1 © 2006 brooks/cole - thomson oxidation-reduction reactions indirect redox reaction a battery...

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© 2006 Brooks/Cole - Thomson

OXIDATION-REDUCTION REACTIONS

OXIDATION-REDUCTION REACTIONS

Indirect Redox ReactionIndirect Redox Reaction

A battery functions by transferring electrons A battery functions by transferring electrons through an external wire from the reducing through an external wire from the reducing

agent to the oxidizing agent.agent to the oxidizing agent.

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ElectrochemistryElectrochemistryAlessandro Volta, Alessandro Volta, 1745-1827, Italian 1745-1827, Italian scientist and scientist and inventor.inventor.

Luigi Galvani, 1737-1798, Luigi Galvani, 1737-1798, Italian scientist and Italian scientist and inventor.inventor.

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

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.

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


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.”

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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 through transfer occurs through an external wire. an external wire.



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

Fe --> FeFe --> Fe2+2+ + 2e- + 2e- CuCu2+2+ + 2e- --> Cu + 2e- --> Cu

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

OxidationOxidationAnodeAnodeNegativeNegative

OxidationOxidationAnodeAnodeNegativeNegative

ReductionReductionCathodeCathodePositivePositive

ReductionReductionCathodeCathodePositivePositive

FeFe

FeFe

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

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Electrochemical Electrochemical CellCell

Electrochemical Electrochemical CellCell

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

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Terms Used for Voltaic Terms Used for Voltaic CellsCells

Figure 20.6Figure 20.6

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CELL POTENTIAL, ECELL POTENTIAL, E

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

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

• Standard reduction potentials are measured at Standard reduction potentials are measured at standard conditions (1 M, 25standard conditions (1 M, 25ooC)C)

Zn and ZnZn and Zn2+2+,,anodeanode

Cu and CuCu and Cu2+2+,,cathodecathode

1.10 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 at 25 ˚C and when [Znand when [Zn2+2+] and [Cu] and [Cu2+2+] = 1.0 M.] = 1.0 M.

• This is the This is the STANDARD CELL STANDARD CELL POTENTIAL, EPOTENTIAL, Eoo

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

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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.

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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

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

Active Active Figure Figure 20.1320.13

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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 betterbetter reducing agent than H reducing agent than H22..

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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 (calc’’d) = +1.10 Vd) = +1.10 V

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

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

++

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Uses of EUses of Eoo Values ValuesUses of EUses of Eoo Values Values

Organize half-reactions Organize half-reactions by relative ability to by relative ability to act as act as oxidizing oxidizing agentsagents

• Use this to predict Use this to predict direction of redox direction of redox reactions and cell reactions and cell potentials.potentials.

CuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s) EEoo = +0.34 V = +0.34 VZnZn2+2+(aq) + 2e- ---> Zn(s) (aq) + 2e- ---> Zn(s) EEoo = –0.76 V = –0.76 V

Note that when a reaction is reversed the Note that when a reaction is reversed the sign of E˚ is reversed!sign of E˚ is reversed!

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Cu(s) | CuCu(s) | Cu2+2+(aq) || H(aq) || H++(aq) | (aq) | HH22(g)(g)

CuCu2+2+ + 2e- --> Cu + 2e- --> CuOrOr

Cu --> CuCu --> Cu2+2+ + 2 e- + 2 e-

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

oror2 H2 H++ + 2e- --> H + 2e- --> H22

CathodeCathodePositivePositive

AnodeAnodeNegativeNegativeElectronsElectrons

<----------<----------

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Cu(s) | CuCu(s) | Cu2+2+(aq) || H(aq) || H++(aq) | (aq) | HH22(g)(g)

CuCu2+2+ + 2e- --> Cu + 2e- --> Cu HH22 --> 2 H --> 2 H++ + 2 e- + 2 e-

CathodeCathodePositivePositive

AnodeAnodeNegativeNegativeElectronsElectrons

<----------<----------

The sign of the electrode in Table 20.1 is the The sign of the electrode in Table 20.1 is the polarity when hooked to the Hpolarity when hooked to the H++/H/H22 half-cell. half-cell.

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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? Calculate Ereaction proceed? Calculate Eoo for this cell. for this cell.

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E at Nonstandard E at Nonstandard ConditionsConditions

• The The NERNST EQUATIONNERNST EQUATION• E = potential under nonstandard conditionsE = potential under nonstandard conditions

• n = no. of electrons exchangedn = no. of electrons exchanged

• F = FaradayF = Faraday’’s constant s constant

• R = gas constantR = gas constant

• T = temp in KelvinsT = temp in Kelvins

• ln = ln = ““natural lognatural log””

• Q = reaction quotientQ = reaction quotient

QnF

RTcello

cell EE ln QnF

RTcello

cell EE ln

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Dry Cell BatteryDry Cell Battery

Anode (-)Anode (-)

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

Cathode (+)Cathode (+)

2 NH2 NH44++ + 2e- ---> + 2e- --->

2 2 NHNH33 + H + H22

Primary batteryPrimary battery — uses — uses redox reactions that cannot redox reactions that cannot be restored by recharge.be restored by recharge.

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Nearly same reactions as in common dry Nearly same reactions as in common dry cell, but under basic conditions.cell, but under basic conditions.

Alkaline BatteryAlkaline Battery

Anode (-): Anode (-): Zn + 2 OHZn + 2 OH-- ---> ZnO + H ---> ZnO + H22O + 2e-O + 2e-

Cathode (+): Cathode (+): 2 MnO2 MnO22 + H + H22O + 2e- ---> O + 2e- --->

MnMn22OO33 + 2 OH + 2 OH--

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Lead Storage BatteryLead Storage Battery

• Secondary batterySecondary battery

• Uses redox Uses redox reactions that can be reactions that can be reversed.reversed.

• Can be restored by Can be restored by rechargingrecharging

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Ni-Cad BatteryNi-Cad Battery

Anode (-)Anode (-)

Cd + 2 OHCd + 2 OH-- ---> Cd(OH) ---> Cd(OH)22 + 2e- + 2e-

Cathode (+) Cathode (+)

NiO(OH) + HNiO(OH) + H22O + e- ---> Ni(OH)O + e- ---> Ni(OH)22 + OH + OH--

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Fuel Cells: HFuel Cells: H22 as a as a FuelFuel

•Fuel cellFuel cell - reactants are - reactants are

supplied continuously supplied continuously

from an external source.from an external source.•Cars can use electricity Cars can use electricity

generated by Hgenerated by H22/O/O22 fuel fuel

cells.cells.

•HH22 carried in tanks or carried in tanks or

generated from generated from

hydrocarbons.hydrocarbons.

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Hydrogen—Hydrogen—Air Fuel CellAir Fuel Cell

Figure 20.12Figure 20.12

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HH22 as a Fuel as a Fuel

Comparison of the volumes of substances Comparison of the volumes of substances required to store 4 kg of hydrogen relative to required to store 4 kg of hydrogen relative to car size. car size. (Energy, p. 290)(Energy, p. 290)

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Storing HStoring H22 as a Fuel as a Fuel

One way to store HOne way to store H22 is to adsorb the gas is to adsorb the gas

onto a metal or metal alloy. onto a metal or metal alloy. (Energy, p. 290)(Energy, p. 290)

1 © 2006 brooks/cole - thomson oxidation-reduction reactions indirect redox reaction a battery...

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