ELECTROCHEMISTRY

transcript

ELECTROCHEMISTELECTROCHEMISTRYRY

The study of the The study of the interchange of chemical interchange of chemical

and electrical energyand electrical energy

Terms to Know: Terms to Know:

OIL RIGOIL RIG

ooxidation xidation iis s lloss, oss, rreduction eduction iis s ggain ain

(of electrons)(of electrons)

OxidationOxidation

the loss of electrons, the loss of electrons,

increase in chargeincrease in charge

ReductionReduction

the gain of electrons, the gain of electrons,

reduction of chargereduction of charge

Oxidation numberOxidation number

the assigned charge on an the assigned charge on an atomatom

Oxidizing agent (OA)Oxidizing agent (OA)

the species that is reduced the species that is reduced

and thus and thus

CAUSES CAUSES oxidationoxidation

Reducing agent (RA)Reducing agent (RA)

the species that is oxidizedthe species that is oxidized

and thus and thus

CAUSESCAUSES reductionreduction

ELECTROCHEMISTRY ELECTROCHEMISTRY INVOLVES TWO MAIN TYPES INVOLVES TWO MAIN TYPES

OF PROCESSESOF PROCESSES

Galvanic (voltaic) cellsGalvanic (voltaic) cells

spontaneous chemical spontaneous chemical reactionsreactions

(battery)(battery)

Electrolytic cellsElectrolytic cells

non-spontaneous and require non-spontaneous and require

external e-source external e-source

(DC power source)(DC power source)

BOTH of these fit into the BOTH of these fit into the category category

entitled: entitled:

Electrochemical CellsElectrochemical Cells

Galvanic CellsGalvanic Cells

Parts of the voltaic or galvanic Parts of the voltaic or galvanic cell… cell…

AnodeAnode

the electrode where oxidation the electrode where oxidation occurs occurs

After a period of time, the anode After a period of time, the anode may may

appear to become smaller as it appear to become smaller as it falls falls

into solution. into solution.

CathodeCathode

the electrode where reduction the electrode where reduction occursoccurs

After a period of time it may After a period of time it may appear appear

larger, due to ions from solution larger, due to ions from solution

plating onto it. plating onto it.

Inert ElectrodesInert Electrodes

used when a gas is involved OR ion used when a gas is involved OR ion

to ion involved such as:to ion involved such as:

FeFe3+3+ being reduced to Fe being reduced to Fe2+2+ rather rather

than Fethan Fe00

made of Pt or graphitemade of Pt or graphite

Salt BridgeSalt Bridge

a device used to maintain electrical a device used to maintain electrical

neutrality in a galvanic cell neutrality in a galvanic cell

This may be filled with agar which This may be filled with agar which

contains a neutral salt or it may be contains a neutral salt or it may be

replaced with a porous cup. replaced with a porous cup.

Electron FlowElectron Flow

always from anode to cathode always from anode to cathode

(through the wire) (through the wire)

Standard Cell Notation Standard Cell Notation (line notation)(line notation)

anode/solution//cathode anode/solution//cathode solution/cathode solution/cathode

Example: Example:

Zn/ZnZn/Zn2+2+ (1.0 M) // Cu (1.0 M) // Cu2+2+ (1.0M) / Cu (1.0M) / Cu

VoltmeterVoltmeter

measures the cell potential measures the cell potential (emf) (emf)

usually is measured in voltsusually is measured in volts

Balance this re-dox reaction:Balance this re-dox reaction:

MnOMnO44-- + Fe + Fe2+2+ Mn Mn2+2+ + Fe + Fe3+3+ [acidic] [acidic]

RED:RED:

OA:OA:

OX:OX:

RA:RA:

Overall rxn:Overall rxn:

If we place MnOIf we place MnO44-- and and

FeFe2+2+ in the same in the same container:container:

TThe electrons are he electrons are transferred transferred directly when the directly when the reactants collide. reactants collide. No useful work is No useful work is obtained from obtained from the chemical the chemical energy involved energy involved which is instead which is instead released as heat!released as heat!

We can harness this energy if We can harness this energy if we separate the oxidizing we separate the oxidizing agent from the reducing agent from the reducing agent, thus requiring the eagent, thus requiring the e-- transfer to occur through a transfer to occur through a wire! wire!

We can harness the energy We can harness the energy that way to run a motor, light a that way to run a motor, light a bulb, etc.bulb, etc.

Sustained electron flow cannot Sustained electron flow cannot occur in this picture. occur in this picture.

Why not?Why not?

Because…Because…

As soon as electrons flow, a As soon as electrons flow, a separation of charge occurs separation of charge occurs which stops the flow of which stops the flow of electrons. electrons.

How do we fix it?How do we fix it?

It’s job is to It’s job is to balance the balance the charge using an charge using an electrolyte electrolyte [usually in a U-[usually in a U-shaped tube shaped tube filled with agar filled with agar that has the that has the salt dissolved salt dissolved into it before it into it before it gels].gels].

It connects the two It connects the two compartments, ions flow from it, compartments, ions flow from it, AND it keeps each “cell” neutral. AND it keeps each “cell” neutral.

Use KNOUse KNO33 as the salt when as the salt when constructing your own diagram so constructing your own diagram so that no precipitation occurs!that no precipitation occurs!

Porous Disk or CupPorous Disk or Cup

… … also allows also allows both cells to both cells to remain remain neutral by neutral by allowing ions allowing ions to flow.to flow.

Cell PotentialCell Potential

Ecell, Emf, or Ecell, Emf, or cell —cell —

a measure of the electromotive a measure of the electromotive force or the “pull” of the force or the “pull” of the electrons as they travel from the electrons as they travel from the anode to the cathode anode to the cathode

[more on that later!][more on that later!]

Volt (V)Volt (V)

the unit of electrical the unit of electrical potential potential

equal to 1 joule of work per equal to 1 joule of work per coulomb of charge coulomb of charge transferredtransferred

VoltmeterVoltmeter

measures electrical potential measures electrical potential

Some energy is lost as heat Some energy is lost as heat [resistance] which keeps the [resistance] which keeps the voltmeter reading a tad lower voltmeter reading a tad lower than the actual or calculated than the actual or calculated voltage. voltage.

Digital voltmeters have less Digital voltmeters have less resistance. resistance.

If you want to get picky and If you want to get picky and eliminate the error introduced by eliminate the error introduced by resistance, you attach a variable-resistance, you attach a variable-external-power source called a external-power source called a potentiometerpotentiometer. .

Adjust it so that zero current flowsAdjust it so that zero current flows—the accurate voltage is then —the accurate voltage is then equal in magnitude but opposite in equal in magnitude but opposite in sign to the reading on the sign to the reading on the potentiometer.potentiometer.

Standard Reduction Standard Reduction PotentialsPotentials

Each half-reaction has a cell potential.Each half-reaction has a cell potential.

Each potential is measured against a Each potential is measured against a

standard which is the standard hydrogen standard which is the standard hydrogen

electrode [consists of a piece of inert electrode [consists of a piece of inert

platinum that is bathed by hydrogen gas platinum that is bathed by hydrogen gas

at 1 atm]. at 1 atm].

The hydrogen electrode is The hydrogen electrode is assigned a value of ZERO assigned a value of ZERO volts.volts.

Standard ConditionsStandard Conditions

1 atm for gases 1 atm for gases

1.0M for solutions 1.0M for solutions

2525C for all (298 K)C for all (298 K)

Naught, °Naught, °

We use the naught to We use the naught to symbolize symbolize

standard conditions standard conditions

[Experiencing a thermo [Experiencing a thermo flashback?] flashback?]

That means EThat means Ecellcell, Emf, or , Emf, or cellcell become Ebecome Ecellcell

oo , Emf , Emfoo , or , or cellcelloo

when measurements are taken when measurements are taken at standard conditions. at standard conditions.

You’ll soon learn how these You’ll soon learn how these change when the conditions change when the conditions are non-standard!are non-standard!

The diagram to The diagram to the right the right illustrates what illustrates what really happens really happens when a Galvanic when a Galvanic cell is cell is constructed from constructed from zinc sulfate and zinc sulfate and copper (II) copper (II) sulfate using the sulfate using the respective respective metals as metals as electrodes.electrodes.

Notice that Notice that 1.0 M 1.0 M solutions of solutions of each salt each salt are used…are used…

Notice an Notice an overall overall voltage of voltage of 1.10 V for 1.10 V for the the process…process…

Reading the reduction Reading the reduction potential chartpotential chart

Elements that have the most positive Elements that have the most positive

reduction potentials are easily reduced reduction potentials are easily reduced

(in general, non-metals).(in general, non-metals).

Elements that have the least positive Elements that have the least positive

reduction potentials are easily oxidized reduction potentials are easily oxidized

(in general, metals).(in general, metals).

The table can also be used to The table can also be used to tell the strength of various tell the strength of various oxidizing and reducing oxidizing and reducing agents.agents.

It can also be used as an It can also be used as an activity series. activity series.

Metals having less positive Metals having less positive reduction potentials are more reduction potentials are more active and will replace metals active and will replace metals with more positive potentials.with more positive potentials.

HOW CAN WE DETERMINEHOW CAN WE DETERMINE

WHICH SUBSTANCE IS WHICH SUBSTANCE IS

BEING REDUCED AND BEING REDUCED AND

WHICH IS BEING WHICH IS BEING

OXIDIZED??OXIDIZED??

The The MORE POSITIVEMORE POSITIVE reduction reduction

potential gets to indeed be potential gets to indeed be reduced reduced

IFIF you are trying to set up a cell you are trying to set up a cell

that can act as a battery.that can act as a battery.

Standard Reduction Standard Reduction Potentials in Aqueous Potentials in Aqueous Solution at 25° CSolution at 25° C

Calculating Standard Cell Calculating Standard Cell PotentialPotential

Symbolized by Symbolized by

EEcellcell OR Emf OR Emf OR OR cellcell

[I’ll mix and match!][I’ll mix and match!]

Decide which element is Decide which element is oxidized or reduced using the oxidized or reduced using the table of reduction potentials. table of reduction potentials.

Remember: Remember:

THE MORE POSITIVE THE MORE POSITIVE REDUCTION POTENITAL REDUCTION POTENITAL GETS TO BE REDUCEDGETS TO BE REDUCED..

Write both equations Write both equations AS ISAS IS from the from thechart with their voltages.chart with their voltages.

Reverse the equation that will be Reverse the equation that will be oxidized oxidized

and change the sign of the voltageand change the sign of the voltage

[this is now E[this is now Eoxidationoxidation].].

Balance the two half reactions.Balance the two half reactions.

**do not multiply voltage values****do not multiply voltage values**

Add the two half reactions and Add the two half reactions and the the

voltages together.voltages together.

EEcell cell = E = Eoxidationoxidation + E + Ereductionreduction

° means standard conditions: ° means standard conditions:

1atm, 1M, 251atm, 1M, 25CC

Terms to know in order to Terms to know in order to

construct a spontaneous construct a spontaneous

cell—one that can act as a cell—one that can act as a

battery:battery:

AN OXAN OX

oxidation occurs at the anode oxidation occurs at the anode

(may show mass decrease)(may show mass decrease)

RED CATRED CAT

reduction occurs at the cathode reduction occurs at the cathode

(may show mass increase)(may show mass increase)

FAT CATFAT CAT

The electrons in a voltaic or The electrons in a voltaic or

galvanic cell galvanic cell ALWAYSALWAYS flow: flow:

FFrom the rom the AAnode node TTo the o the CATCAThodehode

CaCa++hodehode

The cathode is The cathode is ++ in galvanic cells. in galvanic cells.

Bridge between cells Bridge between cells whose purpose is whose purpose is to provide ions to to provide ions to balance the balance the

chargecharge. .

Usually made of a salt Usually made of a salt

filled agar (KNOfilled agar (KNO33) or a ) or a porous cup.porous cup.

ANIONS from the salt move to the anode while CATIONS from the salt move to the cathode!

EPAEPA

In an In an eelectrolytic cell, there is a lectrolytic cell, there is a

ppositive ositive aanode.node.

Exercise 1Exercise 1

A. Consider a galvanic cell based on A. Consider a galvanic cell based on

the reaction:the reaction:

AlAl3+3+(aq) + Mg(s) (aq) + Mg(s) → Al(s) + Ag→ Al(s) + Ag2+2+(aq)(aq)

Give the balanced cell reaction and Give the balanced cell reaction and

calculate E° for the cell.calculate E° for the cell.

SolutionSolution

A. E° = 0.71 VA. E° = 0.71 V

B. A galvanic cell is based on the B. A galvanic cell is based on the reaction [you’ll need a more reaction [you’ll need a more complete table of reduction complete table of reduction potentials!]:potentials!]:

MnOMnO44--(aq) + H(aq) + H++(aq) + ClO(aq) + ClO33

--(aq) → (aq) →

ClOClO44--(aq) + Mn(aq) + Mn2+2+(aq) + H(aq) + H22O(l)O(l)

Give the balanced cell reaction Give the balanced cell reaction and and

calculate E° for the cell.calculate E° for the cell.

SolutionSolution

B. E° = 0.32 VB. E° = 0.32 V

Standard cell notation Standard cell notation (line notation)(line notation)

““Ion sandwich” in alphabetical Ion sandwich” in alphabetical orderorder

AAnode metal | anode ion ||node metal | anode ion ||

cathode ion | cathode ion | CCathode athode metalmetal

For Reaction: For Reaction: M + N M + N++ →→ N + M N + M++

Anode || Cathode (alphabetical Anode || Cathode (alphabetical order!)order!)

M(electrode)M(electrode)||MM++ (solution) || (solution) ||

NN++ (solution) (solution)||N(electrode) N(electrode)

| - | - indicates phase boundaryindicates phase boundary

|| - indicates salt bridge|| - indicates salt bridge

ExampleExample

Zn | ZnZn | Zn2+2+ (1.0M) || Cu (1.0M) || Cu2+2+ (1.0M) | Cu (1.0M) | Cu

Sample ProblemSample Problem

Calculate the cell voltage for the Calculate the cell voltage for the

following reaction. Draw a diagram following reaction. Draw a diagram of of

the galvanic cell for the reaction and the galvanic cell for the reaction and

label completely.label completely.

FeFe3+3+(aq)(aq) + Cu + Cu(s)(s) Cu Cu2+2+

(aq)(aq) + Fe + Fe2+2+(aq)(aq)

Calculate the cell voltage for the Calculate the cell voltage for the

galvanic cell that would utilize silver galvanic cell that would utilize silver

metal and involve iron (II) ion and iron metal and involve iron (II) ion and iron

(III) ion. (III) ion.

Draw a diagram of the galvanic cell for Draw a diagram of the galvanic cell for

the reaction and label completely.the reaction and label completely.

SolutionSolution

E°E°cell cell = 0.03 V= 0.03 V

Cell Potential, Electrical Cell Potential, Electrical Work & Free EnergyWork & Free Energy

Combining the thermodynamics Combining the thermodynamics and and

the electrochemistry, not to the electrochemistry, not to

mention a bit of physics…mention a bit of physics…

The work that can be The work that can be accomplished when electrons are accomplished when electrons are transferred through a wire transferred through a wire depends on the “push” or emf depends on the “push” or emf which is defined in terms of a which is defined in terms of a potential difference [in volts] potential difference [in volts] between two points in the circuit.between two points in the circuit.

JworkVemf

Thus, one joule of work is Thus, one joule of work is

produced [or required] when produced [or required] when one one

coulomb of charge is transferred coulomb of charge is transferred

between two points in the circuit between two points in the circuit

that differ by a potential of one that differ by a potential of one

volt.volt.

IF work flows OUT, it is assigned a IF work flows OUT, it is assigned a

minusminus sign. sign.

When a cell produces a current, the When a cell produces a current, the

cell potential is cell potential is positivepositive and the and the

current can be used to do work.current can be used to do work.

Therefore, Therefore, and work have and work have opposite signs!opposite signs!

= = - w - w

therefore,therefore,

-w = q-w = q

Faraday(F)Faraday(F)

the charge on one MOLE of the charge on one MOLE of

electrons = 96,485 electrons = 96,485 coulombscoulombs

q = # moles of electrons x q = # moles of electrons x FF

For a process carried out at For a process carried out at

constant temperature and constant temperature and pressure, pressure,

wmaxwmax [neglecting the very small [neglecting the very small

amount of energy that is lost as amount of energy that is lost as

friction or heat] is equal to friction or heat] is equal to GG, ,

therefore… therefore…

ΔGΔGoo = -nFE = -nFEoo

G = Gibb’s free energy G = Gibb’s free energy

n = number of moles of electrons n = number of moles of electrons

F = Faraday constant F = Faraday constant

(9.6485309 x 10(9.6485309 x 1044 J/V J/V mol) mol)

So it follows that:So it follows that:

-E-Eoo implies implies nonnonspontaneous spontaneous

+E+Eoo implies spontaneous implies spontaneous (would be a good battery!)(would be a good battery!)

Strongest Oxidizers are Strongest Oxidizers are Weakest ReducersWeakest Reducers

As EAs Eoo reducing strength reducing strength

As EAs Eoo oxidizing strength oxidizing strength

Using the table of standard reduction Using the table of standard reduction

potentials, calculate ∆G° for the potentials, calculate ∆G° for the reaction:reaction:

CuCu2+2+(aq) + Fe(s) → Cu(s) + Fe(aq) + Fe(s) → Cu(s) + Fe2+2+(aq)(aq)

Is this reaction spontaneous?Is this reaction spontaneous?

YesYes

Using the table of standard Using the table of standard

reduction potentials, predict reduction potentials, predict

whether 1 whether 1 MM HNO HNO33 will dissolve will dissolve

gold metal to form a 1 gold metal to form a 1 MM Au Au3+3+

solution.solution.

Dependence of Cell Potential on Dependence of Cell Potential on ConcentrationConcentration

Voltaic cells at NONstandard conditions Voltaic cells at NONstandard conditions ----

LeChatlier’s principle can be applied. LeChatlier’s principle can be applied.

An increase in the concentration of a An increase in the concentration of a

reactant will favor the forward reaction reactant will favor the forward reaction

and the cell potential will increase. and the cell potential will increase.

The converse is also true! The converse is also true!

For the cell reaction: For the cell reaction:

2Al(s) + 3Mn2Al(s) + 3Mn2+2+(aq) (aq) → 2Al→ 2Al3+3+(aq) + (aq) + 3Mn(s)3Mn(s)

E°E°cellcell = ?? = ??

Predict whether EPredict whether Ecellcell is larger or is larger or

smaller than E°smaller than E°cellcell for the following for the following

cases:cases:

a. [Ala. [Al3+3+ ] = 2.0 ] = 2.0 MM, [Mn, [Mn2+2+ ] = 1.0 ] = 1.0 MM

b. [Alb. [Al3+3+ ] = 1.0 ] = 1.0 MM, [Mn, [Mn2+2+] = 3.0 ] = 3.0 MM

A: A: EEcellcell < < E°E°cellcell

B: B: EEcellcell > > E°E°cellcell

For a more quantitative For a more quantitative

approach…..approach…..

When cell is not at standard When cell is not at standard conditions, use conditions, use NernstNernst Equation Equation E = EE = Eoo – – RT RT ln Q ln Q nFnF

R = Gas constant 8.315 J/KR = Gas constant 8.315 J/K mol molF = Faraday constantF = Faraday constantQ = reaction quotient Q = reaction quotient [products[productscoefficientcoefficient]/[reactants]/[reactantscoefficientcoefficient]]E = Energy produced by reactionE = Energy produced by reactionT = Temperature in KelvinsT = Temperature in Kelvinsn = # of electrons exchanged in BALANCED n = # of electrons exchanged in BALANCED redox equationredox equation

Rearranged, another Rearranged, another useful formuseful form

NERNST EQUATION:NERNST EQUATION:

E = E° - E = E° - 0.0592 0.0592 log Q log Q @ @ 25°C(298K)25°C(298K)

As E declines with reactants As E declines with reactants converting converting

to products, E eventually reaches to products, E eventually reaches zero. zero.

Zero potential means reaction is at Zero potential means reaction is at

equilibrium [dead battery]. equilibrium [dead battery].

Also, Q = K AND Also, Q = K AND G = 0 as well.G = 0 as well.

Concentration CellsConcentration Cells

We can construct a cell where We can construct a cell where both both

compartments contain the compartments contain the same same

components BUT at different components BUT at different

concentrations.concentrations.

Notice the Notice the difference in difference in the the concentrations concentrations pictured at the pictured at the left.left.

Because the right Because the right compartment compartment contains 1.0 M contains 1.0 M AgAg++ and the left and the left compartment compartment contains 0.10 M contains 0.10 M AgAg++, there will be , there will be a driving force to a driving force to transfer electrons transfer electrons from left to right.from left to right.

Silver will be deposited on the right Silver will be deposited on the right electrode, thus lowering the electrode, thus lowering the concentrationconcentrationof Agof Ag++ in the right in the right

compartment. In the compartment. In the

left compartment left compartment the the

silver electrode silver electrode

dissolves [producing dissolves [producing

AgAg++ ions] to raise ions] to raise

the concentration of the concentration of

AgAg++ in solution. in solution.

Exercise 6 Exercise 6

Determine the Determine the

direction of direction of

electron flow and electron flow and

designate the designate the

anode and anode and cathode cathode

for the cell for the cell

represented here.represented here.

left left right right

Determine EDetermine Eoocellcell and E and Ecellcell based based

on the following half-reactions:on the following half-reactions:

VOVO22++ + 2H + 2H++ + e + e-- → VO → VO2+2+ + H + H22OO

E° = 1.00 VE° = 1.00 VZnZn2+2+ + 2e + 2e-- → Zn → Zn E° = -0.76V E° = -0.76V

Where:Where:

TT = 25°C = 25°C[VO[VO22

++] = 2.0 ] = 2.0 MM[H[H++] = 0.50 ] = 0.50 MM[VO[VO2+2+] = 1.0 x 10] = 1.0 x 10-2-2 MM[Zn[Zn2+2+] = 1.0 x 10] = 1.0 x 10-1-1 MM

E°E°cellcell = 1.76 V = 1.76 V

EEcellcell = 1.89 V = 1.89 V

Summary of Gibb’s Summary of Gibb’s Free Energy and CellsFree Energy and Cells

-Eo implies NONspontaneous -Eo implies NONspontaneous

+Eo implies spontaneous (would +Eo implies spontaneous (would be a good battery!)be a good battery!)

EE = 0, equilibrium reached = 0, equilibrium reached (dead (dead

battery)battery)

Summary of Gibb’s Summary of Gibb’s Free Energy and Cells, Free Energy and Cells, cont.cont.the larger the voltage, the more the larger the voltage, the more

spontaneous the reactionspontaneous the reaction

G will be negative in spontaneous G will be negative in spontaneous

reactionsreactions

K>1 are favoredK>1 are favored

Two important equationsTwo important equations G = - nFEG = - nFE [“minus nunfe”] [“minus nunfe”] G = - RTlnK [“ratlink”]G = - RTlnK [“ratlink”]

G = Gibbs free energy [Reaction is spontaneous if G = Gibbs free energy [Reaction is spontaneous if ΔGΔG isis

negative]negative]n = number of moles of electrons. n = number of moles of electrons. F = Faraday constant 9.6485309 x 10F = Faraday constant 9.6485309 x 1044 J/V (1 mol of J/V (1 mol of electrons carries 96,500C )electrons carries 96,500C )E = cell potentialE = cell potentialR = 8.31 J/molR = 8.31 J/molKKT = Kelvin temperatureT = Kelvin temperatureK = equilibrium constant [products]K = equilibrium constant [products]coeffcoeff/[reactants]/[reactants]coeffcoeff

Favored conditionsFavored conditions

EEcellcell > 0 > 0 G < 0 K>1G < 0 K>1

For the oxidation-reduction reaction:For the oxidation-reduction reaction:

SS44OO662-2-(aq) + Cr(aq) + Cr2+2+(aq) (aq) → Cr→ Cr3+3+(aq) + S(aq) + S22OO33

2-2-(aq)(aq)

The appropriate half-reactions are:The appropriate half-reactions are:

SS44OO662-2- + 2e + 2e-- → 2S → 2S22OO33

2- 2- E° = 0.17V (1)E° = 0.17V (1)CrCr3+3+ + e + e-- → Cr → Cr2+2+ E° = -0.50 V (2)E° = -0.50 V (2)

Balance the redox reaction, and calculate E° Balance the redox reaction, and calculate E° and and

KK (at 25°C). (at 25°C).

E° = 0.67 VE° = 0.67 V

KK = 10 = 1022.622.6 = 4 x 10 = 4 x 102222

Applications of Galvanic Applications of Galvanic CellsCells

Batteries -- cells connected in Batteries -- cells connected in series series

potentials add together to give a potentials add together to give a total total

voltage.voltage.

Lead-storage batteries (car) --

Pb anode

PbO2 cathode

H2SO4 electrolyte

Examples

Dry cell batteries

Acid versions -- Zn anode, C cathode, MnO2 and NH4Cl paste

Alkaline versions -- some type of basic paste, ex. KOH

Nickel-cadmium -- anode and cathode can be recharged

Fuel cellsFuel cells

Reactants continuously supplied Reactants continuously supplied

(spacecraft – hydrogen and (spacecraft – hydrogen and oxygen)oxygen)

ELECTROLYSIS AND ELECTROLYSIS AND ELECTROLYTIC CELLSELECTROLYTIC CELLS

ElectrolysisElectrolysis

the use of electricity to bring about the use of electricity to bring about

chemical change chemical change

Literal translation: “split with Literal translation: “split with electricity” electricity”

Electrolytic cells Electrolytic cells [NON spontaneous cells][NON spontaneous cells]

used to separate ores or plate used to separate ores or plate out out

metalsmetals

Important differences Important differences between a between a voltaic/galvanic cell and voltaic/galvanic cell and an electrolytic cell:an electrolytic cell:

1)Voltaic cells are spontaneous. 1)Voltaic cells are spontaneous.

Electrolytic cells are forced to Electrolytic cells are forced to

occur by using an electron occur by using an electron pump or pump or

battery or any DC source.battery or any DC source.

2) A voltaic cell is separated into 2) A voltaic cell is separated into

two half cells to generate two half cells to generate electricity. electricity.

An electrolytic cell occurs in a An electrolytic cell occurs in a single single

containercontainer..

3) A voltaic [or galvanic] cell IS 3) A voltaic [or galvanic] cell IS a a

battery. battery.

An electrolytic cell NEEDS a An electrolytic cell NEEDS a

battery.battery.

4) AN OX and RED CAT still apply 4) AN OX and RED CAT still apply BUT the polarity of the electrodes is BUT the polarity of the electrodes is reversed. The cathode is Negative reversed. The cathode is Negative and the anode is Positive and the anode is Positive

(remember (remember E.P.A– electrolytic positive E.P.A– electrolytic positive anodeanode). ).

Electrons still flow Electrons still flow

FATCATFATCAT

(usually use inert (usually use inert electrodes)electrodes)

Predicting the Products Predicting the Products of Electrolysisof Electrolysis

If there is no water present and If there is no water present and

you have a pure molten ionic you have a pure molten ionic

compound, then…compound, then…

The cation will be reduced (gain The cation will be reduced (gain

electrons/go down in charge).electrons/go down in charge).

The anion will be oxidized (lose The anion will be oxidized (lose

electrons/go up in charge).electrons/go up in charge).

If water is present and you If water is present and you have an aqueous solution of have an aqueous solution of

the the ionic compound, then…ionic compound, then…

You’ll need to figure out if the ions You’ll need to figure out if the ions are reacting, or the water is are reacting, or the water is

reacting.reacting.

You can always look at a reduction You can always look at a reduction potential table to figure it out.potential table to figure it out.

But, as a rule of thumbBut, as a rule of thumb

No group IA or IIA metal will be No group IA or IIA metal will be reduced reduced

in an aqueous solution in an aqueous solution

– – water will be reduced instead.water will be reduced instead.

No polyatomic will be oxidized in an No polyatomic will be oxidized in an aqueous solution aqueous solution

– – water will be oxidized instead.water will be oxidized instead.

Since water has the more positive Since water has the more positive

potential, we would expect to see potential, we would expect to see

oxygen gas produced at the anode oxygen gas produced at the anode

because it is easier to oxidize than because it is easier to oxidize than

water or chloride ion.water or chloride ion.

Actually, chloride ion is the first to Actually, chloride ion is the first to

be oxidized. The voltage required in be oxidized. The voltage required in

excess of the expected value (called excess of the expected value (called

the overvoltage) is much greater for the overvoltage) is much greater for

the production of oxygen than the production of oxygen than

chlorine, which explains why chlorine chlorine, which explains why chlorine

is produced first. is produced first.

Causes of overvoltage Causes of overvoltage are are very complexvery complex

Basically, it is caused by Basically, it is caused by

difficulties in transferring electrons difficulties in transferring electrons

from the species in the solution to from the species in the solution to

the atoms on the electrode across the atoms on the electrode across

the electrode-solution interface. the electrode-solution interface.

Therefore, E values must be Therefore, E values must be used used

cautiously in predicting the cautiously in predicting the actual actual

order of oxidation or reduction order of oxidation or reduction of of

species in an electrolytic cell.species in an electrolytic cell.

Half Reactions for the Half Reactions for the electrolysis of water: electrolysis of water: (MUST MEMORIZE!) (MUST MEMORIZE!)

If Oxidized:If Oxidized:

2 H2 H22O O O O22 + 4 H + 4 H++ + 4e + 4e--

If Reduced: If Reduced:

2 H2 H22O + 2eO + 2e-- H H22 + 2 OH + 2 OH--

Calculating the Electrical Calculating the Electrical Energy of ElectrolysisEnergy of Electrolysis

How much metal could be plated out?How much metal could be plated out?

How long would it take to plate out?How long would it take to plate out?

Faraday’s LawFaraday’s Law

The amount of a substance being The amount of a substance being

oxidized or reduced at each oxidized or reduced at each electrode electrode

during electrolysis is directly during electrolysis is directly

proportional to the amount of proportional to the amount of

electricity that passes through the electricity that passes through the

cell.cell.

Use dimensional analysis for Use dimensional analysis for these these

calculations, rememberingcalculations, remembering

# coulombs = It# coulombs = It

1 Volt = 1 Joule/Coulomb1 Volt = 1 Joule/Coulomb

1 Amp = 1 Coulomb/second (current is 1 Amp = 1 Coulomb/second (current is measured in amp, but symbolized by I)measured in amp, but symbolized by I)

Faraday = 96,500 Coulombs/mole of Faraday = 96,500 Coulombs/mole of electronselectrons

Balanced redox equation gives Balanced redox equation gives #moles of e#moles of e--/mole of /mole of

substance.substance.

Formula weight gives Formula weight gives grams/mole.grams/mole.

How long must a current of How long must a current of 5.00 A 5.00 A

be applied to a solution of Ag+ be applied to a solution of Ag+ to to

produce 10.5 g silver metal?produce 10.5 g silver metal?

= 31.3 minutes = 31.3 minutes

An acidic solution contains the ions CeAn acidic solution contains the ions Ce4+4+, ,

VOVO2+2+, and Fe, and Fe3+3+. Using the E° values . Using the E° values

listed in Table 17.1 [Zumdahl], give the listed in Table 17.1 [Zumdahl], give the

order of oxidizing ability of these order of oxidizing ability of these species species

and predict which one will be reduced at and predict which one will be reduced at

the cathode of an electrolytic cell at the the cathode of an electrolytic cell at the

lowest voltage.lowest voltage.

CeCe4+4+ > VO > VO2+2+ > Fe > Fe3+3+

Applications of Applications of electrolytic cellselectrolytic cells

1) Production of pure forms of elements 1) Production of pure forms of elements

from mined oresfrom mined ores

a) Purify copper for wiringa) Purify copper for wiring

b) Aluminum from Hall-Heroult processb) Aluminum from Hall-Heroult process

c) Separation of sodium and chlorine c) Separation of sodium and chlorine

(Down’s cell)(Down’s cell)

b) Aluminum from Hall-b) Aluminum from Hall-Heroult processHeroult process

c) Separation of sodium c) Separation of sodium and chlorine (Down's and chlorine (Down's cell)cell)

2) Electroplating2) Electroplating

applying a thin layer of an expensive applying a thin layer of an expensive

metal to a less expensive onemetal to a less expensive one

a) Jewelry --- 14 K gold plated a) Jewelry --- 14 K gold plated

b) Bumpers on cars --- Chromium b) Bumpers on cars --- Chromium plated plated

3) Charging a battery3) Charging a battery

i.e. your car battery when the i.e. your car battery when the

alternator functionsalternator functions

CorrosionCorrosion

process of returning metals to process of returning metals to their their

natural state, the ores natural state, the ores

involves oxidation of the metal involves oxidation of the metal which which

causes it to lose its structural causes it to lose its structural integrity and attractivenessintegrity and attractiveness

The main component of steel is iron. The main component of steel is iron.

20% of the iron and steel produced 20% of the iron and steel produced annually is used to replace rusted annually is used to replace rusted metal!metal!

Most metals develop a thin oxide Most metals develop a thin oxide coating to protect them -- patina’s, coating to protect them -- patina’s, tarnish, rust, etc.tarnish, rust, etc.

Corrosion of IronCorrosion of Iron

an electrochemical process!an electrochemical process!

Steel has a nonuniform surface Steel has a nonuniform surface since steel is not completely since steel is not completely homogeneous. Physical strains homogeneous. Physical strains leave stress points in the metal as leave stress points in the metal as well, causing iron to be more easily well, causing iron to be more easily oxidized at these points (oxidized at these points (anodicanodic regions) than it is at others regions) than it is at others

((cathodiccathodic regions).regions).

In the anodic region: In the anodic region:

Fe Fe Fe Fe2+2+ + 2 e + 2 e--

The electrons released flow through The electrons released flow through the steel to a cathodic region where the steel to a cathodic region where they react with oxygen.they react with oxygen.

In the cathodic region: In the cathodic region:

OO22 + 2 H + 2 H22O + 4eO + 4e-- 4 OH 4 OH--

The iron (II) ions travel to the The iron (II) ions travel to the cathodic cathodic

regions through the moisture on the regions through the moisture on the surface of the steel [just like ions surface of the steel [just like ions travel through a salt bridge]. travel through a salt bridge].

Another reaction occurs in the Another reaction occurs in the

cathodic region:cathodic region:

4 Fe4 Fe2+2+(aq) + O(aq) + O22(g) + (4 + 2n) H(g) + (4 + 2n) H22O (l) O (l)

2 Fe2 Fe22OO33 n H n H22O (s) + 8 HO (s) + 8 H++ (aq) (aq)

This means rust often forms at This means rust often forms at sites sites

that are remote from those that are remote from those where where

the iron dissolved to form pits the iron dissolved to form pits in the in the

steel.steel.

Hydration of iron affects the Hydration of iron affects the color of color of

the rust: the rust:

black to yellow to the familiar black to yellow to the familiar

reddish brown.reddish brown.

PreventionPrevention

1) paint1) paint

2) coat with zinc [galvanizing] 2) coat with zinc [galvanizing]

3) cathodic protection3) cathodic protection

Cathodic ProtectionCathodic Protection

Insert an active metal like Mg Insert an active metal like Mg connected by a wire to the tank or connected by a wire to the tank or pipeline to be protected. Mg is a pipeline to be protected. Mg is a better reducing agent than iron [so better reducing agent than iron [so is more readily oxidized]. The Mg is more readily oxidized]. The Mg anode dissolves and must be anode dissolves and must be replaced, BUT protects the steel in replaced, BUT protects the steel in the meantime!the meantime!

Ships hulls often have bars of titanium Ships hulls often have bars of titanium attached since in salt water, Ti acts as attached since in salt water, Ti acts as the anode and is oxidized instead of the anode and is oxidized instead of the steel hull.the steel hull.

ELECTROCHEMISTRY

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