Chemical Change

Chapter7

ChemicalChange

Themagicofchemistrycomeswiththrillsandexcitement,flashyreactions

andfireworks,withcolourandsound.Itisnotbondingandstructurethat

grabtheimagination,butspectacleandchange.Hereisthetopicthattells

therealstoryofchemistry.Chemicalchange,morethananything,happens

inacrowdedenvironment.Factorsofimportancearethestateofaggrega-

tion,materialconcentration,temperatureandpressure,collectivelyknown

asthermodynamicconditions.Studentsofchemistry,evenattheelementary

level,shouldbefamiliarwiththermodynamicmodelsofchemicalreactivity.

Foraconciserevisionreferto[15].Abriefsummaryfollows.

7.1ThermodynamicPotentials

Thefundamentalassumptionofthermodynamicsistheconservationofen-

ergy,alsoduringitsconversionfromoneformintoanother.Inchemical

applicationsitiscumbersometoaccountfortotalenergyinallitsforms

andtheproblemisavoidedbyfocussingondifferencesinenergyratherthan

absolutevalues.Asthebasisofcalculationaconvenientenergyzeroisarbi-

trarilydefinedandenergy,relativetothisstate,iscalledthethermodynamic

energy,consistingofthreecomponents:

U=TS−PV+󰀜µjNj(7.1)

oralternatively,

∆U=q+w+t

definingheatflux,mechanicalworkandmatterflow.TheparametersT,S,

P,V,µandNrepresenttemperature,entropy,pressure,volume,chemical

potentialandmolenumber,respectively.Bydiscountingoneormoreofthese

249250CHAPTER7.CHEMICALCHANGE

termsthezeropointmovestoanewlevelandalternativepotentialfunctions

aredefined,suchas:

Enthalpy:H=U+PV=TS+󰀜µjNj

Freeenergy:F=U−TS=−PV+󰀜µjNj

Freeenthalpy:G=H−TS=󰀜µjNj

Theseformulaeexplainthecommonterminologyforone-componentclosed

systems:

H=qP,heatcontent

F=wT,workfunction

G/N=µ,partialmolarfreeenthalpy

Thedifferentialformofthepotentialexpressionsshowsthatchemicalpoten-

tialisdefinedbyafirstderivativeofeachpotential:

µ=󰀏∂U

∂N󰀐

S,P=󰀏∂F

∂N󰀐

T,P

Chemicalreactivity,dependingonthereactionconditions,canbedescribed

equallywellintermsofanyofthesethermodynamicpotentialsandnoeffort

willbemadetodifferentiatebetweentheminthefollowingdiscussion.

7.2ChemicalReactivity

Itispossibletogainsignificantinsightintochemicalreactivityfromafew

simpleprinciples,withoutgettinginvolvedwiththeabstractideasofstatis-

ticalthermodynamics.

Achemicalreactionoccursasthematerialcompositionofareaction

mixturechanges.Shouldthisprocesshappenspontaneously,chemicalenergy

isreleased.Alternatively,supplyofenergyfromanexternalsourcedrivesthe

chemicalchange.Theenergyproducedduringspontaneouschangedoesnot

necessarilycauseanincreaseintemperatureasmostofitmaybedissipatedas

increasedentropy.Thecourseofachemicalreactioncanthereforebefollowed

bymappingchangesintheenergyofasystem.Asageneralprinciplethe

propensityforchemicalchangeinamixtureisconsideredtobeafunctionof

apotential-energyfield,createdbythemassratiosoramountsofsubstance7.2.CHEMICALREACTIVITY251

inthemixture.Thisfieldisrelatedtothequantumpotentialofthesystem

andissaidtoreflectthechemicalpotentialofthereactionmixture.

Thechemicalpotentialofanatomhasbeenshowntodependonitselec-

tronegativity,orquantumpotential,inthevalencestate.Foramolecule,

suchameasure,althoughmoredifficulttoestimate,stillhasthesamemean-

ing.Inthecaseofapuresubstancethechemicalpotentialisanintensive

propertyofthesystem,independentoftheamountofmaterial,butsensi-

tivetothermodynamicchangesintheenvironment.Thequantitythatvaries

withamountofsubstanceisanenergy,u=nµ.Thedimensionlessvariablen

isconvenientlyexpressedinmolesofsubstance,definingµasamolarenergy.

Inareactionmixturethechemicalpotentialofeachcomponentµi=u/niis

equivalenttoitspartialmolarenergy.

Theobservedenergyofreactiondependsonthethermodynamiccondi-

tionsandthechoiceofzeropoint.Theusefulindexinchemicalreactionsis

thereforenottheabsolutevalueoftheenergy,butthechangeinenergyduring

thecourseofthereaction.Thischangedrivesthereactionandisresponsible

forchangingthesupplyofreactantbyanamount∆n:∆u=µ∆n.The

quantity∆u=αisalsoknownastheaffinityofthereaction.Inareaction

mixtureconsistingofseveralreactantsandanumberofreactionproducts,all

ofthesecomponentscontributetotheaffinityatanytimeduringthecourse

ofthereaction:

α=∆u=󰀝󰀜

iniµi󰀟

products−󰀝󰀜

jnjµj󰀟

reactants

Spontaneouschemicalchangeoccurswhen∆u<0andceaseswhen∆u=0.

Chemicalreactionthereforeproceedsinthedirectionthatminimizesthe

affinityanddependsontherateatwhichaffinitychanges.

Becauseoftheirvariablethermodynamicstateandconcentrationeach

reactantorproductischaracterizedatanyinstantbyanintrinsicactivity,

ai,andtheinterplaybetweentheseactivitiesdefinesthechemicalactionA,

atthatinstant.TheactionchangesatarateproportionaltoAandtothe

changeinaffinity,assummarizedbythelinearhomogeneousequation:

dA=βAdαordA