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ElectrochemistryElectrochemistry

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Electricity Electricity

Movt of electronsMovt of electronsMovt of electrons through wire Movt of electrons through wire

connecting 2 half-reactions connecting 2 half-reactions electrochemical cellelectrochemical cell

Also called Also called voltaicvoltaic or or galvanicgalvanic cell cellCell produces current from spont rxnCell produces current from spont rxnEx: copper in soln of AgNOEx: copper in soln of AgNO3 3 is spontis spontElectrolytic cellElectrolytic cell uses uses electrical current electrical current

to drive a to drive a non-spontnon-spont chemical rxn chemical rxn

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Voltaic cellVoltaic cell Solid Zn in zinc ion soln = Solid Zn in zinc ion soln =

half-cellhalf-cell Likewise, Cu/Cu ion solnLikewise, Cu/Cu ion soln Wire attached to each solidWire attached to each solid Salt bridgeSalt bridge = = 1. contains electrolytes, 1. contains electrolytes, 2. connects 2 half-cells, 2. connects 2 half-cells, 3. anions flow to neutralize 3. anions flow to neutralize

accumulated cations at accumulated cations at anode and vice-versa anode and vice-versa (completes circuit)(completes circuit)

““An Ox” = anode An Ox” = anode oxidationoxidation

Has negative charge Has negative charge cuz releases ecuz releases e--

““Red Cat” = Red Cat” = reduction cathodereduction cathode

Has positive charge Has positive charge cuz takes up ecuz takes up e--

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Electrical currentElectrical current

Measured in amperes (A)Measured in amperes (A) 1 A = 6.242 x 101 A = 6.242 x 101818 e e--/s/s Electric current driven by diff in potential Electric current driven by diff in potential

E/unit of chargeE/unit of charge Pot diff (electromotive force or emf) = volt Pot diff (electromotive force or emf) = volt

(V)(V) 1 V = J/C1 V = J/C

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Batteries Batteries

Dry-cell batteriesDry-cell batteries Don’t contain large amts of waterDon’t contain large amts of water Duracell Duracell Anode: Zn oxidizedAnode: Zn oxidized Cathode: 2MnOCathode: 2MnO2(s)2(s) + 2NH + 2NH44

++(aq)(aq) + 2e + 2e--

MnMn22OO3(s)3(s) + 2NH + 2NH3(g)3(g) + H + H22OO(l)(l)

Cathode is Carbon-rod immersed in moist Cathode is Carbon-rod immersed in moist (acidic) paste of MnO(acidic) paste of MnO22 that houses NH that houses NH44ClCl

1.5 V 1.5 V

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Batteries Batteries

More common dry-cell type: More common dry-cell type: alkaline alkaline batterybattery

Anode: ZnAnode: Zn(s)(s) + 2OH + 2OH--(aq)(aq) Zn(OH) Zn(OH)2(s)2(s) + +

2e2e--

Cathode: Cathode: 2MnO2MnO2(s)2(s) + 2H + 2H22OO(l)(l) + 2e + 2e-- 2MnO(OH)2MnO(OH)(s)(s) + 2OH + 2OH--

(aq)(aq)

Longer shelf-life, “live” longerLonger shelf-life, “live” longerCathode in basic pasteCathode in basic paste

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Batteries Batteries

Lead-acid storage batteriesLead-acid storage batteries In carsIn cars 6 electrochemical cells (2V) in series6 electrochemical cells (2V) in series Anode: PbAnode: Pb(s)(s) + HSO + HSO44

--(aq)(aq) PbSO PbSO4(s)4(s) + H + H++

(aq)(aq) + 2e + 2e--

Cathode: Cathode: PbOPbO2(s)2(s) + HSO + HSO44

--(aq)(aq) + 3H + 3H++

(aq)(aq) + 2e + 2e-- PbSO PbSO4(s)4(s) + 2H + 2H22OO(l)(l)

In 30% soln of sulfuric acidIn 30% soln of sulfuric acid If dead, due to excess PbSOIf dead, due to excess PbSO44 covering electrode covering electrode

surfacessurfaces Re-charge (reverse rxn) Re-charge (reverse rxn) converts PbSO converts PbSO44 to Pb to Pb

and PbOand PbO22

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Rechargeable batteries Rechargeable batteries

Ni-CdNi-Cd Anode: CdAnode: Cd(s)(s) + 2OH + 2OH--

(aq)(aq) Cd(OH) Cd(OH)2(s)2(s) + 2e + 2e--

Cathode: Cathode: 2NiO(OH)2NiO(OH)(s)(s) + 2H + 2H22OO(l)(l) + 2e + 2e-- 2Ni(OH) 2Ni(OH)2(s)2(s) + 2OH + 2OH--

(aq)(aq)

KOH, usually, usedKOH, usually, used 1.30 V1.30 V Reverse rxn recharges batteryReverse rxn recharges battery Excess recharging Excess recharging electrolysis of water electrolysis of water EXPLOSION!!!EXPLOSION!!! Muhahahaha!Muhahahaha!

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Rechargeable batteriesRechargeable batteries

Cd toxicCd toxic Ni-MH Ni-MH Hybrid car batteries: high energy densityHybrid car batteries: high energy density Same cathode rxnSame cathode rxn Anode: MHAnode: MH(s)(s) + OH + OH--

(aq)(aq) M M(s)(s) + H + H22OO(l)(l) + e + e--

Commonly, M = ABCommonly, M = AB55, where A is rare earth , where A is rare earth mixture of La, Ce, Nd, Pr, and B is Ni, Co, mixture of La, Ce, Nd, Pr, and B is Ni, Co, Mn, and/or MnMn, and/or Mn

Very few use ABVery few use AB22, where A = Ti and/or V, where A = Ti and/or V

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Rechargeable batteriesRechargeable batteries

Anode made of graphite w/incorporated Anode made of graphite w/incorporated Li-ions btwn carbon layersLi-ions btwn carbon layers

Ions spontaneously migrate to cathodeIons spontaneously migrate to cathodeCathode = LiCoOCathode = LiCoO22 or LiMn or LiMn22OO44

Transition metal reducedTransition metal reducedUsed in laptop computers, cell phones, Used in laptop computers, cell phones,

digital camerasdigital camerasLight weight and high E densityLight weight and high E density

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Fuel cellFuel cell

Reactants flow through batteryReactants flow through batteryUndergo redox rxnUndergo redox rxnGenerate electricityGenerate electricityHydrogen-oxygen fuel cellHydrogen-oxygen fuel cellAnode: 2HAnode: 2H2(g)2(g) + 4OH + 4OH--

(aq)(aq) 4H 4H22OO(l)(l) + 4e + 4e--

Cathode: OCathode: O2(g)2(g) + 2H + 2H22OO(l)(l) + 4e + 4e-- 4OH 4OH--(aq)(aq)

Used in space-shuttle programUsed in space-shuttle program

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Electrolysis Electrolysis

Electrical current Electrical current used to drive used to drive nonspont redox rxnnonspont redox rxn

In electrolytic cellIn electrolytic cell Electrolysis of Electrolysis of

waterwater Metal plating: silver Metal plating: silver

coated on metal, coated on metal, jewelry, etc.jewelry, etc.