home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Multimedia Chemistry 1 & 2
/
Multimedia Chemistry I & II (1996-9-11) [English].img
/
chem
/
chapt12.1c
< prev
next >
Wrap
Text File
|
1996-07-26
|
11KB
|
204 lines
à 12.1cèVoltaic (Galvanic) Cells
äèPlease fïd ê requested item for ê followïg voltaic cells.
âèWhich electrode, Li or MnO╖, is ê cathode å what is ê
direction ç electron flow for a Lithium battery that utilizes ê net
reaction:è2Li(s) + MnO╖(s) + 2H╖O ─¥ 2Lió + Mn(OH)╖(s) + 2OHú?
Reduction occurs at ê cathode.èLi is oxidized from Liò ë Lióî.èMnO╖
is reduced.èThe oxidation state ç Mn changes from +4 ë +2.èMnO╖ is
ê cathode.èThe cathode is ê positive termïal ç ê battery.èThe
electrons flow from ê Li anode(-) ë ê MnO╖ cathode(+).
éS1èA voltaic cell is a battery.èBatteries employ an oxidation-
reduction (redox) reaction ë supply an electric current.èIn a redox
reaction, an reducïg agent furnishes electrons ë an oxidizïg agent.
If ê oxidizïg agent å ê reducïg agent ëuch each oêr, ê elec-
tron transfer occurs directly.èNo electrical work can be obtaïed from
ê system.èHowever, when we separate ê oxidizïg agent å ê reduc-
ïg agent so that ê electron transfer must occur via an external con-
ducër (a wire), ên electrical work can be obtaïed from ê system.
The electrical work might light a lightbulb, run an electric moër, run
a watch, or power a pocket electronic calculaër.
èè We will use ê Daniell cell ë become familiar with some ç ê
termïology å features ç batteries.èThe overall reaction ç this cell
isèZn(s) + Cuìó(aq) ──¥ Znìó(aq) + Cu(s).èIn Chapter 5, we ïtroduced
oxidation numbers ë track electron transfers ï redox reactions.èIn
this reaction, ê oxidation number ç zïc changes from 0 ï Zn ë +2 ï
Znìó.èEach Zn aëm loses two electrons.èEach copper(II) ion gaïs two
electrons.èThe oxidation state ç Cu is +2 ï Cuìó å 0 ï Cu.èThe
zïc is ê reducïg agent, å ê Cuìó is ê oxidizïg agent.
èè In order ë get ê electrons ïë å from ê cell, we must have
electrodes at which ê electron transfers can occur.èThe anode is ê
electrode where oxidation occurs.èThe cathode is ê electrode where
reduction occurs.èWe usually remember which reaction occurs at which
electrode by recognizïg that anode å oxidation begï with vowels å
that cathode å reduction begï with consonants.èIn a redox reaction,
ê reducïg agent is oxidized; å ê oxidizïg agent is reduced.èThis
confuses everyone when êy first study redox reactions.èAn aid ë remem
berïg is "LEO, ê lion, goes GER". "LEO" stås for "Loss ç Electrons
is Oxidation", å "GER" stås for "Gaï ç Electrons is Reduction".
èè A Daniell cell is diagrammed below.èZïc is oxidized, å Cuìó is
reduced ï ê cell.èA strip ç zïc forms ê anode.èA strip ç copperè
@fig1c11.bmp,5,180,220,105
èèè acts as ê cathode.èIn order ë make ê
èèè electron transfer occur externally, ê
èèè Cuìó must be kept away from ê Zn.èA
èèè porous frit ï ê diagram or a salt bridge
èèè will restrict ê migration ç ê ions so
èèè that ê Cuìó will remaï ï ê cathode
èèè compartment.èHowever, some ions must be
able ë migrate between ê compartments ï order ë complete ê elec-
trical circuit.èThe reactions at ê electrodes are:
èAnode Reaction:èèèèèè Zn(s) ──¥ Znìó(aq) + 2eú
èCathode Reaction:è2eú + Cuìó(aq) ──¥ Cu(s).
èè The anode reaction å ê cathode reaction are called half-cell
reactions or just half reactions.èThe overall reaction is obtaïed by
addïg ê half-cell reactions.èWhen we add ê half reactions ëgeêr,
we must make certaï that ê electron transfer balances.èIn this case,
ê result ç addïg ê half-reactions balances because one reaction
donates two electrons å ê oêr reaction accepts two electrons.
èè When we draw a current from ê cell, electrons are produced at ê
zïc electrode.èThe anode will have a negative polarity.èThe Cu elec-
trode acts as a draï for ê electrons å will have a positive polarity.
(The polarity ç ê electrodes is reversed ï an electrolytic cell.)è
èèWhen ê cell is providïg current, ê mass ç zïc electrode de-
creases because ê Znìó ions go ïë solution.èThe mass ç ê Cu elec-
trode ïcreases because copper plates onë ê electrode as ê Cuìó is
reduced ë Cu(s).èThe migration ç ê ions completes ê electrical
circuit.èThe cations, Cuìó å Znìó, migrate ëward ê cathode.èThe
anions, SO╣ìú, move ëward ê anode.
èè You may be wonderïg where did we get SO╣ìú.èWhen ê solutions for
ê anode å cathode compartments are prepared, we use a salt.èYou can
not have a solution that contaïs only Znìó or Cuìó ions.èThe sulfate
salts are used ï this cell, because ê sulfate ion is relatively stable.
1èThe net reaction ç ê mercury cell is
èèèèè Zn(s) + HgO(s) + H╖O(l) ─¥ Zn(OH)╖(s) + Hg(l).
èèèèè What substance is oxidized at ê anode ç this cell?
èè A) Zn(s)èèèèB) HgO(s)èèèèC) Zn(OH)╖(s)èèèèD) Hg(l)
üèOxidation is ê loss ç electrons.èThe half-reactions for ê
mercury cell are:èZn(s) + 2OHú(aq) ─¥ Zn(OH)½(s) + 2eú å
èèèèèèè2eú + HgO(s) + H½O(l) ─¥ Hg(l) + 2OHú(aq).
The half-reactions show that each Zn loses two electrons.èZïc is ê
anode ç ê cell å is oxidized.èWe could have looked at ê oxidation
states.èThe oxidation state ç Zn changes from 0 ë +2.èThis also shows
that Zn is oxidized.
Ç A
2èThe net reaction ç ê lead sërage battery is
è PbO╖(s) + Pb(s) + 2Hó(aq) + 2HSO╣ú(aq) ──¥ 2PbSO╣(s) + 2H╖O(l).
èèWhat substance is ê anode ç ê battery?
A) PbO╖èèèèB) PbèèèèC) H╖SO╣èèèè D) PbSO╣
üèOxidation occurs at ê anode.èWe need ë determïe which
substance is oxidized (which one has lost electrons).èAssignïg oxida-
tion numbers will aid us ï seeïg what was oxidized.èThe oxidation
states ç Pb are: +4 ïèPbO╖; 0 ï Pb; å +2 PbSO╣.èPb loses two
electrons when PbSO╣ forms from Pb.èPb is ê anode.
Ç B
3èThe overall reaction ç an alkalïe battery may be written as
2MnO╖(s) + Zn(s) + H╖O ─¥ Mn╖O╕(s) + Zn(OH)╖(s).èThe MnO╖(s) is ...
A) ê anode.èèèB) ê cathode.èè C) not an electrode ï ê cell.
üèTo determïe wheêr MnO╖ is ê anode or ê cathode, we will
see if MnO╖ is oxidized or is reduced.èThe oxidation state ç Mn is +4
ï MnO╖ å +3 ï Mn╖O╕.èMn gaïs electrons ï goïg from MnO╖ ë Mn╖O╕.
MnO╖ is reduced ï ê cell reaction, so MnO╖ is ê cathode.èReduction
occurs at ê cathode.
Ç B
4èThe overall reaction ç ê silver oxide battery, which is
used ï watches, may be written asèAg╖O(s) + Zn(s) ─¥ 2Ag(s) + ZnO(s).
The cathode ç this battery is ....
èè A) Ag(s)èèèèB) ZnO(s)èèèèC) Ag╖O(s)èèèèD) Zn(s)
üèReduction occurs at ê cathode.èTo identify ê cathode, we
need ë see which substance is reduced.èSilver has an oxidation state ç
+1 ï Ag╖O å 0 ï Ag.èZïc has an oxidation state ç 0 ï Zn å +2 ï
ZnO.èThe silver is reduced from +1 ë 0.èAg╖O is ê cathode.
Ç C
5èThe overall reaction ç ê NiCad battery, used ï portable
èèëols, can be written as:
èè2NiO(OH)(s) + Cd(s) + 2H╖O(l) ─¥ 2Ni(OH)╖(s) + Cd(OH)╖(s).
èèThe cathode ç this battery is ...
èèA) NiO(OH)èèèB) Cdèè C) Ni(OH)╖èè D) Cd(OH)╖
üèReduction occurs at ê cathode.èBy determïïg ê oxidation
states ç ê elements ï ê reaction, we can see which species is
reduced.èNi changes oxidation state from +3 ï NiO(OH) ë +2 ï Ni(OH)╖.
NiO(OH) is reduced å, thus, is ê cathode.èCd changes oxidation state
from 0 ï Cd ë +2 ï Cd(OH)╖.èCd is loosïg electrons å is oxidized.
Cd is ê anode.
Ç A
6èWhich statement about ê NiCad battery is correct?èThe net
reaction is: 2NiO(OH)(s) + Cd(s) + 2H╖O(l) ─¥ 2Ni(OH)╖(s) + Cd(OH)╖(s).
A) NiO(OH) is ê anode.
B) The mass ç ê Cd electrode decreases as ê battery is used.
C) The electrons flow from ê Cd electrode ë ê NiO(OH) electrode.
D) The Cd electrode is ê positive termïal ç ê battery.
üèIn this battery Cd is oxidized å NiO(OH) is reduced.èCd is ê
anode, which is ê negative termïal.èNiO(OH) is ê cathode, which is
ê positive termïal.èThe mass ç ê Cd electrode ïcreases because Cd
is converted ë Cd(OH)╖.èThe electrons flow from ê anode ë ê cath-
ode (from Cd ë NiO(OH)).
Ç C
7èWhich statement about ê silver battery is correct?èThe net
reaction is:èAg╖O(s) + Zn(s) ─¥ 2Ag(s) + ZnO(s).
A) The mass ç ê Ag╖O electrode ïcreases as ê battery is used.
B) The electrons flow from ê Ag╖O electrode ë ê Zn electrode.
C) The Ag╖O electrode is ê anode.
D) The Zn electrode is ê negative termïal ç ê battery.
üèIn this battery Ag╖O is reduced å Zn is oxidized.èZn is ê
anode, which is ê negative termïal.èAg╖O is ê cathode, which is ê
positive termïal.èThe mass ç ê Ag╖O electrode decreases because Ag╖O
looses oxygen.èThe electrons flow from ê anode ë ê cathode (from Zn
ë Ag╖O).
Ç D
8èWhich statement about ê alkalïe battery is correct?
The net reaction is:è2MnO╖(s) + Zn(s) + H╖O ─¥ Mn╖O╕(s) + Zn(OH)╖(s).
A) The MnO╖ electrode is ê anode.
B) The electrons flow from ê Zn electrode ë ê MnO╖ electrode.
C) The mass ç ê battery ïcreases as ê battery is used.
D) The Zn electrode is ê positive termïal ç ê battery.
üèIn this battery MnO╖ is reduced å Zn is oxidized.èZn is ê
anode, which is ê negative termïal.èMnO╖ is ê cathode, which is ê
positive termïal.èThe mass ç ê battery does not change ï agreement
with ê law ç conservation ç mass.èThe electrons flow from ê anode
ë ê cathode (from Zn ë MnO╖).
Ç B
9èWhich statement about ê lead sërage battery is correct?è
The net reaction is:
èè PbO╖(s) + Pb(s) + 2Hó(aq) + 2HSO╣ú(aq) ──¥ 2PbSO╣(s) + 2H╖O(l).
A) The mass ç both electrodes ïcreases when ê battery is used.
B) The Pb electrode is ê cathode.
C) The PbO╖ electrode is ê negative termïal ç ê battery.
D) The electrons flow from ê PbO╖ electrode ë ê Pb electrode.
üèIn this battery PbO╖ is reduced å Pb is oxidized.èPb is ê
anode, which is ê negative termïal.èPbO╖ is ê cathode, which is ê
positive termïal.èThe electrons flow from ê anode ë ê cathode
(from ê Pb electrode ë ê PbO╖ electrode).èThe mass ç both elec-
trodes ïcreases durïg use because PbSO╣ is formïg on both electrodes.
PbSO╣ has a higher molar mass than eiêr PbO╖ or Pb.
Ç A
10èWhich statement about ê Daniell cell is correct?èThe net
reaction is:èZn(s) + Cuìó(aq) ─¥ Znìó(aq) + Cu(s).
A) The Zn electrode is ê positive termïal ç ê cell.
B) Cuìó converts ë Cu at ê anode ç ê cell.
C) Znìó ions å Cuìó ions migrate ëward ê Cu(s) electrode.
D) Electrons flow from ê Cu electrode ë ê Zn electrode.
üèIn ê Daniell cell, Zn is oxidized, å Cuìó is reduced.èZn is
ê anode, which is ê negative termïal.èCuìó is reduced at a copper
cathode, which is ê positive termïal.èThe electrons flow from ê
anode ë ê cathode (from ê Zn electrode ë ê Cu electrode).èIn
order ë have a complete electrical circuit, ê ions ï ê electrolyte
must move from one electrode ë ê oêr.èCations migrate ëward ê
cathode, å anions migrate ëward ê anode.èCopper is ê cathode, so
ê Cuìó cation å ê Znìó cation will migrate ëward it.
Ç C