Download Module 4: Equations and Stoichiometry

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)
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
BalancingChemicalReactions
• The#ofeachtypeofatommustbalance(conservationof
mass)
– Canusecoefficientsinfronttomakethingswork.
• Goodruleofthumb– trytobalancetheatomsthatshowupin
theleast#ofspots(#ofcompounds)1st
• It’sOKtousefractions
– Ifwhole#’sarewanted/needed justmultiplybyLCD
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