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Chapter8LecturepresentationChemicalReactionsandChemicalQuantitie
sCatherineE.MacGowanArmstrongAtlanticStateUniversityClimate
ChangeandCombustionofFossilFuels:TheGreenhouseEffectGreen
housegasesintheatmosphere:Allowsunlighttoentertheatmosph
ereWarmEarth’ssurfacePreventsomeoftheheatgeneratedbythe
sunlightfromescapingThebalancebetweenincomingandoutgoing
energyfromthesundeterminesEarth’saveragetemperature.Globa
lWarmingScientistshavemeasuredanaverage0.6°Criseinatmos
pherictemperaturesince1860.Duringthesameperiod,atmospheric
CO2levelshaverisen25%.Arethetwotrendscausal?PhysicalCha
ngesinMatterWhatisaphysicalchange?Changesthatalteronlyt
hestateorappearance,butnotcomposition,arephysicalchanges
.Theatomsormoleculesthatcomposeasubstancedonotchangeth
eiridentityduringaphysicalchange.Whenwaterboils,itchange
sitsstatefromaliquidtoagas.Thegasremainscomposedofwa
termolecules,sothisisaphysicalchange.ChemicalChangesinM
atterWhatisachemicalchange?Changesthatalterthecomposition
ofmatterarechemicalchanges.Duringachemicalchange,atomsr
earrange,transformingtheoriginalsubstancesintodifferentsub
stances.Arustingnailisachemicalchange.Rustoccurswhenthe
ironatomsexchangeelectronswithoxygenatoms—combiningtoform
aNEWordifferentchemicalsubstance.Ironrustisthecompound
iron(III)oxide(Fe2O3).PracticeProblemonPhysicalandChemica
lChangesWritingandBalancingChemicalReactionsChemicalReactio
nsReactionsinvolvechemicalchangesinmatterresultinginnews
ubstances.Reactionsinvolverearrangementandexchangeofatomst
oproducenewmolecules.Achemicalreactionisawrittenstatemen
t.Itstatesthequantities,thechemicalidentityofthesubstanc
esinthereaction,andtheirphysicalstate.Example:CH4(g)+
O2(g)→CO2(g)+H2O(l)Reactants→ProductsReactants→Product
sChemicalEquationsChemicalEquations:ShorthandforDescribing
aChemicalReactionChemicalequations:Provideinformationaboutt
hereactionMolecularorionicformulasofreactantsandproductsS
tatesofreactantsandproductsGas(g),liquid(l),solid(s),an
daqueous(aq)Relativenumbersofreactantandproductmolecules
thatarerequiredCanbeusedtodetermineweightsofreactantsus
edandproductsthatcanbemadeTheQuantitiesinChemicalReacti
onsTheamountofeverysubstanceusedandmadeinachemicalreac
tionisrelatedtotheamountsofalltheothersubstancesinthe
reaction.LawofconservationofmassBalancingequationsbybalan
cingatomsThestudyofthenumericalrelationshipbetweenchemica
lquantitiesinachemicalreactioniscalledstoichiometry.Pract
iceProblem:BalancingandWritingChemicalEquationsPracticePro
blemcontinuedPracticeProblem:BalancingandWritingChemicalEq
uationsPracticeProblemcontinuedPracticeProblem:Balancingand
WritingChemicalEquationsPracticeProblemcontinuedReactionStoi
chiometry:WhatIsitabout?Thecoefficientsinachemicalreacti
onspecifytherelativeamountsinmolesofeachofthesubstance
sinvolvedinthereaction.2C8H18(l)+25O2(g)→16CO2(g)+18
H2O(g)WhattheaboveBALANCEDchemicalequationsays: 2molecule
sofC8H18reactwith25moleculesofO2toform16moleculesof
CO2and18moleculesofH2O.2molesofC8H18reactwith25moles
ofO2toform16molesofCO2and18molesofH2O.2molC8H18:2
5molO2:16molCO2:18molH2OCookingStoichiometry:MakingP
izzaThenumberofpizzasyoucanmakedependsontheamountofin
gredientsyouuse.Thisrelationshipcanbeexpressedmathematical
ly.1crust:5oz.sauce:2cupscheese:1pizzaWecancompare
theamountofpizzathatcanbemadefrom10cupsofcheese:Since
2cupscheese:1pizza,then,1crust+5oz.tomatosauce+2c
upscheese→1pizzaMole-to-MoleConversions2C8H18(l)+25O2(g
)→16CO2(g)+18H2O(g)Inthesamewaythattheratiowasused
fromthepizzarecipeexample,thestoichiometricratioactsasa
conversionfactorbetweentheamountinmolesofareactanttom
olesofaproduct.Reactanttoproduct:Stoichiometricratio:2m
olesC8H18:16molesCO2Stoichiometricratiocanbebetween:Reac
tanttoreactantStoichiometricratio:2molesC8H18:25molesO
2Producttoproduct:Stoichiometricratio:16molesCO2:18moles
H2OHowManyMolesofCO2FormIf22.0MolesofC8H18Arecombust
ed(Burned)?Set-upandSolution:Balancedchemicalreaction:2C8H
18(l)+25O2(g)→16CO2(g)+18H2O(g)Stoichiometricratiofrom
balancedreaction: 2molesC8H18:16molesCO2Calculation:Answ
er:Thecombustionof22molesofC8H18adds176molesofCO2to
theatmosphere.Mole-to-MassandMass-to-MassConversionsStoichiom
etricratioscanbeusedasaconversionfactorbetweentheamoun
tingrams(mass)ofareactantusedtodeterminemass(grams)of
aproductmade.Strategy:AisreactantandBistheproduct.Ma
ssofA→MolesofA→StoichiometricratioB:A→MolesB→Ma
ssBmassA×(1moleA/mol.massA)×(moleB/moleA)×(mol.mass
B/1molB)Mole-to-MassandMass-to-MassConversions2C8H18(l)+2
5O2(g)→16CO2(g)+18H2O(g)Problem:Determinethemass(grams
)ofCO2producedwhen3.6×1015gramsofC8H18isburnedinex
cessoxygengas.Strategy:NeedabalancedreactionFromthebalance
reaction,usethestoichiometricrelationshipbetweenC8H18and
CO2.MassofC8H18→MolesofC8H18→Stoichiometricratio2C8H1
8:16CO2→MolesofCO2→MassofCO2Problem:Determinethemas
s(grams)ofCO2producedwhen3.6×1015gramsofC8H18isburn
edinexcessoxygengas.Strategy:Needabalancedreaction: 2C8H1
8(l)+25O2(g)→16CO2(g)+18H2O(g)Fromthebalancereaction,
usethestoichiometricrelationshipbetweenC8H18andCO2. 2m
olC8H18:16molCO2MassofC8H18→MolesofC8H18→Stoichiomet
ricratio2C8H18:16CO2→MolesofCO2→MassofCO23.6×1015
gramsC8H18×(1molC8H18/114.22g)×(2molC8H18/16molCO2)
×(44.0g/1molCO2)=1.1×1016gramsofCO2isproducedPracti
ceProblem:StoichiometryLimitingReactant,TheoreticalYieldBack
tothepizzarecipe: 1crust+5oz.tomatosauce+2cupschees
e→1pizzaSupposeyouhave4crusts,10cupsofcheese,and15o
z.tomatosauce.Howmanypizzascanyoumake?Strategy:Wehavee
noughcruststomakeWehaveenoughcheesetomakeWehaveenought
omatosaucetomakeLimitingReactant:PizzaProblemContinuedWeh
aveenoughcrustsfor4pizzas,enoughcheesefor5pizzas,butO
NLYenoughtomatosaucefor3pizzas.Therefore,only3pizzasca
nbemade.Thetomatosaucelimitshowmanypizzascanbemade.Lim
itingReactantandTheoreticalYieldConnectionInthepizzaanalo
gy,thetomatosauceisthelimitingreactant,thereactantthat
makestheleastamountofproduct.Thelimitingreactantisalsok
nownasthelimitingreagent.Themaximumnumberofpizzasthatca
nbemadedependsonthisingredient,thetomatosauce.Inchemi
calreactions,thisiscalledthetheoreticalyield.Theoreticaly
ieldistheamountofproductthatcanbemadeinachemicalreac
tionbasedontheamountoflimitingreactant.Example:Theingred
ientthatmakestheleastamountofpizzadetermineshowmanypiz
zasyoucanmake(theoreticalyield).MoreMakingPizzasAssumetha
twhilemakingpizzas,apizzaisburntordroppedonthefloora
ndonlytwopizzasareavailabletoeat.Theactualamountofpr
oductmadeinachemicalreactioniscalledtheactualyield.Actu
alyieldisaboutefficiency.Todetermineyourefficiencyinmak
ingpizzas,apercentagevaluecanbecalculated.Inchemicalrea
ctions,thisiscalledapercentyield.SummarizingLimitingReact
antandTheoreticalYieldThelimitingreactant(orlimitingreage
nt)isthereactantthatiscompletelyconsumedinachemicalrea
ctionandlimitstheamountofproduct.Thereactantinexcessis
anyreactantthatoccursinaquantitygreaterthanisrequiredt
ocompletelyreactwiththelimitingreactant.Thetheoreticalyie
ldistheamountofproductthatcanbemadeinachemicalreacti
onbasedontheamountoflimitingreactant.Theactualyieldist
heamountofproductactuallyproducedbyachemicalreaction.The
percentyieldiscalculatedas: (actualyield/theoreticalyield
)×100=percentyield%ChemicalReactionInreactionswithmultip
lereactants,itislikelythatoneofthereactantswillbecomp
letelyusedbeforetheothers.Whenthisreactantisusedup,the
reactionstopsandnomoreproductismade.Thereactantthatlimi
tstheamountofproductiscalledthelimitingreactant.Itisso
metimescalledthelimitingreagent.Thelimitingreactantgetsco
mpletelyconsumed.Reactantsnotcompletelyconsumedarecalledex
cessreactants.Thereactantinexcessisanyreactantthatoccur
sinaquantitygreaterthanisrequiredtocompletelyreactwith
thelimitingreactant.Theamountofproductthatcanbemadefro
mthelimitingreactantiscalledthetheoreticalyield.Practice
Problem:Stoichiometry—LimitingReactantandTheoreticalYieldPra
cticeProblem:Stoichiometry—LimitingReactantandTheoreticalYi
eldCombustion:ATypeofChemicalReactionAcombustionreactioni
nvolvesthereactionofasubstancewithO2toformoneormoreo
xygencontainingcompounds.Otherproductsofacombustionreactio
n:Water(H2O)andHeat(energy)Example:CombustionofmethaneCH4(g)
+2O2(g)→CO2(g)+2H2O(g)+HeatPracticeProblem:Combustion
AlkaliMetalReaction:ATypeofChemicalReactionOverviewofAlk
aliMetals:Thealkalimetals(group1A)havens1outerelectronc
onfigurations.Form+1cationto“achieve”noblegasconfiguratio
n.Thereactionsofthealkalimetalswithnonmetalsarevigorous.
Commonreactionforalkalimetals(M)iswithhalogens(X) 2M+
X2→2MXExample:2Na(s)+Cl2(g)→2NaCl(s)Thealkalimetalsr
eactvigorouslywithwatertoformthedissolvedalkalimetalion,
thehydroxideion,andhydrogengas: 2M(s)+2H2O(l)→2M+(aq)+2OH–(aq)+H2(g)Thereactionishighlyexothermicandcanbeexplosivebecausetheheatfromthereactioncanignitethehydrogengas.HalogenReaction:ATypeofChemicalReactionHalogenOverview:Group7elementsthathavens2np5outerelectronconfigurations:Mostlyform–1anions(Fonlyforms–1anion)toachievethe“noblegasconfiguration”MostreactiveofthenonmetalelementsThehalogens(X)tendtoreactwithmetalsespeciallywithGroup1and2Ametalstoformioniccompoundssuchasmetalhalides(MXn). 2M+nX2→2MXnExample:2Fe(s)+3Cl2(g)→2FeCl3(s)Thehalogensreactwithhydrogentoformhydrogenhalides. H2(g)+X2→2HX(g) |
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