ElectrochemistryElectrochemistry

Applications of RedoxApplications of Redox

AP Chemistry

Chapter 20 Notes

ReviewReview

Oxidation reduction reactions involve a Oxidation reduction reactions involve a transfer of electrons.transfer of electrons.

OIL- RIGOIL- RIGOxidation Involves LossOxidation Involves LossReduction Involves GainReduction Involves GainLEO-GER LEO-GER Lose Electrons OxidationLose Electrons OxidationGain Electrons ReductionGain Electrons Reduction

ApplicationsApplicationsMoving electrons is electric current.Moving electrons is electric current. 8H8H+++MnO+MnO44

--+ 5Fe+ 5Fe+2 +2 +5e+5e-- Mn Mn+2 +2 + 5Fe+ 5Fe+3 +3 +4H+4H22OO

Helps to break the reactions into half rxns.Helps to break the reactions into half rxns.

8H8H+++MnO+MnO44--+5e+5e-- Mn Mn+2 +2 +4H+4H22OO

5Fe5Fe+2+2 5Fe 5Fe+3 +3 + 5e+ 5e-- ) )

In the same mixture it happens without doing In the same mixture it happens without doing useful work, but if separateuseful work, but if separate

H+

MnO4-

Fe+2

Connected this way the reaction startsConnected this way the reaction startsStops immediately because charge builds Stops immediately because charge builds

up.up.

H+

MnO4-

Fe+2

Galvanic CellGalvanic Cell

Salt Bridge allows current to flow

H+

MnO4-

Fe+2

e-

Electricity travels in a complete circuitElectricity travels in a complete circuit Instead of a salt bridgeInstead of a salt bridge

H+

MnO4-

Fe+2

Porous Disk

Reducing Agent

Oxidizing Agent

e-

e-

e- e-

e-

e-

Anode Cathode

Cell PotentialCell Potential

Oxidizing agent pushes the electron.Oxidizing agent pushes the electron.Reducing agent pulls the electron. Reducing agent pulls the electron. The push or pull (“driving force”) is called The push or pull (“driving force”) is called

the cell potential the cell potential EEcellcell

Also called the electromotive force (emf) Also called the electromotive force (emf) Unit is the volt(V) Unit is the volt(V) = 1 joule of work/coulomb of charge= 1 joule of work/coulomb of chargeMeasured with a voltmeterMeasured with a voltmeter

Zn+2 SO4-

2

1 M HCl

Anode

0.76

1 M ZnSO4

H+

Cl-

H2 in

Cathode

1 M HCl

H+

Cl-

H2 in

Standard Hydrogen Standard Hydrogen ElectrodeElectrode

This is the reference This is the reference all other oxidations all other oxidations are compared toare compared to

EEº = 0º = 0 º indicates standard º indicates standard

states of 25ºC, 1 atm, states of 25ºC, 1 atm, 1 M solutions.1 M solutions.

Cell PotentialCell PotentialZn(s) + CuZn(s) + Cu+2 +2 (aq)(aq) Zn Zn+2+2(aq)(aq) + Cu(s) + Cu(s)The total cell potential is the sum of the The total cell potential is the sum of the

potential at each electrode.potential at each electrode.

EEºº cellcell = = EEººZnZn Zn Zn+2+2 + + EEºº CuCu+2+2 CuCu

We can look up reduction potentials in a We can look up reduction potentials in a table.table.

One of the reactions must be reversed, so One of the reactions must be reversed, so change it sign.change it sign.

Cell PotentialCell Potential

Determine the cell potential for a galvanic Determine the cell potential for a galvanic cell based on the redox reaction.cell based on the redox reaction.

Cu(s) + FeCu(s) + Fe+3+3(aq)(aq) Cu Cu+2+2(aq)(aq) + Fe + Fe+2+2(aq)(aq)

FeFe+3+3(aq)(aq) + e+ e-- Fe Fe+2+2(aq) (aq) EEº = 0.77 Vº = 0.77 V

CuCu+2+2(aq)+2e(aq)+2e-- Cu(s) Cu(s) EEº = 0.34 Vº = 0.34 V

Cu(s) Cu(s) CuCu+2+2(aq)+2e(aq)+2e-- EEº = -0.34 Vº = -0.34 V

2Fe2Fe+3+3(aq)(aq) + 2e+ 2e-- 2Fe 2Fe+2+2(aq) (aq) EEº = 0.77 Vº = 0.77 V

Line NotationLine Notation

solidsolidAqueousAqueousAqueousAqueoussolidsolidAnode on the leftAnode on the leftCathode on the rightCathode on the rightSingle line different phases.Single line different phases.Double line porous disk or salt bridge.Double line porous disk or salt bridge. If all the substances on one side are If all the substances on one side are

aqueous, a platinum electrode is indicated.aqueous, a platinum electrode is indicated.For the last reactionFor the last reactionCu(s)Cu(s)CuCu+2+2(aq)(aq)FeFe+2+2(aq),Fe(aq),Fe+3+3(aq)(aq)Pt(s)Pt(s)

Galvanic CellGalvanic Cell

The reaction always runs spontaneously in The reaction always runs spontaneously in the direction that produced a positive cell the direction that produced a positive cell potential.potential.

Four things for a complete description.Four things for a complete description.

1)1) Cell PotentialCell Potential

2)2) Direction of flowDirection of flow

3)3) Designation of anode and cathodeDesignation of anode and cathode

4)4) Nature of all the components- electrodes Nature of all the components- electrodes and ionsand ions

PracticePractice

Completely describe the galvanic cell Completely describe the galvanic cell based on the following half-reactions under based on the following half-reactions under standard conditions.standard conditions.

MnOMnO44-- + 8 H + 8 H++ +5e +5e-- Mn Mn+2 +2 + 4H+ 4H22OO

EEº=1.51º=1.51FeFe+3+3 +3e +3e-- Fe(s) Fe(s) EEº=0.036Vº=0.036V

Potential, Work and Potential, Work and GG

emf = potential (V) = work (J) / Charge(C)emf = potential (V) = work (J) / Charge(C)EE = work done by system / charge = work done by system / chargeEE = -w/q = -w/qCharge is measured in coulombs.Charge is measured in coulombs. -w = q-w = qEE Faraday = 96,485 C/mol eFaraday = 96,485 C/mol e--

q = nF = moles of eq = nF = moles of e-- x charge/mole e x charge/mole e--

w = -qw = -qEE = -nF = -nFE E = = G G

Potential, Work and Potential, Work and GG

Gº = -nFGº = -nFE E ºº if if E E º < 0, then º < 0, then Gº > 0 spontaneousGº > 0 spontaneous if if E E º > 0, then º > 0, then Gº < 0 nonspontaneousGº < 0 nonspontaneous In fact, reverse is spontaneous.In fact, reverse is spontaneous.Calculate Calculate Gº for the following reaction:Gº for the following reaction:CuCu+2+2(aq)+ Fe(s) (aq)+ Fe(s) Cu(s)+ FeCu(s)+ Fe+2+2(aq)(aq)

FeFe+2+2(aq)(aq) + e+ e--Fe(s)Fe(s) EEº = 0.44 Vº = 0.44 V

CuCu+2+2(aq)+2e(aq)+2e-- Cu(s) Cu(s) EEº = 0.34 Vº = 0.34 V

Cell Potential and Cell Potential and ConcentrationConcentration

Qualitatively - Can predict direction of change Qualitatively - Can predict direction of change

in in EE from LeChâtelier. from LeChâtelier.2Al(s) + 3Mn2Al(s) + 3Mn+2+2(aq) (aq) 2Al 2Al+3+3(aq) + 3Mn(s)(aq) + 3Mn(s)

Predict if Predict if EEcellcell will be greater or less than will be greater or less than EEººcellcell

if [Alif [Al+3+3] = 1.5 M and [Mn] = 1.5 M and [Mn+2+2] = 1.0 M] = 1.0 M if [Alif [Al+3+3] = 1.0 M and [Mn] = 1.0 M and [Mn+2+2] = 1.5M] = 1.5M if [Alif [Al+3+3] = 1.5 M and [Mn] = 1.5 M and [Mn+2+2] = 1.5 M ] = 1.5 M

The Nernst EquationThe Nernst EquationG = G = Gº +RTln(Q)Gº +RTln(Q) -nF-nFEE = -nF = -nFEEº + RTln(Q)º + RTln(Q)

EE = = EEº - º - RTRTln(Q)ln(Q)

nF nF2Al(s) + 3Mn2Al(s) + 3Mn+2+2(aq) (aq) 2Al 2Al+3+3(aq) + 3Mn(s) (aq) + 3Mn(s)

EEº = 0.48 Vº = 0.48 V Always have to figure out n by balancing.Always have to figure out n by balancing. If concentration can gives voltage, then If concentration can gives voltage, then

from voltage we can tell concentration.from voltage we can tell concentration.

The Nernst EquationThe Nernst EquationAs reactions proceed concentrations of As reactions proceed concentrations of

products increase and reactants decrease.products increase and reactants decrease.

Reach equilibrium where Q = K and Reach equilibrium where Q = K and EEcellcell

= 0= 0

0 = 0 = EEº - º - RTRTln(K)ln(K)

nFnF

EEº = º = RTRTln(K)ln(K)

nF nF

nFnFEEº º = ln(K)= ln(K)

RTRT

Batteries are Galvanic CellsBatteries are Galvanic Cells

Car batteries are lead storage batteries.Car batteries are lead storage batteries.Pb +PbOPb +PbO22 +H +H22SOSO44 PbSOPbSO44(s) +H(s) +H22OO

Dry CellDry Cell Zn + NHZn + NH44

++ +MnO +MnO2 2 Zn Zn+2 +2 + NH+ NH3 3 + H+ H22OO

AlkalineAlkaline Zn +MnOZn +MnO2 2 ZnO+ Mn ZnO+ Mn22OO3 3 (in base)(in base)

NiCadNiCad NiONiO22 + Cd + 2H + Cd + 2H22O O Cd(OH) Cd(OH)2 2 +Ni(OH)+Ni(OH)22

CorrosionCorrosion

Rusting - spontaneous oxidation.Rusting - spontaneous oxidation.Most structural metals have reduction Most structural metals have reduction

potentials that are less positive than Opotentials that are less positive than O22 . .

Fe Fe Fe Fe+2+2 +2e+2e-- EEº= 0.44 Vº= 0.44 V

OO22 + 2H + 2H22O + 4eO + 4e- - 4OH4OH-- EEº= 0.40 Vº= 0.40 V

FeFe+2+2 + O + O2 2 + H+ H22O O FeFe2 2 OO3 3 + H+ H++

Reaction happens in two places.Reaction happens in two places.

Water

Rust

Iron Dissolves- Fe Fe+2

e-

Salt speeds up process by increasing conductivity

Preventing CorrosionPreventing Corrosion

Coating to keep out air and water.Coating to keep out air and water.Galvanizing - Putting on a zinc coatGalvanizing - Putting on a zinc coatHas a lower reduction potential, so it is Has a lower reduction potential, so it is

more. easily oxidized.more. easily oxidized.Alloying with metals that form oxide coats.Alloying with metals that form oxide coats.Cathodic Protection - Attaching large Cathodic Protection - Attaching large

pieces of an active metal like magnesium pieces of an active metal like magnesium that get oxidized instead.that get oxidized instead.

Running a galvanic cell backwards.Running a galvanic cell backwards.Put a voltage bigger than the potential and Put a voltage bigger than the potential and

reverse the direction of the redox reaction.reverse the direction of the redox reaction.Used for electroplating.Used for electroplating.

ElectrolysisElectrolysis

1.0 M

Zn+2

e- e-

Anode Cathode

1.10

Zn Cu1.0 M

Cu+2

1.0 M

Zn+2

e- e-

AnodeCathode

A battery >1.10V

Zn Cu1.0 M

Cu+2

Calculating platingCalculating plating

Have to count charge.Have to count charge.Measure current Measure current II (in amperes) (in amperes)1 amp = 1 coulomb of charge per second1 amp = 1 coulomb of charge per secondq = q = II x t x tq/nF = moles of metalq/nF = moles of metalMass of plated metalMass of plated metalHow long must 5.00 amp current be How long must 5.00 amp current be

applied to produce 15.5 g of Ag from Agapplied to produce 15.5 g of Ag from Ag++

Other usesOther uses

Electroysis of water.Electroysis of water.Seperating mixtures of ions.Seperating mixtures of ions.More positive reduction potential means More positive reduction potential means

the reaction proceeds forward. the reaction proceeds forward. We want the reverse.We want the reverse.Most negative reduction potential is Most negative reduction potential is

easiest to plate out of solution.easiest to plate out of solution.

BalancingRedox

Equations

2. in baseAm3+(aq) + S2O8

2-(aq) ---->

AmO2+(aq) + SO4

2-(aq)

3. MnO4-(aq) + H2C2O4(aq)

Mn2+(aq) + CO2(g)

4. Bi(OH)3 + SnO22-

Bi(s) + SnO32-

ELECTROLYTICCELLS

Electrolytic Cell

a cell that uses electrical energy to produce a chemical change that would

otherwise NOT occur spontaneously

Process referred to

as electrolysis

(+) (-)

M MM+(aq)

X-(aq)

(+) (-) e-e-

M+(aq)

X-(aq)M M

AnodeM M+ + e-

oxidation

CathodeM+ + e- M reduction

Ampere a unit of electrical

current equal to one coulomb of charge

per second

1 amp = 1 coulsec

Coulomba unit of electric

charge equal to the quantity of charge in

about 6 x 1019 electrons

Faradaya constant

representing the charge on one mole

of electrons

1 F = 96,485 C96,500 C

3: It is necessary to replate a silver teapot with 15.0 g of silver. If the electrolytic cell runs at 2.00 amps, how long will it take to plate the teapot?

4: Sodium metal and chlorine gas are prepared industrially in a Down’s Cell from the electrolysis of molten NaCl. What mass of metal and volume of gas can be made per day if the cell operates at 7.0 volts and 4.0 x 104 amps if the cell is 75% efficient?

5: At what current must a cell be run in order to produce 5.0 kg of aluminum in 8.0 hours if the cell produces solid aluminum from molten aluminum chloride?

ELECTROCHEMISTRY,FREE ENERGY, & EQUILIBRIUM

)C(eargch

)J(work)V(emf

q

wE

thus: wmax = - q . Emax

but: wmax = G

and q = nF

thus if: wmax = - q . Emax

then G = - nFE

G = G0 + RT ln Q

G = - nFE

- nFE = - nFE0 + RT ln Q

Q lnnF

RTEE 0

NERNST EQUATION

Q lnnF

RTEE 0

if: aA + bB cC + dD

Q lnnF

RTEE 0

ba

dc

BA

DCQ

IF T = 250C = 298.15 K ln Q = 2.303 log Q R = 8.314 J/mol.K F = 96,485 C/mol

Q lnnF

RTEE 0

Q logn

0592.0EE 0

what if : Q = Keq ?

then: E = 0.0 V

Q lnnF

RTEE 0

K lnnF

RTE0

K logn

0592.0E0

6: Calculate the equilibrium constant at 400C for the cell:

Cd(s) Cd2+ (1M) Pb2+ (1M) Pb(s)

7a: Calculate the standard free energy for the cell:

Cr(s) Cr3+ (1M) Fe2+ (1M) Fe(s)

7a: Calculate the standard free energy for the cell:

Cr(s) Cr3+ (1M) Fe2+ (1M) Fe(s)

7b: What will be the voltage if [Fe2+] = 0.50M and [Cr3+] = 0.30M at 200C?

8: Through electrochemical

calculations, determine the Ksp for silver bromide.

AgBr + e- Ag + Br- E0 = 0.10 V

Review of Redox & Review of Redox & Electrochemical CellsElectrochemical Cells

ReviewOxidation: loss of e-

[increase in ox #] [reducing agent]Reduction: gain of e-

[decrease in ox #] [oxidizing agent]

Reduction Potential

The ease with which a chemical species can

be reduced

Standard ReductionPotential

Appendix MTable 20.1 in text

1. Which of the following elements listed is the best

reducing agent?CuZnFeAgCr

2a. Choosing from among the reactants in the given half reactions, identify the strongest and weakest oxidizing agents.

Anode and CathodeAnode and Cathode OXIDATION OXIDATION occurs at the occurs at the ANODEANODE.. REDREDuction occurs at the uction occurs at the CATCAThode.hode.

Electrochemical Celldevice in which

chemical energy is spontaneously

changed to electrical energy

battery

voltaic cell

galvanic cell

An electrochemical cell consists of ???

M1 M2

M1+(aq)

X-(aq) X-(aq)

M2+(aq)

M1 M2

M1+(aq)

X-(aq) X-(aq)

M2+(aq)

AnodeM1 M1

+ + e-

CathodeM2

+ + e- M2

M1 M2

M1+(aq)

X-(aq) X-(aq)

M2+(aq)

AnodeM1 M1

+ + e-

CathodeM2

+ + e- M2

K+(aq) NO3-(aq)

e- flow is from source of high

“concentration” to source of low “concentration”

M1 M2

M1+(aq)

X-(aq) X-(aq)

M2+(aq)

AnodeM1 M1

+ + e-

CathodeM2

+ + e- M2

K+(aq) NO3-(aq)e-

e-

shorthand notationoxidation reduction

M1 | M1+ || M2

+ | M2 anode cathode

e- flow

this e- flow can accomplish work

)C(eargch

)J(work)V(emf

Electrochemical Standard State

Conditions

[ions] = 1 MT = 250C

Pgas = 1 atm

An electrochemical cell is spontaneous if:

Oxidation-reduction occursEred + Eox > 0

2b. Which of the oxidizing agents listed is (are) capable of oxidizing Br- to BrO3

- ?

Line Notation:

ANODE CATHODENi(s)|Ni2+ (aq, 1 M)||Au3+(aq, 1 M)|Au(s) oxidation reduction

Line Notation:

ANODE CATHODE Al(s) | Al3+(aq, 1 M) || Ni2+(aq, 1 M) | Ni (s)

oxidation reduction