Experiment 9: Determination of Electrode Potentials C. Grefaldia1, C. Hernia2 1
College of Home Economics, University of the Philippines, Diliman, Quezon City 1101 ational !nstitute of "eological #ciences, College of #cience, University of the Philippines, Diliman, Quezon City 1101
2
Performed 30 Jne 201!" #$mitted % Jl& 201!
Answers to Questions Questions 1. Disc Discus usss the the diff differ erenc ences es betw betwee een n galv galvan anic ic and and electrolytic cells. 'ne main difference $et(een t)e t(o cells is t)at *al+anic cell cell con+ con+er erts ts c)em c)emic ical al ener ener*& *& into into elec electr tric ical al ener ener*& *& ()ereas an electrol&tic cell con+erts electrical ener*& into c)emic c)emical al ener*& ener*&.. )e redox redox reacti reaction on in *al+ *al+ani anicc cell cell is spont spontan aneo eos s ()er ()erea eass in elec electr trol ol&t &tic ic cell cell,, it is nonnonsponta spontaneo neos s t)eref t)erefore ore an electr electrica icall ener*& ener*& )as to $e spplied in order to start a reaction. Gal+anic cell ses t(o different containers, one for eac) )alf-cell and a salt $rid*e contai containin nin* * ions ions connec connectt $ot) $ot) contai container ners. s. 'n t)e ot)er ot)er )and, in an electrol&tic cell, $ot) electrodes are placed in t)e t)e same same cont contai aine nerr cont contai aini nin* n* t)e t)e sol solti tion on of t)e t)e elec electr trol ol&t &te. e. n *al+ *al+an anic ic cell cell,, t)e t)e anod anode e is ne*a ne*ati ti+e +e /oxidation and cat)ode /redction is positi+e ()ile in electrol electrol&tic &tic cell, its t)e ot)er (a& arond. ecase t)e reaction in t)e *al+anic cell is spontaneos, electrons are sppli spplied ed $& t)e specie speciess *ettin *ettin* * oxidi oxidied ed ()ere ()ereas as in an electrol&tic cell, t)e $atter& spplies t)e electrons, enterin*
re+erse reaction is *oin* to ta;e place. )e )i*)er t)e +ale of Ecell, t)e more it (ill nder*o redction. 425 !. "om#are the calculated E their literature value.
red
for each half-cell with
)e experimental +ales of Ecell (ere different from t)eir literatre +ale, mostl& lo(er t)an t)eir $oo; +ales. )e calclated percent error ran*es from 12.<-%%.7=. $. %ase %ased d on your your obse observ rvat atio ions ns&& what what are are the the half half reactions which occur at the anode and cathode during the electrolysis )e )e )alf )alf reac eaction tionss t)at t)at occ occr in elec electr trol ol&s &sis is are are respecti+el&: i. node 1. 2 2 > 2e ()erein is eit)er romine or odine, in t)e experiment ii. Cat)ode 1. 2e - > 2H2' 2'H- > H2 as no ot)er species are present. 435 '. %ased on the calculated values& what are the most effective effective reducing and and o(idi)ing agents ?sin* t)e t)eoretical +ales, t)e increasin* trend of t)e oxidiin* propert& is r2@2@Ae3>@Bn2>, romine $ein* t)e most most effect effecti+ i+e e oxidi oxidiin* in* a*ent a*ent ()erea ()ereass Bn is t)e most most effecti+e effecti+e redcin* a*ent. n t)e experimental +ales, Binc is still still t)e most redci redcin*, n*, $t $et(e $et(een en t)e ot)er 3 ions, ions, Ae3>@r2@2 is t)e t)e incr increa easi sin* n* tren trend d of t)ei t)eirr oxid oxidi iin in* * propert& ()ic) is +er& different from t)e t)eoretical trend. )is indicates error in t)e experiment. *. Are values of standard standard reduction reduction #otentials #otentials hel#ful in determining the s#ontaneity of a reaction
t)ro*) t)e cat)ode and comin* ot t)ro*) t)e anode. 415 Figure 1. Electrol&tic cell +s. Gal+anic cell Dia*ram 2. What is the measured value of E cell for each cell and its its rele releva vanc nce e to the the E red for the iron iron half-c half-cell ell and and halogen half-cells )e measred +ale of E cell is t)e total +olta*e of t)e ()ole cell ()ic) is made p of t(o )alf-cells, ()ic) $ot) )a+e differ different ent electr electric ic potent potential ial.. )e condit conditions ions drin* drin* t)e experiment (ere standard /16 soltions, 1 atm, 27 C so it is safe to assme t)at E cell is e8al to t)e E cell so t)e e8ation is: Ecell Eredcat)ode - Eredanode 425 )e measred +ale for Ecell indicates t)at for copper and inc reaction, t)e reaction is spontaneos ()en t)e copper is t)e cat)ode and inc is t)e anode. n t)e copper and iron reacti reaction, on, iron iron nder* nder*oes oes redct redction ion and *ains *ains electr electron on ()ereas copper nder*oes oxidation and loses electrons. posit positi+ i+e e Ecel Ecelll mean meanss t)at t)at t)e t)e reac reacti tion on (ill (ill proc procee eed d spontaneosl& ()ereas a ne*ati+e +ale means t)at t)e
)e spontaneit spontaneit& & of a react reaction ion is *i+en *i+en $& t)e ne*ati ne*ati+e +e c)an*e in Gi$$s Aree ener*&, t)e E cell s)old $e positi+e. ecase ecase t)e standard redction potential potential are elements of t)e Ecell comptation, i.e. E cell E cat)ode- Eanode, so t)e +ales of t)e standard redction potential determine ()et)er a reaction is spontaneos or not. +. What are the #ossible sources of errors and their effect on the calculated #arameters ,ationali)e. Errors drin* t)e soltion preparation ma& $e on e possi$le error ()ic) affects t)e data collected. not)er one is t)e conditions conditions in ()ic) ()ic) t)e experiment experiment (as done, it (as onl& assmed t)at t)e standard conditions of 16, 1 atm and 27 de*ree de*ree Celsi Celsis, s, so t)ese t)ese facto factors rs can contri contri$t $te e to t)e errors. Errors in t)e mltimeter can reslt in error in t)e calclated parameters. ,eferences 415 #;oo*,, Andamentals of nal&tical C)emistr&,
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