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F331­Elementsoflife

Formulae,equationsandamountofsubstance

(a)explainandusetheterms:atomicnumber,massnumber,isotope,Avogadro
constant,relativeisotopicmass,relativeatomicmass,relativeformulamassand
relativemolecularmass
Atomicnumber
Thenumberofprotonsinanatom
Massnumber
Thesumoftheprotonsandneutronsinanatom
Isotope
Atomsofthesameelementthathavethesameatomicnumber(numberofprotons)butadifferent
massnumber,hencehaveadifferentnumberofneutrons
Avogadroconstant
6.02x1023isthenumberofparticlesin1moleofasubstance
Relativeisotopicmass
Themassofanisotope
Relativeatomicmass
Tellsyouthenumberoftimesanatomofanelementisheavierthanone-twelfthofanatomof12C.
Arvalueshavenounits
Relativeformulamass
Thesumoftherelativeatomicmassesforeachatomintheformula.Forasimplemolecular
compound,itissometimescalledrelativemolecularmass.Mrvalueshavenounits

(b)usetheconceptofamountofsubstancetoperformcalculationsinvolving:massesof
substances,empiricalandmolecularformulae,percentagecomposition

Workingouttheformulafromreactingmasses
2.43gofmagnesiumreactswith0.20gofhydrogen.Whatistheformulaofmagnesiumhydride?
!.#$
Step12.43gofMg=!#.$=0.100moles
%.!%
Step20.20gofH= &.% =0.20moles
Step3Dividingthroughbythesmallestnumberofmoles(0.1inthiscase)gives1Mg:2H.The
empiricalformulaisthereforeMgH2

Workingoutthepercentagebymassofanelement
Calculatethepercentagebymassofnitrogeninammoniumsulfate,(NH4)2SO4.
Step1Mr(NH4)2SO4=2x(14.0+1.0+1.0+1.0+1.0)+32.1+(4x16.0)=132.1
Step2Massofnitrogenin1moleof(NH4)2SO4=14.0+14.0=28.0
'())+,-.//0/'/1-21&'+0/ !9.%
Step3%bymass= 3 +,5+'6+718
x100=&$!.&x100=21.2%
4

Workingouttherelativeatomicmassfromgivenrelativeisotopicmasses
Ifthereis75%of35Cland25%of37Clinasampleofchlorine:

($;×=;)?($=×!;)
Averagerelativeatomicmass= &%%…

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Atomicstructure

(d)describeprotons,neutronsandelectronsintermsoftheirmassandrelativecharge
Particle Massonrelativeatomic Charge
massscale
Proton 1 1+
Neutron 1 0
Electron 0.00055 1-

(e)describethestructureofatomsintermsofelectronsandacentralnucleus
containingprotonsandneutrons
Nucleus­protonsandneutrons
Orbitingthenucleus­electrons
(f)explaintheoccurrenceofabsorptionandemissionatomicspectraintermsof
changesinelectronicenergylevels;compareandcontrastthefeaturesofthesespectra:
Similarities:bothlinespectra;linesinsamepositionforagivenelement;linesbecomecloserat
higherfrequencies;setsoflinesrepresentingtransitionstoorfromaparticularlevel.
Differences:bright/colouredlinesonablackbackgroundorblacklinesoncoloured/bright
background;understandtherelationshipbetweentheenergyemittedorabsorbedandthe
frequencyofthelineproducedinthespectra;E=h

Absorptionspectra
· Electronsabsorba`photon'orpackageofenergy
· Excitedelectronsmoveuptoahigherenergylevel­theyarepromoted.Thisiswhatproduces
thelinesinanabsorptionspectrum
· Theelectromagneticradiationabsorbedbyeachofthehydrogenatomshasadefinite
frequency()relatedtothedifferentinenergylevelsbyE=h

Emissionspectra
· Electronsfirstabsorba`photon,orpackageofenergy
· Excitedelectronsmoveuptoahigherenergylevel­theyarepromoted
· Electronsthendropbackdowntolowerenergylevels.Thisiswhatproducesthelinesinan
emissionspectrum
· Theelectromagneticradiationemittedbyeachofthehydrogenatomshasadefinitefrequency
relatedtothedifferenceinenergylevelsbyE=h.SinceEisdifferentforeachtransition,the
frequency()isdifferentforeachtransition,andhencesoisthecolourofthelines

(e)describetheelectronstructureofatomsintermsofmainenergylevels(electron
shells),uptoZ=36
Electronshell…

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(f)recallthatthenucleiofsomeatomsareunstableandthattheseatomsare
radioactive
(g)recallandexplainthedifferentpropertiesof-,-and-radiations
Radiation Whatisit? Relative Howmuchdoes Whatcanitbe Deflectionin
charge thenucleus stoppedby? electricfield?
change
Heliumnuclei, +2 2fewerprotons,2 Paperorskin Low
#
!He fewerneutrons
%
Electrons, D& e -1 1moreproton,1 Aluminumfoil High
fewerneutron
Electromagnetic None Nochange Leadsheet None
radiation

(h)recallthatthetermhalf-lifereferstothetimetakenforhalftheradioactivenucleiin
asampletodecayandthatthehalf-lifeisfixedforanygivenisotope;carryouthalf-life
calculations
Half-lifeisthetimeittakesforhalftheradioactivenucleiinasampletodecay.Thehalf-lifeisfixed
foranygivenradioisotopeandisnotaffectedbytemperature.

Iodine-131hasahalflifeof8days.Ifyoustartwith24goftheisotope,whatmassremainsafter32
days?
Step1After8days,12gofthesampleremains
Step2After16days,6gofthesampleremains
Step3After24days,3gofthesampleremains
Step4after32days,1.5gofthesampleremains

(i)usenuclearsymbolstowriteequationsfornuclearprocesses,bothfusionand
radioactivedecay…

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(k)understandhowradioactiveisotopescanbeusedas`tracers'inthebodyand(given
information)forotheruses;explainthatthehalf-lifeof`tracers'mustbeofan
appropriatelengthtoallowdetectionbutnotcauseunduedamage;understandtheuse
ofradioisotopesinthedatingofarchaeologicalandgeologicalmaterial
Tooshort­decaybeforetracingiscomplete
Toolong­potentiallycauseharmtothepatient
(l)understandthatknowledgeofthestructureoftheatomdevelopedintermsofa
successionofgraduallymoresophisticatedmodels;giveninformation,interpretthese
andotherexamplesofsuchdevelopingmodels













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Bondingandstructure

(m)drawandinterpretsimpleelectron`dot-and-cross'diagramstoshowhowatoms
bondthroughionic,covalentanddativecovalentbondsandbeabletodescribea
simplemodelofmetallicbonding;describesomelimitationsofthesemodels




Ionic­cationsandanionsareheldtogetherinagiantioniclattice.
Thereisanelectrostaticattractionbetweenthecationsandanions.




Covalent­theatomsinvolvedinthecovalentbondareheldtogetherbyan
electrostaticattractionbetweenthepositivenucleiorthetwoatomsandthesharedpairof
negativeelectrons.




Metallic­thebondthatholdstheparticlestogetherinametalisthe
electrostaticattractionbetweenpositivemetalionsandnegativeelectrons.

Limitations
IntheClO3-ionthecentralchlorineatomhasexpandeditsoutershelltohold12electrons.
(n)recallthetypicalphysicalproperties(meltingpoint,solubilityinwater,abilityto
conductelectricity)characteristicofgiant(metallic,ionic,covalentnetwork)and
simplemoleculestructuretypes

Structure Typeofbonding Meltingpoint Solubilityin Electrical
water conductivity
Ioniclattice Ionic High Usuallysoluble Onlyifmoltenof
(NaCl) insolution
Giantcovalent Covalent High Insoluble Wontconduct,
network(SiO2, apartfrom
diamond) graphite
Simplemolecular Covalent Low Usuallyinsoluble Wontconduct
(CO2,H2O)
Metalliclattice Metallic High Insoluble Willconduct…

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(o)usetheelectronpairrepulsionprincipletopredictandexplaintheshapesofsimple
molecules(suchasCH4,NH3,H2OandSF6)andions(suchasNH4+withuptosixouter
pairsofelectrons(anycombinationofbondingpairsandlonepairs)(notreatmentof
hybridizationormolecularorbitalsisexpected)
Molecule Shape Electronpairs
Methane,CH4 Tetrahedral 4bondingpairs
Ammonia,NH3 Pyramidal 3bondingpairs,1lonepair
Water,H2O Bent 2bondingpairs,2lonepairs
Sulfurhexafluoride,SF6 Octahedral 6bondingpairs
Ammoniumion,NH4+ Tetrahedral 4bondingpairs


InorganicchemistryandthePeriodicTable

(p)recallthatthePeriodicTablelistselementsinorderofatomic(proton)numberand
groupselementstogetheraccordingtotheircommonproperties;usegiveninformation
todescribetrendsinagroupofthePeriodicTableandtomakepredictionsconcerning
thepropertiesofanelementinthisgroup;describeperiodictrendsinthepropertiesof
elements,intermsofmeltingpointandboilingpoint
Meltingpoint­increaseandthenadecreaseacrossaperiod(2and3)

(q)recallthatthepositionofanelementinthePeriodicTableisrelatedtoitselectron
structure(mainenergylevelsorelectronshells)andviceversa
1s
2s2p
3s3p3d
4s4p4d4f

(r)describeandcomparethefollowingpropertiesoftheelementsandcompoundsof
Mg,Ca,SrandBainGroup2:reactionsoftheelementswithwater,acid­basecharacter
oftheoxidesandhydroxides,thermalstabilityofthecarbonates,solubilityof
hydroxidesandcarbonates
Group2elementshavesimilarchemicalreactionsastheyallhavetwoelectronsintheirouter
shell.
· Themetalsreactwithwatertogivethemetalhydroxideandhydrogen­thereactionsbecome
morevigorousasyougodownthegroup
· Theoxidesreactwithwatertoproduceanalkalinesolutionofthehydroxide
· Theoxidesreactwithacidssoactasbases
· Thehydroxidesbecomemoresolubleasyougodownthegroup.Thesolutionsproducedare…

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(s)understandhowMendeleevdevelopedthePeriodicTablebyleavinggapsand
rearrangingsomeelementsfromtheiratomicmassorderandhowsubsequent
researchvalidatedthisknowledge;givenrelevantinformation,discussotherexamples
ofhowscientificresearchcanbeusedtoassessthevalidityofadiscovery
· Hearrangedtheknownelementsinorderofrelativeatomicmass
· Elementssimilarsimilarphysicalandchemicalpropertieswereinthesamegroup
· Heswappedelementsoverifhethoughthattheyfittedbetterintoanothergroupbasedontheir
physicalandchemicalproperties
· Heleftgapsforelementswhichhethoughtwereyettobediscovered
· Hemadepredicationsaboutthepropertiesofelementsyettobediscovered.Hispredictions
werefoundtobeveryclosetotheactualproperties­thisvalidatedhisversionoftheperiodic
tableintheopinionofotherchemists
Wenowarrangetheelementsinorderofatomicnumber


Modernanalyticaltechniques

(t)describeandexplainthemainstagesintheoperationofatime-of-flightmass
spectrometer
· Sampleinlet­gasesorliquidsaresimplyinjectedbutsolidsareheatedtovaporisethem
· Ionisationarea­aheatedfilamentproduceshigh-energyelectrons.Theseelectronsbombard
anyatomsormoleculesinthesampleandknockelectronsout.Cationsareformed.X(g) + eD
X ? + 2eD
· Accelerationarea­anelectricfieldisusedtoaccelerateanyionssothattheyallhavethesame
kineticenergy
· Driftregion­thereisavacuumheresothationsdonotcollidewithairmolecules,whichcould
changethedirectionoftheirflightpath.Sincekineticenergy=massxvelocity2andallionshave
thesamekineticenergy,heavierionsmovethroughthisregionmoreslowlythanlightions
· Iondetector­lightionsreachthedetectorbeforeheavierones.Acomputersystemconvertsthe
informationintoamassspectrum.Onlypositiveionsaredetectedafterfragmentation

(u)usedatefromamassspectrometerto:

(i)calculaterelativeatomicmassandtherelativeabundanceofisotopes
Relativeatomicmass=averagemassof100atoms=
('())+,2)+-+6/×(U718(15/)('())+,2)+-+6/×(U718(15/)
&%%



(ii)workouttherelativemoleculemassofmoleculeandunderstandthatother
peaksarecausedbyfragmentsofthemolecule

·…

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DevelopingFuels

Formulae,equationsandamountofsubstance

(a)usetheconceptofamountofsubstancetoperformcalculationsinvolving:volumes
ofgases,balancedchemicalequations,enthalpychanges
Workingoutvolumesofgases

Whatvolumeofcarbondioxideproducedinthecompletecombustionof6.0dm3ofmethane?

Step1Underlinethesubstanceswhosevolumeyou
aregivenandthevolumeyouwanttofind:
CH4+2O2àCO2+2H2O
Step2Indicatethenumberofmolesinvolved:
1mole1mole
Step3Calculatethevolumesofthismanymoles:
24dm324dm3
Step4Converttothevolumegiveninthequestion:
24/24x6.0=6.0dm3
Step5Converttheothervolumeinthesameway

Step6Writedowntheanswer:
6.0dm3ofCO2isproducedinthecomplete
combustionof6.0dm3ofmethane

Workingwithenthalpychanges

Theenthalpyofcombustionformethaneis-890kJmol-1.Calculatetheenergyreleasedwhen3.20
gofmethaneburncompletely.

Step1Underlinethesubstancewhosemassesor
volumesyouaregivenandtheenthalpychangeyou
aregiven:

CH4+202àCO2+2H2O
H=-890kJmol-1
Step2Indicatethenumberofmolesinvolved:
1mole
H=-890kJmol-1

Step3Calculatethemassofthismanymoles:
16.0g

Step4Converttothemassorvolumegiven:
(16.0/16.0)x3.20=3.20g
Step5Converttheenergyreleasedinthesameway:
(-890/16.0)x3.20
=-178kJmol-1
Step6Writedowntheanswer:
178kJofenergyisreleasedwhen3.20gof
methaneburnscompletely

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Energetics

(b)explainandusetheterms:exothermic,endothermic,standardstate,(standard)
enthalpychangeofcombustion(Hc),(standard)enthalpychangeofreaction(Hr),
(standard)enthalpychangeofformation(Hf)
Exothermic­reactiongivesoutenergyfromthesystemtothesurroundings.Thetemperatureof
thesurroundingsincrease,Hisnegative.
Endothermic­reactiontakesinenergyfromthesurroundings.Thetemperatureofthe
surroundingdecreases,Hispositive.
Standardstate­standardconditionsare1atmospherepressureand298K(25)
Enthalpychangeofcombustion-Hc ,istheenthalpychangewhen1moleofasubstanceburns




completelyinoxygenunderstandardconditions.
Enthalpychangeofreaction-Hr istheenthalpychangeforotherreactionsunderstandard




conditions.
Enthalpychangeofformation-Hf istheenthalpychangewhen1moleofasubstanceifformed




fromitsconstituentelements.Boththereactionsandproductsareintheirstandardstate.

(c)describeanddesignsimpleexperimentstomeasuretheenergytransferredwhen
reactionsoccurinsolutionorwhenflammableliquidsburn;explainthelimitationsof
suchpracticalproceduresandtheuncertaintiesofthemeasurementinvolved

Recordthetemperaturerisewhenaknownvolumeofwaterisheatedbythecompletecombustion
ofameasuredmassoffuel.

Energytransferred=mxcxT

Youcannowcalculatetheenthalpychangeforthecombustionof1moleofthefuelused

Thevalueobtainedwillbemuchlowerthanthedatasheetvalueduetoheatlosstothe
surroundings.
(d)calculateenthalpychangesfromexperimentalresults,recallingtheformula:heat
transferred=mass×specificheatcapacity×temperaturechange;describethe
approximationsindensityandspecificheatcapacityofsolutionsmadeinthese
calculations
Specificheatcapacityisatextbookvalue­4.18Jg-1K-1,thismaynotbethespecificheatcapacity
ofthewaterusedinthereaction,thereforeaffectingthevalueoftheenergytransferred.

Otherassumptionsincludethedensityofwater.1cm3=1g,howeverthisisalsoatextbookvalue
andobviouslythewaterusedinthereactionwillnotnecessarilyhavethissamedensity.







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(e)useHess'lawtoexplainhowenthalpycyclescanbeusedtocalculateenthalpy
changesofreaction,includingviaenthalpychangesofformation,combustionandbond
enthalpies;carryoutthesecalculations

Hess'slawstatesthataslongasthestartingandfinishingpointsarethesame,theenthalpychange
forachemicalreactionwillalwaysbethesame,nomatterhowyougofromstarttofinish.Itis
usefulforcalculatingunknownenthalpychangesfromonesforwhichdataisunavailable.
Enthalpycycles




H2bondbreakingisendothermicso+ve
H1bondmakingisexothermicso-ve

(f)explainandusetheterm(average)bondenthalpyandrelatebondenthalpytothe
lengthandstrengthofabond;recallthatbond-breakingisanendothermicprocessand
bond-makingisexothermicandusetheseideastoexplaintheoverallenthalpychange
forareaction
· Thebondenthalpyistheaverageenergyrequiredtobreakthebondsin1moleofgaseous
compounds.
· Thegreaterthebondenthalpy,thestrongerthebond.
· Shortbondsarestrongerthanlongbonds.
· C=CisshorterandstrongerthanC-C.

(g)usethetermentropyinaqualitativemanner,interpretingitasameasureofthe
numberofwaysthatmoleculescanbearranged
· Entropyisameasureofthenumberofwaysinwhichparticlescanbearranged.
· Gaseshavegreaterentropyliquids;liquidshavegreaterentropythansolids.
· Mixtures(solutions)haveagreaterentropythantheunmixedconstituents.
· Ifthenumberofparticlesincreasedduringthecourseofareactionthenentropyusually
increases.

(h)describethedifferencesinmagnitudeoftheentropyofasolid,aliquid,asolution
andagas
Increasesfromsolidtoliquidtogas.









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