ELECTROCHEMISTRY. Electrochemistry Background Many of the things people deal with in real life are...

ELECTROCHEMISTRELECTROCHEMISTRYY

Electrochemistry Electrochemistry BackgroundBackground

• Many of the things people deal with in real life are related to electrochemical reactions.

• Batteries - flashlights, watches, car batteries, calculators, cell phones, garage door openers.

• Aluminum cans – aluminum is extracted by an electrochemical process.

• Chrome – found on cars or motorcycle parts is electroplated on the item.

• Therefore, this field of chemistry is often called Therefore, this field of chemistry is often called ELECTROCHEMISTRYELECTROCHEMISTRY

Electron Transfer Reactions

Electron Transfer Reactions

• Redox reactions – reactions in which there are a simultaneous transfer of electrons from one chemical species to another.

• Composed of two different reactions.

1) Oxidation reaction - loss of electrons

2) Reduction reaction – gain of electrons

• These reactions are coupled, as the electrons that are lost in the oxidation reaction are the same electrons gained in the reduction reaction. Redox reaction.

You can’t have one… You can’t have one… without the other!without the other!

• Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons.

How to remember the How to remember the terminologyterminology

LEOLEO the lion says GERGER!

Oxidation – Losing Oxidation – Losing ElectronsElectrons• Oxidation has three definitions:

• The loss of electrons

• The gain of oxygen atoms

• The loss of hydrogen atoms

In electrochemistry we will deal primarily with the definition that describes the loss of electrons.

• One way to define oxidation is where a chemical substance loses electrons going from reactant to product during a reaction.

• For example, when sodium metal reacts with chlorine gas to form sodium chloride (NaCl), the sodium metal loses electrons, which the chlorine gains.

Na Na+ + e-

The sodium metal has been oxidized

The loss of electrons

Reduction – Gaining Reduction – Gaining ElectronsElectrons

• Reduction has three definitions:

• The gain of electrons

• The loss of oxygen atoms

• The gain of hydrogen atoms

In electrochemistry we will deal primarily with the definition that describes the gain of electrons.

• One way to define reduction is where a chemical substance gains electrons going from reactant to product during a reaction.

• In the process of electroplating silver onto a teapot, the silver cation is reduced to silver metal by the gain of an electron.

Ag+ + e- Ag (metal)

The silver cation has been reduced

The gain of electrons

• Consider the example of the reaction where copper metal reacts with a silver nitrate solution:

Cu(s) + 2 Ag+ Cu+2 + 2 Ag(s)

This overall reaction is really composed of two half-reactions:

Cu(s) Cu+2 + 2e- (oxidation)

2 Ag+ + 2 e- 2 Ag(s) (reduction)

Examples of Redox Reactions

• Consider the example of the reaction where zinc metal reacts with a copper(II) sulfate solution:

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

This overall reaction is really composed of two half-reactions:

Zn(s) Zn+2 + 2e- (oxidation)

Cu+2 + 2e- Cu(s) (reduction)

Examples of Redox Reactions

Terminology for Redox Reactions

Terminology for Redox Reactions

• OXIDATIONOXIDATION—loss of electron(s) by a species—loss of electron(s) by a species

• REDUCTIONREDUCTION—gain of electron(s)—gain of electron(s)

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; substance that is reduced. Copper cation in substance that is reduced. Copper cation in the last slide the last slide allowsallows oxidation of zinc. oxidation of zinc.

• REDUCING AGENTREDUCING AGENT—electron donor; —electron donor; substance that is oxidized. Zinc in the last substance that is oxidized. Zinc in the last slide slide allowsallows reductions of copper. reductions of copper.

Both the oxidizing and reducing agents are on Both the oxidizing and reducing agents are on the left (reactant) side of the redox equationthe left (reactant) side of the redox equation

• OXIDATIONOXIDATION—loss of electron(s) by a species—loss of electron(s) by a species

• REDUCTIONREDUCTION—gain of electron(s)—gain of electron(s)

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; substance that is reduced. Copper cation in substance that is reduced. Copper cation in the last slide the last slide allowsallows oxidation of zinc. oxidation of zinc.

• REDUCING AGENTREDUCING AGENT—electron donor; —electron donor; substance that is oxidized. Zinc in the last substance that is oxidized. Zinc in the last slide slide allowsallows reductions of copper. reductions of copper.

Both the oxidizing and reducing agents are on Both the oxidizing and reducing agents are on the left (reactant) side of the redox equationthe left (reactant) side of the redox equation

Why Study Electrochemistry?Why Study Electrochemistry?

• Batteries

• Corrosion

• Industrial production of chemicals such as Cl2, NaOH, F2 and Al

• Biological redox reactions

The heme groupThe heme group

OXIDATION-REDUCTION REACTIONS

OXIDATION-REDUCTION REACTIONS

Direct Redox Direct Redox ReactionReaction

Oxidizing and reducing agents in direct contact.

Cu(s) + 2 AgCu(s) + 2 Ag++(aq) (aq)

CuCu2+2+(aq) + 2 Ag(s)(aq) + 2 Ag(s)

OXIDATION-REDUCTION REACTIONS

OXIDATION-REDUCTION REACTIONS

Indirect Redox ReactionIndirect Redox Reactiongalvanic or voltaic cellgalvanic or voltaic cell

A battery functions by transferring electrons through an external wire from the reducing

agent to the oxidizing agent.

Galvanic CellsGalvanic CellsGalvanic CellsGalvanic Cells• An apparatus that allows a

redox reaction to occur by transferring electrons through an external connector (wire).

• voltaic or galvanic cell: Chemical reaction produces an electric currentcurrent

• electrolytic cell: Electric current used to cause chemical change.

Batteries are voltaic Batteries are voltaic cellscells

Anode (-)Anode (-) Cathode (+)Cathode (+)

Basic Concepts Basic Concepts of Galvanic Cellsof Galvanic Cells

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

CHEMICAL CHANGE ELECTRIC CHEMICAL CHANGE ELECTRIC CURRENTCURRENTCHEMICAL CHANGE ELECTRIC CHEMICAL CHANGE ELECTRIC CURRENTCURRENT

With time Cu metal plates out and the Zn strip “disappears.” With time Cu metal plates out and the Zn strip “disappears.”

•Zn is oxidized:Zn is oxidized: Zn(s) ZnZn(s) Zn2+2+(aq) + 2e-(aq) + 2e-

•CuCu2+2+ is reduced: is reduced: CuCu2+2+(aq) + 2e- Cu(s)(aq) + 2e- Cu(s)

•To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

CHEMICAL CHANGE CHEMICAL CHANGE ELECTRIC CURRENTELECTRIC CURRENTCHEMICAL CHANGE CHEMICAL CHANGE ELECTRIC CURRENTELECTRIC CURRENT

This is accomplished in a This is accomplished in a GALVANICGALVANIC or or VOLTAICVOLTAIC cell. cell.

A group of such cells is called a A group of such cells is called a batterybattery..

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

••Electrons travel thru external wire.Electrons travel thru external wire.•Salt bridge Salt bridge allows anions and cations to move allows anions and cations to move between electrode compartments.between electrode compartments.

••Electrons travel thru external wire.Electrons travel thru external wire.•Salt bridge Salt bridge allows anions and cations to move allows anions and cations to move between electrode compartments.between electrode compartments.

Zn --> ZnZn --> Zn2+2+ + 2e- + 2e- CuCu2+2+ + 2e- --> Cu + 2e- --> Cu

<--Anions<--AnionsCations-->Cations-->

OxidationOxidationAnodeAnodeNegativeNegative

OxidationOxidationAnodeAnodeNegativeNegative

RedReductionuctionCatCathodehodePositivePositive

RedReductionuctionCatCathodehodePositivePositive

RED CATRED CAT

Terms Used for Voltaic Terms Used for Voltaic CellsCells

CELL POTENTIAL, ECELL POTENTIAL, E

A quantitative measure of the amount of electricity (volts) that the voltaic cell can produce.

E˚E˚cellcell = E˚ = E˚cathode cathode + E˚+ E˚anodeanode

E˚E˚cell cell > 0> 0

CELL POTENTIAL, CELL POTENTIAL, EE

• For Zn/Cu cell, potential is +1.10 V at 25 ˚C and when [Zn2+] and [Cu2+] = 1.0 M.

• This is the This is the STANDARD CELL POTENTIAL, STANDARD CELL POTENTIAL, EEoo

• ——a quantitative measure of the tendency of a quantitative measure of the tendency of reactants to proceed to products when all are in reactants to proceed to products when all are in their standard states at 25 ˚C. their standard states at 25 ˚C.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Calculating Cell VoltageCalculating Cell Voltage

• Balanced half-reactions can be added together to get overall, balanced equation. .

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e- (oxidation)(aq) + 2e- (oxidation)CuCu2+2+(aq) + 2e- ---> Cu(s) (reduction)(aq) + 2e- ---> Cu(s) (reduction)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e- (oxidation)(aq) + 2e- (oxidation)CuCu2+2+(aq) + 2e- ---> Cu(s) (reduction)(aq) + 2e- ---> Cu(s) (reduction)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)

If we know EIf we know Eoo for each half-reaction, we for each half-reaction, we could get Ecould get Eoo for net reaction. for net reaction.

TABLE OF STANDARD TABLE OF STANDARD REDUCTIONREDUCTION POTENTIALS POTENTIALS

TABLE OF STANDARD TABLE OF STANDARD REDUCTIONREDUCTION POTENTIALS POTENTIALS

Eo (V)

Cu2+ + 2e- Cu +0.34

Zn 2+ + 2e- Zn -0.76

oxidizingability of ion

reducing abilityof element

To determine an oxidation To determine an oxidation from a reduction table, just from a reduction table, just take the opposite sign of the take the opposite sign of the reduction!reduction!

Zn/Cu Electrochemical CellZn/Cu Electrochemical Cell

Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e- EEoo = +0.76 V = +0.76 VCuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s) EEoo = +0.34 V = +0.34 V------------------------------------------------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s) (aq) + Cu(s)

EEoo = +1.10 V = +1.10 V

Cathode, Cathode, positivepositive

Anode, Anode, negativenegative

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons ++

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Cd --> CdCd --> Cd2+2+ + 2e- + 2e-oror

CdCd2+2+ + 2e- --> Cd + 2e- --> Cd

Fe --> FeFe --> Fe2+2+ + 2e- + 2e-oror

FeFe2+2+ + 2e- --> Fe + 2e- --> Fe

EEoo for a Voltaic Cell for a Voltaic Cell

All ingredients are present. Which way does All ingredients are present. Which way does reaction proceed?reaction proceed?

From the table, you see • Fe is lower on the list

than Cd (oxidized)• Cd is higher on the list

(reduced)

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

Volts

Cd Salt Bridge

Cd2+

Fe

Fe2+

EEoo for a Voltaic Cell for a Voltaic Cell

Since Fe is being oxidized the half-reaction listed in the table as well as the cell potential listed needs to be reversed.

The table lists reduction half-reactions

Calculating Cell VoltageCalculating Cell Voltage

• Balanced half-reactions can be added together to get overall, balanced equation. .

Fe(s) ---> FeFe(s) ---> Fe2+2+(aq) + 2e- (aq) + 2e- EEoo = +0.44 V = +0.44 V

CdCd2+2+(aq) + 2e- ---> Cd(s) (aq) + 2e- ---> Cd(s) EEoo = -0.40 V = -0.40 V----------------------------------------------------------------------------------------CdCd2+2+(aq) + Fe(s) ---> Fe(aq) + Fe(s) ---> Fe2+2+(aq) + Cd(s)(aq) + Cd(s)

Fe(s) ---> FeFe(s) ---> Fe2+2+(aq) + 2e- (aq) + 2e- EEoo = +0.44 V = +0.44 V

CdCd2+2+(aq) + 2e- ---> Cd(s) (aq) + 2e- ---> Cd(s) EEoo = -0.40 V = -0.40 V----------------------------------------------------------------------------------------CdCd2+2+(aq) + Fe(s) ---> Fe(aq) + Fe(s) ---> Fe2+2+(aq) + Cd(s)(aq) + Cd(s)

If we know EIf we know Eoo for each half-reaction, we for each half-reaction, wecould get Ecould get Eoo for net reaction. for net reaction.

EEoo = +0.04 V = +0.04 V

More About More About Calculating Cell VoltageCalculating Cell Voltage

Assume IAssume I-- ion can reduce water. ion can reduce water.

2 H2O + 2e- ---> H2 + 2 OH- Cathode2 I- ---> I2 + 2e- Anode-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2

2 H2O + 2e- ---> H2 + 2 OH- Cathode2 I- ---> I2 + 2e- Anode-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2

Assuming reaction occurs as written, Assuming reaction occurs as written,

E˚ = E˚E˚ = E˚catcat+ E˚+ E˚anan= (-0.828 V) - (- +0.535 V) = = (-0.828 V) - (- +0.535 V) = -1.363 V-1.363 V

Minus E˚ means rxn. occurs in opposite Minus E˚ means rxn. occurs in opposite

directiondirection

(the connection is backwards or you are (the connection is backwards or you are

recharging the battery)recharging the battery)

Charging a BatteryCharging a BatteryWhen you charge a battery, you are forcing the electrons backwards (from the + to the -). To do this, you will need a higher voltage backwards than forwards. This is why the ammeter in your car often goes slightly higher while your battery is charging, and then returns to normal.

In your car, the battery charger is called an alternator. If you have a dead battery, it could be the battery needs to be replaced OR the alternator is not charging the battery properly.

Dry Cell BatteryDry Cell Battery

Anode (-)

Zn ---> Zn2+ + 2e-

Cathode (+)

2 NH4+ + 2e- --->

2 NH3 + H2

Alkaline BatteryAlkaline Battery

Nearly same reactions as in common dry cell, but under basic conditions.

Anode (-): Anode (-): Zn + 2 OHZn + 2 OH-- ---> ZnO + H ---> ZnO + H22O + 2e-O + 2e-

Cathode (+): Cathode (+): 2 MnO2 MnO22 + H + H22O + 2e- ---> O + 2e- --->

MnMn22OO33 + 2 OH + 2 OH--

Mercury BatteryMercury Battery

Anode:Anode:

Zn is reducing agent under basic conditions

Cathode:Cathode:

HgO + H2O + 2e- ---> Hg + 2 OH-

Common type of battery in watches and pacemakers

Lead Storage BatteryLead Storage Battery

Anode (-) Eo = +0.36 V

Pb + HSO4- ---> PbSO4 + H+ + 2e-

Cathode (+) Eo = +1.68 V

PbO2 + HSO4- + 3 H+ + 2e-

---> PbSO4 + 2 H2O

Ni-Cad BatteryNi-Cad Battery

Anode (-)Anode (-)

Cd + 2 OH- ---> Cd(OH)2 + 2e-

Cathode (+) Cathode (+)

NiO(OH) + H2O + e- ---> Ni(OH)2 + OH-

HH22 as a Fuel as a Fuel

Cars can use electricity generated by HCars can use electricity generated by H22/O/O22

fuel cells.fuel cells.HH22 carried in tanks or generated from carried in tanks or generated from

hydrocarbonshydrocarbons

StopStop

Balancing Equations Balancing Equations for Redox Reactionsfor Redox Reactions

Some redox reactions have equations that must be balanced by special techniques.

MnOMnO44-- + 5 Fe + 5 Fe2+2+ + 8 H + 8 H++

Mn Mn2+ 2+ + 5 Fe+ 5 Fe3+3+ + 4 H + 4 H22OO

Mn = +7Mn = +7 Fe = +2Fe = +2Fe = +3Fe = +3Mn = +2Mn = +2

Balancing Balancing EquationsEquations

Consider the Consider the reduction of Agreduction of Ag++ ions with copper ions with copper metal.metal.

Cu + AgCu + Ag++ --give--> Cu --give--> Cu2+2+ + Ag + Ag

Balancing Balancing EquationsEquations

Step 1:Step 1: Divide the reaction into half-reactions, one for Divide the reaction into half-reactions, one for oxidation and the other for reduction.oxidation and the other for reduction.

OxOx Cu ---> CuCu ---> Cu2+2+

RedRed Ag Ag++ ---> Ag ---> Ag

Step 2:Step 2: Balance each element for mass. Already done Balance each element for mass. Already done in this case.in this case.

Step 3:Step 3: Balance each half-reaction for charge by Balance each half-reaction for charge by adding electrons.adding electrons.

OxOx Cu ---> Cu Cu ---> Cu2+2+ + + 2e-2e-

RedRed Ag Ag++ + + e- e- ---> Ag---> Ag

Balancing Balancing EquationsEquations

Step 4:Step 4: Multiply each half-reaction by a factor so that Multiply each half-reaction by a factor so that the reducing agent supplies as many electrons as the the reducing agent supplies as many electrons as the oxidizing agent requires.oxidizing agent requires.

Reducing agentReducing agent Cu ---> Cu Cu ---> Cu2+2+ + 2e- + 2e-Oxidizing agentOxidizing agent 22 Ag Ag++ + + 22 e- ---> e- ---> 22 Ag AgStep 5:Step 5: Add half-reactions to give the overall equation.Add half-reactions to give the overall equation.Cu + 2 AgCu + 2 Ag++ ---> Cu ---> Cu2+2+ + 2Ag + 2Ag

The equation is now balanced for both The equation is now balanced for both charge and mass.charge and mass.

Balancing Balancing EquationsEquations

Balance the following in acid solution—Balance the following in acid solution—

VOVO22++ + Zn ---> VO + Zn ---> VO2+ 2+ + Zn+ Zn2+2+

Step 1:Step 1:Write the half-reactionsWrite the half-reactions

OxOx Zn ---> ZnZn ---> Zn2+2+

RedRed VOVO22++ ---> VO ---> VO2+2+

Step 2:Step 2:Balance each half-reaction for mass.Balance each half-reaction for mass.

OxOx Zn ---> ZnZn ---> Zn2+2+

RedRedVOVO22

++ ---> VO ---> VO2+2+ + + HH22OO2 H2 H++ ++

Add HAdd H22O on O-deficient side and add HO on O-deficient side and add H++

on other side for H-balance.on other side for H-balance.

Balancing Balancing EquationsEquations

Step 3:Step 3: Balance half-reactions for charge.Balance half-reactions for charge.OxOx Zn ---> ZnZn ---> Zn2+2+ + + 2e- 2e-

RedRed e- e- + 2 H+ 2 H++ + VO + VO22++ ---> VO ---> VO2+2+ + H + H22OO

Step 4:Step 4: Multiply by an appropriate factor.Multiply by an appropriate factor.OxOx Zn ---> ZnZn ---> Zn2+2+ + + 2e-2e-

RedRed 22e-e- + + 44 H H++ + + 22 VO VO22++

---> ---> 22 VO VO2+2+ + + 22 H H22OO

Step 5:Step 5: Add Add balancedbalanced half-reactions half-reactions

Zn + 4 HZn + 4 H++ + 2 VO + 2 VO22++

---> Zn ---> Zn2+2+ + 2 VO + 2 VO2+2+ + 2 H + 2 H22OO

Tips on Balancing Tips on Balancing EquationsEquations

• Never add ONever add O22, O atoms, or , O atoms, or OO2-2- to balance oxygen. to balance oxygen.

• Never add HNever add H22 or H atoms to or H atoms to balance hydrogen.balance hydrogen.

• Be sure to write the correct Be sure to write the correct charges on all the ions.charges on all the ions.

• Check your work at the end Check your work at the end to make sure mass and to make sure mass and charge are balanced.charge are balanced.

• PRACTICE!PRACTICE!