Ch.4: Types of Rxns & Solu5on Stoichiometry
Transcript of Ch.4: Types of Rxns & Solu5on Stoichiometry
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