Electrochemical CellsElectrochemical Cells

By, Marvin Lu and Riley By, Marvin Lu and Riley CorrCorr

PurposePurpose

To determine the effects of To determine the effects of changing the concentrations of changing the concentrations of solutions and the identities of the solutions and the identities of the solutions themselves on the solutions themselves on the voltage in an electrochemical cell.voltage in an electrochemical cell.

Oxidation: Mg(s)---> MgOxidation: Mg(s)---> Mg2+2+(aq)+2e-(aq)+2e-Reduction: CuReduction: Cu2+2+(aq)+2e- --->Cu(s)(aq)+2e- --->Cu(s)----------------------------------------------------------------------------------CuCu2+2+(aq)+Mg(s)--->Mg(aq)+Mg(s)--->Mg2+2+(aq)+Cu(s)(aq)+Cu(s)

Electrons are transferred from Mg to Cu2+, but there is no useful electric current.

Electrons are transferred from Mg to Cu2+, but there is no useful electric current.

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENTCHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

Mg metal

Cu2+ ions

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENTCHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT

To obtain a useful To obtain a useful current, we current, we separated the separated the oxidizing and oxidizing and reducing agents so reducing agents so that electron that electron transfer occurred transfer occurred thru a volt meter.thru a volt meter.

This is accomplished in a This is accomplished in a GALVANICGALVANIC or or VOLTAICVOLTAIC cell. cell.

A group of such cells is called a A group of such cells is called a batterybattery..

Mg

Mg2+ ions

Cu

Cu2+ ions

saltbridge

l

voltmeter

••Electrons travel thru the voltmeterElectrons travel thru the voltmeter..Salt bridge Salt bridge allows anions and allows anions and cations to move between electrode cations to move between electrode compartments.compartments.

••Electrons travel thru the voltmeterElectrons travel thru the voltmeter..Salt bridge Salt bridge allows anions and allows anions and cations to move between electrode cations to move between electrode compartments.compartments.

Mg --> Mg2Mg --> Mg2++ + 2e- + 2e- CuCu2+2+ + 2e- --> Cu + 2e- --> Cu

<--Anions<--AnionsCations-->Cations-->

OxidationOxidationAnodeAnodeNegativeNegative

OxidationOxidationAnodeAnodeNegativeNegative

ReductionReductionCathodeCathodePositivePositive

ReductionReductionCathodeCathodePositivePositiveMg

Mg2+ ions

Cu

Cu2+ ions

wire

saltbridge

ProcedureProcedure

Obtain GogglesObtain Goggles Obtain a 3”x5” piece of copper sheet and a Obtain a 3”x5” piece of copper sheet and a

3” long magnesium ribbon3” long magnesium ribbon Obtain .5M, .1M, and .01M Mg(NOObtain .5M, .1M, and .01M Mg(NO33))2 2 solutionssolutions Obtain .5M, .1M, and .01M Cu(NOObtain .5M, .1M, and .01M Cu(NO33))22 solutions solutions Obtain a .1M CuSOObtain a .1M CuSO44 solution solution Obtain filter paper and .1M KNOObtain filter paper and .1M KNO33 solution for solution for

salt bridge. To make salt bridge, roll filter salt bridge. To make salt bridge, roll filter paper and soak in solution.paper and soak in solution.

Obtain a voltmeter, set it to 40V.Obtain a voltmeter, set it to 40V. Obtain two 50mL beakers, and two 50mL Obtain two 50mL beakers, and two 50mL

graduated cylinders.graduated cylinders.

Trial:Trial: 11 22 33 44 55

[Mg(NO[Mg(NO33))22

] (mol/L)] (mol/L).1.1 .01.01 .5.5 .1.1 .1.1

[Cu(NO[Cu(NO33))22

] (mol/L)] (mol/L).1.1 .1.1 .1.1 .01.01 .5.5

Trial:Trial: 66

[Mg(NO[Mg(NO33))22

] (mol/L)] (mol/L).1.1

[CuSO[CuSO44] ] (mol/L)(mol/L)

.1.1

• Set up electrochemical cells as follows:Set up electrochemical cells as follows: - Measure out 30mL of each solution to be used, pour - Measure out 30mL of each solution to be used, pour into into separate beakers.separate beakers. - Make a salt bridge and place an end into each - Make a salt bridge and place an end into each beaker.beaker. - Place the copper into the solution containing the - Place the copper into the solution containing the copper ion.copper ion. - Place the magnesium into the solution containing - Place the magnesium into the solution containing the the

magnesium ion.magnesium ion.- Connect the voltmeter to the electrodes, the red wire - Connect the voltmeter to the electrodes, the red wire should should attach to the Copper, the black to the Magnesium.attach to the Copper, the black to the Magnesium.

• Repeat the procedure for all combinations of solutions Repeat the procedure for all combinations of solutions below.below.

Procedure Procedure (Continued)(Continued)

AnodeAnode CathodeCathode

Electrochemical CellElectrochemical Cell

Mg

Mg2+ ions

Cu

Cu 2+ ions

Voltmeter

Salt Bridge KNO3

ObservationsObservations

Copper solutions had blue color, the higher the Copper solutions had blue color, the higher the concentration, the darker the color.concentration, the darker the color.

Magnesium ribbon was coated in a white oxide, it Magnesium ribbon was coated in a white oxide, it was scraped off with a scoopula before use.was scraped off with a scoopula before use.

Magnesium solution was clear.Magnesium solution was clear. Solutions were prepared from solid salts: Cu(NOSolutions were prepared from solid salts: Cu(NO33))2 2

•• 3H3H22OO, , Mg(NOMg(NO33))2 2 •• 6H6H22O, KNOO, KNO33

The CuSOThe CuSO44 solution was prepared from a .4M solution was prepared from a .4M solution of unknown quality.solution of unknown quality.

BK Precision VoltmeterBK Precision Voltmeter Voltage readings required some time to stabilize.Voltage readings required some time to stabilize.

DataData

Trial:Trial: 11 22 33 44 55 66

VoltagVoltage (V)e (V)

1.471.47 1.761.76 1.551.55 1.661.66 1.671.67 1.541.54

DiscussionDiscussion

In this lab, Mg(NOIn this lab, Mg(NO33))2 2 and Cu(NOand Cu(NO33))2 2 were were used as solutions in a voltaic cell. used as solutions in a voltaic cell. Concentrations of both were varied Concentrations of both were varied and the differences in voltage were and the differences in voltage were negligible.negligible.

In trial 6, the source of CuIn trial 6, the source of Cu2+2+ ions was ions was changed from changed from Cu(NOCu(NO33))22 to to CuSOCuSO44. . Once again, there was a negligible Once again, there was a negligible change in voltage.change in voltage.

Trial:Trial: 11 22 33 44 55

[Mg(NO[Mg(NO33))22] ] (mol/L)(mol/L)

.1.1 .01.01 .5.5 .1.1 .1.1

[Cu(NO[Cu(NO33))22] ] (mol/L)(mol/L)

.1.1 .1.1 .1.1 .01.01 .5.5

Voltage (V)Voltage (V) 1.471.47 1.761.76 1.551.55 1.661.66 1.671.67

Trial:Trial: 66

[Mg(NO[Mg(NO33))22] (mol/L)] (mol/L) .1.1

[CuSO[CuSO44] (mol/L)] (mol/L) .1.1

Voltage (V)Voltage (V) 1.541.54

ErrorError

The CuSOThe CuSO44 solution was prepared from a solution was prepared from a .4M solution of unknown quality..4M solution of unknown quality.

The Cu(NOThe Cu(NO33))2 2 solution was a hydrate but solution was a hydrate but the exact chemical formula was the exact chemical formula was unknown. It was assumed to be unknown. It was assumed to be Cu(NOCu(NO33))2 2 •• 3H3H22O based on the Flinn O based on the Flinn Scientific chemical and biological Scientific chemical and biological catalog reference manual 2001catalog reference manual 2001

Not all of the impurities of the Not all of the impurities of the magnesium ribbon were removed.magnesium ribbon were removed.

BibliographyBibliographyhttp://bkprecision.com/www/http://bkprecision.com/www/np_searchmodel7.asp?lf=Dual+Display+RSnp_searchmodel7.asp?lf=Dual+Display+RS%2D232+Interface+Multimeters%2D232+Interface+Multimeters

http://www.brookscole.com/cgi-Brookscole/http://www.brookscole.com/cgi-Brookscole/course_products_bc.pl?course_products_bc.pl?fid=M63&product_isbn_issn=003033604X&chaptfid=M63&product_isbn_issn=003033604X&chapter_number=20&resource_id=21&altname=Powerer_number=20&resource_id=21&altname=PowerPointsPoints

www.ivygreen.ctc.edu/knutsen/chem160/www.ivygreen.ctc.edu/knutsen/chem160/vltaicll.html vltaicll.html