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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
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.
CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT
CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC 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
CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT
CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT
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
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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.•Lets revisit my haiku! Lets revisit my haiku!
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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+
Zn Zn2+ + 2e- OXIDATION ANODE
2 H+ + 2e- H2REDUCTIONCATHODE
- +
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+
Zn Zn2+ + 2e- OXIDATION ANODE
2 H+ + 2e- H2REDUCTIONCATHODE
- +
Volts
ZnH2
Salt Bridge
Zn2+ H+
Zn Zn2+ + 2e- OXIDATION ANODE
2 H+ + 2e- H2REDUCTIONCATHODE
- +
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+
Cu2+ + 2e- Cu REDUCTION CATHODE
H2 2 H+ + 2e-OXIDATION ANODE
-+
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+
Cu2+ + 2e- Cu REDUCTION CATHODE
H2 2 H+ + 2e-OXIDATION ANODE
-+
Volts
CuH2
Salt Bridge
Cu2+ H+
Cu2+ + 2e- Cu REDUCTION CATHODE
H2 2 H+ + 2e-OXIDATION ANODE
-+
+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
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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
oxidizingability of ion
reducing abilityof element
Eo (V)
Cu2+ + 2e- Cu +0.34
2 H+ + 2e- H2 0.00
Zn2+ + 2e- Zn -0.76
oxidizingability of ion
reducing abilityof element
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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?
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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
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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?
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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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If you have reached this far, you need a break!