chapter 17 · pdf filechapter 17 spontaneity ... section 17.1 spontaneous processes and...
TRANSCRIPT
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Chapter 17
Spontaneity, Entropy,
and Free Energy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
2013 AP Curriculum Guidelines
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Thermodynamics vs. Kinetics
Domain of Kinetics
Rate of a reaction depends on the pathway from reactants to products.
Thermodynamics tells us whether a reaction is spontaneous based only on the properties of reactants and products.
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Section 17.1 Spontaneous Processes and Entropy
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Spontaneous Processes and Entropy
Thermodynamics lets us predict the direction in which a process will occur but gives no information about the speed of the process.
A spontaneous process is one that occurs without outside intervention.
Thermodynamically favorable
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Section 17.1 Spontaneous Processes and Entropy
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Consider 2.4 moles of a gas contained in a 4.0 L bulb at a constant temperature of 32°C. This bulb is connected by a valve to an evacuated 20.0 L bulb. Assume the temperature is constant.
a) What should happen to the gas when you open the valve?
CONCEPT CHECK!
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Section 17.1 Spontaneous Processes and Entropy
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Consider 2.4 moles of a gas contained in a 4.0 L bulb at a constant temperature of 32°C. This bulb is connected by a valve to an evacuated 20.0 L bulb. Assume the temperature is constant.
b) Calculate ΔH, ΔE, q, and w for the process you described above.
All are equal to zero.
CONCEPT CHECK!
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Section 17.1 Spontaneous Processes and Entropy
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Consider 2.4 moles of a gas contained in a 4.0 L bulb at a constant temperature of 32°C. This bulb is connected by a valve to an evacuated 20.0 L bulb. Assume the temperature is constant.
c) Given your answer to part b, what is the driving force for the process?
Entropy
CONCEPT CHECK!
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Section 17.1 Spontaneous Processes and Entropy
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The Expansion of An Ideal Gas Into an Evacuated Bulb
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Section 17.1 Spontaneous Processes and Entropy
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Entropy
The driving force for a spontaneous process is an increase in the entropy of the universe.
A measure of molecular randomness or disorder.
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Section 17.1 Spontaneous Processes and Entropy
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Entropy
Thermodynamic function that describes the number of arrangements that are available to a system existing in a given state.
Nature spontaneously proceeds toward the states that have the highest probabilities of existing.
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Section 17.1 Spontaneous Processes and Entropy
The Microstates That Give a Particular Arrangement (State)
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Section 17.1 Spontaneous Processes and Entropy
Positional Entropy
A gas expands into a vacuum to give a uniform distribution because the expanded state has the highest positional probability of states available to the system.
Therefore: Ssolid < Sliquid << Sgas
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Section 17.1 Spontaneous Processes and Entropy
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Predict the sign of ΔS for each of the following, and explain:
a) The evaporation of alcohol
b) The freezing of water
c) Compressing an ideal gas at constant temperature
d) Heating an ideal gas at constant pressure
e) Dissolving NaCl in water
+
–
–
+
+
CONCEPT CHECK!
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
2014 AP Exam
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Section 17.1 Spontaneous Processes and Entropy
2014 AP Exam
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Section 17.2 Entropy and the Second Law of Thermodynamics
Second Law of Thermodynamics
In any spontaneous process there is always an increase in the entropy of the universe.
The entropy of the universe is increasing.
The total energy of the universe is constant, but the entropy is increasing.
Suniverse = ΔSsystem + ΔSsurroundings
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Section 17.2 Entropy and the Second Law of Thermodynamics
ΔSsurr
ΔSsurr = +; entropy of the universe increases
ΔSsurr = -; process is spontaneous in opposite direction
ΔSsurr = 0; process has no tendency to occur
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Section 17.2 Entropy and the Second Law of Thermodynamics
2014 AP Exam
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Section 17.2 Entropy and the Second Law of Thermodynamics
2014 AP Exam
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Section 17.3 The Effect of Temperature on Spontaneity
For the process A(l) A(s), which direction involves an increase in energy randomness? Positional randomness? Explain your answer.
As temperature increases/decreases (answer for both), which takes precedence? Why?
At what temperature is there a balance between energy randomness and positional randomness?
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CONCEPT CHECK!
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Section 17.3 The Effect of Temperature on Spontaneity
For the process A(l) A(s), which direction involves an increase in energy randomness? Positional randomness? Explain your answer.
As temperature increases/decreases (answer for both), which takes precedence? Why?
At what temperature is there a balance between energy randomness and positional randomness?
Copyright © Cengage Learning. All rights reserved 32
CONCEPT CHECK!
Since energy is required to melt a solid, the reaction as written is
exothermic. Thus, energy randomness favors the right (product;
solid). Since a liquid has less order than a solid, positional
randomness favors the left (reactant; liquid).
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Section 17.3 The Effect of Temperature on Spontaneity
For the process A(l) A(s), which direction involves an increase in energy randomness? Positional randomness? Explain your answer.
As temperature increases/decreases (answer for both), which takes precedence? Why?
At what temperature is there a balance between energy randomness and positional randomness?
Copyright © Cengage Learning. All rights reserved 33
CONCEPT CHECK!
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Section 17.3 The Effect of Temperature on Spontaneity
Describe the following as spontaneous/non-spontaneous/cannot tell, and explain.
A reaction that is:
a) Exothermic and becomes more positionally random
Spontaneous
b) Exothermic and becomes less positionally random
Cannot tell
a) Endothermic and becomes more positionally random
Cannot tell
a) Endothermic and becomes less positionally random
Not spontaneous
Explain how temperature affects your answers.
CONCEPT CHECK!
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Section 17.3 The Effect of Temperature on Spontaneity
ΔSsurr
The sign of ΔSsurr depends on the direction of the heat flow.
The magnitude of ΔSsurr depends on the temperature.
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Section 17.3 The Effect of Temperature on Spontaneity
ΔSsurr
Copyright © Cengage Learning. All rights reserved 36
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Section 17.3 The Effect of Temperature on Spontaneity
ΔSsurr
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Section 17.3 The Effect of Temperature on Spontaneity
ΔSsurr
Heat flow (constant P) = change in enthalpy = ΔH
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surr =
H
ST
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Section 17.3 The Effect of Temperature on Spontaneity
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.4 Free Energy
Free Energy (G)
A process (at constant T and P) is spontaneous in the direction in which the free energy decreases.
Negative ΔG means positive ΔSuniv.
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univ = (at constant and )
G
S T PT
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Section 17.4 Free Energy
Gibbs Free Energy (G)
ΔG = ΔH – TΔS (at constant T and P)
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Energy Free to do work
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
2014 AP Exam
Hint: Which equation should you use?
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Section 17.1 Spontaneous Processes and Entropy
2014 AP Exam
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.4 Free Energy A liquid is vaporized at its boiling point. Predict the signs of:
w
q
ΔH
ΔS
ΔSsurr
ΔG
Explain your answers.
Copyright © Cengage Learning. All rights reserved 47
–
+
+
+
–
0
CONCEPT CHECK!
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Section 17.4 Free Energy A liquid is vaporized at its boiling point. Predict the signs of:
w
q
ΔH
ΔS
ΔSsurr
ΔG
Explain your answers.
Copyright © Cengage Learning. All rights reserved 48
–
+
+
+
–
0
CONCEPT CHECK!
As a liquid goes to vapor, it does work on
the surroundings (expansion occurs). Heat
is required for this process. Thus, w =
negative; q = H = positive. S = positive
(a gas is more disordered than a liquid), and
Ssurr = negative (heat comes from the
surroundings to the system); G = 0
because the system is at its boiling point
and therefore at equilibrium.
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.1 Spontaneous Processes and Entropy
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Section 17.4 Free Energy
The value of ΔHvaporization of substance X is 45.7 kJ/mol,
and its normal boiling point is 72.5°C.
Calculate ΔS, ΔSsurr, and ΔG for the vaporization of one mole of this substance at 72.5°C and 1 atm.
ΔS = 132 J/K·mol
ΔSsurr = -132 J/K·mol
ΔG = 0 kJ/mol
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EXERCISE!
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Section 17.4 Free Energy
Spontaneous Reactions
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Section 17.4 Free Energy
Effect of ΔH and ΔS on Spontaneity
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Section 17.5 Entropy Changes in Chemical Reactions
Gas A2 reacts with gas B2 to form gas AB at constant temperature and pressure. The bond energy of AB is much greater than that of either reactant.
Predict the signs of:
ΔH ΔSsurr ΔS ΔSuniv
Explain.
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– + 0 +
CONCEPT CHECK!
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Section 17.5 Entropy Changes in Chemical Reactions
Third Law of Thermodynamics
The entropy of a perfect crystal at 0 K is zero.
The entropy of a substance increases with temperature.
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Section 17.5 Entropy Changes in Chemical Reactions
Standard Entropy Values (S°)
Represent the increase in entropy that occurs when a substance is heated from 0 K to 298 K at 1 atm pressure.
ΔS°reaction = ΣnpS°products – ΣnrS°reactants
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Section 17.5 Entropy Changes in Chemical Reactions
Calculate ΔS° for the following reaction:
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Given the following information:
S° (J/K·mol)
Na(s) 51
H2O(l) 70
NaOH(aq) 50
H2(g) 131
ΔS°= –11 J/K Copyright © Cengage Learning. All rights reserved 63
EXERCISE!
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Section 17.1 Spontaneous Processes and Entropy
http://www.bozemanscience.com/ap-chemistry/
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Section 17.6 Free Energy and Chemical Reactions
Standard Free Energy Change (ΔG°)
The change in free energy that will occur if the reactants in their standard states are converted to the products in their standard states.
ΔG° = ΔH° – TΔS°
ΔG°reaction = ΣnpG°products – ΣnrG°reactants
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Section 17.6 Free Energy and Chemical Reactions
A stable diatomic molecule spontaneously forms from its atoms.
Predict the signs of:
ΔH° ΔS° ΔG°
Explain.
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– – –
CONCEPT CHECK!
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Section 17.6 Free Energy and Chemical Reactions
Consider the following system at equilibrium at 25°C.
PCl3(g) + Cl2(g) PCl5(g)
ΔG° = −92.50 kJ
What will happen to the ratio of partial pressure of PCl5 to partial pressure of PCl3 if the temperature is raised? Explain.
The ratio will decrease.
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CONCEPT CHECK!
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Section 17.7 The Dependence of Free Energy on Pressure
Free Energy and Pressure
G = G° + RT ln(P)
or
ΔG = ΔG° + RT ln(Q)
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Section 17.7 The Dependence of Free Energy on Pressure
Sketch graphs of:
1. G vs. P
2. H vs. P
3. ln(K) vs. 1/T (for both endothermic and exothermic cases)
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CONCEPT CHECK!
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Section 17.7 The Dependence of Free Energy on Pressure
The Meaning of ΔG for a Chemical Reaction
A system can achieve the lowest possible free energy by going to equilibrium, not by going to completion.
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Section 17.7 The Dependence of Free Energy on Pressure
http://www.bozemanscience.com/ap-chemistry/
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Section 17.8 Free Energy and Equilibrium
The equilibrium point occurs at the lowest value of free energy available to the reaction system.
ΔG = 0 = ΔG° + RT ln(K)
ΔG° = –RT ln(K)
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Section 17.8 Free Energy and Equilibrium
Change in Free Energy to Reach Equilibrium
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Section 17.8 Free Energy and Equilibrium
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Section 17.9 Free Energy and Work
Maximum possible useful work obtainable from a process at constant temperature and pressure is equal to the change in free energy.
wmax = ΔG
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Section 17.9 Free Energy and Work
Achieving the maximum work available from a spontaneous process can occur only via a hypothetical pathway. Any real pathway wastes energy.
All real processes are irreversible.
First law: You can’t win, you can only break even.
Second law: You can’t break even.
As we use energy, we degrade its usefulness.
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Section 17.9 Free Energy and Work
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Section 17.9 Free Energy and Work
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Section 17.9 Free Energy and Work
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Section 17.9 Free Energy and Work
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Section 17.9 Free Energy and Work