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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.
46
Titrations• Ex/A16.42mLvolumeof0.1327MKMnO4solutionisneededtooxidize25.00mLofaFeSO4solutioninanacidicmedium.WhatistheconcentrationoftheFeSO4solutionifthenetionicequationis5Fe2++MnO4-+8H+àMn2++5Fe3++4H2O
• First,findmolesofKMnO4,andthenusethebalancedequationtofindthemolesofFeSO4
• Second,dividebythevolumeoftheFeSO4solutiontogetmolarity
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