Electrochemistry

Chapter 21Chapter 21

Electrochemistry and Redox

Oxidation-reduction:Oxidation-reduction: “Redox”“Redox”

Electrochemistry:Electrochemistry:

study of the interchange between chemical study of the interchange between chemical change and electrical workchange and electrical work

Electrochemical cells:Electrochemical cells:

systems utilizing a redox reaction to produce systems utilizing a redox reaction to produce or use electrical energyor use electrical energy

Redox Review

Redox reactions:Redox reactions: electron transfer processeselectron transfer processes

Oxidation:Oxidation: loss of 1 or more eloss of 1 or more e--

Reduction:Reduction: gain of 1 or more egain of 1 or more e--

Oxidation numbers:Oxidation numbers: imaginary chargesimaginary charges

(Balancing redox reactions)(Balancing redox reactions)

Oxidation Numbers (O.N.)

1.1. Pure elementPure element O.N. is zeroO.N. is zero

2.2. Monatomic ionMonatomic ion O.N.O.N. is chargeis charge

3.3. Neutral compoundNeutral compound:: sum of O.N. is zerosum of O.N. is zero

Polyatomic ionPolyatomic ion:: sum of O.N. is ion’s chargesum of O.N. is ion’s charge

*Negative O.N. generally assigned to more *Negative O.N. generally assigned to more electronegative elementelectronegative element

Oxidation Numbers (O.N.)

4.4. HydrogenHydrogen

assigned +1assigned +1

(metal hydrides, -1)(metal hydrides, -1)

5.5. OxygenOxygen

assigned -2assigned -2

(peroxides, -1; OF(peroxides, -1; OF22, +2), +2)

6.6. FluorineFluorine

always -1always -1

Oxidation-reduction

OxidationOxidation is loss of e is loss of e--

O.N. increases (more positive)O.N. increases (more positive)

ReductionReduction is gain of e is gain of e--

O.N. decreases (more negative)O.N. decreases (more negative)

Oxidation involves lossOxidation involves loss OILOIL

Reduction involves gainReduction involves gain RIGRIG

Redox

OxidationOxidation is loss of e is loss of e--

causes reductioncauses reduction

““reducing agentreducing agent””

ReductionReduction is gain of e is gain of e--

causes oxidationcauses oxidation

““oxidizing agentoxidizing agent””

Balancing Redox Reactions

1.1. Write separate equations (Write separate equations (half-reactionshalf-reactions) ) forfor oxidation and reductionoxidation and reduction

2.2. For For eacheach half-reaction half-reaction

a. a. Balance elements involved in eBalance elements involved in e-- transfer transfer

b.b. Balance number eBalance number e-- lost and gained lost and gained

3.3. To To balance ebalance e--

multiply each half-reaction by whole numbersmultiply each half-reaction by whole numbers

Balancing Redox Reactions: Acidic

4.4. AddAdd half-reactions/cancel like terms (e half-reactions/cancel like terms (e --))

5. 5. Acidic conditions:Acidic conditions:

Balance oxygen using HBalance oxygen using H22OO

Balance hydrogen using HBalance hydrogen using H++

Basic conditions:Basic conditions:

Balance oxygen using OHBalance oxygen using OH--

Balance hydrogen using HBalance hydrogen using H22OO

6.6. CheckCheck that all atoms and charges balance that all atoms and charges balance

Examples

Acidic conditions:Acidic conditions:

Basic conditions:Basic conditions:

3(aq)

2(aq)

2(aq)

-4(aq) FeMn FeMnO acid

2(aq)2(g)(aq)(s) Ag(CN)OCN Ag base

Types of cells

Voltaic (galvanic) cells:Voltaic (galvanic) cells:

a spontaneous reaction generates electrical energya spontaneous reaction generates electrical energy

Electrolytic cells:Electrolytic cells:

absorb free energy from an electrical source to absorb free energy from an electrical source to drive a nonspontaneous reactiondrive a nonspontaneous reaction

Common Components

Electrodes:Electrodes:conduct electricity between cell and conduct electricity between cell and

surroundingssurroundings

Electrolyte:Electrolyte:mixture of ions involved in reaction or mixture of ions involved in reaction or carrying chargecarrying charge

Salt bridge:Salt bridge:completes circuit (provides charge balance)completes circuit (provides charge balance)

ElectrodesAnode:Anode:

Oxidation occurs at the anodeOxidation occurs at the anode

Cathode:Cathode:

Reduction occurs at the cathodeReduction occurs at the cathode

Active electrodes:Active electrodes: participate in redoxparticipate in redox

Inactive:Inactive: sites of ox. and red.sites of ox. and red.

Voltaic (Galvanic) Cells

A device in which chemical energy A device in which chemical energy is changed to electrical energy.is changed to electrical energy.

Uses a spontaneous reaction.Uses a spontaneous reaction.

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Porous disk

Reducingagent

Oxidizingagent

e –

e –

e – e –

e –

e –

CathodeAnode (b)(a)

Oxidation Reduction

Zn2+(aq) + Cu(s) Cu2+

(aq) + Zn(s)

Zn Zn gives up electrons to Cugives up electrons to Cu— ““pushes harder” on epushes harder” on e--

— greater potential energygreater potential energy— greater “electrical potential”greater “electrical potential”

Spontaneous reactionSpontaneous reaction due to due to — relative difference in metals’ abilities to give erelative difference in metals’ abilities to give e --

— ability of eability of e-- to flow to flow

Cell Potential

Cell PotentialCell Potential / / Electromotive ForceElectromotive Force (EMF): (EMF):

The “pull” or driving force on electronsThe “pull” or driving force on electrons

Measured voltage (potential difference)Measured voltage (potential difference)

VC

J

moved charge ofunit

energy potential electricalor work Ecell

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e–

e– e–

e–

Zn 2+

SO4 2–

Zn(s)

1.0 M Zn 2+

solution

Anode

1.0 M Cu 2+

solution

Cathode

Cu 2+

SO4 2–

Cu(s)

Ecell = +1.10 V

Cell Potential, E0cell

EE00cellcell

cell potential under standard conditionscell potential under standard conditions

elements in standard states (298 K)elements in standard states (298 K)

solutions:solutions: 1 M1 M

gases:gases: 1 atm1 atm

Standard Reduction Potentials

EE values for reduction half-reactions with values for reduction half-reactions with solutes at 1M and gases at 1 atmsolutes at 1M and gases at 1 atm

CuCu2+2+ + 2e + 2e Cu Cu

EE = 0.34 V vs. SHE = 0.34 V vs. SHE

SOSO4422 + 4H + 4H++ + 2e + 2e H H22SOSO33 + H + H22OO

EE = 0.20 V vs. SHE = 0.20 V vs. SHE

E0cell and G0

EE00cellcell > 0 > 0 GG00 < 0 < 0 SpontaneousSpontaneous

EE00cellcell < 0 < 0 GG00 > 0 > 0 NotNot

EE00cellcell = 0 = 0 GG00 = 0 = 0 EquilibriumEquilibrium

Calculating E0cell

EE00cellcell = E = E00

cathodecathode - E - E00anodeanode

BrBr2(aq)2(aq)+2V+2V3+3+ +2H +2H22OO(l)(l) 2VO 2VO2+2+(aq)(aq)+ 4H+ 4H++

(aq)(aq)+ 2Br+ 2Br--(aq)(aq)

Given:Given: EE00cellcell = +1.39 V = +1.39 V

EE00Br2Br2 = +1.07 V = +1.07 V

What isWhat is EE00V3+V3+ and is the reaction spontaneous? and is the reaction spontaneous?

E0 values

More positive:More positive:

Stronger oxidizing agentStronger oxidizing agent

More readily accepts eMore readily accepts e--

More negative:More negative:

Stronger reducing agentStronger reducing agent

More readily gives eMore readily gives e--

Stronger R.A. + O.A. Stronger R.A. + O.A. Weaker R.A. + O.A. Weaker R.A. + O.A.

Free Energy and Cell Potential

n:n: number of moles of enumber of moles of e--

F:F: Faraday’s constantFaraday’s constant

96485 C 96485 C

mol of emol of e--

00max nFEG w

G0, E0, and K

At equilibrium:At equilibrium: GG00 = 0 and K = Q = 0 and K = Q

At 298 K:At 298 K:

00 nFERTlnKG

lnKnF

RTE0 so

logKn

0.0592E0

Nernst Equation

Under nonstandard conditionsUnder nonstandard conditions

RTlnQnFEnFE

RTlnQGG0

0

lnQnF

RTEE 0

cell

lnQn

0.0592EE 0298K

cell

Concentration Cells

. . .. . . a cell in which both a cell in which both compartments have the compartments have the same same componentscomponents but at but at different different concentrationsconcentrations

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e–

e–e–

e–

Ag

1 M Ag+

1 M NO3–

Anode Cathode

Porousdisk

Ag

0.1 M Ag+

0.1 M NO3–

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Reference solution ofdilute hydrochloric acid

Silver wire coated withsilver chloride

Thin-walled membrane

Batteries

A A batterybattery is a galvanic cell or, more is a galvanic cell or, more commonly, a commonly, a groupgroup of galvanic of galvanic cells connected in series.cells connected in series.

Fuel Cells

Galvanic cells Galvanic cells

Reactants are continuously supplied.Reactants are continuously supplied.

2H2H2(2(gg)) + O + O2(2(gg)) 2H 2H22OO((ll))

anodeanode: : 2H2H22 + 4OH + 4OH 4H 4H22O + 4eO + 4e

cathodecathode: : 4e4e + O + O2 2 + 2H + 2H22O O 4OH 4OH

Corrosion

Some metals, such as copper, gold, silver and Some metals, such as copper, gold, silver and platinum, are relatively difficult to oxidize.platinum, are relatively difficult to oxidize.

These are often called These are often called noble metalsnoble metals..

Electrolysis

ForcingForcing a current through a cell to a current through a cell to produce a chemical change for produce a chemical change for which the cell potential is negative.which the cell potential is negative.

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Moltenaluminum

Carbondioxideformed atthe anodes

Carbon-lined iron tank

Plug

Molten Al2O3/Na3AlF6mixture

Electrodes ofgraphite rods

To externalpower source

Stoichiometry

How much How much chemical change chemical change occurs with the flow occurs with the flow of a given current for a specified time?of a given current for a specified time?

current and time current and time quantity of charge quantity of charge moles of electrons moles of electrons moles of analyte moles of analyte grams of analytegrams of analyte