Transcript Chapter 16.1-16.3 in our book - PART 1 Spontaneity, Entropy, and Free Energy.

Chapter 16.1-16.3 in
our book - PART 1
Spontaneity, Entropy,
and Free Energy
Chapter 16
Table of Contents
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16.1
Spontaneous Processes and Entropy
16.2 Entropy and the Second Law of
Thermodynamics
16.3
The Effect of Temperature on Spontaneity
16.4
Free Energy
16.5
Entropy Changes in Chemical Reactions
16.6
Free Energy and Chemical Reactions
16.7
The Dependence of Free Energy on Pressure
16.8
Free Energy and Equilibrium
16.9
Free Energy and Work
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3
Why Study Thermodynamics?
• With a knowledge of thermodynamics and
by making a few calculations before
embarking on a new venture, scientists and
engineers can save themselves a great deal
of time, money, and frustration.
– “To the manufacturing chemist thermodynamics gives information
concerning the stability of his substances, the yield which he may
hope to attain, the methods of avoiding undesirable substances, the
optimum range of temperature and pressure, the proper choice of
solvent.…” - from the introduction to Thermodynamics and the Free
Energy of Chemical Substances by G. N. Lewis and M. Randall
• Thermodynamics tells us what processes are
possible.
• Enthalpy
– heat transfer between the system and
its surroundings under const. press.
– Enthalpy is a guide to whether a
reaction is likely to proceed.
– It is not the only factor that
determines whether a reaction
proceeds.
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Introduction
• Thermodynamics examines the relationship
between heat and work.
• Spontaneity is the notion of whether or not a
process can take place unassisted.
• Entropy is a mathematical concept describing the
distribution of energy within a system.
• Free energy is a thermodynamic function that
relates enthalpy and entropy to spontaneity, and
can also be related to equilibrium constants.
From Chapter 6
Chemical thermodynamics is the
study of energy relationships in
chemistry.
The First law of Thermodynamics
- energy cannot be created or
destroyed only converted from one
form to another. Energy is constant
in the universe.
Spontaneous
Processes
• Occur without outside intervention
• Have a definite direction.
– The reverse process is not spontaneous.
• Temperature has an impact on
spontaneity.
– Ex: Ice melting or forming
– Ex: Hot metal cooling at room temp.
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Spontaneous Change
• A spontaneous process is one that can
occur in a system left to itself; no action
from outside the system is necessary to
bring it about.
• A nonspontaneous process is one that
cannot take place in a system left to itself.
• If a process is spontaneous, the reverse
process is nonspontaneous, and vice versa.
• Example: gasoline combines spontaneously
with oxygen.
Section 16.4
Free Energy
Spontaneous Reactions
http://college.cengage.com/chemistry/discipline/thinkwell/2999.html
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12
Spontaneous Change (cont’d)
• Thermodynamics determines the
equilibrium state of a system.
• Thermodynamics is used to predict the
proportions of products and reactants at
equilibrium.
• Kinetics determines the pathway by which
equilibrium is reached.
• A high activation energy can effectively
block a reaction that is thermodynamically
favored.
KI (aq) + Pb(NO3)2 (aq)  PbI2(s) + KNO3 (aq)
When mixed  Precipitate forms spontaneously.
*It does not reverse itself and become two clear solutions.
Reversible &
Irreversible
• Reversible:
• System changes state and can be restored by
reversing original process.
• Ex: Water (s)
Water (l)
• Irreversible:
• System changes state and must take a
different path to restore to original state.
• Ex: CH4 + O2  CO2 + H2O
• Whenever a system is in equilibrium, the
reaction can go reversibly to reactants or
products (water  water vapor at 100 º C).
• In a Spontaneous process, the path between
reactants and products is irreversible. (Reverse
of spontaneous process is not spontaneous).
• *Scrambled eggs don’t unscramble*
Section 16.1
Spontaneous Processes and Entropy
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 16.1
Spontaneous Processes and Entropy
Spontaneous Processes and Entropy
• Thermodynamics lets us predict whether a
process will occur but gives no information
about the amount of time required for the
process.
• A spontaneous process is one that occurs
without outside intervention.
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•
–
Example
Indicate whether each of the following
processes is spontaneous or nonspontaneous.
Comment on cases where a clear determination
cannot be made.
(a) The action of toilet bowl cleaner, HCl(aq), on
“lime” deposits, CaCO3(s).
(b) The boiling of water at normal atmospheric
pressure and 65 °C.
(c) The reaction of N2(g) and O2(g) to form NO(g)
at room temperature.
(d) The melting of an ice cube.
ANSWERS
• Answers with additional examples
*
*
*Therefore melting ice is spontaneous since heat will
flow to a colder body.
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20
Spontaneous Change (cont’d)
• Early chemists proposed that spontaneous chemical
reactions should occur in the direction of
decreasing energy.
• It is true that many exothermic processes are
spontaneous and that many endothermic reactions
are nonspontaneous.
• However, enthalpy change is not a sufficient
criterion for predicting spontaneous change …
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Spontaneous Change (cont’d)
Water falling (higher to
lower potential energy) is a
spontaneous process.
H2 and O2 combine
spontaneously to form water
(exothermic) BUT …
Conclusion: enthalpy alone is
not a sufficient criterion for
prediction of spontaneity.
… liquid water vaporizes
spontaneously at room
temperature; an
endothermic process.
22
The Concept of Entropy
When the valve
is opened …
… the gases mix spontaneously.
• There is no significant enthalpy
change.
• Intermolecular forces are negligible.
• So … why do the gases mix?
23
The Concept of Entropy (cont’d)
• The other factor that drives reactions is a
thermodynamic quantity called entropy.
• Entropy is a mathematical concept that is difficult
to portray visually.
• The total energy of the system remains unchanged
in the mixing of the gases …
• … but the number of possibilities for the
distribution of that energy increases.
Entropy (S)
• A measure of randomness or disorder
• S = entropy in J/K·mole
• Increasing disorder or increasing
randomness is increasing entropy.
• Three types of movement can lead to
an increase in randomness.
Entropy is a state function
• Change in entropy of a system
S = Sfinal- Sinitial
• Depends only on initial and final states, and
not the pathway.
• -S indicates a more ordered state
(think: < disorder or - disorder)
• Positive (+) S = less ordered state
• (think: > disorder or + disorder)
Section 16.1
Spontaneous Processes and Entropy
Positional Entropy
• A gas expands into a vacuum because the
expanded state has the highest positional
probability of states available to the system.
• Therefore: Ssolid < Sliquid << Sgas
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Section 16.1
Spontaneous Processes and Entropy
The Expansion of An Ideal Gas Into an Evacuated Bulb
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Entropy, S
- a measure of disorder
Ssolid
<
Sliquid
<<
Sgas
Increasing Entropy
Increasing Entropy
Section 16.1
Spontaneous Processes and Entropy
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 16.1
Spontaneous Processes and Entropy
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.

The probability of occurrence of a particular state
depends on the number of ways (microstates) in which
that arrangement can be achieved
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Section 16.1
Spontaneous Processes and Entropy
The Microstates That Give a Particular Arrangement (State)
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Section 16.1
Spontaneous Processes and Entropy
The Microstates That Give a Particular Arrangement (State)
Entropy is a thermodynamic function describing the
number of arrangements that are available to a
system
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Increasing Entropy
“But ma, it’s not my fault… the universe wants my room like this!”
Section 16.1
Spontaneous Processes and Entropy
Concept Check #1a
•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?
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Section 16.1
Spontaneous Processes and Entropy
Concept Check #1b
•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 H, E, q, and w for the
process you described above.
–
All are equal to zero.
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Section 16.1
Spontaneous Processes and Entropy
Concept Check #1c
•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
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Section 16.1
Spontaneous Processes and Entropy
Concept Check #2
•Predict the sign of S 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
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Section 16.2
Atomic Masses
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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The Second Law
of Thermodynamics
- The entropy of the universe always
increases in a spontaneous process
and remains unchanged in an
equilibrium process.
If entropy always increases, how can we account for the fact
that water spontaneously freezes when placed in the freezer?
• Movement of compressor
+
• Evaporation and condensation of refrigerant
+
• Warming of air around container
Net increase in the entropy of the universe
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Formation of an Ideal Solution
Benzene and toluene have similar
intermolecular forces, so there is no
enthalpy change when they are mixed.
They mix completely because
entropy of the mixture is higher
than the entropies of the two
substances separated.
48
Increase in Entropy in the
Vaporization of Water
Evaporation is
spontaneous because of
the increase in entropy.
49
The Concept of Entropy
• The spreading of the energy among states,
and increase of entropy, often correspond
to a greater physical disorder at the
microscopic level (however, entropy is not
“disorder”).
• There are two driving forces behind
spontaneous processes: the tendency to
achieve a lower energy state (enthalpy
change) and the tendency for energy to be
distributed among states (entropy).
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Assessing Entropy Change
• The difference in entropy (S) between two
states is the entropy change (S).
• The greater the number of configurations
of the microscopic particles (atoms, ions,
molecules) among the energy levels in a
particular state of a system, the greater is
the entropy of the system.
• Entropy generally increases when:
• Solids melt to form liquids.
• Solids or liquids vaporize to form gases.
• Solids or liquids dissolve in a solvent to form nonelectrolyte
solutions.
• A chemical reaction produces an increase in the number of
On the AP exam, you will likely be asked to:
1) predict whether a process leads to an increase in entropy or a
decrease in entropy.
2) Determine if ΔS is + or –
3) Determine substances or reactions that have the highest entropy.
Processes that lead to an Increase in Entropy
1) When a solid melts.
2) When a solid dissolves in solution.
3) When a solid or liquid becomes a gas.
4) When the temperature of a substance increases.
5) When a gaseous reaction produces more molecules.
6) If no net change in # of gas molecules, can be + or -, but small.
Predict whether the entropy change is
greater than or less than zero for each of
the following processes:
a) Freezing liquid bromine S<0
b)Evaporating a beaker of
ethanol at room temperature S>0
c) Dissolving sucrose in water S>0
d)Cooling N2 from 80ºC to 20ºC S<0
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• Example
–Predict whether each of the following leads to an
increase or decrease in the entropy of a system. If in
doubt, explain why.
–(a) The synthesis of ammonia:
–
N2(g) + 3 H2(g)  2 NH3(g)
H2O(l)
–(b) Preparation of a sucrose solution:
–
C12H22O11(s)
C12H22O11(aq)
–(c) Evaporation to dryness of a solution of urea,
CO(NH2)2, in water:
–
CO(NH2)2(aq)  CO(NH2)2(s)
Predict whether the entropy
change of the system in each of the
following reactions is positive or
negative:
1.) Ag+(aq)+ Cl-(aq)AgCl(s)
1)S –
2.) NH4Cl(s) NH3(g)+ HCl(g)
2)S+
3.) H2(g) + Br2(g)2HBr(g)
3)S?
According to the 2nd law of thermodynamics; the
entropy of the universe always increases.
?
What if the entire senior class assembles in the auditorium?
Aren’t we decreasing disorder, and therefore decreasing entropy?
If so, how can the second law of thermodynamics be true?
If we consider the senior class as the system, the
ΔS of the system would indeed decrease.
ΔS of the system is –
In order for the students to gather, they would:
• Metabolize food (entropy increase of surroundings)
• Generate heat (entropy increase of surroundings)
The magnitude of the entropy increase of the surroundings will
always be greater than the entropy decrease of the system.
Theoretical values
Suniverse = Ssystem + Ssurroundings
Suniverse = (-10)
+
(+20)
Suniverse = +10
+ means entropy increases
The same can be considered in a chemical process.
When a piece of metal rusts:
4Fe(s) + O2(g)  2Fe2O3(s)
The entropy of the solid slowly decreases.
Although this is a slow process, it is exothermic,
and heat is released into the surroundings causing
an overall increase in entropy of the universe!
The Second Law of Thermodynamics
- The entropy of the universe
increases in a spontaneous process
and remains unchanged in an
equilibrium process.
Suniverse = Ssystem + Ssurroundings
Suniverse > 0 for spontaneous reaction
Suniverse = 0 at equilibrium
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Entropy Change
• Sometimes it is necessary to obtain
quantitative values of entropy changes.
The expansion
can be reversed
•
S = qrxn/T
by allowing the
to return,
• where qrxn is reversible heat, a state sand
one grain at a
time.
function.
A reversible
process can be
reversed by a very small
• as in the expansion
change,
of this gas. A reversible
process is never more than a
tiny step from equilibrium.
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Entropy as a Function of Temperature
Entropy always
increases with
temperature …
… and it increases
dramatically during a
phase change.
Section 16.3
The Effect
Mole 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 16.3
The Effect
Mole of Temperature on Spontaneity
ΔSsurr
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Section 16.3
The Effect
Mole of Temperature on Spontaneity
ΔSsurr
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Section 16.3
The Effect
Mole of Temperature on Spontaneity
ΔSsurr
• Heat flow (constant P) = change in enthalpy =
ΔH
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Section 16.3
The Effect
Mole of Temperature on Spontaneity
Interplay of Ssys and Ssurr in Determining the Sign of Suniv
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Section 16.3
The Effect
Mole of Temperature on Spontaneity
Example
• Determining ∆Ssurr
In the metallurgy of antimony, the pure metal is recovered via different reactions, depending on the
composition of the ore. For example, iron isused to reduce antimony in sulfide ores:
Record and show work for these 2 answers in notes.
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Section 16.3
The Effect
Mole of Temperature on Spontaneity
Example
• Determining ∆Ssurr
In the metallurgy of antimony, the pure metal is recovered via different reactions, depending on the
composition of the ore. For example, iron isused to reduce antimony in sulfide ores:
Record and show work for these 2 answers in notes.
One of the answers is = -2.61 × 103 J/K if done correctly.
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Section 16.3
The Effect
Mole of Temperature on Spontaneity
Concept Check - Do this without writing down.
•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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Section 16.3
The Effect
Mole of Temperature on Spontaneity
Concept Check
• 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
c) Endothermic and becomes more positionally random
–
Cannot tell
d) Endothermic and becomes less positionally random
–
–
Not spontaneous
Explain how temperature affects your answers.
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Section 16.3
The Effect
Mole of Temperature on Spontaneity
Homework
• Section 16.1-16.3 pg. 783 #19-21 and #23-26 due WED
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Section 16.3
The Effect
Mole of Temperature on Spontaneity
STOPPED HERE
• Go to the other presentation instead of finishing with this
one for the rest of the chapter.
• Section 16.7-16.9 are not covered on this unit test.
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