12-lecture 12 (2015-07-22) (student version)(1).ppt
TRANSCRIPT
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1
Housekeeping for Wed. 7/22/15- Endgame for rest of the B term…
- cover h. 1! toda" and tomorro#$ finish #/ h. 1% tomorro#
- take-home &uestions for E'am ( handed out/posted to B)ack*oard and
+oog)e ,rive
- due in c)ass ne't Monday (7/27)
- ptiona)0 cumu)ative fina) Wednesda" 3)) have grades #orked up
*" end of 4onda" night
- HW 1( e)ectrochemistr" and so)3ns posted to B)ack*oard and +oog)e,rive
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Weak acids and *ases have sp)it6 persona)ities
2
H"de and Hare60 1!550
Warner Bros.
http//*it.)"/18B9:!1
http://bit.ly/1LBjP91http://bit.ly/1LBjP91
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0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
Ha)f-e&uiva)ence
point
;t ha)f-e&uiva)ance point0
p? a
Buffering region is p? a @/- 1.
Buffering region
A
; )itt)e more a*out *uffers…
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0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
Ha)f-e&uiva)ence
point
he p? a of a *uffer therefore
represents the pH #here #e have
equal concentrations of acid and
con9ugate *ase or *ase and
con9ugate acid.
Cor a weak acid …
pH D p? a0
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0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
Ha)f-e&uiva)ence
point
5
; )itt)e more a*out *uffers…
pH p? a…
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0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. HCl (mL)
p H
F
itration of GHA #ith H) #eak *ase/strong acid
Ha)f-e&uiva)ence point
GHA @ H2 >>D GH(@
@ H-
Buffering region
r…
GH(@ @ H2 >>D GHA @ HA
@
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0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. HCl (mL)
p H
7
itration of GHA #ith H) #eak *ase/strong acid
Ha)f-e&uiva)ence point
pH D p? a…
D GHA @ HA
@
pH > p? a @ )og
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hinking a*out p?a
%
he p?a is the pH #here #e #i)) have equal amounts of acid and
con9ugate *ase or *ase and con9ugate acid.
So0 if #e kno# the pH and p?a0 #e can determine the re)ative
proportions of acid/con9ugate *ase or *ase/con9ugate acid.
;s a ru)e of thum*…
pH D p?a… the more base component dominates *ase or con9. *ase
pH p?a… the more acid component dominates acid or con9. acid
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onsider the acetic acid/acetate *uffer s"stem.
he p? a of acetic acid is (.7(.
What is the e&ui)i*rium for acetic acid in #aterI
What is the re)ative concentration of acetate at pH 5.7(I +reater than0
)ess than0 or e&ua) to acetic acidI
What is the re)ative concentration of acetic acid at pH A.7AI +reater
than0 )ess than0 or e&ua) to acetateI
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Ho# a*out the ammonia/ammonium *uffer s"stemI
he p? * of ammonia is (.7(. Hint #hat3s the p? aI
What is the e&ui)i*rium of ammonia in #aterI
What is the re)ative concentration of ammonia at pH 1J.2FI +reater
than0 )ess than0 or e&ua) to ammoniumI
What is the re)ative concentration of ammonium at pH %.2FI +reater
than0 )ess than0 or e&ua) to ammoniaI
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h. 1! E)ectrochemistr"
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1A
'idation and reduction redo' redu'
'idation > loss of e)ectrons eduction > gain of e)ectrons
LEO says GER
Loss of e)ectrons > o'idation
G ain of e)ectrons > r eduction
Oil Rig
O'idation > l oss
Reduction > g ainf "ou3re in oi) countr".
*Oida!ion and reduc!ion always occur !oge!"er#*
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C)aming +ummi Bear http//*it.)"/FLAv
2?)A s→ 2?) s @ A2 g
12H2211 s @ A2 g→ ! s @ A2 g @ 11H2 g
http://bit.ly/6V3Ovhttp://bit.ly/6V3Ov
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edo' #ithout om*ustion
2 Ga s @ )2 g → 2 Ga) s
2 Ga → 2 Ga@ @ 2 e−
)2 @ 2 e−
→ 2 )−
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E)ectron Bookkeeping
M for reactions that are not meta) @ nonmeta)0 or do
not invo)ve 20 #e need a method for determiningho# the e)ectrons are transferred
M chemists assign a num*er to each e)ement in a
reaction ca))ed an oxidation state that a))o#s themto determine the e)ectron f)o# in the reaction
N even though the" )ook )ike them0 oxidation states are
not ion charges!
M oxidation states are imaginar" charges assigned *ased on a set of ru)es sorr"0 more ru)es
M ion charges are rea)0 measura*)e charges
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$n general 0 o'idation num*ers for monoatomic ions are
the same as the charges for monoatomic ions. E'cept#here noted a*ove.
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elements of Group 2A
have a +2 charge
elements of
Group 5A have a
3 charge
elements of
Group 6A have a
2 charge
elements of
Group !A have a
1 charge
F.2
he charge on an ion can *e predicted from its position in the periodic
ta*)e.
elements of
Group 1A have a
+1 charge
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:ractice N ;ssign o'idation num*ers to each
e)ement in the fo))o#ing compounds
M 2
M H2O(
M ? A;s(
M 4n(-
M 2(2-
M Ga22
M GH(@
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'idation and eduction ;nother ,efinition
- o'idation occurs #hen an atom3s o'idation state increases during a reaction- reduction occurs #hen an atom3s o'idation state decreases during a reaction
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'idation and eduction ;nother ,efinition
H( @ 2 2 Q 2 @ 2 H2-( @1 J @( N2 @1 -2
Based on the changes in o'idation num*ers0 #hich e)ement
undergoes o'idationI eductionI
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'idationNeductionM o'idation and reduction must occur simultaneously
N if an atom )oses e)ectrons another atom must take them
M the reactant that reduces an e)ement in another reactant is ca))ed
the reducing agent
N the reducing agent contains the e)ement that is o'idiSed
M the reactant that o'idiSes an e)ement in another reactant is ca))edthe oxidizing agent
N the o'idiSing agent contains the e)ement that is reduced
2 Ga s @ )2 g Q 2 Ga@) N s
Ga is o'idiSed0 ) is reduced
Ga is the reducing agent0 )2 is the o'idiSing agent
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dentif" the 'idiSing and educing
;gents in Each of the Co))o#ing
2H2 g @ 2 g→ 2H2 g
Tn s @ H2O( a&→ TnO( a& @ H2 g
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ne more e'amp)e
Lesuvius fire @ :haraoh3s serpent demonstration
http//*it.)"/1A2Ur"
http://bit.ly/132XCryhttp://bit.ly/132XCry
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Lesuvius Cire / :haraoh3s Oerpent reactions
Lesuvius Cire6 decomposition of ammonium dichromate
GH(2r 27 Q r 2A @ G2 @ (H2
:haraoh3s Oerpent6 decomposition of HgOG2
2HgOG2 Q 2HgO @ O2 @ A G(
HgO @ 2 Q Hg @ O2
O2 @ A2 Q 2 @ 2O2A G( Q AG2 @ G2
his is the snake6.
,ecomposition gas
products cause the
snake to gro#6.
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Lesuvius Cire is a redo' reaction
Lesuvius Cire6 decomposition of ammonium dichromate
GH(2r 27 Q r 2A @ G2 @ (H2
Which atom is o'idiSedI Which atom is reducedI
educing agentI 'idiSing agentI
-A @F @A J
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2%
+a)vanic aka Lo)taic eactions
M ;re spontaneous redo' reactions$ no energeticinput re&uired
M e)ease energ" as heat if no resistance is met
M 4a" *e separated into compartments to harnessenerg" as a *atter"
us @ ;g@
a& Q u2@
a& @ ;gs
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2!
+a)vanic aka Lo)taic eactions0 cont.
us @ ;g@a& Q u2@a& @ ;gs
Oince this is a redo' reaction0 e)ectrons are transferred *et#eencopper and si)ver. here are t#o issues #e need to dea) #ith
- #hat is the o&erall reactionI
- ho# can #e harness the transfer of e)ectrons to do #ork for usI
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AJ
Ba)ancing redo' e&uations
us @ ;g@a& Q u2@a& @ ;gs
he on)" difference *et#een *a)ancing redo' e&uations and non-redo' e&uations is that #e have to *a)ance mass and charge in a
redo' e&uation.
Cor this e'amp)e0 mass is *a)anced0 *ut charge isn3t. Here3s ho##e go a*out *a)ancing charge…
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A1
+a)vanic aka Lo)taic eactions0 cont.
orrect e&uation us @ 2;g@a& Q u2@a& @ 2;gs
n this e'amp)e0 e)ectrons are transferred direc!ly *et#een thecopper and si)ver. hat is0 *oth ha)f-reactions occur in the same
space. Ho# can #e harness the transfer of e)ectrons to do #ork
for usI
S i f h )f i i h )f )) ))
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A2
Se'ara!ion of ha)f-reactions into ha)f-ce))s a))o#s us to
harness transfer of e)ectrons
eduction ha)f-ce)) 'idation ha)f-ce))
Red uction >ca! hode node >o idation
Elec!rons low ro anode !o ca!"ode#
e-e-
) f ) * id
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AA
o)e of sa)t *ridge
Oa)t *ridge a))o#s us to comp)ete circuit *et#een the individua)
ha)f-ce))s *" a))o#ing coun!erions to f)o# to the appropriate ha)f-
ce)). ther#ise0 #e *ui)d up im*a)ance of charge in each ha)f-ce)).
) f )t * id t
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A(
o)e of sa)t *ridge0 cont.
) f )t * id t
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A5
o)e of sa)t *ridge0 cont.
;nother #a" to )ook at it…- at cathode0 reduction of ;g@a& to ;gs resu)ts in deficienc" of
positive charge$ in other #ords0 *ui)d up of negative charge- at anode0 o'idation of us to u2@a& resu)ts in *ui)d up of
positive charge
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) f )t * id t
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A7
o)e of sa)t *ridge0 cont.
f #e use a so)ution of ?GA0 ? @ cations #i)) migrate to#ards thecathode to make up for the )oss of ;g@ cations.
GA
- anions #i)) migrate to#ards the anode to counter the *ui)d
up of u2@ cations.
+"e sal! bridge allows or !"e igra!ion o bo!" ions,
? @ GA-
*+i'- anion Q
anode*
*+i'- ca! ion Q
ca!"ode*
Ot d d )) G t ti 8i )) G t ti
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A%
Otandard e)) Gotation 8ine e)) Gotation
M e)) reactions separated *" VV that represents the sa)t *ridge
M E)ectrodes appear at the outsides
M eaction e)ectro)"tes in inner section
M :hases separated #ith V
M Opecies in the same state separated #ith 0
M oncentrations sho#n in
Ot d d )) G t ti 8i )) G t ti
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A!
Otandard e)) Gotation 8ine e)) Gotation
,ra# a diagram that corresponds #ith the fo))o#ing standard ce))
notation
4ns V 4n2@a& VV Gi2@a& V Gis
;ssume ?GA is the e)ectro)"te so)ution.
n the diagram…
- identif" the anode and cathode ce))s$
- the direction of e)ectron f)o# *et#een the anode and cathode$
- direction of ion movement from the e)ectro)"te so)ution
Ot d d )) G t ti 8i )) G t ti
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(J
Otandard e)) Gotation 8ine e)) Gotation
Cor the fo))o#ing e)ectrochemica) ce))0
4ns V 4n2@a& VV Gi2@a& V Gis
- Write the corresponding ha)f-reactions- the format of the )ine notation gives us a hint at the e&uation
for the ha)f-reaction at each e)ectrode
- Write the overa))0 *a)anced reaction
'idation at the anode
4ns Q 4n2@ a& @ 2e-
eduction at the cathode
Gi2@ a& @ 2e- Q Gis
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(1
E)ectrica) :otentia)
M Ever" su*stance has the potentia) to gaine)ectrons0 or *e reduced in o'idation state
M he re)ative ease of gaining e)ectrons is termed the
reduc!ion 'o!en!ial 0 and is s"m*o)iSed E red
M f the matter *eing o*served is in standard state
then E is termed the s!andard reduc!ion 'o!en!ial
and is s"m*o)iSed as E. red
M nits are Lo)t 1 L > 1 X -1
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(2
Otandard eduction :otentia)s E Yred
M E Yred are ta*u)ated for near)" ever"kno#n su*stance
M ; high va)ue of E Yred E Yred D J
means that the su*stance is easi)"
reduced
M E Yred is a re)ative num*er0ar*itrari)" determined
M ;)) su*stances are compared toH@0 #hich has a E Yred of J.JJ L
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(A
e)) :otentia)s
M he standard ce)) potentia) is ca)cu)ated as E Yce)) > E Ycathode - E Yanode
M f the ce)) is non-standard
E ce)) > E cathode - E anode
M n spontaneous redo' reactions0 the cathode
portion of the reaction has a higher )ess
negative reduction potentia) than that of theanode E cathode D E anode
M Cor a spontaneous redo' reaction0 Ece)) D J
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easie
rto
redu
ce
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harder
to
red
uce$
easierto
o i d i / e
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eduction potentia) ref)ects ease of reduction
Ho# can #e use e)ectronic structure to e'p)ain #h" f)uorine is the
easiest to reduce and #h" )ithium is the easiest to o'idiSeI
Which is the *etter o'idiSing agentI Which is the *etter reducing
agentI
a)cu)ating ce)) potentia)
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(7
a)cu)ating ce)) potentia)
,etermine the ce)) potentia) for the fo))o#ing e)ectrochemica) ce))0
4ns V 4n2@a& VV Gi2@a& V Gis
s the reaction spontaneousI
a)cu)ating ce)) potentia)
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(%
a)cu)ating ce)) potentia)
What if #e reverse the po)arit"I
Gis V Gi2@a& VV 4n2@a& V 4ns
What is the ce)) potentia)I s the reaction spontaneousI
C d f th ht
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Cood for thought…
Which ga)vanic ce)) #ou)d "ou e'pect to have the )arger ce)) potentia)I
8is V 8i2@a& VV C-a& V C2:*s V :*2@a& VV H2Og V Os @ 2H
@ @ 2e-
e)) potentia)s e'p)ain activit" series for
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5J
4ore active
meta)s #i))disp)ace )ess
active meta)s
e)) potentia)s e'p)ain activit" series for
sing)e-disp)acement reactions
4 ti t ) i)) di ) ) ti t )
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4ore active meta)s #i)) disp)ace )ess active meta)s
u s @ ;gGA a& Q uGA a& @ ;g s
e)) notation us V u
2@
a& VV ;g
@
a& V ;gs
Ece)) > I s this reaction spontaneous or nonspontaneousI s
this consistent #ith the ce)) potentia) as #e)) as the prediction
from the re)ative activitiesI
Ba)ancing redo' e&uations and #orking out E ))
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Ba)ancing redo' e&uations and #orking out Ece))
us Q u2@a& @ 2e-
;g@ @ e- Q ;gs
-----------------------------
us Q u2@a& @ 2e-
2;g@ @ e- Q ;gs
-----------------------------
us Q u2@a& @ 2e-
2;g@ @ 2e- Q 2;gs
-----------------------------
us @ 2;g@a& Q
u2@a& @ 2;gs
his is e'act)" the same as the #a" #e
*a)anced the redo' e&uation *et#een copper
and si)ver ear)ier on. he &uestion no# is#hat a*out the potentia)sI Ohou)d #e
mu)tip)" the potentia) for si)ver *" 2 since #e
did that to *a)ance the overa)) e&uationI
*0O,* he reason #h" #e don1! is *ecause potentia)s are in!ensi&e$ the"3re independent
of the amount of su*stance. he rea) reason
#h" is *ecause potentia) is defined as a ra!io
of energ" to charge. he ratio sta"s the same
regard)ess of ho# much su*stance "ou have.
+ake"oe essage use the potentia)s as
!"ey are #hen "ou ca)cu)ate Ece)). ,on3t ad9ust
them
8et3s consider the opposite
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8et s consider the opposite…
;gs V ;g@a& VV u2@a& V us
Ece)) > I s this reaction spontaneous or nonspontaneousI s
this consistent #ith the ce)) potentia) as #e)) as the prediction
from the re)ative activitiesI
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Oo)utions to e'ercises
; )itt)e more a*out *uffers…
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55
; )itt)e more a*out *uffers…
onsider the acetic acid/acetate *uffer s"stem.
he p? a of acetic acid is (.7(.
What is the re)ative concentration of acetate at pH 5.7(I +reater than0
)ess than0 or e&ua) to acetic acidI
! '3 4,756 we1re abo&e !"e ' a , 8ce!a!e9 : 8ce!ic acid9
What is the re)ative concentration of acetic acid at pH A.7AI +reater
than0 )ess than0 or e&ua) to acetateI
! '3 ;,756 we1re below !"e ' a , 8ce!ic acid9 : 8ce!a!e9
; )itt)e more a*out *uffers…
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5F
Ho# a*out the ammonia/ammonium *uffer s"stemI
he p? * of ammonia is (.7(. Hint #hat3s the p? aI
$ ' b < 5,756 b < an!ilog(5,75) < =>5,75 < =,? => 4 ,
a < w / b < =,> =>=5 / (=,? => 4 ) < 4,@ => =>
' a < log (4,@ => => ) < A,2@
What is the re)ative concentration of ammonia at pH 1J.2FI +reater
than0 )ess than0 or e&ua) to ammoniumI
! '3 =>,2@6 we1re abo&e !"e ' a , 8onia9 : 8oniu9
What is the re)ative concentration of ammonium at pH %.7FI +reater
than0 )ess than0 or e&ua) to ammoniaI
! '3 ?,2@6 we1re below !"e ' a , 8oniu9 : 8onia9
:ractice N ;ssign o'idation num*ers to each
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:ractice ;ssign o'idation num*ers to each
e)ement in the fo))o#ing compounds
M 2
M H2O(
M?
A
;s(
M 4n(-
M 2(2-
M Ga22
M GH(@
@($ -2
H @1$ O @F$ -2
? @1$ ;s @5$ -2
4n @7$ -2
@A$ -2
Ga @1$ -1 G -A$ H @1
id ti d d ti ; th , fi iti
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'idation and eduction ;nother ,efinition
H( @ 2 2 Q 2 @ 2 H2-( @1 J @( N2 @1 -2
o'idation
reduction
Based on the changes in o'idation num*ers0 #hich e)ement
undergoes o'idationI eductionI
dentif" the 'idiSing and educing
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dentif" the 'idiSing and educing
;gents in Each of the Co))o#ing
2H2 g @ 2 g→ 2H2 g
Tn s @ H2O( a&→ TnO( a& @ H2 g
J J @1 -2
J @1 @F -2 @2 @F -2 J
o'idation
reduction
o'idation
reduction
H2 is reducing agent
2 is oidiing agent
Tn is reducing agent
H2 is oidiing agent
Ba)ancing redo' e&uations
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FJ
Ba)ancing redo' e&uations
us @ ;g@a& Q u2@a& @ ;gs
- Op)it the reaction into ha)f-reactions there #i)) a)#a"s *e ano'idation ha)f-reaction and a reduction ha)f-reaction
- se o'idation num*ers
us @ ;g@a& Q u2@a& @ ;gs
J @1 @2 J
u is o'idiSed
;g is reduced
Ba)ancing redo' e&uations
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F1
Ba)ancing redo' e&uations
us @ ;g@a& Q u2@a& @ ;gs
- Write each ha)f-reaction separate)"- Ba)ance each ha)f-reaction *" mass and charge
'idation ha)f-reaction
us Q u 2@a& mass is *a)anced
charge is not
us Q u 2@a& @ 2e-
'idation ha)f-reaction is fu))"
*a)anced
eduction ha)f-reaction
;g@a& Q ;gs mass is *a)anced
charge is not
;g@a& @ e- Q ;gs
eduction ha)f-reaction is fu))"
*a)anced
Ba)ancing redo' e&uations
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F2
g &
us @ ;g@a& Q u2@a& @ ;gs
- 8ine up ha)f-reactions and prepare to add6 them to o*tain overa))
reaction
us Q u2@a& @ 2e- P of e- gained and )ost
;g@ @ e- Q ;gs must *e the same
-----------------------------
us Q u2@a& @ 2e-
2;g@ @ e- Q ;gs
-----------------------------
us Q u2@a& @ 2e-
2;g@ @ 2e- Q 2;gs
-----------------------------
us @ 2;g@a& Q u2@a& @ 2;gs
4ass and charge are fu))"
*a)anced
Otandard e)) Gotation 8ine e)) Gotation
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FA
Otandard e)) Gotation 8ine e)) Gotation
? @ from ?GA
e)ectro)"te so)3n
f)o#s to#ards
cathode.
GA- f)o#s
to#ards anode.
eduction potentia) ref)ects ease of reduction
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p
Which is the *etter o'idiSing agentI Which is the *etter reducing
agentI
Bluorine has the )arger reduction potentia). t #ants to *e
reduced . Higher reduction potentia)0 easier to reduce. herefore0
it #i)) *e a strong oidiing agen! .
Li!"iu has the )o#er reduction potentia). t #ants to *e oidied .
8o#er reduction potentia)0 easier to o'idiSe. herefore0 it #i)) *e
a strong reducing agen! .
a)cu)ating ce)) potentia)
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F5
a)cu)ating ce)) potentia)
,etermine the ce)) potentia) for the fo))o#ing e)ectrochemica) ce))0
4ns V 4n2@a& VV Gi2@a& V Gis
Ece)) > Ecathode - Eanode
Crom the ce)) notation… the anode is the o'idation of 4n to 4n2@
the cathode is the reduction of Gi2@ to Gi
Ecathode > -J.25 L
Eanode > -1.FF L
Ece)) > Ecathode - Eanode > -J.25 L - -1.FF L
> @1.(1 L$ Ece)) D J… the reaction is s'on!aneous
a)cu)ating ce)) potentia)
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FF
g p
,etermine the ce)) potentia) for the fo))o#ing e)ectrochemica) ce))0
Gis V Gi2@a& VV 4n2@a& V 4ns
Ece)) > Ecathode - Eanode
Crom the ce)) notation… the anode is the o'idation of Gi to Gi2@
the cathode is the reduction of 4n2@ to 4n
Ecathode > -1.FF L
Eanode > -J.25 L
Ece)) > Ecathode - Eanode > -1.FF L - -J.25 L
> -1.(1 L$ Ece)) J… the reaction is no! spontaneous
Cood for thought…
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8is V 8i2@a& VV C-a& V C2 Ece)) > 2.%7 L - -A.J5 L > @ 5.!2 L
:*s V :*2@a& VV H2Og V Os @ 2H@ @ 2e- Ece)) > J.1( L - -J.1A L
> @ J.27 L
* +"e ar!"er a'ar! !"e anode and ca!"ode 'o!en!ials are ro eac"
o!"er6 !"e larger !"e cell 'o!en!ial, *
4ore active meta)s #i)) disp)ace )ess active meta)s
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4ore active meta)s #i)) disp)ace )ess active meta)s
u s @ ;gGA a& Q uGA a& @ ;g s
e)) notation us V u2@a& VV ;g@a& V ;gs
Ecathode > J.%J L$ Eanode > J.A( L
Ece)) > Ecathode - Eanode > J.%J L - J.A( L > @J.(F L
Ece)) DJ$ process is spontaneous. onsistent #ith the fact that
u is more active than ; .
8et3s consider the opposite…
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8et s consider the opposite…
;gs V ;g@a& VV u2@a& V us
Ece)) > I s this reaction spontaneous or nonspontaneousI s
this consistent #ith the ce)) potentia) as #e)) as the prediction
from the re)ative activitiesI
Ecathode > J.A( L$ Eanode > J.%J LEce)) > Ecathode - Eanode > J.A( L - J.%J L > -J.(F L
Ece)) J… nons'on!aneous#
onsistent #ith the fact that ;g is less active than u.