Ch. 17: Spontaneity, Entropy, and Free Energy
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