F331 - Chemistry for Life

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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= &%%
=35.5
(c)writeandinterpretbalancedchemicalequationsincludingstatesymbols

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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.…read more

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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
Radioactivedecay
particle- !$9 !$# #
L!U L%Th + !He
particle-&# &# %
PC =N + D&e
radiation­thereisnochangeintheatoms;justareleaseofenergyfromthenucleus.radiation
oftenaccompaniestheemissionofalphaand/orbetaparticles.
Nuclearfusion
& ! $
&H + &H !He
(j)recallthat,infusionreactions,lighteratomsjointogiveheavieratoms(under
conditionsofhightemperatureandpressure)andunderstandthatthisishowcertain
elementsareformed
Nuclearfusionisthejoiningtogetheroftwoormorenucleitoformaheaviernucleusofanew
element.Hightemperaturesand/orpressuresarerequiredtoprovidetheenergyneededto
overcometherepulsionbetweentwopositivenuclei.Nuclearfusionoccursduringstarformation.…read more

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Tooshort­decaybeforetracingiscomplete
Toolong­potentiallycauseharmtothepatient
(l)understandthatknowledgeofthestructureoftheatomdevelopedintermsofa
successionofgraduallymoresophisticatedmodels;giveninformation,interpretthese
andotherexamplesofsuchdevelopingmodels…read more

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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.…read more

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CH4,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.…read more

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MendeleevdevelopedthePeriodicTablebyleavinggapsand
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
· Theionwiththegreatestmasscorrespondstothemolecularmassofthesamplecompound.
Thisiscalledthemolecularion,andcorrespondstotheparentmoleculeminusanelectron.
· Themostintensepeakiscalledthebasepeak
· Themolecularionbreaksdownintofragments.…read more

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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.…read more

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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.…read more

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Hess'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.…read more

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