lec. 3 (electrochemistry) prep. (3).pdf

Electrochemistry

1. Chemistry (1)
CHE 011
Preparatory year
Lecture (2)
Electrochemistry
Ass. Prof./ HebaAbdEl Gawad
The Higher Institute of Engineering
Chemical Engineering Department
2022-2023

2. Definition of electrochemistry:
Itisdefinedasthebranchofchemistrywhichdealswithinterconversionofchemical
energytoelectricalenergyandviceversa.
Forexample:
I.Inbatteryandfullcell,chemicalenergyisconvertedtoelectricalenergy.
Spontaneous
I.Inelectroplating/electrolysisandbatterycharging,electricalenergyisconvertedto
chemicalenergy. Non-Spontaneous

3. Electrochemical cell:
Itisasystemordevicefortheconversionofelectricalenergyintochemicalenergyor
vice-versa.
Inwhichtwoelectrodesarefittedinthesameelectrolyteorintwodifferent
electrolytes,whicharejoinedbyasaltbridge.
Typesofelectrochemicalcell:
1.Galvaniccellorvoltaiccell
Thedeviceusedtoconvertthechemicalenergyproducedonaredoxreactioninto
electricalenergyintheformofanelectricalcurrent.
Chemical energy Electrical energy
Spontaneous

4. 2.Electrolyticcell
Itisadeviceinwhichelectrolysis(chemicalreactioninvolvingoxidationand
reduction)iscarriedoutbyusingelectricityorinwhichtheconversionofelectrical
energyintochemicalenergy.
Electrical energy Chemical energy
Non-Spontaneous

5. Redoxreaction:
Itisachemicalreactioninwhichelectronsaretransferredfromonespecietoanother.
Aspecieslosingelectronsissaidtobeoxidized;onegainelectronsissaidtobe
reduced.Thetwoprocessestogetherarecalledredox(ie,oxidationandreduction
reactionscanoccur).
Oxidation Isthelossofelectrons
(Increaseinoxidationnumber) Redoxreaction
Reduction Isthegainofelectrons
(Decreaseinoxidationnumber)

6. Oxidation reaction
Example:
Reduction reaction
Redox reaction
Oxidizing agent
Reducing agent

7. Galvanic cell
AtypicalexampleofagalvaniccellisthatofaDaniellcell.
DaniellcellisagalvaniccellinwhichZinc(Zn)andCopper(Cu)areused
fortheredoxreactiontotakeplace.
Itisformedbycombinationoftwohalfcells.Oneisoxidationhalfcellor
anodichalfcellandotheroneisreductionhalfcellorcathodichalfcell.

8. TheoxidationhalfcellcontainsazincmetalelectrodedippinginZnSO4
solutionandthereductionhalfcellconsistsofCumetalelectrodein
CuSO4solution.
Boththehalfcellsareconnectedexternallybyametallicwireandinternally
connectedbyasaltbridge.
AsaltbridgeisaUshapedtubecontainingasaltsolutionsuchasKCl,
KNO3,Na2SO4orNH4NO3thatpreventsthemechanicalmixingofthe
solutionjoinsthehalfcells.

10. Thefollowingreactionstakeplaceinthecell:
Atanode: Zn Zn2++2e-(Oxidationreaction)
Atcathode: Cu+2+2e- Cu(Reductionreaction)
Theoverallcellreaction:
Zn + Cu+2 Zn2+ + Cu(Redox reaction)
Presentationofgalvaniccell:
Zn Zn2+(C1) Cu+2(C2) Cu

11. Ingalvaniccell,twoelectrodesarerepresented;atoneelectrodeoxidationtakesplace
andatotherelectrodereductiontakesplace.
Thetendencyofanelectrode(M)togetoxidizedorreducedwhenitiscontactwithits
ownionic(Mn+)solution,knownaselectrodepotential.ItisrepresentedbyE.
Ifthemetalelectrodeissuspendedinaionicsolution1M(molar)concentrationor
unityactivityandtemperatureiskeptat25
o
Cunder1atmpressure,thepotential
developedbetweenelectrodeandionicsolutionisknownasstandardelectrode
potential.ItrepresentedbyE
o
.
Electrode potential

13. Potentialdifferencesetupbetweenmetalrodandsolutioncontainingthemetal
ions,anddependsuponmetalnatureandmetalionconcentration.
Theelectricalpotentialdifferencebetweenthecathodeandtheanodeisknownas:
1.Cellpotential:isknownasreductionpotential(E)
2.Electromotiveforce(EMF)

14. Electromotive force (EMF)
Theforcewhichcausestheflowofelectronsfromoneelectrodetoanother
electrodeandthusresultsintheflowofcurrentfromelectrodeathigher
potentialtoelectrodeatlowerpotentialiscalledelectromotiveforce.
Standard EMF or standard cell potential = (Standard reduction potential of R.H.S. electrode –
(Standard reduction potential of L.H.S. electrode)
Oxidation potential = -reduction potential

15. Electrochemical series
Series in which elements are arranged in increasing order of their standard electrode
potential is known as electrochemical series.
Thestandardelectrodepotentialforanygivenelementtobeoxidizedor
reducedinagalvaniccellcanbemeasuredbycouplingitwithareference
electrode.
Standardhydrogenelectrodeisthereferenceelectrode,.

17. Example:

19. Example:

20. Nernst equation
E
cell= E
o
-RT/nF Ln [K]
Where:
Eistheelectrodepotential
E
o
isthestandardelectrodepotential
Ristheuniversalgasconstant
nisthenumberofelectrons
Fisthefaraday(96,500coulombs)
Kisequilibriumconstant
K = concentration of product / concentration of reactant

21. mM
n+
nN
n+
[N
n+
]
n
P
K=
[M
n+
]
m
R
E
cell= E
o
-0.059/n Log ??????
Thisequationusedtomeasurethecellpotentialattemperature
notequal25oCandconcentrationofelectrolytenotequal1
molar.(nonstandardconditions).

22. Nernstequationcanbeusedtocalculate:
1.Half-cellpotential
2.Cellpotential

23. Example:

25. Spontaneity of cell reaction
IfE
cell=+veand∆G<zero(-ve) reactionisspontaneous
IfE
cell=-veand∆G>zero(+ve) reactionisnon-spontaneous
IfE
cell=zeroand∆G=zero noreaction(reachtoequilibrium)
E
o
= 0.059/n Log ??????
Where:∆G=Freeenergy