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PRINCIPLES OF REACTIVITY:
ENTROPY AND FREE ENERGY
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CHAPTER OVERVIEW
• This chapter examines factors that
determine whether a reaction is
spontaneous, product-favored,
non-spontaneous, reactants-
favored.
• Review thermodynamic basics
(next)
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Thermodynamics
Thermodynamics is the science
of heat (energy) transfer.
Heat energy is associated
with molecular motions.
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CHEMICAL REACTIVITY
What drives chemical reactions?
How do they occur?
The first is answered by THERMODYNAMICS and
the second by KINETICS.
Have already seen a number of “driving forces”
for reactions that are PRODUCT-FAVORED.
• formation of a precipitate
• gas formation
• H2O formation (acid-base reaction)
• electron transfer in a battery
CHEMICAL REACTIVITY
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But ENERGY TRANSFER also allows us to predict
reactivity.
In general, reactions that transfer energy to their
surroundings are product-favored.
So, let us consider heat transfer in chemical
processes.
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19.1 SPONTANEOUS REACTIONS AND
SPEED:
THERMODYNAMICS VERSUS KINETICS
• A spontaneous or product-favored reaction
is one in which most of the reactants can
eventually be converted into products,
given sufficient time.
• A non-spontaneous or reactant-favored
reaction is one in which little of the
reactants will be converted into products,
regardless of the time allowed.
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THERMODYNAMICS VERSUS KINETICS
• This can also be expressed in another way.
• Reactant-favored reactions are those in
which the products will be converted to
reactants, given sufficient time.
• Notice that the speed of the reaction is not
an issue.
• Reaction speed is kinetics, Chapter 15.
• We are studying thermodynamics.
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Entropy and Free Energy
How to predict if a reaction
can occur, given enough
time? THERMODYNAMICS
How to predict if a reaction
can occur at a reasonable
rate? KINETICS
Thermodynamics
• Is the state of a chemical system such
that a rearrangement of its atoms and
molecules would decrease the energy of
the system?
• If yes, system is favored to react — a
product-favored system.
• Most product-favored reactions are
exothermic.
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Thermodynamics
• Most product-favored reactions
are exothermic.
• Often referred to as spontaneous
reactions.
• Spontaneous does not imply
anything about time for reaction to
occur.
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Thermodynamics and Kinetics
Diamond is
thermodynamically
favored to convert to
graphite, but not
kinetically favored.
Paper burns — a productfavored reaction. Also
kinetically favored once
reaction begins.
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Product-Favored
Reactions
In general, productfavored reactions are
exothermic.
Fe2O3(s) + 2 Al(s) --->
2 Fe(s) + Al2O3(s)
ΔH = - 848 kJ
Thermite Reaction
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Product-Favored
Reactions
But many spontaneous reactions or processes
are endothermic or even have ΔH = 0.
NH4NO3(s) + heat
NH4NO3(aq)
19.2 DIRECTIONALITY OF
REACTIONS: ENTROPY
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• Spontaneous reactions occur because they
generate a final state that is lower in energy,
that is energy dispersed, and/or a final state
that is more random or more disordered.
• The first condition is met by reactions that are
exothermic.
–These reactions release heat to the universe
resulting in more particles, molecules and/or
atoms, having the energy that was originally
concentrated on the reactants.
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Entropy, S
One property common to
product-favored processes is
that the final state is more
DISORDERED or RANDOM
than the original.
Reaction of K
with water
SPONTANEITY IS
RELATED TO AN
INCREASE IN
RANDOMNESS.
The thermodynamic property related to
randomness is ENTROPY, S.
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Entropy: A Measure of Matter
Dispersal or Disorder
• A perfect crystal at 0 Kelvin has no
randomness or disorder. This statement is
called the third law of thermodynamics.
• The thermodynamic function that represents
the randomness of matter is called entropy
and is given the symbol S.
• If energy is added to matter in such a way
that there is essentially no temperature
change we can calculate the change in
entropy: ΔS = q/T
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Entropy
• By adding up all these small changes from
absolute zero to any temperature, T, the
absolute entropy, So of the substance at that
temperature can be calculated.
• Appendix L has a list of these values for
several pure substances at 298.15 K.
• The units on So are e.u. or J/mole K.
• Table 20.1, page 917, is useful in identifying
some general trends in entropy.
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Entropy For similar substances:
• S gas > S liquid > S solid
• S complex molecules > S simple molecules
• S weak ionic bonds > S strong ionic bonds
• S solution of solid or liquid > S solute
• *S solution of gas < S solute
+ solvent
+ solvent
*volume of area is constricted for a gas when it is in a liquid
note the less than symbol
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The entropy of
liquid water is
greater than
the entropy of
solid water
(ice) at 0° C.
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Directionality of
Reactions
How probable is it that reactant
molecules will react?
PROBABILITY suggests that a
product-favored reaction will
result in the dispersal of
energy or of matter or both.
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Directionality of
Reactions
Probability suggests that a product-favored
reaction will result in the dispersal of
energy or of matter or both.
Matter Dispersal
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Directionality of Reactions
Probability suggests that a productfavored reaction will result in the
dispersal of energy or of matter or both.
Energy Dispersal
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Directionality of Reactions
—
Energy Dispersal
Exothermic reactions involve a release of
stored chemical potential energy to the
surroundings.
The stored potential energy starts out in a few
molecules but is finally dispersed over a great
many molecules.
The final state—with energy dispersed—is
more probable and makes a reaction productfavored.
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Entropy, S
S (gases) > S (liquids) > S (solids)
So (J/K•mol)
H2O(liq)
69.91
H2O(gas) 188.8
Entropy, S
Entropy of a substance
increases with temperature.
Molecular motions
of heptane, C7H16
Molecular motions of
heptane at different
temperatures.
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Entropy, S
Increase in molecular complexity
generally leads to increase in S.
So (J/K•mol)
CH4
248.2
C2H6
336.1
C3H8
419.4
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Entropy, S
Entropies of ionic solids depend
on coulombic attractions.
So (J/K•mol)
MgO
26.9
NaF
51.5
Entropy, S
Entropy usually increases
when a pure liquid or solid
dissolves in a solvent.
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Entropy Changes for Phase Changes
For a phase change,
ΔS = q/T
where q = heat
transferred in phase
change
For H2O (liq) ---> H2O(g)
ΔH = q = +40,700 J/mol
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Entropy Changes for Phase Changes
For a phase change,
ΔS = q/T
where q = heat transferred in
phase change
For H2O (liq) ---> H2O(g)
ΔH = q = +40,700 J/mol
q
40, 700 J/mol
=
= + 109 J/K • mol
DS =
T
373.15 K
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Entropy
• The entropy change for a change of state is
calculated using the equation q/T, which
becomes ΔHfus / To for the fusion process.
• See O.H. # 89 for graphical and equation
information.
• The ΔHovap for Al is 326 kJ/mole. The normal
boiling point is 2467oC. Calculate the entropy
of vaporization, ΔSovap , for Al. 119 e.u.
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Entropy
Predict the sign of ΔS for each reaction below:
X(g)
===> X(liq) -
X(s)
===> X(liq) +
X(g)
===> X(aq) -
X(liq) ===> X(aq) +
X(g) ===> X(s)
-
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Calculating DS for a Reaction
D So =
 So (products) -  So (reactants)
Consider 2 H2(g) + O2(g) ---> 2 H2O(liq)
ΔSo = 2 So (H2O) - [2 So (H2) + So (O2)]
ΔSo = 2 mol (69.9 J/K•mol) [2 mol (130.7 J/K•mol) +
1 mol (205.3 J/K•mol)]
ΔSo = -326.9 J/K
Note that there is a decrease in S because 3 mol
of gas give 2 mol of liquid.
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Figure 20.7
2 NO + O2
2 NO2
3 moles gas form
2 moles gas.
DS is negative.
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Entropy: Second Law of
Thermodynamics
• The second law states that the
entropy of the universe is increasing.
• For spontaneous, product-favored,
reactions, ΔSouniverse > 0 .
• This entropy change is calculated by
considering the two terms that make
up this entropy.
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Second Law of Thermodynamics
ΔSouniverse = ΔSo surroundings + ΔSosystem , where
ΔSosurroundings = qsurroundings / T = - ΔHosystem / T
and
Δ Sosystem= ΔSo(products) - ΔSo(reactants),
Equation 20.1.
Be sure to include the stoichiometric coefficient
with each term.
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2nd Law of Thermodynamics
A reaction is spontaneous (product-favored) if ΔS
for the universe is positive.
ΔSuniverse = ΔSsystem + ΔSsurroundings
ΔSuniverse > 0 for product-favored process
First calculate entropy created by matter dispersal
(ΔSsystem)
Next, calculate entropy created by energy
dispersal (ΔSsurround)
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2nd Law of Thermodynamics
Dissolving NH4NO3 in
water—an entropy
driven process.
2nd Law of Thermodynamics
2 H2(g) + O2(g) ---> 2 H2O(liq)
ΔSosystem = -326.9 J/K
DS
o
surroundings
qsurroundings
-DH system
=
=
T
T
ΔHorxn = Δ Hosystem = -571.7 kJ
DS
o
surroundings
- (-571.7 kJ)(1000 J/kJ)
=
298.15 K
ΔSosurroundings = +1917 J/K
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2nd Law of Thermodynamics
2 H2(g) + O2(g) ---> 2
H2O(liq)
ΔSosystem = -326.9 J/K
ΔSosurroundings = +1917 J/K
ΔSouniverse = +1590. J/K
The entropy of the
universe is increasing,
so the reaction is
product-favored.
Enthalpy driven.
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Second Law of Thermodynamics
• Table 19.2, page 804, shows how
ΔHsystem and ΔSsystem can be used to
predict the spontaneity of a reaction
(product-favored).
• There are four possible cases which we
will consider in another format using a
new thermodynamic function G.