chapter 19 chemical thermodynamics. spontaneity of physical & chemical changes thermodynamics is...

Chapter 19 Chapter 19

Chemical ThermodynamicsChemical Thermodynamics

Spontaneity of Physical Spontaneity of Physical & Chemical Changes& Chemical Changes

Thermodynamics is concerned with the Thermodynamics is concerned with the question: can a reaction occur?question: can a reaction occur?First Law of Thermodynamics: energy is conserved.First Law of Thermodynamics: energy is conserved.

Any process that occurs without outside Any process that occurs without outside intervention is spontaneous.intervention is spontaneous.When two eggs are dropped they spontaneously When two eggs are dropped they spontaneously

break.break.The reverse reaction (two eggs leaping into your The reverse reaction (two eggs leaping into your

hand with their shells back intact) is not hand with their shells back intact) is not spontaneous.spontaneous.

We can conclude that a spontaneous We can conclude that a spontaneous process has a direction.process has a direction.

Spontaneity of Physical Spontaneity of Physical & Chemical Changes& Chemical Changes

Spontaneous changesSpontaneous changes happen without any happen without any continuing outside influences. A continuing outside influences. A spontaneous change has a natural direction.spontaneous change has a natural direction.

rusting of iron - rusting of iron - occurs spontaneouslyoccurs spontaneouslyHave you ever seen rust turn into iron metal Have you ever seen rust turn into iron metal

without man made interference?without man made interference?

melting of ice at room temperature - melting of ice at room temperature - occurs occurs spontaneouslyspontaneouslyWill water spontaneously freeze at room Will water spontaneously freeze at room

temperature?temperature?

The Two Parts of SpontaneityThe Two Parts of Spontaneity

Exothermicity does not ensure spontaneityExothermicity does not ensure spontaneity

Freezing of waterFreezing of waterexothermicexothermic

spontaneous only below 0spontaneous only below 0ooCC

Spontaneous ProcessesSpontaneous ProcessesA process that is spontaneous in one direction is not A process that is spontaneous in one direction is not spontaneous in the opposite direction.spontaneous in the opposite direction.The direction of a spontaneous process can depend The direction of a spontaneous process can depend on temperature: Ice turning to water is spontaneous on temperature: Ice turning to water is spontaneous at at TT > 0 > 0C, Water turning to ice is spontaneous at C, Water turning to ice is spontaneous at TT < 0< 0C.C.

The Second Law of The Second Law of ThermodynamicsThermodynamics

In spontaneous changes the universe In spontaneous changes the universe tends towards a state of greater tends towards a state of greater disorder.disorder.

Spontaneous processes require:Spontaneous processes require:

free energy change of system must be free energy change of system must be negativenegative

entropy of entropy of universeuniverse must increase must increase– system must be capable of doing useful system must be capable of doing useful

work on surroundingswork on surroundings

Entropy, SEntropy, S

Entropy is a measure of the Entropy is a measure of the disorderdisorder or or randomnessrandomness of a system. of a system.

As with As with H, entropies have been H, entropies have been measured and tabulated in Appendix C measured and tabulated in Appendix C as Sas Soo

298298. When:. When:

S is S is positivepositive disorder increases (favors disorder increases (favors spontaneity)spontaneity)

S is S is negativenegative disorder decreases disorder decreases (disfavors spontaneity)(disfavors spontaneity)


From Second Law of From Second Law of Thermodynamics, for a spontaneous Thermodynamics, for a spontaneous processprocess

S S Suniverse system surroundings 0

Entropy, SFor a reversible process: For a reversible process: SSunivuniv = 0. = 0.For a spontaneous process (i.e. irreversible): For a spontaneous process (i.e. irreversible):

SSunivuniv > 0 > 0

Note: the second law states that the entropy of Note: the second law states that the entropy of the universe must increase in a spontaneous the universe must increase in a spontaneous process. It is possible for the entropy of a process. It is possible for the entropy of a system to decrease as long as the entropy of system to decrease as long as the entropy of the surroundings increases.the surroundings increases.

The Molecular Interpretation of The Molecular Interpretation of EntropyEntropy

A gas is less ordered than a liquid that is A gas is less ordered than a liquid that is less ordered than a solid.less ordered than a solid.

In general:In general:SSgasgas> S> Sliquidliquid > S > Ssolidsolid

Any process that increases the number of Any process that increases the number of gas molecules leads to an increase in gas molecules leads to an increase in entropy.entropy.

The Molecular Interpretation of Entropy

NO (g) and ONO (g) and O22(g) react to form NO(g) react to form NO22(g)(g)


There are three atomic There are three atomic modes of motion:modes of motion:

translationtranslation (the moving of a (the moving of a molecule from one point in space to molecule from one point in space to another),another),

vibrationvibration (the shortening and (the shortening and lengthening of bonds, including the lengthening of bonds, including the change in bond angles),change in bond angles),

rotationrotation (the spinning of a (the spinning of a molecule about some axis).molecule about some axis).


Energy is required to get a molecule to Energy is required to get a molecule to translate, vibrate or rotate.translate, vibrate or rotate.

The more energy stored in translation, vibration and The more energy stored in translation, vibration and rotation, the greater the degrees of freedom and rotation, the greater the degrees of freedom and the higher the entropy.the higher the entropy.

In a perfect crystal at 0 K there is no In a perfect crystal at 0 K there is no translation, rotation or vibration of molecules.translation, rotation or vibration of molecules.

Therefore, this is a state of perfect order.Therefore, this is a state of perfect order.

Third Law of Thermodynamics: the entropy of a Third Law of Thermodynamics: the entropy of a perfect crystal at 0 K is zero.perfect crystal at 0 K is zero.


As we heat a substance from absolute zero, As we heat a substance from absolute zero, the entropy must increase.the entropy must increase.

Entropy will increase whenEntropy will increase when•liquids or solutions are formed from solids,liquids or solutions are formed from solids,•gases are formed from solids or liquids,gases are formed from solids or liquids,•the number of gas molecules increase,the number of gas molecules increase,•the temperature is increased.the temperature is increased.

Entropy, SEntropy, SThird Law of Thermodynamics states that the Third Law of Thermodynamics states that the

entropy of a pure, perfect, crystalline solid at entropy of a pure, perfect, crystalline solid at 0 K is zero.0 K is zero.– allows us to measure absolute values of entropy for allows us to measure absolute values of entropy for

substancessubstances– cool them down to 0 K, or as close as possible, then cool them down to 0 K, or as close as possible, then

measure entropy increase as substance warms upmeasure entropy increase as substance warms up

Entropy changes for reactions can be Entropy changes for reactions can be determined similarly to determined similarly to H for reactions.H for reactions.

Entropy, SEntropy, SThird Law of Thermodynamics states that the entropy Third Law of Thermodynamics states that the entropy

of a pure, perfect, crystalline solid at 0 K is zero.of a pure, perfect, crystalline solid at 0 K is zero.allows us to measure absolute values of entropy for allows us to measure absolute values of entropy for

substancessubstances

cool them down to 0 K, or as close as possible, then cool them down to 0 K, or as close as possible, then measure entropy increase as substance warms upmeasure entropy increase as substance warms up

Entropy changes for reactions can be determined Entropy changes for reactions can be determined similarly to similarly to H for reactions.H for reactions.

Units: J/mol-K. Note units of Units: J/mol-K. Note units of H: kJ/mol.H: kJ/mol.

Standard molar entropies of elements are not zero.Standard molar entropies of elements are not zero.

S n S n S298o

productso

reactantso

Entropy, SEntropy, SCalculate the entropy change for the Calculate the entropy change for the

following reaction at 25following reaction at 25ooC. Use appendix.C. Use appendix.

)()(2 422 gONgNO


2 NO N O

S n S n S

S S

mol mol

or - 0.1758

2 g 2 4 g

298o

productso

reactantso

N Oo

NOo

Jmol K

Jmol K

JK

kJK

2 4 g 2 g

2

1 304 2 2 240 0

1758

. .

.

negative sign indicates system is more orderednegative sign indicates system is more ordered reverse the reaction and sign changesreverse the reaction and sign changes SSoo

298298== +0.1758 kJ/K+0.1758 kJ/K where the + sign where the + sign indicates system is more disorderedindicates system is more disordered


Calculate Calculate SSoo298298 for the reaction below. for the reaction below.

Use appendix.Use appendix.

g2g2g NOONNO 3


Changes in Changes in S are usually quite small S are usually quite small compared to compared to H.H.

K

kJK

J

KJ

0NO

0NO

0ON

0298

g2g2g

0.1724-or 4.172

210.43 - 240.0 7.219

S 3SSS

NO ONNO 3

gg2g2

Free Energy Change, Free Energy Change, G, G, and Spontaneityand Spontaneity

J. Willard Gibbs determined the J. Willard Gibbs determined the relationship of enthalpy and entropy relationship of enthalpy and entropy that best describes the that best describes the maximum maximum useful energy obtainable useful energy obtainable in the form of in the form of work from a process at constant T & P.work from a process at constant T & P.

The relationship also describes the The relationship also describes the spontaneity of a system.spontaneity of a system.

The relationship is a new state function, The relationship is a new state function, G, the G, the Gibbs Free EnergyGibbs Free Energy..


The relationship is a new state The relationship is a new state function, function, G, the G, the Gibbs Free Gibbs Free EnergyEnergy..

G = H - T S (at constant T & P)


The change in the Gibbs Free Energy is a The change in the Gibbs Free Energy is a reliable indicator of spontaneity of a reliable indicator of spontaneity of a physical process or chemical reaction.physical process or chemical reaction.does notdoes not tell us the speed of the process tell us the speed of the process

That is kineticsThat is kinetics

When:When:

G is > 0 G is > 0 reaction is nonspontaneousreaction is nonspontaneous

G is = 0 G is = 0 system is at equilibriumsystem is at equilibrium

G is < 0 G is < 0 reaction is spontaneousreaction is spontaneous


Changes in free energy obey the same type of Changes in free energy obey the same type of relationship we have described for enthalpy and relationship we have described for enthalpy and entropy changes.entropy changes.

G = n G n G298o

productso

reactantso


Calculate Calculate GGoo298298 for the reaction: for the reaction:

)(4)(3)(5)( 22283 lOHgCOgOgHC


C H + 5 O 3 CO + 4 H O

G G G G G

kJ

3 8 g 2 g 2 g 2

298o

f COo

f H O o

f C H o

f O o

2 g 2 3 8 g 2 g

l

l 3 4 5

3 394 4 4 237 3 2349 5 0

2108 5

( . ) ( . ) ( . ) ( )

.GGoo

298 298 is is negativenegative, so the reaction is , so the reaction is spontaneousspontaneous at at standard state conditions.standard state conditions.

If we reverse the reaction:If we reverse the reaction:

GGoo298 298 is is positivepositive, so the reaction is , so the reaction is nonspontaneousnonspontaneous at at

standard state conditions.standard state conditions.

The Temperature The Temperature Dependence of SpontaneityDependence of Spontaneity

The general relationship of The general relationship of G, G, H, and H, and S isS is

G = H - T S


The general relationship of The general relationship of G, G, H, and H, and S isS is

This gives us 4 possibilities among the This gives us 4 possibilities among the signs signs

G = H - T S


HH SS GG ThereforeTherefore

-- + + -- forward rxn spontaneous at all forward rxn spontaneous at all T’sT’s

-- - - ?? forward rxn spontaneous at low T’sforward rxn spontaneous at low T’s

++ + + ?? forward rxn spontaneous at high forward rxn spontaneous at high T’sT’s

++ - - ++ forward rxn nonspontaneous at all forward rxn nonspontaneous at all T’sT’s

The Temperature Dependence of Spontaneity


Example: Calculate Example: Calculate SSoo298298 for the following reaction. for the following reaction.

We found that We found that HHoo298298= = -2219.9 kJ-2219.9 kJ, and we found that , and we found that

GGoo298298= = -2108.5 kJ-2108.5 kJ..

C H + 5 O CO + 4 H O3 8 g 2 g 2 g 2 3 l


Example: Calculate Example: Calculate SSoo298298 for the following reaction. for the following reaction.

We found that We found that HHoo298298= = -2219.9 kJ-2219.9 kJ, we found that , we found that

GGoo298298= = -2108.5 kJ-2108.5 kJ..

C H + 5 O CO + 4 H O

G H T S

T S H G

3 8 g 2 g 2 g 2

o o o

o o o

3 l


C H + 5 O CO + 4 H O

G H T S

T S H G

SH G

T kJ

K or - 374

3 8 g 2 g 2 g 2

o o o

o o o

oo o

kJK

JK

3

2219 9 2108 5

2980 374

l

. ( . )

.


SSoo298 298 = = -374 J/K-374 J/K which indicates that the which indicates that the

disorder of the system disorder of the system decreasesdecreases ..For the reverse reaction,For the reverse reaction,

3 CO3 CO2(g)2(g) + 4 H + 4 H22OO(l) (l) CC33HH8(g)8(g) + 5 O + 5 O2(g)2(g)

SSoo298 298 = = +374 J/K+374 J/K which indicates that the which indicates that the

disorder of the system disorder of the system increasesincreases ..

Free Energy and the Equilibrium Constant

For a spontaneous process, the free energy For a spontaneous process, the free energy of the system decreases until it reaches a of the system decreases until it reaches a minimum value. When this minimum value minimum value. When this minimum value is reached the system is in a state of is reached the system is in a state of Equilibrium. Equilibrium.


GG and and KK (equilibrium constant) apply to (equilibrium constant) apply to standard conditions.standard conditions.

GG and and QQ (equilibrium quotient) apply to (equilibrium quotient) apply to any conditions.any conditions.

It is useful to determine whether It is useful to determine whether substances under any conditions will react:substances under any conditions will react:

R = Ideal gas law constant 8.314 J/molKR = Ideal gas law constant 8.314 J/molK

QRTGG ln


At equilibrium, At equilibrium, QQ = = KKcc and and GG = 0, so = 0, so

From the above we can conclude:From the above we can conclude:If If GG < 0, then < 0, then KKcc > 1. > 1.

The more negative The more negative G is the larger the equilibrium constant.G is the larger the equilibrium constant.

If If GG = 0, then = 0, then KKcc = 1. = 1.If If GG > 0, then > 0, then KKcc < 1. < 1.

.ln

.ln0

ln

KRTG

KRTG

QRTGG

Synthesis QuestionSynthesis QuestionWhen it rains an inch of rain, that means When it rains an inch of rain, that means

that if we built a one inch high wall that if we built a one inch high wall around a piece of ground that the rain around a piece of ground that the rain would completely fill this enclosed would completely fill this enclosed space to the top of the wall. Rain is space to the top of the wall. Rain is water that has been evaporated from a water that has been evaporated from a lake, ocean, or river and then lake, ocean, or river and then precipitated back onto the land. How precipitated back onto the land. How much heat must the sun provide to much heat must the sun provide to evaporate enough water to rain 1.0 evaporate enough water to rain 1.0 inch onto 1.0 acre of land?inch onto 1.0 acre of land?

1 acre = 43,460 ft1 acre = 43,460 ft22

Synthesis QuestionSynthesis Question

38

27

27

2

22

222

cm 1003.1

cm 54.2cm 1004.4volume

cm 1004.4

ft 1

cm 930ft 43,460 acre 1

cm 930cm 30.5 ft 1

cm 30.5 ft 1 cm 2.54 in 1

Synthesis QuestionSynthesis Question

kJ 1051.2

molkJ 0.44mol 1071.5supplymust sun heat

molkJ 0.44H

mol 1071.5

g 18

mol 1g 1003.1 waterof moles

waterof g 1003.1

cm

g 1cm 1003.1 waterof mass

8

6

waterofon vaporizati

6

8

8

338

Group QuestionGroup QuestionWhen Ernest Rutherford, introduced in Chapter When Ernest Rutherford, introduced in Chapter

5, gave his first lecture to the Royal Society 5, gave his first lecture to the Royal Society one of the attendees was Lord Kelvin. one of the attendees was Lord Kelvin. Rutherford announced at the meeting that he Rutherford announced at the meeting that he had determined that the earth was at least 1 had determined that the earth was at least 1 billion years old, 1000 times older than Kelvin billion years old, 1000 times older than Kelvin had previously determined for the earth’s age. had previously determined for the earth’s age. Then Rutherford looked at Kelvin and told him Then Rutherford looked at Kelvin and told him that his method of determining the earth’s age that his method of determining the earth’s age based upon how long it would take the earth to based upon how long it would take the earth to cool from molten rock to its present cool, solid cool from molten rock to its present cool, solid formform

Group QuestionGroup Question was essentially correct. But there was a new, was essentially correct. But there was a new,

previously unknown source of heat that Kelvin previously unknown source of heat that Kelvin had not included in his calculation and therein had not included in his calculation and therein lay his error. Kelvin apparently grinned at lay his error. Kelvin apparently grinned at Rutherford for the remainder of his lecture. Rutherford for the remainder of his lecture. What was this “new” source of heat that What was this “new” source of heat that Rutherford knew about that had thrown Kelvin’s Rutherford knew about that had thrown Kelvin’s calculation so far off?calculation so far off?

chapter 19 chemical thermodynamics. spontaneity of physical & chemical changes thermodynamics is...

Documents

spontaneous processentropy

reversible process

natural direction

law states

opposite direction

sgas sliquid ssolidany

state of greater disorder

negative disorder