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ElectrochemistryElectrochemistry Generating Voltage (Potential)

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HistoricallyHistorically

Historically oxidation involved reaction with O2.i.e., Rusting

4 Fe(s) + 3O2 (g) Fe2O3 (s)

Other exampleZn(s) + Cu2+

(aq) Zn2+(aq) + Cu(s)

In this reaction:Zn(s) Zn2+

(aq) Oxidation

Cu2+(aq) Cu(s) Reduction

In a redox reaction, one process can’t occur without the other. Oxidation-Reduction reaction must simultaneously occurs.

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Redox Between Redox Between If Zn(s) and Cu2+

(aq) is in the same solution, then the electron is a transferred directly between the Zn and Cu.

No useful work is obtained. However if the reactants are separated No useful work is obtained. However if the reactants are separated and the electrons shuttle through an external path...and the electrons shuttle through an external path...

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Electrochemical CellsElectrochemical CellsVoltaic / Galvanic Cell Apparatus which produce

electricity

Electrolytic Cell Apparatus which consumes electricity

Consider: Initially there is a Initially there is a flow of e- flow of e- After some time the After some time the process stopsprocess stopsElectron transport Electron transport stops because of stops because of charge build upcharge build up

Build up of Build up of positive chargepositive charge

Build up of negative charge

The charge separation will lead The charge separation will lead to process where it cost too to process where it cost too much energy to transfer much energy to transfer electron.electron.

ZnZn CuCu

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Completing the CircuitCompleting the CircuitElectron transfer can occur if the circuit is closedParts: Two conductorsElectrolyte solutionSalt Bridge / Porous membrane

3 process must happen if e- is to flow.A. e- transport through external circuitB. In the cell, ions a must migrateC. Circuit must be closed (no charge build up)

Anode (-)

Black

Negative electrode generates electron

Oxidation Occur

Cathode (+)

Red

Positive electrode accepts electron

Reduction Occur

A

B

C

Anode/Anion (-)

Cathode/Cation(+)

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Voltaic CellVoltaic CellElectron transfer can occur if the circuit is closedParts: Two conductorsElectrolyte solutionSalt Bridge / Porous membrane

3 process must happen if e- is to flow.A. e- transport through external circuitB. In the cell, ions a must migrateC. Circuit must be closed (no charge build up)

Anode (-)

Black

Negative electrode generates electron

Oxidation Occur

Cathode (+)

Red

Positive electrode accepts electron

Reduction Occur

Anode/Anion (-)

Cathode/Cation(+)

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Completing the Circuit: Salt Completing the Circuit: Salt BridgeBridge

In order for In order for electrons to move electrons to move through an through an external wire, external wire, charge must not charge must not build up at any build up at any cell. This is done cell. This is done by the salt bridge by the salt bridge in which ions in which ions migrate to different migrate to different compartments compartments neutralize any neutralize any charge build up.charge build up.

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Sign Convention of Voltaic CellSign Convention of Voltaic Cell

@ Anode: Negative Terminal (anions).

Source of electron then repels electrons. Oxidation occurs.

Zn(s) Zn+2(aq) + 2e- : Electron source

@ Cathode: Positive Terminal (cation)

Attracts electron and then consumes electron. Reduction occurs.

Electron target: 2e- + Cu+2(aq) Cu(s)

Overall:

Zn(s) + Cu+2(aq) Zn+2

(aq) + Cu(s) E° = 1.10 V

Note when the reaction is reverse: Cu(s) + Zn+2(aq) Cu+2

(aq) + Zn(s)

Sign of E ° is also reversed E° = -1.10 V

Oxidation: Zn(s) Zn+2(aq) E° = 0.76 V

Reduction: Cu+2(aq) Cu(s) E° = 0.34 V

1.10 V = E°CELL

or E°CELL = E°red (Red-cathode) - E°red (Oxid-anode)

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Other Voltaic CellOther Voltaic CellZn(s) + 2H+

(aq) Zn+2(aq) + H2 (g) E° = 0.76 V

@ Anode: Negative Terminal (anions): ZnNegative Terminal (anions): Zn(s)(s) Zn Zn+2+2(aq)(aq) + 2e + 2e-- : :

Source of electron then repels electrons. Oxidation occurs.

@ Cathode: Positive Terminal (cation): 2ePositive Terminal (cation): 2e-- + 2H + 2H++(aq)(aq) HH2 (g)2 (g)

Attracts electron and then consumes electron. Reduction occurs.

Net: ZnNet: Zn(s)(s) + 2H + 2H++ (aq)(aq) Zn Zn2+2+ (aq)(aq) + H + H22 (g)(g)

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Other Voltaic CellOther Voltaic CellZn(s) + 2H+

(aq) Zn+2(aq) + H2 (g) E° = 0.76 V

@ Anode: Negative Terminal (anions): ZnNegative Terminal (anions): Zn(s)(s) Zn Zn+2+2(aq)(aq) + 2e + 2e-- : :

Source of electron then repels electrons. Oxidation occurs.

@ Cathode: Positive Terminal (cation): 2ePositive Terminal (cation): 2e-- + 2H + 2H++(aq)(aq) HH2 (g)2 (g)

Attracts electron and then consumes electron. Reduction occurs.

Net: ZnNet: Zn(s)(s) + 2H + 2H++ (aq)(aq) Zn Zn2+2+ (aq)(aq) + H + H22 (g)(g)

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Line notation: Convenient convention for electrochemical cell

Schematic Representation1. Anode Cathode

[oxidation (-) ] [reduction (+)]

2. “ “ phase boundary (where potential may develop)

33. “ “ Liquid junction

4. Concentration of component

Zn(s) ZnSO4 (aq,1.0M) CuSO4 (aq,1.0M) Cu(s)

Line Notation ConventionLine Notation Convention

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4

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Line Notation ExamplesLine Notation ExamplesConsider : ZnZn(s)(s) + Cu + Cu+2+2

(aq)(aq) Zn Zn+2+2(aq)(aq) + Cu + Cu(s(s

Anode: Zn Zn+2 + 2e-

Cathode: Cu+2 + 2e- Cu

Shorthand “Line” notation

Zn (s) Zn+2 (aq)(1.0M) Cu+2(aq) (1.0M) Cu(s)

2nd Example : ZnZn(s)(s) + 2H + 2H+ + (aq)(aq) Zn Zn+2+2

(aq)(aq) + H + H2(g)2(g)

Anode: Zn Zn+2 + 2e-

Cathode: 2H+ + 2e- H2 (g)

Shorthand “Line” notation

Zn (s) Zn+2 (aq)(1.0M) H+(aq) (1.0M), H2(g, 1atm) Pt(s)

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Other Voltaic Cell & Their Line Other Voltaic Cell & Their Line NotationNotation

Zn(s) | Zn+2 (aq)||H+(aq) , H2 (g,1atm)|Pt

C(s)| I-(aq) , I2 (g,1atm) || MnO4

-(aq) , Mn+2 (aq)| C(s)

Cr(s) | Cr+3 (aq)||Ag+(aq) | Ag(s)

Oxidation half-reactionZn(s) Zn+2

(aq) + 2e-

Oxidation half-reaction2I- (aq) I2 (s) + 2e-

Oxidation half-reactionCr(s) Cr+3

(aq) + 3e-

Reduction half-reactionAg+

(aq) + e- Ag (s)

Reduction half-reactionMnO4

-(aq)

+ 8H+(aq)

+ 5e-

Mn 2+(aq) +4H2O(l)

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Line Notation ExamplesLine Notation Examples

Example 1: B&L 20.13Zn(s) + Ni2+

(aq) Zn+2(aq) + Ni

(aq)

Example 2: B&L 20.19Tl+3

(aq) + 2Cr2+(aq) Tl+

(aq) + Cr+3(aq)

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Voltage of Galvanic / Voltaic CellVoltage of Galvanic / Voltaic CellTransport of any object requires a net force.

Consider water flowing through pipes. This occurs because of pressure gradient.

Pressure ()

Pressure ()

Flow (Fluid Transport)

Or

Object Object falling or falling or transport transport down due to down due to hh

Similarly, electron are Similarly, electron are transported through transported through wires because of the wires because of the electromotive force electromotive force EMF or Ecell.EMF or Ecell.(-) e (-) e -- (+)(+)

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EMF - ElectroMotive ForceEMF - ElectroMotive Force

Potential energy of electron is higher at the anode. This is the driving force for the reaction (e- transfer)

eeAnode (-)

e- flow toward cathode(+)

Cathode

P.E. = V = J e - C

Larger the gap, the greater the potential (Voltage)

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ElectroMotive ForceElectroMotive ForceEMF - Electro Motive Force

Potential energy difference between the two electrodesPotential energy difference between the two electrodesThe larger the The larger the P.E. the larger EMF value.P.E. the larger EMF value.The magnitude of P.E. for the reaction (half reaction) is The magnitude of P.E. for the reaction (half reaction) is an intensive property)an intensive property)i.e., Size independent: i.e., Size independent: , T, Tbptbpt, C, Css..

Therefore EMF is also an intensive property.Therefore EMF is also an intensive property.Analogy:

Size of rock not important, only the height from ground.(Electron all have the same mass)

Unit: EMF: V - Volts : 1V - 1 Joule / Coulomb1 Joule of work per coulomb of charge transferred.

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Stoichiometry Relationship Stoichiometry Relationship to E°to E°

EMF - Intensive PropertyE°cell Standard state conditions 25°C, 1atm, 1.0 M

E°cell Intensive property, Size Independent

Consider: Li+ + e- Li (s) E°Cell = -3.045 V

x 2 2 Li+ + 2 e- 2 Li (s) E°Cell = (-3.045 V) x 2 = ??

But E° = Voltage per electronE° ‘ = E° x 2 = ? - 3.045 V • 2 = -3.045 V

2 e-

Stoichiometry does not change E°, but Stoichiometry does not change E°, but reversing the reaction does change the reversing the reaction does change the sign of E°.sign of E°.

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Standard Reduction Standard Reduction PotentialPotentialCell Potential is written as a reduction

equation. M+ + e- M E° = std red. potential

Wri

tten

as r

ed

ucti

on

Most spontaneous <Reduction occurs> Oxidizing Agent

Most non-spontaneous Spontaneous in the reverse direction. <Oxidation occurs> Reducing Agent

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Zoom View of Std. Reduction Zoom View of Std. Reduction PotentialPotentialCell Potential is written as a reduction

equation.

M+ + e- M E° =

F2 (g) + 2e- 2 F - (aq) 2.87 V

Ce4+ + e- Ce3+

(aq) 1.61 V

2H+ + 2e- H2 (g) 0.00 V

Li+(aq) + e- Li(s) -3.045 V

Wri

tten

as r

ed

ucti

on Most

spontaneous Reduction Oxidizing Agent

Most non-spontaneous Spontaneous in the reverse direction. Oxidation Reducing Agent

All reaction written as reduction reaction. But in electrochemistry, there can’t be just a reduction reaction. It must be coupled with an oxidation reaction.

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E°E°CellCell Evaluation EvaluationE°Cell Function of the reaction

Oxidation Process (Anode reaction)

Reduction Process (Cathode reaction)

orE°Cell = E°Cathode & E°Anode

Cathode (+)

Anode (-) Most Negative Reduction reaction

Therefore,

E°Cell = E°red (Cathode) - E°red (anode)

Neg Minus (Large negative)

(Very Positive Value)Very Positive Very Spontaneous

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Standard Reduction Standard Reduction PotentialPotential

How is E°red (Cathode) and E°red (Anode)

determine.E° (EMF) - State Function; there is no absolute scaleAbsolute E° value can’t be measured experimentallyThe method of establishing a scale is to measure the difference in potential between two half-cells.Consider:

Zn Zn+2 + 2e- E°=?

Can’t determine because the reaction must be coupled

How can a scale of reduction potential be determine ?

Use a half reaction as reference and assign it a potential of zero.

Electrochemical reaction more spontaneous than this reference will have positive E°, and those less spontaneous will have negative E°.

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Side-Bar: Relative ScaleSide-Bar: Relative Scale

Consider a baby whose weight is to be determine but will not remain still on top of a scale. How can the

parents determine the babies weight?

Carry the child in arms and weight both child and parent then subtract the weight of the parent from the total to yield the baby weight.

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Reference PotentialReference PotentialSelected half reaction is:

H+ / H2 (g) couple half reaction: 2H+ (aq, 1.0M) + 2e- H2 (g,1atm)

by definition E° = 0.0 V, the reverse is also 0.0 V

H+/H2 couple - Standard Hydrogen Electrode (SHE)

To determine E° for a another half reaction, the reaction of interest needs to be coupled to this SHE. The potential measured is then assigned to the half-reaction under investigation.

E°Cell = 0.76 V = E°red (Cat) - E°red

(Anode)

0.0 V - (?)

E°red (Anode) = - 0.76 V

Zn+2/Zn E° = -0.76 V Reduction rxn

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Determining Other Half-Cell Determining Other Half-Cell PotentialPotential

Now consider the reaction:Zn(s)|Zn+2 (1.0 M)||Cu+2(1.0 M)|Cu(s)E°Cell = 1.10 V

E°Cell = E°red (Cat) - E°red (Anode)

recall, E° Zn+2/Zn = - 0.76 V

Therefore, E°Cell = E°Cu+2/Cu - E° Zn+2/Zn

1.10 V = (?) - (- 0.76 V)

E°Cu+2/Cu = + 0.34 V

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Example: Half-Cell PotentialExample: Half-Cell Potential

Example BBL20.19: For the reaction: Tl+3 + 2Cr2+ Tl+ + 2Cr3+ E°Cell = 1.19 V

i) Write both half reaction and balanceii) Calculate the E°Cell Tl+3 Tl+

iii) Sketch the voltaic cell and line notation

i) Tl+3 + 2e- Tl+

(Cr2+ 2Cr3+ + 2e- ) x 2 E° = 0.41 V

ii) E°Cell = 1.19 V = E°red (Cat) - E°red (Anode)

1.19 V= E°red (Cat) - 0.041 V

for Tl+3 + 2e- Tl+ : 1.19 V - 0.41 = E°red (Cat) = 0.78 V

Pt

Pt

Cr2+ Cr3+ Tl3+ Tl+

1.19 V

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Voltaic Vs. Electrolytic CellsVoltaic Vs. Electrolytic Cells

General characteristics of voltaic and electrolytic cells. A voltaic cell generates energy from a spontaneous reaction (G<0), whereas an electrolytic cell requires energy to drive a nonspontaneous reaction (G>0). In both types of cell, two external circuits provides the means or electrons to flow. Oxidation takes place all the anode, and reduction takes place at the cathode, but the relative electrode changes are opposite in the two cells.

Oxidation ReactionA- A + e-

Oxidation ReactionX X+ + e-

Reduction Reactione- + Y+ Y

Reduction Reactione- + B+ B

Overall (Cell) ReactionX + Y+ X+ + Y, G

= 0

Overall (Cell) ReactionA- + B+ A + B, G> 0

Voltaic CellEnergy is released from

spontaneous redox reaction

System does work on load (surroundings)

Electrolytic CellEnergy is absorbed to drive

nonspontaneous redox reaction

Surrounding (power supply) do work on system (cell)

Anode (Oxidation)