Ch.4:TypesofRxns&Solu5onStoichiometry

Recall

Overview• Wecancategorizereac2onsintosomegeneraltypes:– Combina2on/Synthesis– Decomposi2on– Combus2on– RedoxReac2ons– Singledisplacement/replacement– Doubledisplacement/replacement• Precipita2onReac2on• Acid-Base/Neutraliza2on

Oxida2on-Reduc2onReac2ons

Oxida2on-Reduc2on•  Acid-basereac2onscanbeclassifiedasproton-transferprocesses

NH3(aq)+H2O(l)ßàNH4+(aq)+OH-

(l)

Oxida2on-Reduc2on•  Oxida2on-Reduc2on(redox)reac2onscanbeconsideredelectron-transferreac2ons

•  Overallreac2on:2Mg(s)+O2(g)à2MgO(s)

•  Half-Reac2ons(showtheelectronsinvolved):2Mgà2Mg2++4e-

O2+4e-à2O2-

Oxida2onStates

Oxida2on-Reduc2on•  First,weneedtobeabletoassignoxida+onstatestodeterminehowelectronsarebeingtransferredinareac2on

•  Oxida2onstates(oxida2onnumbers)=indicatesthenumberofchargestheatomwouldhaveinamolecule/compoundifelectronsweretransferredcompletely

•  Rules:#1:Elementsintheirfreestatehaveanoxida2onstateofZEROEx/N2,Na

Oxida2on-Reduc2on•  Rules:

#2:Monoatomicionshaveoxida2onstatesequaltotheirchargeEx/Li+hasanoxida2onstateof+1,O2-hasanoxida2onstateof-2#3:Incompounds,fluorinealwayshasanoxida2onstateof-1Ex/NaF

Oxida2on-Reduc2on•  Rules:

#4:Theoxida2onnumberofoxygeninmostcompoundsis-2Ex/MgOButinhydrogenperoxideandperoxide,itis-1Ex/O2

2-

#5:Theoxida2onnumberofhydrogenis+1exceptwhenbondedtometalsinbinarycompoundsEx/H2Ovs.LiH

Oxida2on-Reduc2on•  Rules:

#6:Thesumoftheoxida2onstatesmustbezeroforanelectricallyneutralcompound.Forpolyatomicions,itmustequalthechargeoftheion.Ex/CaH2Ex/NH4

+

– Generallyspeaking,metallicelementshaveposi2veoxida2onnumbers,whereasnonmetallicelementscanhaveposi2veornega2veoxida2onnumbers

–  Thehighestoxida2onnumberanelementinGroups1A=7Acanhaveisitsgroupnumber

–  Transi2onmetalshaveseveralpossibleoxida2onnumbers.– And,yes,it’spossibletohavenon-integeroxida2onstates

Oxida2on-Reduc2on•  Assignoxida2onstatestothefollowing:a)  Li2O

b)  HNO3c)  Cr2O7

2-

RedoxReac2ons

Oxida2on-Reduc2on•  Oxida2on–involvesthelossofelectrons•  Reduc2on–involvesthegainofelectrons•  LEOthelionsaysGER

Oxida2on-Reduc2on•  Whenlookingatreac2ons,wecanassignoxida2onstatestoseewhichsubstanceisoxidized,andwhichisreduced

CH4(g)+2O2(g)àCO2(g)+2H2O(g)

Oxida2on-Reduc2on•  Wecanalsolookalidlemorecloselyatthetransferofelectrons.Remember:LEOsaysGER.

CH4(g)+2O2(g)àCO2(g)+2H2O(g)

CH4àCO2+8e-

2O2+8e-àCO2+2H2O

Oxida2on-Reduc2on•  Termstouse:Oxidized,reduced,oxidizingagent,reducingagent.

CH4(g)+2O2(g)àCO2(g)+2H2O(g)

CH4àCO2+8e-

2O2+8e-àCO2+2H2O

Oxida2on-Reduc2on•  Given2Al(s)+3I2(s)à2AlI3,iden2fytheatomsthatareoxidizedandreduced,andalsostatetheoxidizingandreducingagents.

BalancingRedoxReac2ons

Balancing1.  Separatetherxnintohalf-reac2ons2.  Balanceallatomsexcepthydrogenandoxygen3.  Balanceoxygenatomsbyaddingonewater

moleculeforeachoxygenatomneeded4.  Acidicsolu2onsàbalancehydrogenbyaddingH+.

Basicsolu2onsàaddonewatermoleculeforeachhydrogenatomneeded,andthenaddthesamenumberofOH-atomsontheoppositeside.

5.  Balancethechargesbyaddinge-stotheposi2veside.

6.  Mul2plerxnsbytheappropriatenumbertogettheelectronstobalanceoutandcancel.

7.  Addreac2onstogether,cancelingouttermsthatappearonoppositesides

Balancing•  Ex/Balancethereac2onbetweenpermanaganateandiron(II)ionsinacidicsolu2on:

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Balancing•  First,assignoxida2onstates

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Balancing•  Second,separateintohalfreac2ons

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Reduc2on:MnO4-àMn2+

Oxida2on:Fe2+àFe3+

Balancing•  Third,balancehalf-reac2onsaccordingtoourrules

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Reduc2on:MnO4-àMn2+

MnO4-àMn2++4H2O

8H++MnO4-àMn2++4H2O

5e-+8H++MnO4-àMn2++4H2O

Balancing•  Third,balancehalf-reac2onsaccordingtoourrules

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Oxida2on:Fe2+àFe3+Fe2+àFe3++e-

Balancing•  Third,balancehalf-reac2onsaccordingtoourrules

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)

AnotherNoteonTitra2ons

Titra2ons•  Recall:Titra2onsareavolumetricanalysiswecanusetodeterminetheamountofacertainsubstance

•  Ina2tra2on,astandardsolu2on(ofKNOWNconcentra2on)isaddedgraduallytoasolu2onofunknownconcentra2onun2lthechemicalreac2oniscomplete.

Titra2ons•  Acid-BaseTitra2onMethod:1.  Analytesolu2on(ofunknownM)isplacedina

flaskorbeaker2.  Asmallamountofindicatorisadded3.  Titrantisplacedinaburedeandslowlyaddedto

theanalyteandindicatormixture4.  Theprocessisstoppedwhentheindicatorcauses

achangeinthecolorofthesolu2on5.  Thechangeinvolumeisusedtodeterminethe

volumeoftheanalytesolu2on

Titra2ons•  Justasanacidcanbe2tratedagainstabase,wecan2trateanoxidizingagentagainstareducingagent.

•  Wecancarefullyaddasolu2oncontaininganoxidizingagenttoasolu2oncontainingareducingagent

•  Theequivalencepointisreachedwhenthereducingagentiscompletelyoxidizedbytheoxidizingagent

•  Wes2lluseanindicatorthatchangescolor.•  Theindicatorhasacharacteris2ccolorofthereducedformandoxidizedform.Atorneartheequivalencepoint,asharpchangeincolorwilloccur.

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Titra2ons•  Ex/A16.42mLvolumeof0.1327MKMnO4solu2onisneededtooxidize25.00mLofaFeSO4solu2oninanacidicmedium.Whatistheconcentra2onoftheFeSO4solu2onifthenetionicequa2onis5Fe2++MnO4

-+8H+àMn2++5Fe3++4H2O•  First,findmolesofKMnO4,andthenusethebalancedequa2ontofindthemolesofFeSO4

•  Second,dividebythevolumeoftheFeSO4solu2ontogetmolarity