Whatisstoichiometry?
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(Probably)themostimportanttopicinchemistry!
Thisisthebasisformany subsequentchapters
Relatedtotheamount ofaspeciesorsubstance
Sometimesreferredtoasthemathematicsofchemistry
Somedefinitions
• Molarmass(akamolecularweight)– sumofatomicmasses
(weights)foralltheatomsinagivenmolecule.
– Usetheperiodictableandthemolecularformulatodeterminethis
• Formulamass(formulaweight)– sumofthemassesforallthe
ionsinagivenformulaunit
Theperiodictable
AnImportantInterpretation
• Thestoichiometriccoefficientsthatarepresentinabalanced
chemicalreactionarerelatedtotheratios ofreactantsand
productsinachemicalreaction
• Thisratioisonlyintermsofmoles(ormolecules),BUTNOT
mass!
Example
• Thefinalstepintheproductionofnitricacidinvolvesthe
reactionofnitrogendioxidewithwater;nitrogenmonoxideis
alsoproduced.Howmanygramsofnitricacidareproducedfor
every100.0gofnitrogendioxidethatreacts?
Solution
• Step1:Writedownthechemicalreaction
– NO2 +H2Oà HNO3 +NO
• Step2:Balancethechemicalreaction
– 3NO2 +H2Oà 2HNO3 +NO
• Step3:DeterminethemolesofNO2 thatwillreact
– 100gNO2 /46.006g/mol=2.174molNO2
Solution(continued)
• Step4:UsestoichiometrytodeterminethemolesofHNO3
thatwillbeproduced
Solvingforx,x=1.449molHNO3
• Step5:Converttogramsofnitricacid
– 1.449molHNO3 *63.013g/mol=91.31gHNO3
Using the periodic table,
predict whether the
following chlorides are
ionic or covalent: KCl,
NCl3, ICl, MgCl2,PCl5, and
CCl4.
LimitingandExcessReagents(Reactants)
• Equivalent– amathematicallyequalamountof
achemicalsubstance(intermsofmoles)
• Sometimesyoudon’thavethe
“stoichiometricallycorrect”numberof
equivalents
– Cost
– Availability
– Reactionconditions
• Limiting– getsusedupentirely
• Excess– remaining(leftover)
Moreonlimitingreagents
• Thisisbasedonthe#ofmolesinabalanced
chemicalreaction
–Youcannot simplylookat#ofmolesdirectly,or
themass(grams)thataregiven.
• Thelimitingreagentalways determinesthe
outcomeofachemicalreaction
–#ofmoles(orgrams)ofproductthatcanbe
formed
Example
• Magnesiumnitridecanbeformedbythe
reactionofmagnesiummetalwithnitrogen
gas.
• A)Howmanygramsofmagnesiumnitridecan
bemadeinthereactionof35.00gof
magnesiumand15.00gofnitrogen?
• B)Howmanygramsoftheexcessreactant
remainafterthereaction?
Solution(parta)
• Step1:Writedown(andbalance)thechemical
reaction
– 3Mg+N2 à Mg3N2
• Step2:Findthe#ofmolesofeachreactant.This
representsthemolesyouHAVE.
– molMg=35.00g/24.305g/mol=1.440molMg
– molN2 =15.00g/28.013g/mol=0.5355molN2
Solution(parta)
• Step3:Pickone reactant,andfindthe
numberofmolesoftheother using
stoichiometry.Thisrepresentsthemolesyou
NEED.
Solvingforx,x=0.4800molN2
Solution(parta)
• Step4:ComparethemolesyouHAVEwith
themolesyouNEED.IfHAVE>NEED,thisisin
excess. IfyouHAVE<NEED,thisislimiting.
– Wehave0.5355molN2 andneed0.4800mol
ofN2,soN2 mustbeinexcess.ThereforeMg
islimiting.
Solution(parta)
• Step5:Usingthelimitingreactantand
stoichiometry,determinethenumberof
molesofproduct.
Solvingforx,x=0.4800molMg3N2.
• Step6:Findthemassoftheproduct
– 0.4800molMg3N2*100.93g/mol=48.45g
Mg3N2
Solution(partb)
• Step1:DeterminehowmuchN2 (theexcess
reagent)isactuallyused.
– 0.4800molN2 *28.013g/mol=13.45gN2
• Step2:Determinetheamountofexcess.
– 15.00g– 13.45g=1.55gN2
Analternatesolutiontopartb
• Conservationofmass
– Thetotalmassbefore thechemicalreactionmustbethe
sameasthetotalmassafter thechemicalreaction
– massMg+massN2 =35.00g+15.00g=50.00g
– MassofMg3N2 =48.45g
– ThereforemassofexcessN2 mustbe50.00– 48.45g=
1.55g
Yield
• Thisisrelatedtotheefficiencyofachemical
reaction(howwellitworked)
• Actualreferstoanexperimentalquantity
• Theoreticalreferstotheamountcalculatedusing
stoichiometry
• Theamountsusedcanbemassormoles,aslongas
youareconsistent(andboth#’srefertothe
product)
• Engineersusuallyalsoareconcernedwithselectivity
andconversion.
In an accident, a solution
containing 2.5 kg of nitric
acid was spilled. Two
kilograms of Na2CO3 was
quickly spread on the
area and CO2 was
released by the reaction.
Was sufficient Na2CO3
used to neutralize all of
the acid?
BalancingChemicalReactions
• The#ofeachtypeofatommustbalance(conservationof
mass)
– Canusecoefficientsinfronttomakethingswork.
• Goodruleofthumb– trytobalancetheatomsthatshowupin
theleast#ofspots(#ofcompounds)1st
• It’sOKtousefractions
– Ifwhole#’sarewanted/neededjustmultiplybyLCD
Example:CombustionofEthane
• Step1:Writedownthereaction
– C2H6 +O2 à CO2 +H2O
• Step2:BalancetheC’s
– C2H6 +O2 à 2CO2 +H2O
• Step3:BalancetheH’s
– C2H6 +O2 à 2CO2 +3H2O
• Step4:BalancetheO’s
– C2H6 +7/2O2 à 2CO2 +3H2O
• Step5:Usewholenumbercoefficients(optional)
– 2C2H6 +7O2 à 4CO2 +6H2O
Balancingredox reactions
• Key: Thenumberofelectrons“lost”(inanoxidation)mustbethesameasthenumberof
electrons“gained”(inareduction)
1) Determineoxidationnumbersandwritedownthehalf-reactions.
2) Balancetheatomsineachhalf-reaction(exceptOandH)
3) Balancethechargeineachhalf-reactionbyaddingelectrons.
4) Balancethetotalnumberofelectronsforbothhalf-reactionsandaddthetwo
reactions.
5) AddH2OtobalancetheO’s(andH’s).
6) Ifacidic,addH+ tobalancetheH’s.
7) Ifbasic,addH+ tobalancetheH’s,thenaddanequalnumberofOH- tobothsides(H+ +
OH- à H2O),andsimplify.
Check:Thetotalchargeontheleftsidemustbeequaltothetotalchargeontherightside
oftheoverallreaction.
Example
• Inbasicsolution,Br2 disproportionates tobromideionsand
bromate ions.Usethehalf-reactionmethodtobalancethe
equationforthisreaction:
Br2(l)à Br-(aq)+BrO3-
Solution
• FirstassignoxidationnumberstotheBr’s:Br2 =0,Br- =-1,BrinBrO3- =+5
(sinceOhasanoxidationnumberof-2)
• Sothehalf-reactionsareBr2 +2e- à 2Br- andBr2 à 2BrO3- +10e• Balancethenumberofelectronsbymultiplyingthereductionreactionby5,
andaddthetworeactions:6Br2 +10e- à 10Br- +2BrO3- +10e• Simplify:3Br2 à 5Br- +BrO3• AddH2OtobalancetheO’s:3Br2 +3H2Oà 5Br- +BrO3• AddH+ andOH- (sincethesolutionisbasic)tobalancetheH’s:3Br2 +3H2O+
6OH- à 5Br- +BrO3- +6H+ +6OH• Simplify:3Br2 +6OH- à 5Br- +BrO3- +3H2O
• Check: Totalchargeontheleft=3(0)+6(-1)=-6,andthetotalchargeonthe
rightis5(-1)+-1+3(0)=-6
Balance the following
equation according to
the half-reaction
method:
Zn(s) + NO3−(aq) ⟶
Zn2+(aq) + NH3(aq) (in
base)
ArrheniusTheoryofDissociation
• Dissociationhappensspontaneouslywhenionic(soluble)
compoundsdissolveinH2O.
• Themoreionsarepresent(i.e.thebetteritdissociates),the
moreelectricityisconducted.
http://antoine.frostburg.edu/chem/senese/101/reactions/slides/sld006.htm
Classificationofelectrolytes
• Strong– solubleionicsubstances(salts),mineralacids,bases
– Acids:HCl,HBr,HI,HNO3,H2SO4,HClO4
– Bases:LiOH,NaOH,KOH,RbOH,CsOH,Ca(OH)2,Sr(OH)2,Ba(OH)2
• Weak– carboxylicacids,amines
• Non-electrolytes– mostorganiccompounds
• Thewords“strong”and“weak”referonly tohowwell
somethingdissociatesandformsions,NOTifitisdangerous,
reactive,etc.
Determiningconcentrationsofionicsolutions
• Forthe[]ofions,weneedtoconsiderboth theformulaand
whetherornotitdissociatescompletely(strongelectrolyte)