;,aq6't
B. 't6;282,aq/ ;,aq6;2866;,aq/ +n2,aq6+n82, s't
C. Both are correct
. 4either is correct
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19.2 Cell potentials can be related to reduction potentials 1
Electrical 'otential
) Every substance has the potential to gainelectrons/ or be reduced in oxidation state
) he relative ease o" gaining electrons is ter$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
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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" =.==
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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
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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.%%
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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 $ost – E red.
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19. Standard reduction potentials can predict spontaneous reactions 1&
'redicting Cells in a Spontaneous (eaction
Cathode ,Gains e-
*node ,oses e-
) Dsually a non-$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
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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
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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
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19.! Cell potentials are related to "ree energy changes 22
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 $ol-1 Q ,-=.#5 C-1 F 211 7
4ot spontaneous
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19.! Cell potentials are related to "ree energy changes 2
our urn
Given that RGH F #5. 7 / 0hat is the value "or E Hcell"or the reaction Cu2,aq ;g,l : ;g2,aq Cu, s3
*. #.5
B. .=.& C. -=.&
. -=.#%#
E. 4one o" these
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19.! Cell potentials are related to "ree energy changes 2!
(elating E H to the E@uilibriu$ 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
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19.! Cell potentials are related to "ree energy changes 25
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 =.!#
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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
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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 $ol U 29&.15 U
. 4one o" the above
÷ ÷×
−−
−−
×××
− K K Jmol
JC Cmol mol e 15.29&1!.&
.!9#5==1211
11
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19.5 Concentrations in a galvanic cell a""ect the cell potential 2&
he 4ernst E@uation
ln−
= + RT
E E QnF
o
) Oor non- standardconditions/
) Substituting "or the "reeenergy/ NG/ ter$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
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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
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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
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19.5 Concentrations in a galvanic cell a""ect the cell potential 1
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$6't
E cell F =.===5%1
2
=.=592 KxL=.!9! , =.25 , =.%! log
# K=.5L= − − − −
;J;2 =.== Cr JCr -=.%!
4i2J4i -=.25 Cu2JCu =.!
=.9%& M F x? p; F =.==9#5
2
2
=.=592 KxL=.===5%1 ,=.== =.== log
2 K1L= − −
=.222 M F x
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19.5 Concentrations in a galvanic cell a""ect the cell potential 2
our urn
Consider the reaction sho0n. >hat is the value o" E cell3'b S8!2-,=.1 M 'bS8! 4i2,=.2 M 4i
*. =.99&
B. -=.99&
C. -=.1=1
. 4one o" these
4i2 4i -=.25 'bS8! 'b -=.#
=.=#=
cell
=.=592 1, =.25 , =.# log
2 K=.1LK=.2L= − − − − E
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19.5 Concentrations in a galvanic cell a""ect the cell potential
earning Chec7
Consider the "ollo0ing reaction. Is "avored at high or lo0te$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
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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$ed in one
container/ 0ithout resistance
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19.# Electrolysis uses electrical energy to cause che$ical reactions 5
Electrolysis vs. Electrolytic vs. Galvanic
) 4ote that electrolysis $eans the application o"
electricity
) Electrolytic $eans that the particular reaction
is not spontaneous
Electrolytic Cell Galvanic Cell
Cathode is negative ,reduction
*node is positive ,oxidation 4on-spontaneous
(e@uires a battery
Cathode is positive ,reduction
*node is negative ,oxidationSpontaneous
Is a battery
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19.# Electrolysis uses electrical energy to cause che$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$petes
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19.# Electrolysis uses electrical energy to cause che$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
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19.# Electrolysis uses electrical energy to cause che$ical reactions &
our urn
*n electrode is dipped into an a@ueous solutioncontaining U ,aq / 48-,aq/ ;28,l and Cl2,aq.
>hich is the cathode3
*. U ,aq
B. 48-,aq
C. ;28,l
. Cl2,aq
E Hred
!;28 2e- : 2;2 82 -=.&
48-
;
e-
: ;482 ;28 =.9!U e- : U -2.92
Cl2 2e- : 2Cl- 1.#
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19.% Stoichio$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 $ol-1
) Dsing units tell us ho0 these @uantities are related
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19.% Stoichio$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
$ass n++ coe""icient
= ÷ ÷
ee
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19.% Stoichio$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
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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= $*3
*. 1.2 Q 1=# s
B. !.1 Q 1=5
sC. #.5 Q 1=% s
. 1.% Q 1=5 s
E redY
Cr e- : Cr ,s -=.%!!
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19.& 'ractical applications o" electroche$istry !
Batteries - Galvanic Cells
'ri$ary Cell Secondary Cell
4on-rechargeable
*l7aline dry cell
(echargeable
'b storage Battery
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19.& 'ractical applications o" electroche$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
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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
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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$s in solids
such as graphite WintercalationX
) he i ion battery is based on the transport o" i ions
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19.& 'ractical applications o" electroche$istry !%
;ydrogen Ouel Cell
) Clean burning) 4o electrode loss
) Easily replenished
);igh operationalte$perature
) ;ighly e""icient
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19.& 'ractical applications o" electroche$istry !&
*pplication< (e"ining
) *l28 is electrolyed in cryolite/ *lO#.
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19.& 'ractical applications o" electroche$istry !9
'uri"ication o" Copper
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*pplication< 'roduction o" 4a8;