Ch.17:Spontaneity,Entropy,andFreeEnergy

Enthalpy

Enthalpy•  Enthalpyofformation(ΔHf)=HeatabsorbedorreleasedwhenONEmoleofcompoundisformedfromelementsintheirstandardstatesinkJ/mol– ΔHf=0forelementsinstandardstates

Recall •  Thermodynamics: •  The study of energy changes that

accompany chemical and physical processes

TheFirstLawofThermodynamics

The First Law of Thermodynamics •  Law of conservation of energy

– The 1st Law of Thermodynamics: – Application of the law of conservation of

energy to heat and thermodynamic processes.

– Energy cannot be created or destroyed, but it can be changed (the energy of the universe is constant)

The First Law of Thermodynamics •  Law of conservation of energy

– ΔH = Change in enthalpy – ΔH0

f = Standard Molar Enthalpy of Formation

ReversibleReactions

Reversible Reactions •  Reactions can occur in the forward and

reverse directions. •  Ex/ ClNO2 + NO ßà NO2 + ClNO

SpontaneousProcesses

Spontaneous Processes – Spontaneous = naturally occurs under a

given set of conditions (occurs without intervention). Can be fast or slow.

Spontaneous Processes – Nonspontaneous = does not occur under a

given set of conditions.

Spontaneous Processes •  If a process is spontaneous in the forward

direction, then the reverse process will be nonspontaneous.

Spontaneous Processes •  What makes a process spontaneous?

– Systems like to be at a lower state of energy • Most (not all) spontaneous processes are also exothermic (ΔH < 0) and release energy

Spontaneous Processes •  What makes a process spontaneous?

– Systems like to be become more disordered/random • High entropy

Spontaneous Processes •  Spontaneity is determined by ΔH (enthalpy)

and ΔS (entropy)

Equations

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Entropy

Entropy •  Entropy (S) = Thermodynamic function that

describes the number of arrangements that are available to a system existing in a given state

Entropy •  Entropy (S) is a measure of the disorder or

randomness of a system. – Entropy increases as the number of possible

microstates increases

TheExpansionofanIdealGasintoanEvacuatedBulb

•  Naturespontaneouslyproceedstowardthestatesthathavethehighestprobabilitiesofexisting

Microstate

•  Eachconfigurationthatgivesaparticulararrangement

•  Probabilityofoccurrenceofastatedependsonthenumberofmicrostatesinwhichthearrangementcanbeachieved

Entropy •  ΔS=changeinentropy•  ΔS>0meansthesystemincreasedindisorder.

Entropy •  Physical States

– Solids are highly ordered à lowest entropy

– Gas molecules move rapidly and randomly à highest entropy

PositionalProbabilityandChangesofState

•  Positionalentropyincreaseswhengoingfromsolidtogaseousstate

Entropy •  Trends for Standard Entropy values

– Solids < liquids < gases – More complex molecules have higher

entropies (greater vibrational energy)

PositionalEntropyandSolutions•  Entropychangewhenmixingtwopuresubstancesisexpectedtobepositive– Resultofthepresenceofmoremicrostatesforthemixedcondition

– Causedowingtotheincreasedvolumeavailabletoagivenparticleaftermixingoccurs

•  Formationofsolutionsisfavoredbyanincreaseinpositionalentropythatisassociatedwithmixing

Think•  Foreachofthefollowingpairs,choosethesubstancewiththehigherpositionalentropy(permole)atagiventemperaturea.  SolidCO2andgaseousCO2b.  N2gasat1atmandN2gasat1.0×10–2atm

Think•  Wouldentropyincreaseordecrease…?

a.  Solidsugarisaddedtowatertoformasolutionb.  Iodinevaporcondensesonacoldsurfacetoform

crystals

Pair-Share-Respond1.  Distinguishbetweenspontaneousandnonspontaneousprocesses

2.  Systemsliketobeata______energystateand______disordered/randomstate

3.  Whatdeterminesspontaneity?4.  Define“entropy”5.  Whichwouldhaveahigherentropy:iceorvapor?Explain.

The2ndLawofThermodynamics

The Second Law of Thermodynamics •  Second Law of Thermodynamics =

– The entropy of the universe always increases for a spontaneous process

– Recall: The First Law of Thermodynamics. Energy is conserved, but entropy is not

The Second Law of Thermodynamics •  Changes in Entropy of the Universe •  ΔSunivispositive

– Entropyoftheuniverseincreases– Processisspontaneousinthedirectionwritten

•  ΔSunivisnegative– Processisspontaneousintheoppositedirection

•  ΔSuniviszero– Processhasnotendencytooccur– Systemisatequilibrium

The Second Law of Thermodynamics •  ΔS is usually positive (ΔS > 0) when:

1.  Solid à Liquid à Gas 2.  The total number of gas molecules

increases (look at your products vs. reactants)

3.  A larger molecule is broken into 2 or more smaller molecules

The Second Law of Thermodynamics •  Predict whether ΔS is + or –

– 2SO2(g) + O2(g) ßà 2SO3 • ΔS is…

– CaCO3(s) ßà CaO(s) + CO2(g) • ΔS is…

– AgBr(s)ßà Ag+(aq) + Br-

(aq) • ΔS is …

SampleQuestion•  ForwhichprocessisΔSpositive?

a.  Waterfreezingat-20°Cb.  Idealgasbeingcompressedreversiblyataconstant

temperatureandpressurec.  Precipitationreactiond.  Spontaneousendothermicprocessataconstant

temperatureandpressure

SampleQuestion•  Foranyspontaneousprocess,

a.  ΔSuniv,ΔSsys,andΔSsurrmustallbepositiveb.  ΔSunivandΔSsysmustbepositivec.  ΔSsysandΔSsurrmustbepositived.  OnlyΔSunivmustbepositivee.  OnlyΔSsysmustbepositive

GibbsFreeEnergy