Electrochemistry BS I Physical Chemistry.pdf

Electrochemistry, Electrode

1. PHYSICAL CHEMISTRY BS-I
Course code (CHEM –371)

2. 2
Electrolytesaresubstancesthatformionsinsolutionwhichconductanelectriccurrent.
Sodiumchloride,copper(II)sulphateandpotassiumnitrateareexamplesofelectrolytes.
Nonelectrolytesarecovalentsubstanceswhichfurnishneutralmoleculesinsolution.Their
solutionsdonotconductanelectriccurrent.Sugar,alcoholandglycerolareexampleof
nonelectrolytes.
Electrolysis:Thephenomenonofdecompositionofanelectrolytebypassingelectric
currentthroughitssolutionistermedaselectrolysis
Electrochemistry the branch ofchemistrythat deals with the chemical changes produced
by electricity and the production of electricity by chemical changes.
The process of electrolysis is carried in an apparatus called the Electrolytic cell.

3. 3
Current:
Theflowofelectricchargeiscalledcurrent,usuallyelectronsorelectron-
deficientatoms.ThecommonsymbolforcurrentistheuppercaseletterI.
Ampere(A)
Therateofflowofelectricchargeperunittimeiscalledcurrent.Aunitofelectric
current(A)
Aunitofelectriccurrentequaltoaflowofonecoulombpersecond.
Coulomb(C)
theSIunitofelectriccharge,Itisthecharge(symbol:QorqorC)transportedby
aconstantcurrentofoneampereinonesecond:
Acoulombisaunitquantityofelectricity.

4. Potentialdifference
Itisthedifferenceintheamountofenergythatcharge
carriershavebetweentwopointsinacircuit....
ApotentialdifferenceofoneVoltisequaltooneJoule
ofenergybeingusedbyoneCoulombofchargewhenit
flowsbetweentwopointsinacircuit.

5. 5
Volt
Avoltisaunitofelectromotiveforce.Itisthedifferenceinelectrical
potentialrequiredtosendacurrentofoneamperethrougharesistance
ofoneohm.
Ohm(Ω)
Anohmisaunitofelectricalresistance.theSIunitofelectrical
resistance,Oneohmisequaltotheresistanceofaconductorthrough
whichacurrentofoneampereflowswhenapotentialdifferenceofone
voltisappliedtoit.
Electrochemicalreactionsthetransferofelectronstoorfromamolecule,
atom,orionataninterfacebetweenanelectronicconductor,the
electrode(throughwhichtheelectronsreachorleavetheinterface),and
anionicconductor(throughwhichtheionstravel).

6. 6
Oxidationisthelossofelectronsduringareaction
byamolecule,atomorion
Reductionisachemicalreactionthatinvolvesthe
gainingofelectronsbyoneoftheatomsinvolvedin
thereactionbetweentwochemicals.

7. 7
Electrochemistry
Electrolytic
Cell
E.E C.E
Galvanic
Cell
C.E E.E
Conduction
In metals there is a flow of electrons and in solution
there is a flow of ions.
Home appliances based on flow of e but in solution
conductance based on flow of ions

8. 8
Howtheelectrolysisactuallytakesplace,is
illustratedinFigure.
Thecationsmigratetothecathodeandforma
neutralatombyacceptingelectronsfromit.
Theanionsmigratetotheanodeandyielda
neutralparticlebytransferofelectronstoit.
Asaresultofthelossofelectronsbyanions
andgainofelectronsbycationsattheir
respectiveelectrodeschemicalreactiontakes
place.
Example.Letusconsidertheelectrolysisof
hydrochloricacidasanexample.Insolution,
HClisionised,
The mechanism of electrolysis.

9. 9
IntheelectrolyticcellCl
–
ionswillmovetowardtheanodeandH
+
ionswillmovetowardthe
cathode.Attheelectrodes,thefollowingreactionswilltakeplace.
Atcathode:
Reduction
Asyousee,eachhydrogenionpicksupanelectronfromthecathodetobecomeahydrogen
atom.Pairsofhydrogenatomsthenunitetoformmoleculesofhydrogengas,H
2.
At Anode :
Oxidation
Afterthechlorideionlosesitselectrontotheanode,pairofchlorineatomsunitetoform
chlorinegas,Cl
2.TheneteffectoftheprocessisthedecompositionofHClintohydrogen
andchlorinegases.Theoverallreactionis:
Decomposition

10. 10
Electrolytic cell:
in which electrical energy is converted into chemical energy
Nonspontaneous
Gibbs free energy +ve
ΔG =-nEF
•ΔG = Free energy (Indicator of spontaneous and non
spontaneous if the value is +vethe process is non spontaneous
and if the value is –vethe process is spontaneous.
•n = No of electron transfer
•E = EMF of cell
•F = Faraday constant =96500 C

11. 11

12. 12
AVoltaiccell,alsoknownasagalvaniccellisoneinwhichelectricalcurrentisgeneratedby
aspontaneousredoxreaction.AsimplevoltaiccellisshowninFig.Herethespontaneous
reactionofzincmetalwithanaqueoussolutionofcoppersulphateisused.
Abarofzincmetal(anode)isplacedinzinc
sulphatesolutionintheleftcontainer.Abarof
copper(cathode)isimmersedincopper
sulphatesolutionintherightcontainer.The
zincandcopperelectrodesarejoinedbya
copperwire.Asaltbridgecontaining
potassiumsulphatesolutioninterconnectsthe
solutionsintheanodecompartmentandthe
cathodecompartment.
A simple voltaic (galvanic) cell.

13. 13
Whenthecellissetup,electronsflowfromzincelectrodethroughthewiretothecopper
cathode.Asaresult,zincdissolvesintheanodesolutiontoformZn
2+
ions.TheCu
2+
ionsin
thecathodehalf-cellpickupelectronsandareconvertedtoCuatomsonthecathode.At
thesametime,SO
4
-2
ionsfromthecathodehalf-cellmigratetotheanodehalf-cellthrough
thesaltbridge.Likewise,Zn
2+
ionsfromtheanodehalf-cellmoveintothecathodehalf-cell.
Thisflowofionsfromonehalf-celltotheothercompletestheelectricalcircuitwhich
ensurecontinuoussupplyofcurrent.Thecellwilloperatetilleitherthezincmetalor
copperioniscompletelyusedup.

14. 14
Thisproblemcanbesolvedverysimply.
Thesolutionsinthetwocompartments
maybeconnected,say,byasaltbridge.
ThesaltbridgeisaU-tubefilledwithan
electrolytesuchasNaCl,KCl,orK
2SO
4.It
providesapassagetoionsfromone
compartmenttotheothercompartment
withoutextensivemixingofthetwo
solutions.Withthisionflow,thecircuit
iscompleteandelectronspassfreely
throughthewiretokeepthenetcharge
zerointhetwocompartments.

15. 15

16. 16

17. Electrochemical Series:
Electrochemical Series is the arrangement of various
electrodes in the increasing order of their standard
oxidiationpotential or decreasing order of their
standard reduction potential. A metal lower in the
series is replace by ones above it while the one higher
in the series is coated by the below it for eg,
Zn + CuSO
4 Cu + ZnSO
4
Zn + Cu
2+
Cu + Zn
2+

18. 18

19. 19

20. 20
Thecellpotential,Ecell,isthemeasureofthepotentialdifference
betweentwohalfcellsinanelectrochemicalcell.Thepotential
differenceiscausedbytheabilityofelectronstoflowfromonehalf
celltotheother.Electronsareabletomovebetweenelectrodes
becausethechemicalreactionisaredoxreaction.Aredoxreaction
occurswhenacertainsubstanceisoxidized,whileanotheris
reduced.Duringoxidation,thesubstancelosesoneormore
electrons,andthusbecomespositivelycharged.Conversely,during
reduction,thesubstancegainselectronsandbecomesnegatively
charged.Thisrelatestothemeasurementofthecellpotential
becausethedifferencebetweenthepotentialforthereducingagent
tobecomeoxidizedandtheoxidizingagenttobecomereducedwill
determinethecellpotential.Thecellpotential(E
cell
)ismeasuredin
voltage(V),whichallowsustogiveacertainvaluetothecell
potential.

21. 21
Theanodehalf-cellisdescribedfirst;thecathodehalf-cellfollows.Withinagivenhalf-
cell,thereactantsarespecifiedfirstandtheproductslast.Thedescriptionofthe
oxidationreactionisfirst,andthereductionreactionislast;whenyoureadit,youreyes
moveinthedirectionofelectronflow.Spectatorionsarenotincluded.
Asingleverticalline(|)isdrawnbetweentwochemicalspeciesthatareindifferent
phasesbutinphysicalcontactwitheachother(e.g.,solidelectrode|liquidwith
electrolyte).Adoubleverticalline(||)representsasaltbridgeorporousmembrane
separatingtheindividualhalf-cells.
Thephaseofeachchemical(s,l,g,aq)isshowninparentheses.Iftheelectrolytesinthe
cellsarenotatstandardconditions,concentrationsand/orpressure,theyareincludedin
parentheseswiththephasenotation.Ifnoconcentrationorpressureisnoted,the
electrolytesinthecellsareassumedtobeatstandardconditions(1.00Mor1.00atmand
298 K)
Using these rules, the notation for the cell we put together is:
Cd(s) | Cd
2+
(aq, 0.15 M) || Ag
+
(aq, 0.20 M) | Ag(s)

22. 22
for writing cell diagrams of Zinc-Copper cell.
The symbol for an inert electrode, like the platinum electrode is often enclosed in a
bracket. For example,
The value of emfof a cell is written on the right of the cell diagram. Thus a zinc-copper
cell has emf1.1V and is represented as

23. 23
Themagnitudeoftheemfofacellreflectsthetendencyofelectronstoflowexternally
fromoneelectrodetoanother.Theelectronsaretransportedthroughthecellsolutionby
ionspresent.Thiscorrespondstoaclockwiseflowofelectronsthroughtheexternal
circuit.Thustheemfofthecellisgiventhe+vesign.Iftheemfactsintheopposite
directionthroughthecellcircuit,itisquotedas–vevalue.Forexample,Danielcellhasan
emfof1.1Vandthecopperelectrodeispositive.Thiscanbeexpressedintwoways:
Thenegativesignindicatesthatthecellisnotfeasibleinthegivendirection.Thereaction
willtakeplaceinthereversedirection.

24. 24
Theemfofacellcanbecalculatedfromthehalf-cellpotentialsofthetwocells(anodeand
cathode)byusingthefollowingformula
whereE
RandE
Larethereductionpotentialsoftheright-handandleft-handelectrodes
respectively.Itmaybenotedthatabsolutevaluesofthesereductionpotentialscannotbe
determined.Thesearefoundbyconnectingthehalf-cellwithastandardhydrogen
electrodewhosereductionpotentialhasbeenarbitrarilyfixedaszero.

25. 25
TheemfgeneratedbyanelectrochemicalcellisgivenbythesymbolE.Itcanbemeasured
withthehelpofapotentiometer.Thevalueofemfvarieswiththeconcentrationofthe
reactantsandproductsinthecellsolutionsandthetemperatureofthecell.Whentheemfof
acellisdeterminedunderstandardconditions,itiscalledthestandardemf.Thestandard
conditionsare
(a)1Msolutionsofreactantsandproducts;and
(b)temperatureof25°C.Thusstandardemfmaybedefinedas:
theemfofacellwith1Msolutionsofreactantsandproductsinsolutionmeasuredat25°C.
StandardemfofacellisrepresentedbythesymbolE°.Withgases1atmpressureisa
standardconditioninsteadofconcentration.
ForasimpleZn-Cuvoltaiccell,thestandardemf,E°,is1.10V.Thismeansthattheemfofthe
celloperatedwith[Cu2+]and[Zn2+]bothat1Mand25°Cis1.10V.Thatis,

26. 26

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