Atoms Moles and Stoichiometry Part II.docx
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Atoms Moles and Stoichiometry Part II.docx
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AtomsMolesandStoichiometryPartII
LECTURE1–PartII
REDOXREACTIONS
LectureContent
1RedoxReactions
1.1Definitions
1.2AssigningOxidationNumber
2BalancingRedoxEquations
3RedoxTitrations
3.1PotassiumManganate(VII)titrations
3.2Iodometric(iodine/thiosulfate)titrations
4ApplicationofOxidationNumberandElectronTransferinRedoxReactions
LearningOutcomes
Candidatesshouldbeableto:
a)describeandexplainredoxprocessesintermsofelectrontransferand/orofchangesinoxidationnumber(oxidationstate)
b)constructredoxequationsusingtherelevanthalf-equations
c)performcalculations,usingthemoleconcept,involvingredoxreactions
d)deducestoichiometricrelationshipsfromcalculationssuchasthosein(c)
References
∙Chemistryforadvancedlevel,PeterCannandPeterHughes.
Chemistry:
TheMolecularNatureofMatterandChange,MartinS.Silberberg.
1REDOXREACTIONS
1.1Definitions
Redoxreaction:
AreactionthatinvolvesReductionandOxidationsimultaneously.
Reductionandoxidationprocessesmaybedefinedintermsofelectrontransferand/orchangesinoxidationnumber(oxidationstate).
Reduction:
Aprocesswherebyasubstancegainselectrons,resultinginadecreaseinoxidationnumber.
e.g.Cu2++2eCu
Oxidation:
Aprocesswherebyasubstanceloseselectrons,resultinginanincreaseinoxidationnumber.
e.g.ZnZn2++2e
Acronym:
OILRIG
Reducingagent(reductant):
Asubstancethatgiveselectronstoanother,itselfbeingoxidisedintheprocess.
e.g.Fe2+,C2O42–,I–,S2O32−
Oxidisingagent(oxidant):
Asubstancethattakesinelectronsfromanother,itselfbeingreducedintheprocess.
e.g.MnO4–,Cr2O72–,H2O2,I2
Disproportionation:
Aredoxreactioninwhichthesamereactantisbothoxidised
andreduced.
e.g.3Cl2+6OH5Cl+ClO3+3H2O
Oxidationnumber:
Theoxidationnumber(oxidationstate)ofanelementindicatesthenumberofelectronslostorgainedasaresultofchemicalbonding.
∙Whenwritingtheoxidationnumber(O.N.),the+/–signsmustbestatedbeforethenumber.
∙Foracovalentcompound,thecovalentbondis'converted'toionicbyassigningthesharedelectronstothemoreelectronegative1atom,suchthattheoxidationnumbersoftheatomsarethereforethe'charges'onthe'ions'.
1.2AssigningOxidationNumber
∙Foranatominitselementalform,oxidationnumber=0
e.g.Na,O2
∙Foramonoatomicion,oxidationnumber=chargeontheion
e.g.Mg2+,I–
∙Forapolyatomicion,sumofalltheoxidationnumbers
=chargeonthepolyatomicion
e.g.MnO4–
∙Foraneutralcompound,sumofalltheoxidationnumbers=0
e.g.Fe2O3,Na2S2O3
∙Foracompound,themoreelectronegativeelementisgiventhenegativeoxidationnumber.
e.g.ClF,NaH
Someelementsnearlyalwaysexhibitthesameoxidationnumberintheircompounds.Hence,theyareusedasreferencepointsinassigningoxidationnumberstootherelements.Forexample,
Hydrogen,H
+1,exceptinmetalhydrides(e.g.NaH)whereO.N.is–1
Oxygen,O
–2,exceptinperoxides(e.g.H2O2)whereO.N.is–1
GroupImetals:
Li,Na,K,Rb,Cs,Fr
+1
GroupIImetals:
Be,Mg,Ca,Sr,Ba,Ra
+2
Aluminium,Al
+3
Fluorine,F
–1
1Electronegativityistheabilityofanatomtoattractthepairofelectronsinacovalentbondtoitself.
ElectronegativityincreasesacrossthePeriodsanddecreasesdowntheGroups.(TobediscussedunderPhysicalPeriodicity)
Workedexample1
Statetheoxidationnumberoftheunderlinedelementinthefollowing:
(i)CH3OH(ii)S8(iii)BrF3(iv)HNO2(v)Ca(HCO3)2
WorkedExample2
Identifythetypeofreactionsrepresentedinthefollowingequations.
Whicharethereducingandoxidizingagentsfortheredoxreactions?
(i)NO3–(aq)+4Zn(s)+7OH–(aq)+6H2O(l)4Zn(OH)42–(aq)+NH3(aq)
Typeofreaction:
(ii)KCl(aq)+AgNO3(aq)KNO3(aq)+AgCl(s)
Typeofreaction:
(iii)NaOH(aq)+HCl(aq)NaCl(aq)+H2O(l)
Typeofreaction:
(iv)H2O2(aq)+2Fe2+(aq)+2H+(aq)2Fe3+(aq)+2H2O(l)
Typeofreaction:
2BALANCINGREDOXEQUATIONS
Inaredoxreaction,thereisatransferofelectronsfromthereducingagent(whichloseselectronsandisoxidised)totheoxidizingagent(whichgainselectronsandisreduced).
Hence,inabalancedredoxequation,thenumberofelectronslostbythereducingagentmustbeequaltothenumberofelectronsgainedbytheoxidizingagent,i.e.therearenofreeelectronsintheoverallbalancedequation.
Thefollowingisonemethodthatcanbeusedtobalanceredoxequations:
1.Writeanunbalancedhalf-equationforeachoftheoxidationandreductionhalf-reactions.
2.Balanceeachhalf-equationasfollows:
∙Workouttheoxidationnumberoftherelevantreactantandproduct.
∙Balancetheelementsundergoingoxidationorreduction.
∙Addtheappropriatenumberofelectronsaccordingtothechangeinoxidationnumbersofelements,takingintoaccountthenumberofeachelementintheformulaunit.
∙Balancethechargesonbothsidesofthehalf-equationbyaddingtheappropriatenumberofH+(underacidicconditions)orOH−(underalkalineconditions).Underneutralconditions,eitherH+orOH−canbeused.
∙BalancetheHatomsbyaddingH2Oontheappropriateside.
∙CheckthenumberofOatomsonbothsides–itshouldbebalanced.
3.Multiplyeachhalf-equationbyanappropriateinteger,ifnecessary,tomakethenumberofelectronsthesameineachhalf-equation(becausenumberofelectronslostbyreducingagent=numberofelectronsgainedbyoxidisingagent).
4.Addthetwohalf-equations(theelectronswillcancelout).
5.Simplifythebalancedredoxequationifpossible.Forexample:
∙CombineH+andOH−toformH2O.
∙Cancelanyextraspeciesappearingoneitherside.
6.Makeafinalcheckthattheredoxequationisbalancedintermsofatomsandcharge.
WorkedExample3
Balancethefollowingredoxreactions.
(a)Cr2O72+Fe2+Cr3++Fe3+(acidic)
(b)TcO4+AsTc+AsO2(basic)
(c)Cl2Cl–+ClO3–(basic)
3REDOXTITRATIONS
Inaredoxtitration,aknownconcentrationofoxidizingagentisusedtofindanunknownconcentrationofreducingagent(orviceversa).Redoxtitrationsarealsousedtoestablishthestoichiometryofredoxreactions.Thecalculationsinvolvedarebasedonthemoleconcept,similartoacid-basetitrations.
Commonoxidizingagents
Oxidant
ReactionMedium
Product
MnO4(purple)
Acidic
Mn2+(colourless)
Alkaline
MnO2(brownsolid/ppt)
Cr2O72(orange)
Acidic
Cr3+(green)
I2(browninaqueoussolution)
Neutral
I–(colourless)
H2O2(colourless)
Acidic
H2O
Commonreducingagents
Reductant
ReactionMedium
Product
Fe2+(palegreen)
Acidic
Fe3+(yellow)
I–(colourless)
Acidic/Alkaline/Neutral
I2(browninaqueoussolution)
S2O32–(colourless)
Neutral
S4O62–(colourless)
H2O2(colourless)
Acidic
O2
C2O42–(colourless)
Acidic
CO2
3.1PotassiumManganate(VII)titrations
1.TheKMnO4solutionisusuallyplacedintheburetteandaddedintotheconicalflaskcontainingthereductant(e.g.Fe2+determination)
2.TypicalreducingagentsthatcanreactwithKMnO4are:
Fe2+,C2O42,H2O2andI
3.Manganate(VII)titrationsareusuallycarriedoutinacidicconditions.Theacidusedissulfuricacid(usuallyat1moldm–3)andaddedintotheconicalflask.Nitricacidisnotsuitableasitisitselfanoxidisingagentwhilehydrochloricacidwillbeoxidisedbymanganate(VII)iontogivechlorine.
Relevanthalf-equation:
MnO4+8H++5eMn2++4H2O
4.Duringthetitration,thepurpleMnO4–turnscolourlessasitisreducedtoMn2+ions(colourless/palepink).
5.Whenallthereductantisusedup,thefirstextradropofKMnO4makesthesolutionintheconicalflaskturnpermanentlypink.Thissharpcolourchangeindicatesthattheend-pointisreached.Hencenootherindicatorisrequired.
3.2Iodometrictitrations–iodine/thiosulfatetitration
Iodineisoftenusedtooxidizethiosulfateionstotetrathionateions,itselfreducedtoiodide.
Relevantequation:
2S2O32+I22I+S4O62
1.Thethiosulfatesolutionisusuallyplacedintheburetteandthebrowniodinesolutionintheconicalflask.
2.Duringthetitration,thethiosulfateisaddedfromtheburetteintotheflaskandreducesbrowniodineintocolourlessiodideions.Thesolutiongraduallyfadestopaleyellow.
3.Atthispoint,starchindicatorisaddedintotheflask.Solutionturnsblue-blackasstarchformsanintensebluecomplexwiththeremainingiodine.
4.Titrationthencontinuestillalliodineisusedup.Colourchangeattheend-pointisfromblue-blacktocolourless.
Pointstonotewhenconductingiodometrictitrations
a)Theiodinesolutionshouldbetitratedassoonaspossibleonceitisprepared.Why?
Iodineisvolatileatroomtemperaturehencetitrationmustbecarriedoutassoonaspossible.Ifthisisnotpossible,coverallflaskscontainingiodine.
b)Whyisthestarchindicatoraddedonlytowardstheendoftitrationwhentheiodinesolutionispaleyellow,insteadofrightatthestart?
Starchformsadeep-bluewater-solublecomplexwithiodineinwhichtheiodineistrappedwithinthestarchmolecules.Hence,starchshouldnotbeaddedatthebeginningofthetitrationwhenthereisahighconcentrationofiodinesincesomeiodinemayremaintrappedinthestarchevenattheend-point.
c)Aftertitrationiscomplete,aslowreturnofthebluecolourisobserved.Whyisthisso?
Shouldwecontinueourtitration?
Ignoreanyslowreturnofbluecolourafterthetitrationisdone.ThisisduetoatmosphericoxidationofIbacktoI2.
Iodometricbacktitrationsareoftenusedtoanalyzeredoxreactionsinvolvings
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