12-Lecture 12 (2015-07-22) (student version)(1).ppt

8/17/2019 12-Lecture 12 (2015-07-22) (student version)(1).ppt

1/69

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


2/69

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


3/69

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…


4/69

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


5/69

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


pH p? a…


6/69

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

@


7/69

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


8/69

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


9/69!

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


10/691J

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


11/69


12/69

h. 1! E)ectrochemistr"


13/69

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#*


14/69

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


15/69

edo' #ithout om*ustion

2 Ga s @ )2 g → 2 Ga) s

2 Ga → 2 Ga@ @ 2 e−

)2 @ 2 e−

→ 2 )−


16/69

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


17/69

$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.


18/69

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


19/69

: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(@


20/69

'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


21/69


22/69


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


23/69

'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


24/69

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


25/69

ne more e'amp)e

Lesuvius fire @ :haraoh3s serpent demonstration

http//*it.)"/1A2Ur"

http://bit.ly/132XCryhttp://bit.ly/132XCry


26/69

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.


27/69

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


28/69

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


29/69

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


30/69

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…


31/69

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 )) ))


32/69

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


33/69

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


34/69

A(

o)e of sa)t *ridge0 cont.

) f )t * id t


35/69

A5


;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


36/69

) f )t * id t


37/69

A7


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


38/69

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



39/69

A!


,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



40/69

(J


Cor the fo))o#ing e)ectrochemica) ce))0


- 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


41/69

(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


42/69

(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


43/69

(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


44/69

easie

rto

redu

ce


45/69

harder

to

red

uce$

easierto

o i d i / e


46/69

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)


47/69

(7


,etermine the ce)) potentia) for the fo))o#ing e)ectrochemica) ce))0


s the reaction spontaneousI



48/69

(%


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


49/69

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


50/69

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 )


51/69

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 ))


52/69

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


53/69

8et s consider the opposite…

;gs V ;g@a& VV u2@a& V us





54/69

Oo)utions to e'ercises



55/69

55


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



56/69

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


57/69

: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


58/69


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



59/69


;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



60/69

FJ


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



61/69

F1


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



62/69

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



63/69

FA


? @ 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


64/69

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! .



65/69

F5




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



66/69

FF

g p


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…


67/69

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, *



68/69


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…


69/69

8et s consider the opposite…

;gs V ;g@a& VV u2@a& V us




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.

12-Lecture 12 (2015-07-22) (student version)(1).ppt

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Transcript of 12-Lecture 12 (2015-07-22) (student version)(1).ppt