1 AnodeCathode Basic Concepts of Electrochemical Cells

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AnodeAnode CathodeCathode

Basic Concepts Basic Concepts of Electrochemical Cellsof Electrochemical Cells

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

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


Zn metal

Cu2+ ions

Zn metal

Cu2+ ions

With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”

With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”

•Zn is oxidized Zn is oxidized and is the reducing agent and is the reducing agent Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-•CuCu2+2+ is reduced is reduced and is the oxidizing agentand is the oxidizing agentCuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)

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Oxidation: Zn(s) ---> ZnOxidation: Zn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-Reduction: CuReduction: Cu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)----------------------------------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

Zn metal

Cu2+ ions

Zn metal

Cu2+ ions

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

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



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•To obtain a useful To obtain a useful current, we separate the current, we separate the oxidizing and reducing oxidizing and reducing agents so that electron agents so that electron transfer occurs thru an transfer occurs thru an external wire. external wire.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons



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..

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Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

••Electrons travel thru external wire.Electrons travel thru external wire.•Salt bridge Salt bridge allows anions and cations to move allows anions and cations to move between electrode compartments.between electrode compartments.

••Electrons travel thru external wire.Electrons travel thru external wire.•Salt bridge Salt bridge allows anions and cations to move allows anions and cations to move between electrode compartments.between electrode compartments.

Zn --> ZnZn --> Zn2+2+ + 2e- + 2e- CuCu2+2+ + 2e- --> Cu + 2e- --> Cu

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

OxidationOxidationAnodeAnodeNegativeNegative

OxidationOxidationAnodeAnodeNegativeNegative

ReductionReductionCathodeCathodePositivePositive

ReductionReductionCathodeCathodePositivePositive

66The Cu|CuThe Cu|Cu2+2+ and Ag|Ag and Ag|Ag++ CellCell

Electrons Electrons move from move from anode to anode to cathode in cathode in the wire.the wire.Anions & Anions & cations move cations move thru the salt thru the salt bridge.bridge.

Electrons Electrons move from move from anode to anode to cathode in cathode in the wire.the wire.Anions & Anions & cations move cations move thru the salt thru the salt bridge.bridge.

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Anode, site Anode, site of oxidation,of oxidation,negativenegative

Cathode, site Cathode, site of reduction, of reduction, positivepositive

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

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CELL POTENTIAL, ECELL POTENTIAL, E

• Electrons are “driven” from anode to cathode by Electrons are “driven” from anode to cathode by an electromotive force or an electromotive force or emfemf..

• For Zn/Cu cell, this is indicated by a voltage of For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn1.10 V at 25 ˚C and when [Zn2+2+] and [Cu] and [Cu2+2+] = 1.0 M.] = 1.0 M.

Zn and ZnZn and Zn2+2+,,anodeanode

Cu and CuCu and Cu2+2+,,cathodecathode

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons1.10 V1.10 V

1.0 M1.0 M 1.0 M1.0 M

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CELL POTENTIAL, CELL POTENTIAL, EE

• For Zn/Cu cell, For Zn/Cu cell, potentialpotential is is +1.10 V+1.10 V at 25 ˚C and when at 25 ˚C and when [Zn[Zn2+2+] and [Cu] and [Cu2+2+] = 1.0 M.] = 1.0 M.

• This is the This is the STANDARD CELL POTENTIAL, STANDARD CELL POTENTIAL, EEoo

• ——a quantitative measure of the tendency of reactants a quantitative measure of the tendency of reactants to proceed to products when all are in their standard to proceed to products when all are in their standard states at 25 ˚C. states at 25 ˚C.

• This means pure solids or in solution at a This means pure solids or in solution at a concentration of 1M!!!!concentration of 1M!!!!

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

1010

Calculating Cell VoltageCalculating Cell Voltage

• Balanced half-reactions can be added Balanced half-reactions can be added together to get overall, balanced together to get overall, balanced equation. equation.

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

If we know EIf we know Eoo for each half-reaction, we for each half-reaction, we could get Ecould get Eoo for net reaction. for net reaction.•Lets revisit my haiku! Lets revisit my haiku!

1111

Oxidation Haiku!Oxidation Haiku!

• Lost an electron

• But now feeling positive

• Oxidized is cool!

• What is that? You want a reduction Haiku?

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Reduction Haiku!!!

• Gained some electrons

• Gave me a negative mood!

• Now I can say Ger!

• Thank you… Enjoy the buffet… Don’t eat the chemicals or furniture kids!

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CELL POTENTIALS, ECELL POTENTIALS, Eoo

Can’t measure 1/2 reaction Eo directly. Therefore, measure it relative to a

STANDARD HYDROGEN CELL, SHE.STANDARD HYDROGEN CELL, SHE.

2 H2 H++(aq, 1 M) + 2e- <----> H(aq, 1 M) + 2e- <----> H22(g, 1 atm)(g, 1 atm)2 H2 H++(aq, 1 M) + 2e- <----> H(aq, 1 M) + 2e- <----> H22(g, 1 atm)(g, 1 atm)

EEoo = 0.0 V = 0.0 V

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Zn/Zn2+ half-cell hooked to a SHE.Eo for the cell = +0.76 V

Zn/Zn2+ half-cell hooked to a SHE.Eo for the cell = +0.76 V

Volts

ZnH2

Salt Bridge

Zn2+ H+

Zn Zn2+ + 2e- OXIDATION ANODE

2 H+ + 2e- H2REDUCTIONCATHODE

- +

Volts

ZnH2

Salt Bridge

Zn2+ H+



- +

Negative Negative electrodeelectrode

Supplier Supplier of of

electronselectrons

Acceptor Acceptor of of

electronselectrons

Positive Positive electrodeelectrode

2 H2 H++ + 2e- --> H + 2e- --> H22

ReductionReductionCathodeCathode

Zn --> ZnZn --> Zn2+2+ + 2e- + 2e- OxidationOxidation

AnodeAnode

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Reduction of HReduction of H++ by Zn by Zn

Figure 20.10Figure 20.10

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Volts

ZnH2

Salt Bridge

Zn2+ H+



- +

Volts

ZnH2

Salt Bridge

Zn2+ H+



- +

Overall reaction is reduction of HOverall reaction is reduction of H++ by Zn metal. by Zn metal.

Zn(s) + 2 HZn(s) + 2 H++ (aq) --> Zn (aq) --> Zn2+2+ + H + H22(g)(g) E Eoo = +0.76 V = +0.76 V

Therefore, Therefore, EEoo for for Zn ---> ZnZn ---> Zn2+2+ (aq) + 2e- (aq) + 2e- is is +0.76 V+0.76 V

Zn is a Zn is a (better) (poorer)(better) (poorer) reducing agent than H reducing agent than H22..

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Volts

CuH2

Salt Bridge

Cu2+ H+

Cu2+ + 2e- Cu REDUCTION CATHODE

H2 2 H+ + 2e-OXIDATION ANODE

-+

Volts

CuH2

Salt Bridge

Cu2+ H+



-+

Cu/CuCu/Cu2+2+ and H and H22/H/H++ Cell Cell

EEoo = +0.34 V = +0.34 V

Acceptor Acceptor of of

electronselectrons

Supplier Supplier of of

electronselectrons

CuCu2+2+ + 2e- --> Cu + 2e- --> CuReductionReductionCathodeCathode

HH22 --> 2 H --> 2 H++ + 2e- + 2e-

OxidationOxidationAnodeAnode

PositivePositive NegativeNegative

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Cu/CuCu/Cu2+2+ and H and H22/H/H++ Cell Cell

Overall reaction is reduction of CuOverall reaction is reduction of Cu2+2+ by H by H22 gas. gas.

CuCu2+2+ (aq) + H (aq) + H22(g) ---> Cu(s) + 2 H(g) ---> Cu(s) + 2 H++(aq)(aq)

Measured Measured EEoo = +0.34 V = +0.34 V

Therefore, Therefore, EEoo for Cu for Cu2+2+ + 2e- ---> Cu + 2e- ---> Cu is is

Volts

CuH2

Salt Bridge

Cu2+ H+



-+

Volts

CuH2

Salt Bridge

Cu2+ H+



-+

+0.34 V+0.34 V

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Zn/Cu Electrochemical CellZn/Cu Electrochemical Cell

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e- EEoo = +0.76 V = +0.76 VCuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s) EEoo = +0.34 V = +0.34 V------------------------------------------------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s) (aq) + Cu(s)

EEoo (calc’d) = +1.10 V (calc’d) = +1.10 V

Cathode, Cathode, positive, positive, sink for sink for electronselectrons

Anode, Anode, negative, negative, source of source of electronselectrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons ++

2020

Yes It is finally time for a DEMO!!

•Do you feel like bridging that salt?

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TABLE OF STANDARD TABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALS

TABLE OF STANDARD TABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALS

2

Eo (V)

Cu2+ + 2e- Cu +0.34

2 H+ + 2e- H 0.00

Zn 2+ + 2e- Zn -0.76

oxidizingability of ion

reducing abilityof element

2222

Potential Ladder for Reduction Half-ReactionsPotential Ladder for Reduction Half-ReactionsFigure 20.11Figure 20.11

2323Table 21.1 Page 970Table 21.1 Page 970Table 21.1 Page 970Table 21.1 Page 970

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Standard Redox Potentials, EStandard Redox Potentials, Eoo

Cu2+ + 2e- Cu +0.34

+2 H + 2e- H2 0.00

Zn2+ + 2e- Zn -0.76

Northwest-southeast rule:Northwest-southeast rule: product-favored product-favored reactions occur between reducing agent at reactions occur between reducing agent at

southeast corner (anode) and oxidizing agent southeast corner (anode) and oxidizing agent at northwest corner (cathode). at northwest corner (cathode).

Any substance on the right will reduce any Any substance on the right will reduce any substance higher than it on the left.substance higher than it on the left.

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Standard Redox Potentials, EStandard Redox Potentials, Eoo

Any substance on the right Any substance on the right will reduce any substance will reduce any substance higher than it on the left.higher than it on the left.

• Zn can reduce HZn can reduce H++ and and CuCu2+2+..

• HH22 can reduce Cu can reduce Cu2+2+ but but

not Znnot Zn2+2+

• Cu cannot reduce HCu cannot reduce H++ or or ZnZn2+2+..

Eo (V)

Cu2+ + 2e- Cu +0.34

2 H+ + 2e- H2 0.00

Zn2+ + 2e- Zn -0.76



Eo (V)

Cu2+ + 2e- Cu +0.34

2 H+ + 2e- H2 0.00

Zn2+ + 2e- Zn -0.76



2626

Using Standard Potentials, EUsing Standard Potentials, Eoo

Table 20.1Table 20.1

• In which direction do the following reactions In which direction do the following reactions

go?go?

• Cu(s) + 2 AgCu(s) + 2 Ag++(aq) ---> Cu(aq) ---> Cu2+2+(aq) + 2 Ag(s)(aq) + 2 Ag(s)

• 2 Fe2 Fe2+2+(aq) +(aq) + Sn2+(aq) ---> 2 Fe3+(aq) + Sn(s)

• What is Eonet for the overall reaction?

2727

Standard Redox Standard Redox Potentials, EPotentials, Eoo

E˚E˚netnet = “distance” from “top” half-reaction = “distance” from “top” half-reaction

(cathode)(cathode) to “bottom” half-reaction to “bottom” half-reaction (anode)(anode)

E˚E˚netnet = E˚ = E˚cathodecathode - E˚ - E˚anodeanode

EEoonetnet for Cu/Ag+ reaction = +0.46 V for Cu/Ag+ reaction = +0.46 V

2828

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Cd --> CdCd --> Cd2+2+ + 2e- + 2e-oror

CdCd2+2+ + 2e- --> Cd + 2e- --> Cd

Fe --> FeFe --> Fe2+2+ + 2e- + 2e-oror

FeFe2+2+ + 2e- --> Fe + 2e- --> Fe

EEoo for a Voltaic Cell for a Voltaic Cell

All ingredients are present. Which way does All ingredients are present. Which way does reaction proceed?reaction proceed?

2929

From the table, you see From the table, you see

•• Fe is a better reducing Fe is a better reducing agent than Cdagent than Cd

•• CdCd2+2+ is a better is a better oxidizing agent than oxidizing agent than FeFe2+2+

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

EEoo for a Voltaic Cell for a Voltaic Cell

Overall reactionOverall reactionFe + CdFe + Cd2+2+ ---> Cd + Fe ---> Cd + Fe2+2+

EEoo = E˚ = E˚cathodecathode - E˚ - E˚anodeanode

= (-0.40 V) - (-0.44 V) = (-0.40 V) - (-0.44 V) = +0.04 V= +0.04 V

3030More About More About Calculating Cell VoltageCalculating Cell Voltage

Assume IAssume I-- ion can reduce water. ion can reduce water.

2 H2O + 2e- ---> H2 + 2 OH- Cathode2 I- ---> I2 + 2e- Anode-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2

2 H2O + 2e- ---> H2 + 2 OH- Cathode2 I- ---> I2 + 2e- Anode-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2

Assuming reaction occurs as written, Assuming reaction occurs as written,

E˚E˚netnet = E˚ = E˚cathodecathode - E˚ - E˚anodeanode

= (-0.828 V) - (+0.535 V) = = (-0.828 V) - (+0.535 V) = -1.363 V-1.363 V

Minus E˚ means rxn. occurs in opposite Minus E˚ means rxn. occurs in opposite

directiondirection

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Documents

1 AnodeCathode Basic Concepts of Electrochemical Cells