Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 AP Chem h/w 16.1 9, 17, 19, 23,...
-
Upload
alvin-lawrence -
Category
Documents
-
view
220 -
download
1
Transcript of 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
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
2
ChemistryFIFTH EDITION
Chapter 16Spontaneity, Entropy,
and Free Energy
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
3
Figure 16.1Methane and Oxygen React
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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)
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
8
Figure 16.3 The Expansion of an Ideal Gas into an Evacuated Bulb
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
9
Figure 16.4Three Possible Arrangements (microstates) of Four Molecules in a Two-Bulbed Flask
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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 #
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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?
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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)
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
14
∆Suniv = ∆Ssystem + ∆Ssurrounding
Consider this process:
H2O(l) → H2O(g)
What happens to SSYS here?
What about SSURR ?
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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?
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
16
∆Suniv = ∆Ssystem + ∆Ssurrounding
So for H2O(l) → H2O(g)
SSYS is +
SSURR is - and
∆Suniv = ∆Ssystem + ∆Ssurrounding
So will this rxn occur. It depend on ____?
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
18
The Effect of Temperature on Spontaneity
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
19
The Effect of Temperature on Spontaneity
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
20
The Effect of Temperature on Spontaneity
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
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
21
∆Suniv = ∆Ssystem + ∆Ssurrounding
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
23
The Effect of Temperature on Spontaneity
See S/E 16.4, p. 802, and Table 16.3, p. 803.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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)
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
32
Figure 16.7Schematic Representations of Balls
Rolling Down into Two Types of Hills
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
33
Free Energy and PressureFree Energy and Pressure
G = G + RT ln(Q)
Q = reaction quotient from the law of mass action.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
34
Figure 16.8The Dependence of Free Energy on Partial Pressure
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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.
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
36
Figure 16.9Free Energy and Equilibrium
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
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).
Copyright©2000 by Houghton Mifflin Company. All rights reserved.
40
The Effect of Temperature on Spontaneity
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.