Oxidation and ReductionOxidation and Reduction

Reactions that involve electron Reactions that involve electron transfertransferBatteries and chemistryBatteries and chemistry

What’s in a batteryWhat’s in a battery

A battery does work with electricityA battery does work with electricity

A circuit is requiredA circuit is required

An electric current flows through the circuitAn electric current flows through the circuit

A chemical reaction provides the electricityA chemical reaction provides the electricity

Energy and electricityEnergy and electricity

All chemical reactions involve energy All chemical reactions involve energy changechangeReactions where energy is given out can Reactions where energy is given out can be made to provide the energy in the form be made to provide the energy in the form of electricityof electricityVolta made the first battery (Voltaic cell)Volta made the first battery (Voltaic cell)All batteries involve electron transferAll batteries involve electron transferElectron transfer involves Electron transfer involves oxidation/reductionoxidation/reduction

Oxidation-Reduction (Redox)Oxidation-Reduction (Redox)

Oxidation is Oxidation is lossloss of electrons of electronsNa Na → Na→ Na++ + + ee--

Reduction is Reduction is gaingain of electrons of electronsCl + Cl + ee-- = Cl= Cl--

Formation of NaCl from elements is redoxFormation of NaCl from elements is redox

Single displacement is redoxSingle displacement is redox

Zn atoms Zn atoms → Zn→ Zn2+2+ ions ions (oxidized)(oxidized)

CuCu2+2+ ions → Cu atoms ions → Cu atoms (reduced)(reduced)

Agents of redoxAgents of redox

Oxidizing Oxidizing agentagent: causes oxidation of : causes oxidation of another substanceanother substance– CuCu2+2+ ions oxidize the Zn atoms ions oxidize the Zn atoms

Reducing Reducing agent:agent: causes reduction of causes reduction of another substanceanother substance– Zn atoms reduce the CuZn atoms reduce the Cu2+2+ ions ions

Nuggets of redox processesNuggets of redox processes

Where there is oxidation there is Where there is oxidation there is alwaysalways reductionreduction

Oxidizing agentOxidizing agent Reducing agentReducing agent

Is Is itselfitself reduced reduced Is Is itselfitself oxidized oxidized

GainsGains electrons electrons LosesLoses electrons electrons

Causes oxidationCauses oxidation Causes reductionCauses reduction

Identifying oxidation and reductionIdentifying oxidation and reduction

With elements forming ionic compounds With elements forming ionic compounds identifying oxidation and reduction is identifying oxidation and reduction is usually straightforwardusually straightforward– Follow path of electrons from reactant to Follow path of electrons from reactant to

productproduct

What about covalent molecules and What about covalent molecules and reactions involving only compounds?reactions involving only compounds?System of oxidation numbers is usedSystem of oxidation numbers is usedOxidation numbers keep track of electronsOxidation numbers keep track of electrons

A numbers gameA numbers game

Daniell cellDaniell cell

An electrolytic cell which uses the An electrolytic cell which uses the reduction of Cureduction of Cu2+2+ by Zn to produce a by Zn to produce a voltagevoltage– In left beaker is Cu and CuSOIn left beaker is Cu and CuSO44

– In right beaker is Zn and ZnSOIn right beaker is Zn and ZnSO44

– Adding a “salt bridge” completes the circuit Adding a “salt bridge” completes the circuit and the reaction occurs and the reaction occurs

Zn + CuZn + Cu2+2+ = Zn = Zn2+2+ + Cu + Cu

Galvanic cell: long distance Galvanic cell: long distance chemistrychemistry

Each metal in touch with a solution of its own ionsEach metal in touch with a solution of its own ionsExternal circuit carries electrons transferred during the External circuit carries electrons transferred during the redox processredox processA “salt bridge” containing neutral ions completes the A “salt bridge” containing neutral ions completes the internal circuit.internal circuit.With no current flowing, a potential develops – the With no current flowing, a potential develops – the potential for workpotential for workUnlike the reaction in the beaker, the energy released by Unlike the reaction in the beaker, the energy released by the reaction in the cell can perform useful work – like the reaction in the cell can perform useful work – like lighting a bulblighting a bulb

Odes to a galvanic cellOdes to a galvanic cell

CathodeCathode– Where reduction Where reduction

occursoccurs– Where electrons are Where electrons are

consumedconsumed– Where positive ions Where positive ions

migrate tomigrate to– Has positive signHas positive sign

AnodeAnode– Where oxidation Where oxidation

occursoccurs– Where electrons are Where electrons are

generatedgenerated– Where negative ions Where negative ions

migrate tomigrate to– Has negative signHas negative sign

Cell notationCell notation

Anode on left, cathode on rightAnode on left, cathode on rightElectrons flow from left to rightElectrons flow from left to rightOxidation on left, reduction on rightOxidation on left, reduction on rightSingle vertical = electrode/electrolyte boundarySingle vertical = electrode/electrolyte boundaryDouble vertical = salt bridgeDouble vertical = salt bridge

Anode:Zn →Zn2+ +

2e

Cathode:Cu2+ + 2e

→Cu

Volts and ampsVolts and amps

VoltVolt is the measure of potential – the driving is the measure of potential – the driving force to move electrons. Voltage depends on force to move electrons. Voltage depends on the type of chemical process and not on the size the type of chemical process and not on the size of the batteryof the batteryOther forms of potential:Other forms of potential:– Pressure moves air or liquidsPressure moves air or liquids– Temperature moves heatTemperature moves heat– Chemical potential moves reactionsChemical potential moves reactions

AmpAmp is the flow of current. The size of the is the flow of current. The size of the current flowing can be increased by making the current flowing can be increased by making the electrodes larger (more reaction per second)electrodes larger (more reaction per second)

Measuring tendency for reductionMeasuring tendency for reduction

Reduction potentialReduction potential measures the tendency for a measures the tendency for a substance to reduce another substancesubstance to reduce another substance

The tendency is measured relative to some The tendency is measured relative to some standard – taken to be hydrogenstandard – taken to be hydrogen

2H2H+ + + 2e = H+ 2e = H22

Standard reduction potentials are all measured Standard reduction potentials are all measured under the same conditionsunder the same conditions– Negative value means that process is Negative value means that process is unfavourableunfavourable– Positive value means that process is Positive value means that process is favourablefavourable

Making predictionsMaking predictions

Will the reaction Will the reaction

Zn + ClZn + Cl22 = ZnCI = ZnCI22 proceed? proceed?

Zn + ClZn + Cl22 = Zn = Zn2+2+ + 2Cl + 2Cl--

PositivePositive cell voltage means cell voltage means reaction happensreaction happensWhat are the reduction potentials?What are the reduction potentials?– ZnZn2+2+ + 2e = Zn -0.76 V + 2e = Zn -0.76 V– CICI22 + 2e = 2CI + 2e = 2CI-- +1.36 V +1.36 V

But…in the reaction Zn is oxidized But…in the reaction Zn is oxidized (reverse sign)(reverse sign)– Zn = ZnZn = Zn2+2+ + 2e +0.76 V + 2e +0.76 V

Overall voltage:Overall voltage:0.76 V + 1.36 V = 2.12 V0.76 V + 1.36 V = 2.12 V

Verdict: reaction proceedsVerdict: reaction proceeds

A rusty nail: corrosion and A rusty nail: corrosion and electrochemistryelectrochemistry

The rusting of a nail is an electrochemical The rusting of a nail is an electrochemical processprocess– Anode: Fe is oxidized to FeAnode: Fe is oxidized to Fe2+2+

– Cathode: OCathode: O22 is reduced to H is reduced to H22OO

Why do things rust quicker in salt water?Why do things rust quicker in salt water?

Lithium batteriesLithium batteries

Lithium has a very large negative Lithium has a very large negative reduction potentialreduction potential

Li = LiLi = Li++ + e….E = 3.04 V + e….E = 3.04 V

The basis for light-weight, high energy The basis for light-weight, high energy density batteriesdensity batteries– Low atomic mass of lithiumLow atomic mass of lithium– High reduction potentialHigh reduction potential– Ability to make rechargeable batteriesAbility to make rechargeable batteries

Lead-acid batteries – a unique Lead-acid batteries – a unique systemsystem

Lead battery technology is 100 years oldLead battery technology is 100 years old– Provides high currentProvides high current– RechargeableRechargeable– InexpensiveInexpensive– Rugged Rugged

Oxidation: Pb + HOxidation: Pb + H22SOSO44 = PbSO = PbSO44 + 2H + 2H++ + 2e + 2e

Reduction: PbOReduction: PbO22 + H + H22SOSO44 + 2H + 2H++ + 2e = PbSO + 2e = PbSO44 + 2H + 2H22OO

Overall: Pb + PbOOverall: Pb + PbO22 + 2H + 2H22SOSO44 = 2PbSO = 2PbSO44 + 2H + 2H22O O Discharge

Recharge

Dry cell batteriesDry cell batteries

Acid dry cellAcid dry cell– Zn anodeZn anodeZn Zn → Zn→ Zn2+2+ + 2e + 2e

– MnOMnO22 cathode cathode2MnO2MnO22 + 2NH + 2NH44

++ + 2e → Mn + 2e → Mn22OO33 + 2NH + 2NH33 + H + H22OO

Alkali cellAlkali cell– Zn anodeZn anodeZn + 2OHZn + 2OH-- → ZnO + H → ZnO + H22O + 2eO + 2e

– MnOMnO22 cathode cathode2MnO2MnO22 + H + H22O + 2e O + 2e → → MnMn22OO33 + 2OH + 2OH--

Fuel cells and the hydrogen Fuel cells and the hydrogen economyeconomy

A battery with the “electrolyte” supplied A battery with the “electrolyte” supplied from withoutfrom without– Cathode:Cathode:

2H2H22 → 4H → 4H++ + 4e + 4e

– Anode:Anode:

OO22 + 4H + 4H++ +4e → 2H +4e → 2H22OO

– OverallOverall

2H2H22 + O + O22 → 2H → 2H22OO

Electrolysis – driving against the Electrolysis – driving against the streamstream

In the spontaneous process (left): electrons flow from left In the spontaneous process (left): electrons flow from left to right (battery discharge powers cell phone)to right (battery discharge powers cell phone)Nonspontaneous process (right): apply voltage to Nonspontaneous process (right): apply voltage to electrodes: electrons flow from right to left, reversing the electrodes: electrons flow from right to left, reversing the chemical reaction, restores potential energy to the bonds chemical reaction, restores potential energy to the bonds (plugging phone into charger restores the battery)(plugging phone into charger restores the battery)

Predicting spontaneity and the Predicting spontaneity and the activity seriesactivity series

A more active metal A more active metal will will reducereduce a less a less active metal active metal ionion

A less active metal A less active metal ionion will will oxidizeoxidize a a more active metalmore active metal

Redox in lifeRedox in life

CorrosionCorrosionFe + OFe + O2 2 → Fe→ Fe22OO33

CombustionCombustionCHCH44 + O + O22 → CO→ CO22 + H + H22OO

BleachingBleachingCl + Cl + ee-- = Cl = Cl--

Biological systemsBiological systems

RespirationRespirationCytochrome Cytochrome cc (Fe (Fe3+3+) + ) + ee-- = cytochrome = cytochrome cc (Fe (Fe2+2+))

Followed by:Followed by:

OO22 + 4 + 4ee-- + 4H + 4H+ + → 2H→ 2H22OO

MetabolismMetabolismEthanol Ethanol → acetaldehyde → acetic acid → CO→ acetaldehyde → acetic acid → CO22 + H + H22OO

Vitamin C and oxidationVitamin C and oxidation

Vitamins are organic compounds important for Vitamins are organic compounds important for maintaining healthmaintaining healthVitamin C is also easily oxidized (it is a reducing Vitamin C is also easily oxidized (it is a reducing agent)agent)Body produces free radicals which oxidize – Body produces free radicals which oxidize – aging, cancer, cardiovascular diseaseaging, cancer, cardiovascular diseaseAntioxidants (like vitamin C) defend against Antioxidants (like vitamin C) defend against radicalsradicalsQuestion: should we take antioxidant Question: should we take antioxidant supplements?supplements?