By: Maggie DangBy: Maggie Dang

Electrochemical Cells

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Background InformationBackground Information A chemical reaction that involves the transfer of electrons from one A chemical reaction that involves the transfer of electrons from one

substance to another is an substance to another is an oxidation-reduction (redox) reactionoxidation-reduction (redox) reaction.. Experimentally, when copper wire is placed into a silver ion solution Experimentally, when copper wire is placed into a silver ion solution

copper atoms spontaneously lose electrons (copper atoms are oxidized) copper atoms spontaneously lose electrons (copper atoms are oxidized) to silver ions ( which are reduced). Silver ions migrate to the copper to silver ions ( which are reduced). Silver ions migrate to the copper atoms to pick up electrons and form silver atoms at the copper atoms to pick up electrons and form silver atoms at the copper metal/solution interface; the copper ions that form then move into the metal/solution interface; the copper ions that form then move into the solution away from the interface. The overall reaction that occurs at the solution away from the interface. The overall reaction that occurs at the interface is interface is

Cu (s) + AgCu (s) + Ag++ (aq) -> 2Ag(s) + Cu (aq) -> 2Ag(s) + Cu 2+2+ (aq) (aq) This redox reaction can be divided into an oxidation and a reduction This redox reaction can be divided into an oxidation and a reduction

half-reaction. Each half-reaction, called a redox couple, consists of the half-reaction. Each half-reaction, called a redox couple, consists of the reduced state and the oxidized state of the substance.reduced state and the oxidized state of the substance.

Cu(s)Cu(s) Cu Cu2+2+ (aq) + 2e (aq) + 2e-- (Oxidation half-reaction) (Oxidation half-reaction) 2Ag2Ag++ (aq) + 2e (aq) + 2e-- 2Ag (s) (Reduction half-reaction)2Ag (s) (Reduction half-reaction)

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A voltaic cell A voltaic cell (galvanic cell) is designed to take advantage of this (galvanic cell) is designed to take advantage of this spontaneous transfer of electrons. A voltaic cell separates the spontaneous transfer of electrons. A voltaic cell separates the copper metal from the silver ions and forces the electrons to pass copper metal from the silver ions and forces the electrons to pass externally through a wire, an external circuit.externally through a wire, an external circuit.

The two redox couples are placed in separate compartments, called The two redox couples are placed in separate compartments, called half-cells. Each half-cell consists of an electrode, usually the metal half-cells. Each half-cell consists of an electrode, usually the metal (reduced state) of the redox couple, and a solution containing the (reduced state) of the redox couple, and a solution containing the corresponding cation (oxidized state) of the redox couple. The corresponding cation (oxidized state) of the redox couple. The electrodes of the half-cells are connected by a wire; this is where electrodes of the half-cells are connected by a wire; this is where the electrons flow, providing current for the external circuit.the electrons flow, providing current for the external circuit.

A salt bridgeA salt bridge, which connects the two half-cells, completes the , which connects the two half-cells, completes the construction of the voltaic cell (and the circuit). The salt bridge construction of the voltaic cell (and the circuit). The salt bridge permits limited movement of ions from one half-cell to the other, the permits limited movement of ions from one half-cell to the other, the internal circuit, so that when the cell operates, electrical neutrality is internal circuit, so that when the cell operates, electrical neutrality is maintained in each half-cell.maintained in each half-cell.

The electrode at which reduction occurs is called the The electrode at which reduction occurs is called the cathodecathode; the electrode at which oxidation occurs is ; the electrode at which oxidation occurs is called the called the anodeanode. .

Because oxidation releases electrons to the electrode to Because oxidation releases electrons to the electrode to provide a current in the external circuit, the anode is provide a current in the external circuit, the anode is designated the negative electrode in a galvanic cell. The designated the negative electrode in a galvanic cell. The reduction process draws electrons from the circuit and reduction process draws electrons from the circuit and supplies them to the ions in solution; the cathode is the supplies them to the ions in solution; the cathode is the positive electrode. positive electrode.

The cell potential of a galvanic cell is due to the The cell potential of a galvanic cell is due to the difference in tendencies of the two metals to oxidize or of difference in tendencies of the two metals to oxidize or of their ions to reduce.their ions to reduce.

A A measured reduction potentialmeasured reduction potential, the tendency , the tendency for a substance to gain electrons, is the value for a substance to gain electrons, is the value used to identify the relative ease of reduction for used to identify the relative ease of reduction for a half-reaction.a half-reaction.

A A voltmetervoltmeter, placed in the external circuit , placed in the external circuit between the two electrodes, measures the cell between the two electrodes, measures the cell potential, Epotential, Eoocell, a value that represents the cell, a value that represents the difference between the tendencies of the metal difference between the tendencies of the metal ions in their respective half-cells to undergo ions in their respective half-cells to undergo reduction.reduction.

PurposePurpose

To determine how closely the voltages found in To determine how closely the voltages found in the experiment compare to the Ethe experiment compare to the Ecellcell

oo voltages voltages

calculated using the standard reduction potential calculated using the standard reduction potential chartchart

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MaterialsMaterials Copper(II) nitrate Cu(NOCopper(II) nitrate Cu(NO33))22 1.0M 1.0M Iron(III) nitrate Fe(NOIron(III) nitrate Fe(NO33))33, 1.0M, 1.0M Lead(II) nitrate Pb(NOLead(II) nitrate Pb(NO33))22 1.0M 1.0M Magnesium nitrate Mg (NOMagnesium nitrate Mg (NO33))221.0M1.0M Silver nitrate AgNOSilver nitrate AgNO33 1.0M 1.0M Zinc nitrate Zn(NOZinc nitrate Zn(NO33))22 1.0M 1.0M Sodium chloride NaCl 1.0MSodium chloride NaCl 1.0M 2 beakers (50 mL )2 beakers (50 mL ) VoltmeterVoltmeter 2 Alligator Clips2 Alligator Clips A salt bridge of 1.0M KNO3A salt bridge of 1.0M KNO3 Strips of Zn, Cu, and MgStrips of Zn, Cu, and Mg Iron nailsIron nails Lead Piping Lead Piping Strips of filter paperStrips of filter paper

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Setup Setup

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ProceduresProcedures

Prepare a test cell to measure the voltage of the Prepare a test cell to measure the voltage of the copper and zinc half-cells.copper and zinc half-cells.

Put approximately 2 mL of 1.0 M Cu(NOPut approximately 2 mL of 1.0 M Cu(NO33))22 in in one beaker and 2 mL of 1.0 M Zn(NOone beaker and 2 mL of 1.0 M Zn(NO33))2 2 in the in the other beaker.other beaker.

Polish small strips of zinc and copper metal, and Polish small strips of zinc and copper metal, and place the metal in the appropriate beaker place the metal in the appropriate beaker containing the solution of the ions of that metal. containing the solution of the ions of that metal.

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Copper

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ZINC

Take a small strip of filter paper that has been Take a small strip of filter paper that has been soaked in KNO3 solution, and drape it across soaked in KNO3 solution, and drape it across the wells so that one end dips in the solution in the wells so that one end dips in the solution in each well. This will act as the salt bridge.Use a each well. This will act as the salt bridge.Use a voltmeter to measure the voltages between the voltmeter to measure the voltages between the two half-cells.two half-cells.

Connect the meter so the voltage reading is Connect the meter so the voltage reading is positive. When the voltmeter reads a positive positive. When the voltmeter reads a positive voltage, the electrode connected to the positive voltage, the electrode connected to the positive terminal is the cathode and is undergoing terminal is the cathode and is undergoing reduction, while oxidation is occurring at the reduction, while oxidation is occurring at the electrode connected to the negative terminal, electrode connected to the negative terminal, the anode.the anode.

Record data.Record data.

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KNO3

Rinse out the beakers and measure the voltages Rinse out the beakers and measure the voltages of the other electrodes the same way with zinc of the other electrodes the same way with zinc since it is designated to be the standard since it is designated to be the standard electrode. electrode.

The electrodes to be tested are : Ag, AgThe electrodes to be tested are : Ag, Ag++; Cu, ; Cu, CuCu++; Fe, Fe; Fe, Fe3+3+; Mg,Mg; Mg,Mg2+2+; Pb, Pb; Pb, Pb2+2+ ; Zn, Zn ; Zn, Zn2+2+

Record data.Record data.

Voltmeter

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Voltage of each half-cell versus the Voltage of each half-cell versus the zinc electrodezinc electrode

VoltageVoltage AnodeAnode CathodeCathode

Zn versus AgZn versus Ag 1.401.40 ZnZn AgAg

Zn versus CuZn versus Cu .99.99 ZnZn CuCu

Zn versus FeZn versus Fe .55.55 ZnZn FeFe

Zn versus MgZn versus Mg .60.60 MgMg ZnZn

Zn versus PbZn versus Pb .48.48 ZnZn PbPb

Write reduction equations for each metal Write reduction equations for each metal ion. ion.

Record the standard potentials for all of Record the standard potentials for all of the electrodes using the standard the electrodes using the standard reduction potentials chart, and calculate reduction potentials chart, and calculate the potential energy of the entire cell by the potential energy of the entire cell by using the equation using the equation EEoocell=Ecell=Eoo

redred (cathode) – E (cathode) – Eooredred (anode) (anode)

CalculationsCalculations Standard Electrode Standard Electrode Zinc with Zinc with EEoo= -.76= -.76 Ag+ + e- Ag+ + e- Ag E Ag Eoo=.80=.80

.80 - .80 - --.76.76=1.56=1.56 Cu2+2e-Cu2+2e-Cu ECu Eoo=.34=.34

.34 - .34 - --.76.76=1.1=1.1 Fe+3 +3- Fe+3 +3- Fe EFe Eoo= -.44= -.44

-.44 - -.44 - --.76.76= .32= .32 Mg2+ 2e- Mg2+ 2e- Mg E Mg Eoo= -.76= -.76

-.76 - -.76 - --2.37 2.37 =1.61=1.61 Pb2+ 2e- Pb2+ 2e- Pb EPb Eoo= -.13= -.13

-.13 - -.13 - --.76.76=.63=.63 Zn2+ 2e- Zn2+ 2e- Zn EZn Eoo= -.76= -.76

-.76 - -.76 - --.76 .76 = 0= 0

Reduction Equations for Each Ion Arranged in Decreasing Reduction Equations for Each Ion Arranged in Decreasing Order of PotentialOrder of Potential

Reduction Reduction ReactionReaction

Voltages Voltages using Zinc using Zinc as the as the Standard, Standard, EEZnZn

Eored Voltages Using Standard Reduction Potential ChartEored Voltages Using Standard Reduction Potential Chart

AgAg++ + e + e-- AgAg

1.401.40 1.561.56

CuCu2+ 2+ + 2e+ 2e--

CuCu.99.99 1.101.10

FeFe3+3+ + 3e + 3e-- FeFe

.55.55 .32.32

MgMg2+2+ + 2e + 2e-- Mg Mg

.60.60 1.611.61

PbPb2+2+ + 2e + 2e-- PbPb

.48.48 .63.63

ZnZn2+ 2+ ++ 2e2e- -

ZnZn0.000.00 0.000.00

ConclusionConclusion

Comparing toComparing to thethe EEooredred voltages calculated from voltages calculated from

using the standard reduction potential chart,using the standard reduction potential chart, the the voltages measured using Zinc as the standard, voltages measured using Zinc as the standard, EEZnZn, in the experiment are relatively close. , in the experiment are relatively close.

Therefore, this is an accurate experiment that Therefore, this is an accurate experiment that can be used to measure potential energies can be used to measure potential energies among different types of electrodes.among different types of electrodes.

SourcesSources

Vonderbrink, Sally Ann. Laboratory Vonderbrink, Sally Ann. Laboratory Experiments for Advanced Placement Experiments for Advanced Placement Chemistry Student Edition. Flinn Scientific, Chemistry Student Edition. Flinn Scientific, Inc. Batavia, IL. 1995Inc. Batavia, IL. 1995

Beran, J.A. Laboratory Manual for Beran, J.A. Laboratory Manual for Principles of General Chemistry. John Principles of General Chemistry. John Wiley & Sons, Inc. Hoboken, NJ. 2004.Wiley & Sons, Inc. Hoboken, NJ. 2004.