22CYT12-Unit_I_Electrochemistry - EMF Series & its Applications.ppt

Electrochemistry:Introduction – cells – types - representation of galvanic cell - electrode potential - Nernst equation (derivation of cell EMF) - calculation of cell EMF from single electrode potential - reference electrode: construction, working and applications (Determination of potential of the unknown electrode and pH of the unknown electrode) of standard hydrogen electrode, standard calomel electrode - glass electrode – EMF series and its applications - potentiometric titrations (redox) - conductometric titrations - mixture of weak and strong acid vs strong base.

1. DEPARTMENT OF CHEMISTRY
WELCOMES YOU ALL
22CYT12 &Chemistry for Computer Systems
(Electrochemical series and its Applications)
Prepared By
Krishnaveni K
Assistant Professor
Department of Chemistry
Kongu Engineering College,
Perundurai, Erode-638060

2. ELECTROCHEMISTRY
17-Dec-22

3. Introduction–cells–types-representationofgalvaniccell-electrode
potential-Nernstequation(derivationofcellEMF)-calculationofcellEMFfrom
singleelectrodepotential-referenceelectrode:construction,workingand
applicationsofstandardhydrogenelectrode,standardcalomelelectrode-glass
electrode–EMFseriesanditsapplications-potentiometrictitrations(redox)-
conductometrictitrations-mixtureofweakandstrongacidvsstrongbase.
UNIT-I
ELECTROCHEMISTRY

4. History of Electrochemistry
16 thCentury -William Gilbert –Father of Magnetism
18 thCentury –William Nicholson & Wilhelm Ritter –Decomposition of water –Electrolysis
SvanteArrhenius -Dissociation of electrolytes
Walther Hermann Nernst –Theory of Electromotive Force
Conductance? Ability to conduct current , mho

5. ELECTROCHEMISTRY
INTRODUCTION
Itisabranchofchemistry
Thestudyofprocessinvolvedintheinterconversionof
chemicalandelectricalenergy.
KEY TERMS IN ELECTROCHEMISTRY
Conductor:Materialwhichconductelectriccurrent
Nonconductor:Materialwhichdonotconductelectriccurrent
Current:Theflowofelectronsthroughawireoranyconductor
Oxidation:Lossofelectrons
Reduction:Gainofelectrons
Redoxreaction:oxidationandreductionreactionsoccursimultaneously
Reducingagent:Areactantinwhichdonatesanelectrontothereducedspecies.(Thereducingagent
isoxidized)
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6. Oxidizingagent:Areactantinwhichacceptsanelectronfromtheoxidizedspecies.(Theoxidizingagent
isreduced)
Anode:Theelectrodeatwhichoxidationoccurs
Cathode:Theelectrodeatwhichreductionoccurs
Electrolyte:Awatersolublesubstanceandconductanelectriccurrent
Halfcell:Asingleelectrodeimmersedinanelectrolyticsolutionanddevelopingadefinitepotential
difference.
Cell:Twohalfcellsareconnectedthroughonewire
OxidationPotential:Itisthetendencyofanelectrodetolosselectrons
Reductionpotential:Itisthetendencyofanelectrodetogainelectrons
ElectrodePotential:Itisthetendencyofanelectrodetolossorgainelectrons
SingleElectrodePotential:Itisthetendencyofanelectrodetolossorgainelectronswhenitisdippedin
itsownsaltsolution.(Standard-1Mconcentrationat25
0
C).
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7. 17-Dec-22
LEOGERBOARD

9. ELECTROCHEMICAL CELL
Introduction
Anelectrochemicalcellisadevicein
whicharedoxreactionisutilizedtoget
electricalenergy.
Anelectrochemicalcellisalsocommonly
referredtoasvoltaicorgalvaniccell.
Theelectrodewherereductionoccursis
calledcathode.
Theelectrodewhereoxidationoccursis
calledanode.
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10. Construction
ElectrochemicalCellsaremadeupoftwohalf-cells,eachconsistingofanelectrode
whichisdippedinanelectrolyte.Thesameelectrolytecanbeusedforbothhalfcells.
Thesehalfcellsareconnectedbyasaltbridgewhichprovidestheplatformforionic
contactbetweenthem.Asaltbridgeminimizesoreliminatestheliquidjunction
potential.
ThepracticalapplicationofanelectrochemicalorgalvaniccellistheDanielcell.
ItconsistsofaZnelectrodedippinginZnSO
4solutionandaCuelectrodedippingin
CuSO
4solution.
EMF= E
oxi+ E
Red
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11. Cell reaction
Anode : Zn → Zn
2+
+ 2e
-
(Oxidation) {0.76V}
Cathode : Cu
2+
+ 2e
-
→ Cu (reduction) {0.34V}
Overall : Zn + Cu
2+
→ Zn
2+
+ Cu (Redox)
Representation of Daniel cell : Zn / Zn
2+
|| Cu
2+
/ Cu
 Zn / ZnSO
4 (1M)// CuSO
4 (1M) / Cu
Cell EMF : 1.1 V
EMF= E
oxi+ E
Red
= E
Zn+ E
cu= 0.76+0.34
CuSO
4-Cu
2+
+ SO
4
2-

12. Electrolytic cells
Electrical Energy --Chemical Energy
Anode positive Charge -oxidation ---2Cl
-
Cl
2+ 2e-
Cathode negative charge reduction ---2Na
+
+ 2e-Na
Overall reaction ---2Na
+
+ 2Cl
-
2NaCl

13. Representation of Galvanic Cell
Anode : Zn Cathode : Cu
Zn Zn
2+
ZnSO
4 CuSO
4Cu
2+
Cu
Metal and the electrolyte or metal ion can be separated by , / ;
Zn / Zn
2+
Cu
2+
/ Cu
Concentration of the electrolyte should be in ()
Zn / ZnSO
4 (1M) CuSO
4(1M) / Cu
Zn , Zn
2+
Cu
2+
, Cu
Salt bridge can be represented by ||
Zn / Zn
2+
|| or // Cu
2+
/ Cu

14. Electrochemical Series
Thestandardelectrodepotentialsofanumberofelectrodesarearrangedinthe
increasingorderofreductionpotentialat25°Cisreferredtoasemforelectrochemical
series.
Characteristicsofelectrochemicalseries:
Lithiumisthefirstmemberoftheseries.
Highlyreactivemetalsystemsareatthetopoftheseries.
Inotherwords,goodreducingagentsareatthetopoftheseries,havingthenegativesignandactas
anode.
Allgoodoxidizingagentsareatthebottomoftheseries,havingthepositivesignandactascathode.
Hydrogensystemisatthemiddleoftheseries.Alltheelementswhichdisplacehydrogenfromdilute
acidsareplacedaboveit.

17. ApplicationsofElectrochemicalSeries
ToFindReactivityofMetals
Aswemovedownintheelectrochemicalseriesreactivityofmetal
decreases
Alkalimetalsandalkalineearthmetalsatthetoparehighlyreactive.
Theycanreactwithcoldwaterandevolvehydrogen.Theydissolvein
acidsformingsalts.
MetalslikeFe,Pb,Sn,NiandCowhichliealittledownintheseries,
donotreactwithcoldwaterbutreactwithsteamandevolvehydrogen.
MetalslikeCu,AgandAuwhichliebelowthehydrogenareless
reactiveanddonotevolvehydrogenfromwater.

18. For Studying displacement reaction
Elementshavinghigherreductionpotentialwillgainelectronsandthathavinglower
reductionpotentialwillloseelectrons.Henceelementhigherinelectrochemicalseries
candisplaceanelementplacedlowerinelectrochemicalseriesfromitssaltsolution.
Example
Canzincdisplacescopperfromitssaltsolution?
ZndisplacesCufromCuSO
4,because,zincisplacedhigherinelectrochemicalseries
whileCuisplacedlowerinelectrochemicalseries.Hencezinccaneasilydisplace
copperfromCuSO
4.
Zn+CuSO
4--------> ZnSO
4+ CuE
0
Zn= -0.76 volts
Cu+ZnSO
4--------> No recationE
0
Cu= +0.34 volts

19. For choosing elements as Oxidizing Agents
Theelementswhichhavemoreelectron-acceptingtendencyareoxidizingagents.The
strengthofanoxidizingagentincreasesasthevalueofreductionpotentialbecomesmore
andmorepositive.Elementsatthebottomoftheelectrochemicalserieshavehigher(+ve)
reductionpotential.Sotheyaregoodoxidizingagents.Thus,oxidizingpowerincreases
fromtoptobottomintheseries.
Example-F
2isastrongeroxidantthanCl
2,Br
2andI
2.
Cl
2isastrongeroxidantthanBr
2andI
2.

20. For choosing elements as Reducing Agents
Theelementswhichhavemoreelectronlosingtendencyarereducingagents.The
powerofreducingagentincreasesasthevalueofreductionpotentialbecomesmoreand
morenegative.Elementsatthetopoftheelectrochemicalserieshavehigher(-ve)
reductionpotential.Sotheyaregoodreducingagents.Thus,reducingpowerdecreases
fromtoptobottomintheseries.
Example-
TheelementlikeZn,K,Na,Fe,etc.aregoodreducingagent.

21. Displacement of hydrogen from dilute acids by metals
ThemetalwhichcanprovideelectronstoH
+
ionspresentindiluteacidsforreductionevolvehydrogen
fromdiluteacids.Themetalhavingnegativevaluesofreductionpotentialpossessesthepropertyof
losinganelectronorelectrons.
Thus,themetalsoccupyingtoppositionsintheelectrochemicalseriesreadilyliberatehydrogenfrom
diluteacidsandondescendingintheseries,tendencytoliberatehydrogengasfromdiluteacids
decreases.
ThemetalswhicharebelowhydrogenintheelectrochemicalserieslikeCu,Hg,AuandPtdonotevolve
hydrogenfromdiluteacids.
Example
Zinc reacts with dil.H
2SO
4 to give H
2but Ag does not. Why?
Zn+H
2SO
4--------> ZnSO
4+ H
2; E
0
Zn= -0.76 volts
Ag+H
2SO
4--------> No reaction; E
0
Ag= +0.80 volts
The metal with a positive reduction potential will not displace hydrogen from an acid solution.

22. Displacement of hydrogen from water
Ironandthemetalsaboveironarecapableofliberatinghydrogenfromwater.Thetendency
decreasesfromtoptobottomintheelectrochemicalseries.
AlkalimetalsandalkalineearthmetalsliberatehydrogenfromcoldwaterbutMg,ZnandFe
liberatehydrogenfromhotwaterorsteam.
ForCalculationofStandardemfofthecell
Standardreductionpotentialvaluesaregiveninemfseries.FromthevaluesE
0
celliscalculated
usingformula
E
0
cellorstandardemfofacell=E
0
oxi(cathode)-E
0
red(anode)

23. Calculation of standard EMF of the cell
EMF= E
oxi+ E
Red
Zn & Cu Couple
EMF= E
oxi+ E
Red
= E
Zn+ E
Cu
= 0.76+ + 0.34
= 1.1V
Fe & H
2
EMF= E
oxi+ E
Red
EMF= E
Fe+ E
H2
 = 0.441+ 0
0.441V

24. Ni & Hg Couple
Ni –Anode
Hg -Cathode
EMF= E
oxi+ E
Red
 = E
Ni+ E
Hg
=0.236 + 0.61= 0.846V
EMF = Standard reduction potential of R.H.S electrode-Standard reduction potential of L.H.S
electrode
E
0
= E
0
RHS -E
0
LHS
 = E
0
Hg-E
0
Ni
= 0.61 –(-0.236)
= 0.61+0.236 = 0.846V
= E
Ni+ E H
g
= 0.236+0.61
= 0.846V

25. Cr & Sn Couple
Cr –Anode
Sn -Cathode
EMF= E
oxi+ E
Red
EMF= E
Cr+ E
Sn
= -0.74+(-0.14)
= 0.60V
EMF = Standard reduction potential of R.H.S electrode-
Standard reduction potential of L.H.S electrode
E
0
= E
0
RHS -E
0
LHS
= E
0
Sn-E
0
Cr
= –0.14 -(-0.74)
= -0.14+0.74 = 0.60V

26. For predicting spontaneity of the cell reaction
E
0
cell >0cell reaction is spontaneous
E
0
cell <0cell reaction is non-spontaneous
E
0
cell =0cell reaction is in equilibrium
Fordeterminationofequilibriumconstantforareaction
Weknowthat
-∆G
0
=RTlnK
=2.303RTlogK
logK=
logK= (-∆G
0
=nFE
0
)
Thus,fromthevalueofE
0
foracellreaction,itsequilibriumconstantcanbecalculated.

27. REFERENCES:
1.PalanisamyP.N.,ManikandanP.,GeethaA.&ManjulaRaniK,“Applied
Chemistry”,6thEdition,TataMcGrawHillEducationPrivateLimited,New
Delhi,2019.
2.PayaPayalB.Joshi,ShashankDeep.,“EngineeringChemistry”,Oxford
UniversityPress,NewDelhi,2019.
3.PalannaO.,“EngineeringChemistry”,McGrawHillEducation,New
Delhi,2017.
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YOU
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