18
Thermodynamics
Thursday Lecture November 10, 2011
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Chapter Seventeen
Thermodynamics: Spontaneity,
Entropy, and Free Energy
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Outline Lecture 18
What is Thermodynamics?
Why study thermodynamics?
Think about those food labels, calories etc.
Some thermodynamics definitions
Entropy
Free energy
Enthalpy
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Thermodynamics vs. Kinetics
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Introductory Concepts
Thermodynamics examines the relationship
between heat (q) and work (w)
Spontaneity is the notion of whether or
not a process can take place unassisted
Entropy is a measure of how energy
is spread out among the particles of a
system
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Introduction
Free energy is a thermodynamic function
that relates enthalpy and entropy to
spontaneity, and can also be related to
equilibrium constants.
G = H-TS
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First Law of Thermodynamics
conservation of energy
For an exothermic reaction, lost heat from
the system goes into the surroundings.
two ways energy lost from a system
converted to heat, q
used to do work, w
Energy conservation requires that the energy
change in the system equal the heat released
plus the work done.
E = q + w
E = H + P V
E is a state function.
internal energy change independent of how
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done
First Law of Thermodynamics Conclusion:
You cant win!
First law of thermodynamics: Energy
cannot be created or destroyed.
The total energy of the universe cannot
change.
though you can transfer it from one place
to another
Euniverse = 0 = Esystem + Esurroundings
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The Energy Tax
You cant break even!
Recharging a battery with 100
kJ of useful energy will require
more than 100 kJ.
Every energy transition results
in a loss of energy.
conversion of energy to heat, which
is lost by heating up the
surroundings
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Why study thermodynamics?
Ever wonder why the FDA requires
food labels with calories?
How about when you exercise and
want to know how much fat you burn?
How about the heat you get from
burning natural gas?
How much work can 1 litre of gasoline
do?
Will a reaction occur spontaneously?
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Why Study Thermodynamics 2?
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.
(Kinetics tells us whether the process is practical.)
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Another reason to study thermo.
Lavoisier studying human respiration as a form of combustion.
A whole-body calorimeter.
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Introductory Concepts
Free energy (G) is a thermodynamic
function that relates enthalpy and entropy
to spontaneity and will be discussed later
lecture.
Free energy is connected with the
ability to do work
e.g., the chemical reaction in a
battery generates electricity to light a
flashlight bulb
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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 the
change about
Example: spontaneous combustion
of damp hay or grain silo explosions
from gases evolved from
decomposing grain
If a process is spontaneous, the
reverse process is nonspontaneous
and vice versa
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Spontaneous Change
(contd)
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.
Example: combustion reactions are
thermodynamically favored, but (fortunately for
life on Earth!) most such reactions also have a high
activation energy.
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Spontaneity
A general rule: that exothermic reactions are
spontaneous and endothermic reactions are
nonspontaneous works in many cases
However,
enthalpy
change is not
a sufficient
criterion for
predicting
spontaneous
change
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Spontaneous Change
Water falling (higher to
lower potential energy)
is a spontaneous
process.
(contd)
Conclusion: enthalpy alone
is not a sufficient criterion
for prediction of
spontaneity.
liquid water
vaporizes
spontaneously at room
temperature; an
endothermic process.
H2 and O2 combine
spontaneously to form
water (exothermic) BUT
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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?
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The Concept of Entropy
Consider mixing two gases: this occurs
spontaneously, and the gases form a
homogeneous mixture.
There is essentially
no enthalpy change
involved, so why is
the process
spontaneous?
The driving force is a thermodynamic quantity
called entropy, a mathematical concept that is
difficult to portray visually
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Entropy
The total energy of a system remains
unchanged in the mixing of the gases but the
number of possibilities for the distribution of
that energy increases.
This spreading of the
energy and increase of
entropy correspond to a
greater physical
disorder at the
microscopic level.
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Entropy
There are two natural tendencies behind
spontaneous processes: the tendency to
achieve a lower energy state and the
tendency toward a more disordered state
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The Concept of Entropy (S)
Entropy refers to the state of order.
A change in order is a change in the number of ways of arranging the
particles, and it is a key factor in determining the direction of a
spontaneous process.
more order
solid
liquid
more order
crystal + liquid
less order
ions in solution
more order
crystal + crystal
less order
gas
less order
gases + ions in solution
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Entropy (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 the
entropy of the system
Entropy (S) is a state
function: it is path
independent
Sfinal Sinit = S
S = qrev/T
EOS
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Reversibility of Process
Any spontaneous process is irreversible.
It will proceed in only one direction.
A reversible process will proceed back
and forth between the two end
conditions.
equilibrium
results in no change in free energy
If a process is spontaneous in one
direction, it must be nonspontaneous in
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Reversible Process
A process that is never more than step an
infinitesimal away from equilibrium
The process can
reverse directions
by a miniscule
change in a
variable
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Entropy
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.
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Fig. L18.2
Low entropy (low disorder or randomness)
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High entropy (high disorder)
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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
<<
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Sgas
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Enthalpy
H is generally measured in kJ/mol.
stronger bonds = more stable molecules
A reaction is generally exothermic if the
bonds in the products are stronger than the
bonds in the reactants.
exothermic = energy released, H is negative
A reaction is generally endothermic if the
bonds in the products are weaker than the
bonds in the reactants.
endothermic = energy absorbed, H is positive
The enthalpy is favorable for exothermic
reactions and unfavorable for endothermic
29reactions.
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Entropy
Entropy, S, is a thermodynamic function
that increases as the number of
energetically equivalent ways of arranging
the components increases.
S is generally measured in J/mol.
S = k lnW
k = Boltzmann constant = 1.38 x 1023 J/K
W is the number of energetically equivalent
ways
unitless
Random systems require less energy than
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ordered systems. Veterans
Changes in Entropy, S
Entropy change is favorable when the result is
a more random system.
S is positive
Some changes that increase the entropy are
reactions whose products are in a more
random state.
solid more ordered than liquid more ordered than
gas
reactions that have larger numbers of product
molecules than reactant molecules
increase in temperature
solids dissociating into ions upon dissolving
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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 (more randomness)
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
molecules of gases.
A substance is heated.
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Increases in Entropy
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S
For a process in which the final
condition is more random than the
initial condition, Ssystem is positive.
favorable entropy
For a process in which the final
condition is more orderly than the
initial condition, Ssystem is negative.
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unfavorable entropy
Ssystem = Sreaction = (Sprod) (Sreact)
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PracticePredict whether Ssystem is + or
for each of the following.
Heating air in a balloon S is +
Water vapor condensing S is
Separation of oil and vinegar salad dressing S is
S is +
Dissolving sugar in tea
2 HgO(s) 2 Hg(l) + O2(g) S is +
2 NH3(g) N2(g) + 3 H2(g) S is +
Ag+(aq) + Cl(aq) AgCl(s) S is
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Temperature Dependence of
Ssurroundings
When a system process is exothermic, it
adds heat to the surroundings, increasing the
entropy of the surroundings.
When a system process is endothermic, it
takes heat from the surroundings, decreasing
the entropy of the surroundings.
The amount the entropy of the surroundings
changes depends on its initial temperature.
The higher the original temperature, the less effect
addition or removal of heat has.
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Example 17.2(a) Calculate the entropy change of the surroundings at 25C for the
reaction:
C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g); Hrxn = 2044 kJ.
Given:
Find:
Conceptual
Plan:
Hsystem = 2044 kJ, T = 25C = 298 K
Ssurroundings (J/K)
T , H
S
Relationships:
Solution:
Check:
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Combustion is largely exothermic, so the entropy
of the surroundings should increase significantly.
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The Third Law of Thermodynamics
Absolute Entropy
The absolute entropy of a
substance is the amount of
energy it has due to dispersion
of energy through its
particles.
The third law states that for
a perfect crystal at absolute
zero, the absolute entropy = 0
J/molK.
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Therefore, every substance that is
not a perfect crystal at absolute
zero has some energy from
entropy.
Therefore, the absolute entropy
of substances is always +.
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Standard Molar Entropies
According to the Third Law of Thermodynamics,
the entropy of a pure, perfect crystal can be
taken to be zero at 0 K.
The standard molar entropy, S , is the entropy
of one mole of a substance in its standard
state.
Since entropy increases with temperature,
standard molar entropies are positiveeven for
elements.
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Relative Standard EntropiesStates
The gaseous state has a larger entropy
than the liquid state at a particular
temperature.
The liquid state has a larger entropy
than the solid state at a particular
temperature.
Substance
S
(J/molK)
H2O(g)
H2O(l)
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188.8
70.0
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Relative Standard Entropies
Molar Mass
The larger the molar
mass, the larger the
entropy.
Available energy
states are more
closely spaced,
allowing more
dispersal of energy
through the states.
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Relative Standard Entropies
Allotropes
The less
constrained
the structure
of an
allotrope, the
larger its
entropy.
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Relative Standard Entropies
Molecular Complexity
Larger, more
Substance
complex molecules
generally have
Ar(g)
larger entropy.
NO(g)
More available
energy states,
allowing more
dispersal of energy
through the states Veterans
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Molar
S
Mass (J/molK)
39.948 154.8
30.006
210.8
Relative Standard Entropies
Dissolution
Dissolved solids
generally have
larger entropy.
Substance
S
(J/molK)
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143.1
KClO3(aq)
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KClO3(s)
265.7
Changes in Entropy
The standard entropy change is the
difference in absolute entropy
between the reactants and products
under standard conditions.
Sreaction = (npSproducts) (nrSreactants)
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Example L21.3 Calculate S for the
reaction:
NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g)
Given:
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NH3(g) + 5 O2(g) 4 NO(g) + 6 H2O(g).
Given: standard entropies from Appendix C.1
Find: S (J/K)
Conceptual
SNH3, SO2, SNO, SH2O,
Plan:
S
Relationships:
Solution:
Check:
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S is +, as you would expect for a reaction with more gas
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product molecules than reactant molecules.
PracticeCalculate S for the reaction
2H2(g) + O2(g) 2 H2O(g)
and for
2H2(g) + O2(g) 2 H2O(l).
Please get use to using Appendix C.1 Thermodynamic
properties in text on page A13.
Substance
H2(g)
O2(g)
H2O(g)
H2O(l)
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S, J/molK
130.7
205.2
188.8
110.0
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Lecture Summary
Key terms in thermodynamics,
First, second, and third laws,
Entropy change,
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