Recall

Overview• Wecancategorizereactionsintosomegeneraltypes:– Combination/Synthesis– Decomposition– Combustion– RedoxReactions– Singledisplacement/replacement– Doubledisplacement/replacement• PrecipitationReaction• Acid-Base/Neutralization

Recall:Brønsted-LowryDefinition•  Acid-basereactionscanbeclassifiedasproton-transferprocesses

•  Ex1:

HCl(aq)+H2O(l)àH3O+(aq)+Cl-(aq)

•  Ex2:

NH3(aq)+H2O(l)ßàNH4+(aq)+OH-(aq)

Donated(Lost)H+

Donated(Lost)H+

Accepted(gained)H+

Accepted(gained)H+

Acid

Acid

Base

Base

Conj.acid

Conj.base

Conj.acid

Conj.base

Oxidation-ReductionReactions

Oxidation-Reduction•  Oxidation-Reduction(redox)reactionscanbeconsideredelectron-transferreactions

•  Overallreaction:2Mg(s)+O2(g)à2MgO(s)

2Mg(s)+O2(g)à2MgO(s)

EachOgains2e-tobecomeO2-

EachMgloses2e-tobecomeMg2+

•  Half-Reactions(showtheelectronsinvolved):

2Mgà2Mg2++4e- O2+4e-à2O2-

OxidationStates

Oxidation-Reduction•  First,weneedtobeabletoassignoxidationstatestodeterminehowelectronsarebeingtransferredinareaction

•  Oxidationstates(oxidationnumbers)=indicatesthenumberofchargestheatomwouldhaveinamolecule/compoundifelectronsweretransferredcompletely

•  Rules:#1:ElementsintheirfreestatehaveanoxidationstateofZEROEx/N2,Na

Oxidation-Reduction•  Rules:

#2:MonoatomicionshaveoxidationstatesequaltotheirchargeEx/Li+hasanoxidationstateof+1,O2-hasanoxidationstateof-2#3:Incompounds,fluorinealwayshasanoxidationstateof-1Ex/NaF

Oxidation-Reduction•  Rules:

#4:Theoxidationnumberofoxygeninmostcompoundsis-2Ex/MgOButinhydrogenperoxideandperoxide,itis-1Ex/O22-

#5:Theoxidationnumberofhydrogenis+1exceptwhenbondedtometalsinbinarycompoundsEx/H2Ovs.LiH

Oxidation-Reduction•  Rules:

#6:Thesumoftheoxidationstatesmustbezeroforanelectricallyneutralcompound.Forpolyatomicions,itmustequalthechargeoftheion.Ex/CaH2Ex/NH4+– Generallyspeaking,metallicelementshavepositiveoxidationnumbers,whereasnonmetallicelementscanhavepositiveornegativeoxidationnumbers

–  ThehighestoxidationnumberanelementinGroups1A=7Acanhaveisitsgroupnumber

–  Transitionmetalshaveseveralpossibleoxidationnumbers.– And,yes,it’spossibletohavenon-integeroxidationstates

Oxidation-Reduction•  Assignoxidationstatestothefollowing:a)  Li2O

b)  HNO3c)  Cr2O72-

RedoxReactions

Oxidation-Reduction•  Oxidation–involvesthelossofelectrons•  Reduction–involvesthegainofelectrons•  LEOthelionsaysGER

Oxidation-Reduction•  Whenlookingatreactions,wecanassignoxidationstatestoseewhichsubstanceisoxidized,andwhichisreduced

CH4(g)+2O2(g)àCO2(g)+2H2O(g)

Loste-s(oxidized)

Gainede-s(reduced)

C:-4H:+1

O:0 C:+4O:-2

H:+1O:-2

Oxidation-Reduction•  Wecanalsolookalittlemorecloselyatthetransferofelectrons.Remember:LEOsaysGER.

CH4(g)+2O2(g)àCO2(g)+2H2O(g)

CH4àCO2+8e-

2O2+8e-àCO2+2H2O

Oxidation-Reduction•  Termstouse:Oxidized,reduced,oxidizingagent,reducingagent.

CH4(g)+2O2(g)àCO2(g)+2H2O(g)

CH4àCO2+8e-

2O2+8e-àCO2+2H2O

Oxidation-Reduction•  Given2Al(s)+3I2(s)à2AlI3,identifytheatomsthatareoxidizedandreduced,andalsostatetheoxidizingandreducingagents.Labelyourequationtosupportyouranswer.

BalancingRedoxReactions

Balancing1.  Separatetherxnintohalf-reactions2.  Balanceallatomsexcepthydrogenandoxygen3.  Balanceoxygenatomsbyaddingonewater

moleculeforeachoxygenatomneeded4.  AcidicsolutionsàbalancehydrogenbyaddingH+.

Basicsolutionsàaddonewatermoleculeforeachhydrogenatomneeded,andthenaddthesamenumberofOH-atomsontheoppositeside.

5.  Balancethechargesbyaddinge-stothepositiveside.

6.  Multiplerxnsbytheappropriatenumbertogettheelectronstobalanceoutandcancel.

7.  Addreactionstogether,cancelingouttermsthatappearonoppositesides

Balancing•  Ex/Balancethereactionbetweenpermanaganateandiron(II)ionsinacidicsolution:

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Balancing•  First,assignoxidationstates

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Balancing•  Second,separateintohalfreactions

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Reduction:MnO4-àMn2+Oxidation:Fe2+àFe3+

Balancing•  Third,balancehalf-reactionsaccordingtoourrules

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Reduction:MnO4-àMn2+MnO4-àMn2++4H2O

8H++MnO4-àMn2++4H2O5e-+8H++MnO4-àMn2++4H2O

Balancing•  Third,balancehalf-reactionsaccordingtoourrules

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Oxidation:Fe2+àFe3+Fe2+àFe3++e-

Balancing•  Third,balancehalf-reactionsaccordingtoourrules

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

5e-+8H++MnO4-àMn2++4H2O5x(Fe2+àFe3++e-)

--------------------------------5e-+8H++MnO4-+5Fe2+àMn2++4H2O+5Fe3++5e-8H++MnO4-+5Fe2+àMn2++4H2O+5Fe3+

Last,doublecheckthateverythingbalances(atoms,charges)

Balancing•  Trythisone:H+(aq)+Cr2O72-(aq)+C2H5OH(l)àCr3+(aq)+CO2(g)+H2O(l)

AnotherNoteonTitrations

Titrations•  Recall:Titrationsareavolumetricanalysiswecanusetodeterminetheamountofacertainsubstance

•  Inatitration,astandardsolution(ofKNOWNconcentration)isaddedgraduallytoasolutionofunknownconcentrationuntilthechemicalreactioniscomplete.

Titrations•  Acid-BaseTitrationMethod:1.  Analytesolution(ofunknownM)isplacedina

flaskorbeaker2.  Asmallamountofindicatorisadded3.  Titrantisplacedinaburetteandslowlyaddedto

theanalyteandindicatormixture4.  Theprocessisstoppedwhentheindicatorcauses

achangeinthecolorofthesolution5.  Thechangeinvolumeisusedtodeterminethe

volumeoftheanalytesolution

Titrations•  Justasanacidcanbetitratedagainstabase,wecantitrateanoxidizingagentagainstareducingagent.

•  Wecancarefullyaddasolutioncontaininganoxidizingagenttoasolutioncontainingareducingagent

•  Theequivalencepointisreachedwhenthereducingagentiscompletelyoxidizedbytheoxidizingagent

•  Westilluseanindicatorthatchangescolor.•  Theindicatorhasacharacteristiccolorofthereducedformandoxidizedform.Atorneartheequivalencepoint,asharpchangeincolorwilloccur.

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Titrations•  Ex/A16.42mLvolumeof0.1327MKMnO4solutionisneededtooxidize25.00mLofaFeSO4solutioninanacidicmedium.WhatistheconcentrationoftheFeSO4solutionifthenetionicequationis5Fe2++MnO4-+8H+àMn2++5Fe3++4H2O

•  First,findmolesofKMnO4,andthenusethebalancedequationtofindthemolesofFeSO4

•  Second,dividebythevolumeoftheFeSO4solutiontogetmolarity