Chem 1B Chapter 18 Exercises Exercises #1: Redox Reactions 1.
$.
Determine Determine the oxidation oxidation number number (o.n.) (o.n.) of each atom in the followin followin formula formula (compounds (compounds or pol!atomic ions): ions): "nswer: (a) $%&': 1* % +* & ,$* (b) -aBr&: -a 1* Br /* & ,$* (c) -a$%$&: -a 1* % $* & ,$*
(d) Ca(&Cl) $: Ca $* Cl 1* & ,$*
(e) 0n&',: 0n * & ,$*
(f) Cr$& $,: Cr +* & ,$
Balance Balance the the followi followin n e2uat e2uations ions and then then write write balanced balanced net net ionic ionic e2uation e2uation for each each reaction. reaction. 3n each reaction4 identif! the oxidi5in aent and the reducin aent. "lso indicate the element that is oxidi5ed and the element reduced in the reaction. (a) Cu(s) $ "-& (aq) aq) → Cu(-& )(aq) aq) $ "(s) (b) $ "l(s)
.
Cu%&'(aq) aq) → "l$(%&')(aq) aq)
Cu(s)
Balance Balance the the followin followin redox redox reactions reactions in acidic acidic solut solution ion usin usin half,r half,reacti eaction on method method.. 3n each case4 identif! the oxidi5in aent4 the reducin aent4 the element that is oxidi5ed and the element that is reduced. (a)
,
$ Ca(&Cl)$(aq) aq) $ Cl (aq) aq) ' (a2) → $ Cl$(a2) Ca (a2) $ $&
,
(b) Br& (aq) aq)
,
/ Br (aq) aq)
+ (a2) → Br $(a2)
$&
,
(c) $ Cu$(a2) ' 3 (a2) → $ Cu3(s) 3$(a2)
'.
Balanced Balanced the the followi followin n redox redox reactio reactions ns in in basic basic soluti solution. on. 3n each case4 case4 ident identif! if! the the oxidi5 oxidi5in in aent aent44 the reducin aent4 the element bein oxidi5ed4 and the element bein reduced. , $, (a) $ Cr(&)',(aq) aq) $&$(aq) aq) $ & (a2) → $ Cr&' (aq) aq)
,
,
8 $&(l )
,
(b) Br $(aq) aq) + & (aq) aq) → Br& (aq) aq) / Br (aq) aq) $&(l )
Exercises #$: Electrochemical Cells Cells and Batteries 1.
6hat 6hat are the the diff differe erence ncess and7or and7or simil similari aritie tiess betwee between n an electrochemical cell and and an electrolytic cell 6here do oxidation and reduction reactions occur in each t!pe of cell ("nswer: 3n electrochemical cells4 chemical ener! is con9erted to electrical ener! (to produce electricit!)* in electrol!tic cell4 electrical ener! is used to cause nonspontaneous processes to occur. 3n both t!pes of cells4 oxidation occurs at anode and reduction occurs at cathode.)
1
Chem 1B Chapter 18 Exercises $.
i9en the followin reduction potentials: o
E
alf,;eaction
(<) ==
3$ $e, → $3,*
>./'
?e e, → ?e$*
>.
Br $ $e, → $Br *
,
1.>@
0n&', 8 /e, → 0n$ '$&*
1./1
Aredict whether each of the followin redox reactions is spontaneous or non,spontaneous. ,
(a) $?e(a2) $Br (a2) → $?e$(a2) Br $(a2)* (Eocell ,>.$ <* reaction is not spontaneous) ,
(b) $?e(a2) $3 (a2) → $?e$(a2) 3$(a2)* ,
(Eocell >.$ <* reaction is spontaneous)
,
(c) $0n&' (a2) 1>Br (a2) 1+(a2) → $0n$(a2) /Br $(a2) 8$&(l )* (Eocell >.'$ <) ,
(d) 0n&' (a2) /?e$(a2) 8(a2) → 0n$(a2) /?e(a2) '$&(l )* (Eocell >.' <) ,
,
(e) Br $(a2) $3 (a2) → $Br (a2) 3$(a2) ,
(E
,
>.// <* spontaneous reaction)
(Eocell ,>.// <* not spontaneous)
(e) 3$(a2) $Br (a2) → Br $(a2) $3 (a2)* .
o cell
he followin diaram represents a al9anic cell composed of copper metal in 1.> M Cu(-&)$ solution and sil9er metal in 1.> M "-& solution. he two half,cells are connected b! -& salt bride. (a) Determine the identit! of metal 0 and based on the fact that the followin reaction is spontaneous. Cu(s) $ "-& (a2) → Cu(-&)$(a2) $ "(s) (b) 6hich is the anode and cathode half,cell4 respecti9el! 6hich metal (" or Cu) ser9es as the anode and which metal ser9es as the cathode 6rite the anode half,cell reaction and the cathode half, cell reaction. (c) %how the directions in which and -&, ions flow in the salt bride. i9en that: Cu $(a2) $ e, → Cu(s)* "(a2) e, → "(s)*
o
>.' <
o
>.8> <
E E
(d) Calculate the cell potential ( Eocell) for this electrochemical cell. (e) Calculate the cell potential ( Ecell) when Cu$F 1.> 0 and " F >.>>1> 0. ("nswer: (b) "node half,cell: Cu |Cu$(a2)* Cu(s) → Cu$(a2) $e ,* (oxidation) Cathode half,cell: " |"(a2)* "(a2) e, → "* (reduction) (c) flows into cathode half,cell* -& , flows into anode half,cell. (d) Eocell >.'+ <* (e) Ecell >.$8 <) '.
Consider the followin half,cell reactions: o
Gn$(a2) $ e, → Gn(s)*
E ,>.+ <
Cu$(a2) $ e, → Cu(s)*
E >.' <
o
$
Chem 1B Chapter 18 Exercises
(a) %Hetch a 9oltaic cell consistin of Gn (s)|Gn%&'(a2) and Cu(s)|Cu%&'(a2) half,cells usin -a $%&' as salt,bride. Iabel the followin: ( i) the anode and cathode half,cells* ( ii) the positi9e and neati9e electrodes* (iii) the direction electron flows in the circuit and ions flow in the salt bride. (b) 6rite the cell notation for the 9oltaic cell. (c) 6rite net ionic e2uation for: ( i) anode half,cell reaction* ( ii) cathode half,cell reaction4 and ( iii) the o9erall cell reaction. (d) Calculate standard cell potential ( E ocell) at $/oC and the E cell when the concentrations are Gn $F 1.> M and Cu$F >.>>1> M (e) 6hat is the 9alue of ∆Go for the o9erall reaction at $/ oC (f) Calculate the maximum free ener! a9ailable for worH if /.> of 5inc is reacted at an a9erae cell potential of 1.>> < "nswer: (a) (i) "node half,cell: Gn (s)|Gn%&'(a2)* Cathode half,cell: Cu(s)|Cu%&'(a2)* (ii) Aositi9e electrode (Cu)* neati9e electrode (Gn)* ( iii) electron flows from Gn (anode) to Cu (cathode)* (b) Cell notation: Gn (s)|Gn%&'(a2)||Cu%&'|Cu(s)* (c) Gn(s) Cu$(a2) → Gn$(a2) Cu(s)* (d) Eocell 1.1> <* Ecell 1.>1 <* (e) ∆Go ,$1$ HJ* (f) ?ree ener! from /.> of Gn ,1/ HJ
/.
Consider the followin standard reduction (half,cell) potentials: "l(a2) e, → "l(s)*
E o ,1.++ <
Cu$(a2) $e, → Cu(s)*
E o >.' <
(a) %Hetch a 9oltaic cell consistin of the abo9e half,cells usin -& as salt,bride. Iabel the followin: (i) the anode and cathode half,cells* (ii) the positi9e and neati9e electrodes* ( iii) the direction electron flows in the circuit and ions flow in the salt bride. (b) 6rite the net e2uation for the o9erall cell reaction and calculate the standard cell potential ( E ocell) at $/oC. (c) Calculate ∆Go for the o9erall reaction at $/ oC. (d) ow much ener! (in HJ) can be deri9ed from the oxidation of +./ of aluminum at an a9erae cell potential of $.>> < ("nswer: (a) (i) "node half,cell: "l(s) |"l(a2)* Cathode half,cell: Cu(s) |Cu$(a2)* (ii) Aositi9e electrode (Cu)* neati9e electrode ("l)* ( iii) electrons flow from "l (anode) to Cu (cathode)* (b) $ "l(s) Cu $(a2) → $ "l(a2) Cu(s)* Eocell $.>> <* (c) ∆Go ,1.1+ x 1> HJ* (d) 1'/ HJ)
+.
"n automobile batter! uses the folloin cell reaction: Ab(s) Ab&$(s) $$%&'(a2) → $Ab%&'(s) $$&(l ) (a) Calculate the standard cell potential ( Eocell) for this reaction usin the followin half,cell potentials4: ,
Ab%&'(s) (a2) $e, → Ab(s) %&' (a2)*
,
Ab&$(s) %&' (a2) (a2) $e, → Ab%&'(s) $$&(l )*
Eo ,>./+ < Eo 1.+8/ <
(b) 6hat is the cell potential ( E cell) if the concentration of sulfuric acid in the batter! is +.> 0 ,
("ssume $%&' onl! dissociates as follows: $%&'(a2) → (a2) %&' (a2)) ("nswer: (a)
E
o cell
$.>' <* (b) Ecell $.1 <)
Chem 1B Chapter 18 Exercises
Exercises #: Electrical Energy and Electrolysis 1.
he followin reaction has an a9erae cell potential of .>> <: $ Ii (s) Ii3 (s)
→
Ii3(s)
ow much ener! can be deri9ed from a reaction that consumes 1.>> of Ii (i9en: 1 J 1 C<* ?arada!Ks constant4 F @+'8/ Coulomb7mol) ("nswer: '1. HJ) $.
" concentration cell is set up b! connectin a half,cell containin a lead electrode in >.1> 0 Ab(-&)$(a2) and another half,cell containin lead electrode in a saturated solution of AbC& . 3f the measure cell potential at $/ oC is >.$> <4 calculate the K sp of AbC& at $/oC ("nswer: K sp .> x 1> ,1+)
.
6hat products are formed at the anode and cathode4 respecti9el!4 when each of the followin is electrol!5ed Explain. (a) IiCl(l ): (Ii formed at cathode (reduction): Ii e,
→ Ii*
Cl$ formed at anode (oxidation): $Cl , → Cl$ $e,*
(b) -aCl(aq): $ is formed at cathode and Cl $ formed at anode (c) Cu%&'(a2): Cu is formed at cathode and & $ formed at anode* (d) $%&'(a2)4 usin platinum as electrodes: $ formed at cathode and & $ at anode. '.
ow man! rams of copper will be produced if a current of 1./> " is passed throuh Cu%& ' solution for >.> minutes ("nswer: >.8@ of Cu)
/.
ow lon (in minutes) does it taHe to produce +.// of copper from a solution containin Cu $ usin a cell that operates at +.> < and produces a constant current of 1./ " ow much ener! (in HJ) is consumed to produce this amount of copper ("nswer: it will taHe .+ hrs* ener! consumed 1$> HJ)
+.
ow man! rams of -a and Cl $4 respecti9el!4 can be produced in 1.>> hour b! electrol!sis of molten -aCl in a Downs cell that operates at './ < and ./ x 1> ' " ow man! Hilowatt,hours (H6h) of ener! are consumed to produce this amount of sodium and chlorine (?arada!Ks constant @+4'8/ C7mol e ,* 1 H6h .+ x 1> + J) ("nswer: +' H of -a and @@ H of Cl $* '> H6h)
.
0olten manesium chloride is electrol!5ed in a cell that operates at .> < and 1./ x 1> / ". (a) ow man! Hilorams of manesium are produced in an 8.>,hour shift (b) 6hat is the b!,product in this electrol!sis and how man! H of this product is formed (c) ow man! H6h of ener! is consumed durin this 8,hr process ("nswer: (a) /'> H of 0* (b) Cl$* 1+>> H of Cl $* (c) +>> H6h)
8.
Electrol!sis of "l $& dissol9ed in molten cr!olite produces aluminum metal. 3f the electrol!tic cell operates at .> < and $.> x 1> + "4 how lon does it taHe to produce 1.>> H of aluminum ow much H6h of ener! is consumed to produce this much aluminum '
Chem 1B Chapter 18 Exercises ("nswer: /.' seconds* 8.@ H6h)
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