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Harnessing the Power Harnessing the Power of Voltaic Cellsof Voltaic Cells

Batteries and CorrosionBatteries and Corrosion

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Commercial Voltaic CellsCommercial Voltaic CellsVoltaic Cells are convenient energy sourcesBatteries is a self-contained group of voltaic cells arranged in series.

Advantage: PortableDisadvantage: Very Expensive (.80€ / Kwatt-h)

Need cells in series to provide power

The Processes occurring during the discharge and recharge of a lead-acid battery. When the lead-acid battery is discharging (top) it behaves like a voltaic cell: the anode is negative (electrode-1) and the cathode is positive (electrode-2). When it is recharging (bottom), it behaves like an electrolytic cell; the anode is positive (electrode-2) and the cathode is negative (electrode-1).

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Dry Cell or LeClanche CellDry Cell or LeClanche CellDry CellsInvented in the 1860’s the common dry cell or LeClanche cell, has become a familiar household item. An active zinc anode in the form of a can house a mixture of MnO2 and an acidic electrolytic paste, consisting of NH4Cl, ZnCl2, H2O and starch powdered graphite improves conductivity. The inactive cathode is a graphite rod.

Anode (oxidation)Zn(s) Zn2+

(aq) = 2e-Cathode (reduction). The cathodic half-reaction is complex and even today, is still being studied. MnO2(s) is reduced to Mn2O3(s) through a series of steps that may involve the presence of Mn2+ and an acid-base reaction between NH4

+ and OH- :

2MnO2 (s) + 2NH4+

(aq) + 2e- Mn2O3(s) + 2NH3(aq) + H2O (l)

The ammonia, some of which may be gaseous, forms a complex ion with Zn2+, which crystallize in contact Cl- ion:

Zn2+(aq) + 2NH3

(aq) + 2Cl-(aq) Zn(NH3)2Cl2(s)

Overall Cell reaction:Overall Cell reaction:2MnO2 (s) + 2NH4Cl(aq) + Zn(s) Zn(NH3)2Cl2(s) + H2O (l) + Mn2O3(s) Ecell = 1.5 V

Uses: common household items, such as portable radios, toys, flashlights,Advantage;Advantage; Inexpensive, safe, available in many sizesDisadvantages:Disadvantages: At high current drain, NH3(g) builds up causing drop in voltage, short shelf life because zinc anode reacts with the acidic NH4+ ions.

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Dry Cell or LeClanche CellDry Cell or LeClanche CellInvented by George Leclanche, a French Invented by George Leclanche, a French Chemist.Chemist.

Acid version:Acid version: Zinc inner case that acts as the anode Zinc inner case that acts as the anode and a carbon rod in contact with a moist paste of solid MnOand a carbon rod in contact with a moist paste of solid MnO22 , , solid NHsolid NH44Cl, and carbon that acts as the cathode. As battery Cl, and carbon that acts as the cathode. As battery wear down, Conc. of Znwear down, Conc. of Zn+2+2 and NH and NH3 (aq)3 (aq) increases thereby increases thereby decreasing the voltage.decreasing the voltage.

Half reactions: E°Cell = 1.5 V

Anode: Zn(s) Zn+2(aq) + 2e-

Cathode: 2NH4+

(aq) + MnO2(s) + 2e- Mn2O3(s) + 2NH3(aq) + H2O(l)

Advantage:Inexpensive, safe, many sizes

Disadvantage:High current drain, NH3(g) build up, short shelf life

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Alkaline BatteryAlkaline BatteryAlkaline BatteryThe alkaline battery is an improved dry cell. The half-reactions are similar, but the electrolyte is a basic KOH paste, which eliminates the buildup of gases and maintains the Zn electrode.

Anode (oxidation)Zn(s) + 2OH- (aq) ZnO(s) + H2O (l) + 2e-Cathode (reduction). 2MnO2 (s) + 2H2O (l) + 2e- Mn(OH)2(s) + 2OH-

(aq)

Overall Cell reaction:Overall Cell reaction:2MnO2 (s) + H2O (l) + Zn(s) ZnO(s) + Mn(OH)2(s) Ecell = 1.5 V

Uses:Uses: Same as for dry cell.Advantages:Advantages: No voltage drop and longer shell life than dry cell because of alkaline electrolyte; sale ,amu sizes.Disadvantages;Disadvantages; More expensive than common dry cell.

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Alkaline BatteryAlkaline BatteryLeclanche Battery: Alkaline VersionLeclanche Battery: Alkaline Version

In alkaline version; solid NHIn alkaline version; solid NH44Cl is replaced with KOH or NaOH. Cl is replaced with KOH or NaOH. This makes cell last longer mainly because the zinc anode This makes cell last longer mainly because the zinc anode corrodes less rapidly under basic conditions versus acidic corrodes less rapidly under basic conditions versus acidic conditions.conditions.

Half reactions: E°Cell = 1.5 V

Anode: Zn(s) + 2OH-(aq) ZnO(s) + H2O(l) + 2e-

Cathode: MnO2 (s) + H2O(l) + 2e- MnO3 (s) + 2OH-(aq)

Nernst equation: E = E° - [(0.592/n)log Q], Q is constant !!Advantage:No voltage drop, longer shelf life.

Disadvantage:More expensive

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Mercury Button BatteryMercury Button BatteryMercury and Silver batteries are similar.Mercury and Silver batteries are similar.

Like the alkaline dry cell, both of these batteries use zinc in a Like the alkaline dry cell, both of these batteries use zinc in a basic medium as the anode. The solid reactants are each basic medium as the anode. The solid reactants are each compressed with KOH, and moist paper acts as a salt bridge.compressed with KOH, and moist paper acts as a salt bridge.

Half reactions: E°Cell = 1.6 V

Anode: Zn(s) + 2OH-(aq) ZnO(s) + H2O(l) + 2e-

Cathode (Hg): HgO (s) + 2H2O(l) + 2e- Hg(s) + 2OH-

(aq)

Cathode (Ag): Ag2O (s) + H2O(l) + 2e- 2Ag(s) + 2OH-

(aq)Advantage:Small, large potential, silver is nontoxic.

Disadvantage:Mercury is toxic, silver is expensive.

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Lead Storage Lead Storage BatteryBatteryLead-Acid Battery.Lead-Acid Battery. A typical 12-V lead-acid battery has six cells A typical 12-V lead-acid battery has six cells connected in series, each of which delivers about 2 V. Each cell contains connected in series, each of which delivers about 2 V. Each cell contains two lead grids packed with the electrode material: the anode is spongy two lead grids packed with the electrode material: the anode is spongy Pb, and the cathode is powered PbO2. The grids are immersed in an Pb, and the cathode is powered PbO2. The grids are immersed in an electrolyte solution of 4.5 M Helectrolyte solution of 4.5 M H22SOSO44. Fiberglass sheets between the grids . Fiberglass sheets between the grids prevents shorting by accidental physical contact. When the cell prevents shorting by accidental physical contact. When the cell discharges, it generates electrical energy as a voltaic cell.discharges, it generates electrical energy as a voltaic cell.

Half reactions: E°Cell = 2.0 V

Anode: Pb(s) + SO42- PbSO4 (s) +2 e- E° = 0.356

Cathode (Hg): PbO2 (s) + SO42- + 4H+ + 2e-

PbSO4 (s) + 2 H2O E° = 1.685V

Net: PbO2 (s) + Pb(s) + 2H2SO4 PbSO4 (s) + 2 H2O E°Cell = 2.0 V Note hat both half-reaction produce Pb2+ ion, one through oxidation of Pb, the other through reduction of PbO2. At both electrodes, the Pb2+ react with SO4

2- to form PbSO4(s)

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Nickel-Cadmium BatteryNickel-Cadmium BatteryBattery for the Technological AgeBattery for the Technological AgeRechargeable, lightweight “ni-cad” are used for variety of cordless Rechargeable, lightweight “ni-cad” are used for variety of cordless appliances. Main advantage is that the oxidizing and reducing agent appliances. Main advantage is that the oxidizing and reducing agent can be regenerated easily when recharged. These produce constant can be regenerated easily when recharged. These produce constant potential.potential.

Half reactions: E°Cell = 1.4 V

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

Cathode: 2Ni(OH) (s) + 2H2O(l) + 2e- Ni(OH)2 (s) + 2 OH-(aq)

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Fuel CellsFuel Cells

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Fuel Cells; BatteriesFuel Cells; Batteries

Fuel Cell also an electrochemical device for Fuel Cell also an electrochemical device for converting chemical energy into electricity.converting chemical energy into electricity.

In contrast to storage battery, fuel cell does not need to In contrast to storage battery, fuel cell does not need to involve a reversible reaction since the reactant are supplied involve a reversible reaction since the reactant are supplied to the cell as needed from an external source. This to the cell as needed from an external source. This technology has been used in the Gemini, Apollo and Space technology has been used in the Gemini, Apollo and Space Shuttle program.Shuttle program.

Half reactions: E°Cell = 0.9 V

Anode: 2H2 (g) + 4OH-(aq) 4H2O(l) + 4e-

Cathode: O2 (g) + 2H2O(l) + 4e- 4OH-(aq) Advantage:

Clean, portable and product is water. Efficient (75%) contrast to 20-25% car, 35-40% from coal electrical plant

Disadvantage:Cannot store electrical energy, needs continuous flow of reactant, Electrodes are short lived and expensive.

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CorrosionCorrosionNot all spontaneous redox reaction are Not all spontaneous redox reaction are beneficial.beneficial.

Natural redox process that oxidizes metal to their oxides Natural redox process that oxidizes metal to their oxides and sulfides runs billions of dollars annually. Rust for and sulfides runs billions of dollars annually. Rust for example is not the direct product from reaction between example is not the direct product from reaction between iron and oxygen but arises through a complex iron and oxygen but arises through a complex electrochemical process.electrochemical process.Rust: FeRust: Fe22OO3 3 • X H• X H22OO

Anode: Fe(s) Fe+2 + 2e- E° = 0.44 V

Cathode: O2 (g) + 4H+ + 4e- 2H2O (l) E° = 1.23 V

Net:Net: FeFe+2+2 will further oxidized to Fe will further oxidized to Fe22OO3 3 • X H• X H22OO

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Conditions for CorrosionConditions for Corrosion

Conditions for Iron Oxidation:

Iron will oxidize in acidic mediumIron will oxidize in acidic medium

SOSO22 H H22SOSO44 H H++ + HSO + HSO44++

Anions improve conductivity for oxidation.Anions improve conductivity for oxidation.

ClCl-- from seawater or NaCl (snow melting) enhances rusting from seawater or NaCl (snow melting) enhances rusting

Conditions for Prevention:Conditions for Prevention:

Iron will not rust in dry air; moisture must be presentIron will not rust in dry air; moisture must be present

Iron will not rust in air-free water; oxygen must be presentIron will not rust in air-free water; oxygen must be present

Iron rusts most rapidly in ionic solution and low pH (high HIron rusts most rapidly in ionic solution and low pH (high H++))

The loss of iron and deposit of rust occur at different placm on The loss of iron and deposit of rust occur at different placm on objectobject

Iron rust faster in contact with a less active metal (Cu)Iron rust faster in contact with a less active metal (Cu)

Iron rust slower in contact with a more active metal (Zn)Iron rust slower in contact with a more active metal (Zn)

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Iron Corrosion; ChemistryIron Corrosion; Chemistry

Iron will not rust in dry air; moisture must be present.

Iron will not rust in air-free water; oxygen must be present

Iron rusts most rapidly in ionic solutions and at low pH (High H+)

Most common and economically Most common and economically destructive form of corrosion is the destructive form of corrosion is the rusting of iron. Rust is not a direct rusting of iron. Rust is not a direct product of the reaction between iron product of the reaction between iron and oxygen but arises through and oxygen but arises through complex electrochemical process. The complex electrochemical process. The features of a voltaic cell can help features of a voltaic cell can help explain this process.explain this process.

The loss of iron and the depositing of rust often occur at different places on the same object.

Iron rust faster in contact with a less active metal (such as Cu) and more slowly in contact with a more active metal (such as Zn).

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Corrosion PreventionCorrosion Prevention