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REDOXREDOX
BY SUWAHONO
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Electron Transfer ReactionsElectron Transfer Reactions
• Electron transfer reactions are Electron transfer reactions are oxidation-oxidation-
reductionreduction or or redoxredox reactions. reactions.
• Results in the generation of an electric Results in the generation of an electric
current (electricity) or be caused by current (electricity) or be caused by
imposing an electric current. imposing an electric current.
• Therefore, this field of chemistry is often Therefore, this field of chemistry is often
called called ELECTROCHEMISTRY.ELECTROCHEMISTRY.
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Terminology for Redox Terminology for Redox ReactionsReactions
• OXIDATIONOXIDATION—loss of electron(s) by a species; —loss of electron(s) by a species; increase in oxidation number; increase in increase in oxidation number; increase in oxygen.oxygen.
• REDUCTIONREDUCTION—gain of electron(s); decrease in —gain of electron(s); decrease in oxidation number; decrease in oxygen; oxidation number; decrease in oxygen; increase in hydrogen.increase in hydrogen.
• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.
• REDUCING AGENTREDUCING AGENT—electron donor; species is —electron donor; species is oxidized.oxidized.
• OXIDATIONOXIDATION—loss of electron(s) by a species; —loss of electron(s) by a species; increase in oxidation number; increase in increase in oxidation number; increase in oxygen.oxygen.
• REDUCTIONREDUCTION—gain of electron(s); decrease in —gain of electron(s); decrease in oxidation number; decrease in oxygen; oxidation number; decrease in oxygen; increase in hydrogen.increase in hydrogen.
• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.
• REDUCING AGENTREDUCING AGENT—electron donor; species is —electron donor; species is oxidized.oxidized.
44You can’t have one… without the other!
• Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons.
• You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation
LEOLEO the lion says the lion says GERGER!!
GER!GER!
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J Deutsch 2003
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LEO growls GER
Losing Electrons Oxidation
Gaining Electrons Reduction
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Another way to remember
•OIL RIG
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OXIDATION-REDUCTION REACTIONS
Direct Redox ReactionDirect Redox Reaction
Oxidizing and Oxidizing and reducing agents in reducing agents in
direct contact.direct contact.
Cu(s) + 2 AgCu(s) + 2 Ag++(aq) ---> (aq) ---> CuCu2+2+(aq) + 2 Ag(s)(aq) + 2 Ag(s)
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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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Why Study Electrochemistry?Why Study Electrochemistry?
• BatteriesBatteries
• CorrosionCorrosion
• Industrial Industrial production of production of chemicalschemicals such as such as Cl Cl22, NaOH, F, NaOH, F22 and and AlAl
• Biological redox Biological redox reactionsreactions
The heme groupThe heme group
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Electrochemical CellsElectrochemical Cells• An apparatus that allows An apparatus that allows
a redox reaction to occur a redox reaction to occur by transferring electrons by transferring electrons through an external through an external connector.connector.
• Product favored reaction Product favored reaction ---> ---> voltaic or galvanic cellvoltaic or galvanic cell ----> electric current----> electric current
• Reactant favored reaction Reactant favored reaction ---> ---> electrolytic cellelectrolytic cell ---> ---> electric current used to electric current used to cause chemical change.cause chemical change.
Batteries are voltaic Batteries are voltaic cellscells
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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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•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
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..
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf
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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
RedReductionuctionCatCathodehodePositivePositive
RedReductionuctionCatCathodehodePositivePositive
RED CATRED CAT
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Terms Used for Voltaic Terms Used for Voltaic CellsCells
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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.
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.
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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
To determine an oxidation To determine an oxidation from a reduction table, just from a reduction table, just take the opposite sign of the take the opposite sign of the reduction!reduction!
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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 = +1.10 V = +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
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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
2323More 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˚E˚ = E˚catcat+ E˚+ E˚anan= (-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
(the connection is backwards or you are (the connection is backwards or you are
recharging the battery)recharging the battery)
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Charging a BatteryCharging a BatteryWhen you charge a battery, you are When you charge a battery, you are forcing the electrons backwards (from forcing the electrons backwards (from the + to the -). To do this, you will the + to the -). To do this, you will need a higher voltage backwards than need a higher voltage backwards than forwards. This is why the ammeter in forwards. This is why the ammeter in your car often goes slightly higher your car often goes slightly higher while your battery is charging, and then while your battery is charging, and then returns to normal.returns to normal.
In your car, the battery charger is In your car, the battery charger is called an alternator. If you have a called an alternator. If you have a dead battery, it could be the dead battery, it could be the battery needs to be replaced OR battery needs to be replaced OR the alternator is not charging the the alternator is not charging the battery properly.battery properly.
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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
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Alkaline BatteryAlkaline Battery
Nearly same reactions as Nearly same reactions as in common dry cell, but in common dry cell, but under basic conditions.under basic conditions.
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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Mercury BatteryMercury Battery
Anode:Anode:
Zn is reducing agent under basic conditionsZn is reducing agent under basic conditions
Cathode:Cathode:
HgO + HHgO + H22O + 2e- ---> Hg + 2 OHO + 2e- ---> Hg + 2 OH--
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Lead Storage BatteryLead Storage Battery
Anode (-) Anode (-) EEoo = +0.36 V = +0.36 V
Pb + HSOPb + HSO44-- ---> PbSO ---> PbSO44 + H + H++ + 2e- + 2e-
Cathode (+) Cathode (+) EEoo = +1.68 V = +1.68 V
PbOPbO22 + HSO + HSO44-- + 3 H + 3 H++ + 2e- + 2e-
---> PbSO---> PbSO44 + 2 H + 2 H22OO
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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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HH22 as a Fuel as a Fuel
Cars can use electricity generated by HCars can use electricity generated by H22/O/O22
fuel cells.fuel cells.HH22 carried in tanks or generated from carried in tanks or generated from
hydrocarbonshydrocarbons
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Balancing Equations Balancing Equations for Redox Reactionsfor Redox Reactions
Some redox reactions have equations that must be balanced by Some redox reactions have equations that must be balanced by special techniques.special techniques.
MnOMnO44-- + 5 Fe + 5 Fe2+2+ + 8 H + 8 H++
---> Mn---> Mn2+ 2+ + 5 Fe+ 5 Fe3+3+ + 4 H + 4 H22OO
Mn = +7Mn = +7 Fe = +2Fe = +2Fe = +3Fe = +3Mn = +2Mn = +2
3232
Balancing Balancing EquationsEquations
Consider the Consider the reduction of Agreduction of Ag++ ions with copper ions with copper metal.metal.
Cu + AgCu + Ag++ --give--> Cu --give--> Cu2+2+ + Ag + Ag
3333Balancing Balancing EquationsEquations
Step 1:Step 1: Divide the reaction into half-reactions, one for Divide the reaction into half-reactions, one for oxidation and the other for reduction.oxidation and the other for reduction.
OxOx Cu ---> CuCu ---> Cu2+2+
RedRed Ag Ag++ ---> Ag ---> Ag
Step 2:Step 2: Balance each element for mass. Already done Balance each element for mass. Already done in this case.in this case.
Step 3:Step 3: Balance each half-reaction for charge by Balance each half-reaction for charge by adding electrons.adding electrons.
OxOx Cu ---> Cu Cu ---> Cu2+2+ + + 2e-2e-
RedRed Ag Ag++ + + e- e- ---> Ag---> Ag
3434Balancing Balancing EquationsEquations
Step 4:Step 4: Multiply each half-reaction by a factor so that Multiply each half-reaction by a factor so that the reducing agent supplies as many electrons as the the reducing agent supplies as many electrons as the oxidizing agent requires.oxidizing agent requires.
Reducing agentReducing agent Cu ---> Cu Cu ---> Cu2+2+ + 2e- + 2e-Oxidizing agentOxidizing agent 22 Ag Ag++ + + 22 e- ---> e- ---> 22 Ag AgStep 5:Step 5: Add half-reactions to give the overall equation.Add half-reactions to give the overall equation.Cu + 2 AgCu + 2 Ag++ ---> Cu ---> Cu2+2+ + 2Ag + 2Ag
The equation is now balanced for both The equation is now balanced for both charge and mass.charge and mass.
3535Balancing Balancing EquationsEquations
Balance the following in acid solution—Balance the following in acid solution—
VOVO22++ + Zn ---> VO + Zn ---> VO2+ 2+ + Zn+ Zn2+2+
Step 1:Step 1:Write the half-reactionsWrite the half-reactions
OxOx Zn ---> ZnZn ---> Zn2+2+
RedRed VOVO22++ ---> VO ---> VO2+2+
Step 2:Step 2:Balance each half-reaction for mass.Balance each half-reaction for mass.
OxOx Zn ---> ZnZn ---> Zn2+2+
RedRedVOVO22
++ ---> VO ---> VO2+2+ + + HH22OO2 H2 H++ ++
Add HAdd H22O on O-deficient side and add HO on O-deficient side and add H++
on other side for H-balance.on other side for H-balance.
3636Balancing Balancing EquationsEquations
Step 3:Step 3: Balance half-reactions for charge.Balance half-reactions for charge.OxOx Zn ---> ZnZn ---> Zn2+2+ + + 2e- 2e-
RedRed e- e- + 2 H+ 2 H++ + VO + VO22++ ---> VO ---> VO2+2+ + H + H22OO
Step 4:Step 4: Multiply by an appropriate factor.Multiply by an appropriate factor.OxOx Zn ---> ZnZn ---> Zn2+2+ + + 2e-2e-
RedRed 22e-e- + + 44 H H++ + + 22 VO VO22++
---> ---> 22 VO VO2+2+ + + 22 H H22OO
Step 5:Step 5: Add Add balancedbalanced half-reactions half-reactions
Zn + 4 HZn + 4 H++ + 2 VO + 2 VO22++
---> Zn ---> Zn2+2+ + 2 VO + 2 VO2+2+ + 2 H + 2 H22OO
3737
Tips on Balancing Tips on Balancing EquationsEquations
• Never add ONever add O22, O atoms, or , O atoms, or OO2-2- to balance oxygen. to balance oxygen.
• Never add HNever add H22 or H atoms to or H atoms to balance hydrogen.balance hydrogen.
• Be sure to write the correct Be sure to write the correct charges on all the ions.charges on all the ions.
• Check your work at the end Check your work at the end to make sure mass and to make sure mass and charge are balanced.charge are balanced.
• PRACTICE!PRACTICE!
