copyright©2000 by houghton mifflin company. all rights reserved. 1 ap chem h/w 16.1 9, 17, 19, 23,...

Copyright©2000 by Houghton Mifflin Company. All rights reserved.

1

AP Chem h/w 16.1

9, 17, 19, 23, 24, 26, 28, 30, 31


2

ChemistryFIFTH EDITION

Chapter 16Spontaneity, Entropy,

and Free Energy


3

Figure 16.1Methane and Oxygen React


4

Spontaneous Processes Spontaneous Processes

A spontaneous process is one that occurs without outside intervention.

Thermodynamics : predicts whether a process will occur spontaneously. gives no info about time required for the process (kinetics)


5

Figure 16.2 Rate of Reaction


6

EntropyEntropy

The driving force for a spontaneous process is an increase in the entropy of the universe.

Entropy, S, can be viewed as a measure of randomness, or disorder.


7

Entropy and probabilty

System tends towards disorder simply because there are more “disordered states”

than “ordered” states.

Nature spontaneously goes to states with a higher probability of existing.

Ex.: deck of cards


8

Figure 16.3 The Expansion of an Ideal Gas into an Evacuated Bulb


9

Figure 16.4Three Possible Arrangements (microstates) of Four Molecules in a Two-Bulbed Flask


10

Probability of finding all molecules in left bulb

# molecules prob.

1 1/2

2 1/2 x1/2 = 1/4

3 1/2 x 1/2 x 1/2 = 1/8

4 1/16

n 1/2n

One mole 1/2avogadro = small #


11

Positional EntropyPositional Entropy

A gas expands into a vacuum because the expanded state has the highest positional probability of states available to the system.

Other ex:

1. Ssolid < Sliquid << Sgas

2. Formation of a solution entropy?


12

The Second Law of Thermodynamics

The Second Law of Thermodynamics

. . . in any spontaneous process there is always an increase in the entropy of the universe.

Suniv > 0

for a spontaneous process.

(notice, unlike mass and energy, entropy is NOT conserved)


13

∆Suniverse

∆Suniv = ∆Ssystem + ∆Ssurrounding

∆Suniv + for spontaneous

∆Suniv - spontaneous in opp direction

∆Suniv 0 process no tendency to occur

. We must know the entropy changes in both the sys and the surr.


14


Consider this process:

H2O(l) → H2O(g)

What happens to SSYS here?

What about SSURR ?


15

∆Ssurr

∆Ssurr mainly determined by heat flow to or from the sys.

If system is exo, heat flows to surr and

∆Ssurr is pos

If system is endo, heat flows out of surr and

∆Ssurr is neg

Why?


16


So for H2O(l) → H2O(g)

SSYS is +

SSURR is - and


So will this rxn occur. It depend on ____?


17

The Effect of Temperature on Spontaneity

We know that if T > 100˚C, the process is spontaneous as written.

If T < 100˚C, the process is spontaneous in the reverse direction.


18


Why does T matter?

Think exothermic rxn.

Recall the $50 dollar bill story.

So, the impact of a transfer of heat to or from the surroundings will be greater at lower temperatures.


19


To summarize:

entropy flow in the surroundings:

• The SIGN of SSURR depends on the direction of heat flow.

• The MAGNITUDE of SSURR depends on the temperature.


20


Combining these concepts shows that for conditions of constant T and P,

SSURR = - H

T

•minus sign : we’re looking from the system’s POV, and this equation shows a property of the surroundings.

ΗΤ

ΗΤ


21


Finally for H2O(l) → H2O(g)

SSYS is +

SSURR is - and

∆Suniv = ∆Ssystem - H

T

Sys favors sponteneous (positional), but surrounding do not (endo). The higher the T the better.


22

One more thing

∆Ssurrounding = - H TAt low T, exothermicity will be most import. driving forceFor endothermic, higher T will minimize the neg effect on entropy (think vaporization example


23


See S/E 16.4, p. 802, and Table 16.3, p. 803.


24

Free EnergyFree Energy

G = H TS (from the standpoint of the system)

A process (at constant T, P) is spontaneous in the direction in which free energy decreases:

G means +Suniv


25

Free Energy

• See Table 16.4. Spend some time with this.• Study it closely. • Have your calculator out and run the

numbers.• Satisfy yourself that these numbers explain

why ice melts at +10˚C, freezes at -10˚C, and is in equilibrium with liquid water at 0˚C.


26

Effect of H and S on Spontaneity

Effect of H and S on Spontaneity

H ΔS Result

− + spontaneous at all temps

+ + spontaneous at high temps

− − spontaneous at low temps

+ − not spontaneous at any temp


27

The Third Law of ThermodynamicsThe Third Law of Thermodynamics

. . . the entropy of a perfect crystal at 0 K is zero.

Because S is explicitly known (= 0) at 0 K, S values at other temps can be calculated.


28

Figure 16.5Entropy


29

Because Entropy is a State Function,

S˚RXN = ΣnpS˚products - ΣnpS˚reactants

• This time, we must include values for elements in their standard states, because we have a real “floor” value from which to compare them.

• See S/E 16.7, p. 809 and S/E 16.8, pp 809 – 810.


30

Figure 16.6H2O Molecule


31

Free Energy Change and Chemical Reactions

Free Energy Change and Chemical Reactions

G = standard free energy change that occurs if reactants in their standard state are converted to products in their standard state.

G = npGf(products) nrGf(reactants)


32

Figure 16.7Schematic Representations of Balls

Rolling Down into Two Types of Hills


33

Free Energy and PressureFree Energy and Pressure

G = G + RT ln(Q)

Q = reaction quotient from the law of mass action.


34

Figure 16.8The Dependence of Free Energy on Partial Pressure


35

Free Energy and EquilibriumFree Energy and Equilibrium

G = RT ln(K)

K = equilibrium constant

This is so because G = 0 and Q = K at equilibrium.


36

Figure 16.9Free Energy and Equilibrium


37

Temperature Dependence of KTemperature Dependence of K

y = mx + b

(H and S independent of temperature over a small temperature range)

ln( )KHR

T S=− ° + ° ( / )1


38

Reversible v. Irreversible Processes

Reversible v. Irreversible Processes

Reversible: The universe is exactly the same as it was before the cyclic process.

Irreversible: The universe is different after the cyclic process.

All real processes are irreversible -- (some work is changed to heat).


39

Figure 16.10A Battery


40


Since SSYS and SSURR are opposed here, the temperature must have an effect on the relative importance of these two terms.

The idea here is that entropy changes in the surroundings are determined by heat flow.

So, exothermicity is an important driving force for spontaneity.

copyright©2000 by houghton mifflin company. all rights reserved. 1 ap chem h/w 16.1 9, 17, 19, 23,...

Documents

houghton mifflin company

ssurr ssurr

process kineticscopyright2000

spontaneoussuniv spontaneous

spontaneous processes

free energy copyright2000

entropy changes

rate of reaction copyright2000