11 entropy and free energy how to predict if a reaction can occur, given enough time? thermodynamics...

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Entropy and Free Entropy and Free EnergyEnergy

Entropy and Free Entropy and Free EnergyEnergy

How to predict if a How to predict if a reaction can occur, reaction can occur, given enough time?given enough time?

THERMODYNAMICSTHERMODYNAMICS

How to predict if a How to predict if a reaction can occur at reaction can occur at a reasonable rate?a reasonable rate?

KINETICSKINETICS

2222ThermodynamicsThermodynamics• If the state of a chemical system is such that a If the state of a chemical system is such that a

rearrangement of its atoms and molecules would rearrangement of its atoms and molecules would

decrease the energy of the system--- decrease the energy of the system---

• ANDAND the K is greater than 1, the K is greater than 1,

• then this is a then this is a product-favoredproduct-favored system.system.

• Most product-favored reactions are Most product-favored reactions are

exothermicexothermic

—but this is not the only criterion—but this is not the only criterion

3333ThermodynamicsThermodynamics• Both product- and reactant-favored reactions can proceed to Both product- and reactant-favored reactions can proceed to

equilibrium in a equilibrium in a spontaneousspontaneous process.process.

AgCl(s) AgCl(s) e e AgAg++(aq) + Cl(aq) + Cl––(aq) (aq) K = 1.8 x 10K = 1.8 x 10-10-10

Reaction is not product-favored, but it moves Reaction is not product-favored, but it moves spontaneously toward equilibrium.spontaneously toward equilibrium.

• Spontaneous does not imply anything about time for reaction Spontaneous does not imply anything about time for reaction to occur.to occur.

4444Thermodynamics and Thermodynamics and KineticsKinetics

Thermodynamics and Thermodynamics and KineticsKinetics

Diamond is Diamond is thermodynamically thermodynamically favoredfavored to convert to to convert to graphite, but graphite, but not not kinetically favoredkinetically favored..

Paper burns — a Paper burns — a product-favoredproduct-favored reaction. reaction. Also Also kinetically favoredkinetically favored once once reaction is begun.reaction is begun.

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Spontaneous ReactionsSpontaneous Reactions

In general, spontaneous In general, spontaneous reactions are reactions are exothermicexothermic..

FeFe22OO33(s) + 2 Al(s) ---> (s) + 2 Al(s) --->

2 Fe(s) + Al2 Fe(s) + Al22OO33(s)(s)

∆∆H = - 848 kJH = - 848 kJ

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Spontaneous ReactionsSpontaneous ReactionsBut many spontaneous reactions or But many spontaneous reactions or

processes are endothermic or even processes are endothermic or even have ∆H = 0.have ∆H = 0.

NHNH44NONO33(s) + heat ---> NH(s) + heat ---> NH44NONO33(aq)(aq)

7777

Entropy, SEntropy, SOne property common to One property common to

spontaneous processes is spontaneous processes is that the final state is more that the final state is more DISORDEREDDISORDERED or or RANDOMRANDOM than the original.than the original.

Spontaneity is related to an Spontaneity is related to an increase in randomness.increase in randomness.

The thermodynamic property The thermodynamic property related to randomness is related to randomness is ENTROPY, SENTROPY, S..

Reaction of K Reaction of K with waterwith water

8888

The entropy of liquid water is greater than the entropy of solid water (ice) at 0˚ C.

9999

How probable is it that reactant How probable is it that reactant molecules will react? molecules will react?

PROBABILITYPROBABILITY suggests that a suggests that a spontaneous reaction will result in spontaneous reaction will result in

the the dispersaldispersal

** ofof energyenergy

** or ofor of mattermatter

** or of or of energy & matterenergy & matter..

Directionality of Directionality of ReactionsReactions

10101010

Probability suggests that a spontaneous Probability suggests that a spontaneous reaction will result in the dispersal of reaction will result in the dispersal of energy or of matter or both.energy or of matter or both.

Matter DispersalMatter Dispersal


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Probability suggests that a spontaneous Probability suggests that a spontaneous reaction will result in the dispersal of reaction will result in the dispersal of energy or of matter or both.energy or of matter or both.

Energy DispersalEnergy Dispersal


12121212


Energy DispersalEnergy Dispersal

Exothermic reactions involve a release of Exothermic reactions involve a release of stored chemical potential energy to the stored chemical potential energy to the surroundings. surroundings.

The stored potential energy starts out in a few The stored potential energy starts out in a few molecules but is finally dispersed over a molecules but is finally dispersed over a great many molecules. great many molecules.

The final state—with energy dispersed—is The final state—with energy dispersed—is more probable and makes a reaction more probable and makes a reaction spontaneous.spontaneous.

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S (gases) > S (liquids) > S (solids)S (gases) > S (liquids) > S (solids)

SSoo (J/K•mol) (J/K•mol)

HH22O(liq)O(liq) 69.9569.95

HH22O(gas)O(gas) 188.8 188.8


HH22O(liq)O(liq) 69.9569.95

HH22O(gas)O(gas) 188.8 188.8

Entropy, SEntropy, S

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Entropy and States of Entropy and States of MatterMatter

S˚(BrS˚(Br22 liq) < S˚(Br liq) < S˚(Br22 gas) gas) S˚(HS˚(H22O sol) < S˚(HO sol) < S˚(H22O liq)O liq)

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Entropy of a substance increases Entropy of a substance increases with temperature.with temperature.

Molecular motions Molecular motions of heptane, Cof heptane, C77HH1616

Molecular motions of Molecular motions of heptane at different temps.heptane at different temps.


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Increase in molecular Increase in molecular complexity generally complexity generally leads to increase in S.leads to increase in S.


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Entropies of ionic solids depend on Entropies of ionic solids depend on coulombic attractions.coulombic attractions.


MgOMgO 26.926.9

NaFNaF 51.551.5


MgOMgO 26.926.9

NaFNaF 51.551.5


MgMg2+2+ & O & O2-2- NaNa++ & F & F--

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Entropy usually increases when a Entropy usually increases when a pure liquid or solid dissolves in a pure liquid or solid dissolves in a solvent.solvent.

Entropy, SEntropy, SEntropy, SEntropy, S

19191919

Standard Molar EntropiesStandard Molar Entropies

20202020Entropy Changes for Phase Entropy Changes for Phase ChangesChanges

For a phase change, For a phase change, ∆S = q/T∆S = q/T

where q = heat transferred in where q = heat transferred in phase changephase change

For HFor H22O (liq) ---> HO (liq) ---> H22O(g)O(g)

∆∆H = q = +40,700 J/molH = q = +40,700 J/mol

S = qT

= 40, 700 J/mol

373.15 K = + 109 J/K • molS =

qT

= 40, 700 J/mol

373.15 K = + 109 J/K • mol

21212121Entropy and TemperatureEntropy and TemperatureEntropy and TemperatureEntropy and Temperature

S increases S increases slightly with Tslightly with T

S increases a S increases a large amount large amount with phase with phase changeschanges

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Consider 2 HConsider 2 H22(g) + O(g) + O22(g) ---> 2 H(g) ---> 2 H22O(liq)O(liq)

∆∆SSoo = 2 S = 2 Soo (H (H22O) - [2 SO) - [2 Soo (H (H22) + S) + Soo (O (O22)])]

∆∆SSoo = 2 mol (69.9 J/K•mol) - = 2 mol (69.9 J/K•mol) - [2 mol (130.7 J/K•mol) + [2 mol (130.7 J/K•mol) +

1 mol (205.3 J/K•mol)]1 mol (205.3 J/K•mol)]

∆∆SSoo = -326.9 J/K = -326.9 J/K

Note that there is a Note that there is a decrease in S decrease in S because 3 because 3 mol of gas give 2 mol of liquid.mol of gas give 2 mol of liquid.

Calculating ∆S for a Calculating ∆S for a ReactionReaction

∆∆SSoo = = S Soo (products) - (products) - S Soo (reactants) (reactants)∆∆SSoo = = S Soo (products) - (products) - S Soo (reactants) (reactants)

232323232nd Law of 2nd Law of ThermodynamicsThermodynamics

A reaction is spontaneous if ∆S for the A reaction is spontaneous if ∆S for the universeuniverse is is positive.positive.

∆∆SSuniverseuniverse = ∆S = ∆Ssystemsystem + ∆S + ∆Ssurroundingssurroundings

∆∆SSuniverseuniverse > 0 for spontaneous process > 0 for spontaneous process

First calc. entropy created by First calc. entropy created by mattermatter dispersal dispersal (∆S(∆Ssystemsystem))

Next, calc. entropy created by Next, calc. entropy created by energyenergy dispersal dispersal (∆S(∆Ssurroundsurround))

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Dissolving NH4NO3 in water—an entropy driven process.


∆∆SSuniverseuniverse = =

∆∆SSsystemsystem + ∆S + ∆Ssurroundingssurroundings

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2 H2 H22(g) + O(g) + O22(g) ---> 2 H(g) ---> 2 H22O(liq)O(liq)

∆∆SSoosystemsystem = -326.9 J/K = -326.9 J/K

Sosurroundings =

qsurroundings

T =

-Hsystem

TSo

surroundings = qsurroundings

T =

-Hsystem

T

Sosurroundings =

- (-571.7 kJ)(1000 J/kJ)

298.15 KSo

surroundings = - (-571.7 kJ)(1000 J/kJ)

298.15 K


∆∆SSoosurroundingssurroundings = +1917 J/K = +1917 J/K

Can calc. that ∆HCan calc. that ∆Hoorxnrxn = ∆H = ∆Hoo

systemsystem = -571.7 kJ = -571.7 kJ

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2 H2 H22(g) + O(g) + O22(g) ---> 2 H(g) ---> 2 H22O(liq)O(liq)

∆∆SSoosystemsystem = -326.9 J/K = -326.9 J/K

∆∆SSoosurroundingssurroundings = +1917 J/K = +1917 J/K

∆∆SSoouniverse universe = +1590. J/K= +1590. J/K

• The entropy of The entropy of the universe is the universe is increasing, so increasing, so the reaction is the reaction is product-favored.product-favored.


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Spontaneous or Not?Spontaneous or Not?

Remember that –∆H˚Remember that –∆H˚syssys is proportional to ∆S˚ is proportional to ∆S˚surrsurr

An exothermic process has ∆S˚An exothermic process has ∆S˚surrsurr > 0. > 0.

11 entropy and free energy how to predict if a reaction can occur, given enough time? thermodynamics...

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