ElectrochemistryElectrochemistry

Thermodynamics at the electrodeThermodynamics at the electrode

Learning objectivesLearning objectives

You will be able to:You will be able to: Identify main components of an electrochemical cellIdentify main components of an electrochemical cell Write shorthand description of electrochemical cellWrite shorthand description of electrochemical cell Calculate cell voltage using standard reduction potentialsCalculate cell voltage using standard reduction potentials Apply Nernst equation to determine free energy changeApply Nernst equation to determine free energy change Apply Nernst equation to determine pHApply Nernst equation to determine pH Calculate K from electrode potentialsCalculate K from electrode potentials Calculate amount of material deposited in electrolysisCalculate amount of material deposited in electrolysis

Energy in or energy outEnergy in or energy out

GalvanicGalvanic (or (or voltaicvoltaic) cell relies on ) cell relies on spontaneous process to generate a potential spontaneous process to generate a potential capable of performing work – energy outcapable of performing work – energy out

ElectrolyticElectrolytic cell performs chemical reactions cell performs chemical reactions through application of a potential – energy inthrough application of a potential – energy in

Redox ReviewRedox Review

Oxidation is...Oxidation is... Loss of electronsLoss of electrons

Reduction is...Reduction is... Gain of electronsGain of electrons

Oxidizing agents oxidize and are reducedOxidizing agents oxidize and are reduced Reducing agents reduce and are oxidizedReducing agents reduce and are oxidized

Redox at the heart of the matterRedox at the heart of the matter

Zn displaces Cu from CuSOZn displaces Cu from CuSO44(aq)(aq)

In direct contact the enthalpy of reaction is In direct contact the enthalpy of reaction is dispersed as heat, and no useful work is donedispersed as heat, and no useful work is done

Redox process: Redox process: Zn is the reducing agentZn is the reducing agent CuCu2+2+ is the oxidizing agent is the oxidizing agent

eaqZnsZn 2)()( 2

)(2)(2 sCueaqCu

Separating the combatantsSeparating the combatants

Each metal in touch with a solution of its own ionsEach metal in touch with a solution of its own ions External circuit carries electrons transferred during the redox processExternal circuit carries electrons transferred during the redox process A “salt bridge” containing neutral ions completes the internal circuit.A “salt bridge” containing neutral ions completes the internal circuit. With no current flowing, a potential develops – the potential for workWith no current flowing, a potential develops – the potential for work Unlike the reaction in the beaker, the energy released by the reaction Unlike the reaction in the beaker, the energy released by the reaction

in the cell can perform useful work – like lighting a bulbin the cell can perform useful work – like lighting a bulb

Labelling the partsLabelling the parts

Odes to a galvanic cellOdes to a galvanic cell

CathodeCathode Where reduction occursWhere reduction occurs Where electrons are Where electrons are

consumedconsumed Where positive ions Where positive ions

migrate tomigrate to Has positive signHas positive sign

AnodeAnode Where oxidation occursWhere oxidation occurs Where electrons are Where electrons are

generatedgenerated Where negative ions Where negative ions

migrate tomigrate to Has negative signHas negative sign

The role of inert electrodesThe role of inert electrodes

Not all cells start with elements as the redox Not all cells start with elements as the redox agentsagents

Consider the cellConsider the cell

Fe can be the anode but FeFe can be the anode but Fe3+3+ cannot be the cannot be the cathode.cathode.

Use the FeUse the Fe3+3+ ions in solution as the ions in solution as the “cathode” with an inert metal such as Pt“cathode” with an inert metal such as Pt

)(3)(2)( 23 aqFeaqFesFe

Anode Cathode

Oxidation

Reduction

Cell notationCell notation

Anode on left, cathode on rightAnode on left, cathode on right Electrons flow from left to rightElectrons flow from left to right Oxidation on left, reduction on rightOxidation on left, reduction on right Single vertical = electrode/electrolyte boundarySingle vertical = electrode/electrolyte boundary Double vertical = salt bridgeDouble vertical = salt bridge

Anode:Zn →Zn2+ +

2e

Cathode:Cu2+ + 2e

→Cu

Vertical │denotes different phaseVertical │denotes different phase

Fe(s)Fe(s)│Fe│Fe2+2+(aq)║Fe(aq)║Fe3+3+(aq),Fe(aq),Fe2+2+(aq)│Pt(s)(aq)│Pt(s)

Cu(s)Cu(s)│Cu│Cu2+2+(aq)║Cl(aq)║Cl22(g)│Cl(g)│Cl--(aq)│C(s)(aq)│C(s)

Connections: cell potential and free Connections: cell potential and free energyenergy

The cell in open circuit generates an The cell in open circuit generates an electromotive force (emf) or potential or electromotive force (emf) or potential or voltage. This is the potential to perform voltage. This is the potential to perform workwork

Energy is charge moving under applied Energy is charge moving under applied voltagevoltage

VCJ 111

Relating free energy and cell Relating free energy and cell potentialpotential

The Faraday: The Faraday:

F = 96 485 C/mol eF = 96 485 C/mol e

Standard conditions (1 M, 1 atm, 25Standard conditions (1 M, 1 atm, 25°C)°C)

nFEG

nFEG

Standard Reduction PotentialsStandard Reduction Potentials

The total cell potential is the sum of the potentials The total cell potential is the sum of the potentials for the two half reactions at each electrodefor the two half reactions at each electrode

EEcellcell = E = Ecathcath + E + Eanan

From the cell voltage we cannot determine the From the cell voltage we cannot determine the values of either – we must know one to get the values of either – we must know one to get the otherother

Enter the Enter the standard hydrogen electrode (SHE)standard hydrogen electrode (SHE) All potentials are referenced to the SHE (=0 V)All potentials are referenced to the SHE (=0 V)

Unpacking the SHEUnpacking the SHE

The SHE consists of a Pt electrode in contact with The SHE consists of a Pt electrode in contact with HH22(g) at 1 atm in a solution of 1 M H(g) at 1 atm in a solution of 1 M H++(aq). (aq).

The voltage of this half-cell is defined to be 0 VThe voltage of this half-cell is defined to be 0 V An experimental cell containing the SHE half-cell An experimental cell containing the SHE half-cell

with other half-cell gives voltages which are the with other half-cell gives voltages which are the standard potentials for those half-cellsstandard potentials for those half-cells

EEcellcell = 0 + E = 0 + Ehalf-cellhalf-cell

Zinc half-cell with SHEZinc half-cell with SHE

Cell measures 0.76 VCell measures 0.76 V Standard potential for Zn(s) = ZnStandard potential for Zn(s) = Zn2+2+(aq) + 2e = 0.76 (aq) + 2e = 0.76

VV

Where there is no SHEWhere there is no SHE

In this cell there is no SHE and the In this cell there is no SHE and the measured voltage is 1.10 Vmeasured voltage is 1.10 V

)()()()( 22 sCuaqZnaqCusZn

CuaquCaqZnZn )()( 22

VEeaqZnsZn o 76.0,2)()( 2

VEsCueaqCu o 34.0),(2)(2

Standard reduction potentialsStandard reduction potentials

Any half reaction can be written in two ways:Any half reaction can be written in two ways: Oxidation:Oxidation:

M = MM = M++ + e (+V) + e (+V) Reduction:Reduction:

MM++ + e = M (-V) + e = M (-V) Listed potentials are standardListed potentials are standard reduction reduction

potentialspotentials

Applying standard reduction Applying standard reduction potentialspotentials

Consider the reaction Consider the reaction

What is the cell potential?What is the cell potential? The half reactions are:The half reactions are:

EE° = 0.80 V – (-0.76 V) = 1.56 V° = 0.80 V – (-0.76 V) = 1.56 V NOTE: Although there are 2 moles of Ag NOTE: Although there are 2 moles of Ag

reduced for each mole of Zn oxidized, we do not reduced for each mole of Zn oxidized, we do not multiply the potential by 2.multiply the potential by 2.

)(2)()(2)( 2 sAgaqZnaqAgsZn

eaqZnsZn 2)()( 2 )()( sAgeaqAg

Extensive Extensive v v intensiveintensive

Free energy is Free energy is extensiveextensive property so need to property so need to multiply by no of moles involvedmultiply by no of moles involved

But to convert to E we need to divide by no of But to convert to E we need to divide by no of electrons involvedelectrons involved

E is an E is an intensiveintensive property property

nFEG

nFGE

The Nernst equationThe Nernst equation

Working in nonstandard conditions Working in nonstandard conditions

QRTnFEnFE ln

QRTGG ln

QnFRTEE ln

QnEE log0592.0

Electrode potentials and pHElectrode potentials and pH

For the cell reactionFor the cell reaction

The Nernst equationThe Nernst equation

Half-cell potential is proportional to pHHalf-cell potential is proportional to pH

222

2

22log

06.0

HHHHH p

H

n

VEE

eaqHgH 2)(2)(2

2

2log

06.02

Hn

VE

HH

The pH meter is an electrochemical cellThe pH meter is an electrochemical cell

Overall cell potential is proportional to pHOverall cell potential is proportional to pH

In practice, a hydrogen electrode is In practice, a hydrogen electrode is impracticalimpractical

refcell EpHVE 06.0

V

EEpH refcell

06.0

Calomel reference electrodesCalomel reference electrodes

The potential of the calomel electrode is known vs The potential of the calomel electrode is known vs the SHE. This is used as the reference electrode the SHE. This is used as the reference electrode in the measurement of pHin the measurement of pH

The other electrode in a pH probe is a glass The other electrode in a pH probe is a glass electrode which has a Ag wire coated with AgCl electrode which has a Ag wire coated with AgCl dipped in HCl(aq). A thin membrane separates dipped in HCl(aq). A thin membrane separates the HCl from the test solutionthe HCl from the test solution

CllHgesClHg 2)(22)(22

Cell potentials and equilibriumCell potentials and equilibrium

Lest we forget…Lest we forget…

So thenSo then

andand

KRTnFE ln

nFEGKRTG ln

KnF

RTKnF

RTE 10log303.2

ln

Cell potential a convenient way to Cell potential a convenient way to measure Kmeasure K

Many pathways to one endingMany pathways to one ending

Measurement of K from different Measurement of K from different experimentsexperiments Concentration dataConcentration data

Thermochemical dataThermochemical data

Electrochemical dataElectrochemical data

ba

dc

BA

DC

KRTG ln

KRTnFE ln

BatteriesBatteries

The most important application of galvanic The most important application of galvanic cellscells

Several factors influence the choice of Several factors influence the choice of materialsmaterials VoltageVoltage WeightWeight CapacityCapacity Current densityCurrent density Rechargeability Rechargeability

Running in reverseRunning in reverse

Recharging a battery requires to run the Recharging a battery requires to run the process in reverse by applying a voltageprocess in reverse by applying a voltage

In principle any reaction can be reversedIn principle any reaction can be reversed In practice it will depend upon many factorsIn practice it will depend upon many factors Reversibility depends on kinetics and not Reversibility depends on kinetics and not

thermodynamicsthermodynamics Cell reactions that involve minimal structural Cell reactions that involve minimal structural

rearrangement will be the easiest to reverserearrangement will be the easiest to reverse

Lithium batteriesLithium batteries

Lightweight (Molar mass Li = 6.94 g)Lightweight (Molar mass Li = 6.94 g) High voltageHigh voltage Reversible processReversible process

Fuel cells – a battery with a Fuel cells – a battery with a differencedifference

Reactants are not contained within a sealed Reactants are not contained within a sealed container but are supplied from outside container but are supplied from outside sourcessources

elOHaqOHgHanode 4)(4)(4)(2: 22

)(44)(2)(: 22 aqOHelOHgOcathode

)(2)()(2: 222 lOHgOgHoverall

Store up not treasures on earth Store up not treasures on earth where moth and rust…where moth and rust…

An electrochemical mechanism for corrosion of iron. The metal and a surface An electrochemical mechanism for corrosion of iron. The metal and a surface water droplet constitute a tiny galvanic cell in which iron is oxidized to Fewater droplet constitute a tiny galvanic cell in which iron is oxidized to Fe2+2+ in a in a region of the surface (anode region) remote from atmospheric Oregion of the surface (anode region) remote from atmospheric O22, and O, and O22 is is

reduced near the edge of the droplet at another region of the surface (cathode reduced near the edge of the droplet at another region of the surface (cathode region). Electrons flow from anode to cathode through the metal, while ions flow region). Electrons flow from anode to cathode through the metal, while ions flow through the water droplet. Dissolved Othrough the water droplet. Dissolved O22 oxidizes Fe oxidizes Fe2+2+ further to Fe further to Fe3+3+ before it is before it is

deposited as rust (Fedeposited as rust (Fe22OO33·H2O).·H2O).

MechanismsMechanisms

Why does salt enhance rusting?Why does salt enhance rusting? Improves conductivity of electrolyteImproves conductivity of electrolyte

Standard reduction potentials indicate which Standard reduction potentials indicate which metals will “rust”metals will “rust”

Aluminium should corrode readily. It Aluminium should corrode readily. It doesn’t. Is thermodynamics wrong?doesn’t. Is thermodynamics wrong? No, the AlNo, the Al22OO33 provides an impenetrable barrier provides an impenetrable barrier

No greater gift than to give up your No greater gift than to give up your life for your friendlife for your friend

A layer of zinc protects iron from oxidation, even when the A layer of zinc protects iron from oxidation, even when the zinc layer becomes scratched. The zinc (anode), iron zinc layer becomes scratched. The zinc (anode), iron (cathode), and water droplet (electrolyte) constitute a tiny (cathode), and water droplet (electrolyte) constitute a tiny galvanic cell. Oxygen is reduced at the cathode, and zinc is galvanic cell. Oxygen is reduced at the cathode, and zinc is oxidized at the anode, thus protecting the iron from oxidized at the anode, thus protecting the iron from oxidation.oxidation.

ElectrolysisElectrolysis Electrolysis of a molten salt using inert electrodesElectrolysis of a molten salt using inert electrodes Signs of electrodes:Signs of electrodes:

In electrolysis, anode is positive because electrons are removed In electrolysis, anode is positive because electrons are removed from it by the batteryfrom it by the battery

In a galvanic cell, the anode is negative because is supplies In a galvanic cell, the anode is negative because is supplies electrons to the external circuitelectrons to the external circuit

egCllClAnode 2)()(2: 2

)()(2)(2)(2: 2 gCllNalCllNaOverall

)(22)(2: lNaelNaCathode

Electrolysis in aqueous solutions – a Electrolysis in aqueous solutions – a choice of processchoice of process

There are (potentially) There are (potentially) competing processes competing processes in the electrolysis of an in the electrolysis of an aqueous solutionaqueous solution CathodeCathode

AnodeAnode

VElNaelNaCathode 71.2)...(22)(2:

VEaqOHgHelOHCathode 83.0)...(2)(2)(2: 22

VEegCllClAnode 36.1...2)()(2: 2

VEeHgOlOHAnode 23.1...44)()(2: 22

Thermodynamics or kinetics?Thermodynamics or kinetics?

On the basis of thermodynamics we choose On the basis of thermodynamics we choose the processes which are favoured the processes which are favoured energeticallyenergetically

But…chlorine is evolved at the anodeBut…chlorine is evolved at the anode

VEeHgOlOHAnode 23.1...44)()(2: 22

VEaqOHgHelOHCathode 83.0)...(2)(2)(2: 22

The role of The role of overpotentialsoverpotentials

Thermodynamic quantities prevail only at Thermodynamic quantities prevail only at equilibrium – no current flowingequilibrium – no current flowing

When current flows, kinetic considerations When current flows, kinetic considerations come into playcome into play

Overpotential represents the additional Overpotential represents the additional voltage that must be applied to drive the voltage that must be applied to drive the processprocess

In the NaCl(aq) solution the overpotential for In the NaCl(aq) solution the overpotential for evolution of oxygen is greater than that for evolution of oxygen is greater than that for chlorine, and so chlorine is evolved chlorine, and so chlorine is evolved preferentiallypreferentially

Overpotential will depend on the electrolyte Overpotential will depend on the electrolyte and electrode. By suitable choices, and electrode. By suitable choices, overpotentials can be minimized but are never overpotentials can be minimized but are never eliminatedeliminated

The limiting process in electrolysis is usually The limiting process in electrolysis is usually diffusion of the ions in the electrolyte (but not diffusion of the ions in the electrolyte (but not always)always)

Driving the cell at the least current will give Driving the cell at the least current will give rise to the smallest overpotentialrise to the smallest overpotential

Electrolysis of waterElectrolysis of water

In aqueous solutions of In aqueous solutions of most salts or acids or most salts or acids or bases the products will bases the products will be Obe O22 and H and H22

VEeHgOlOHAnode 23.1...44)()(2: 22

VEaqOHgHelOHCathode 83.0)...(2)(2)(2: 22

Quantitative aspects of electrolysisQuantitative aspects of electrolysis

Quantitative analysis Quantitative analysis uses the current uses the current flowing as a measure flowing as a measure of the amount of of the amount of materialmaterial

Charge = current x Charge = current x timetime

Moles = Moles = charge/Faradaycharge/Faraday