Spontaneity, Entropy,
&
Gibbs Free Energy
GHS Honors Chem
Spontaneous Processes
Spontaneous processes
are those that can
proceed without any
outside intervention.
‹ The gas in vessel B will
spontaneously effuse
into vessel A, but once
the gas is in both
vessels, it will not
spontaneously reverse
‹
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What is a Spontaneous
Process?
• Melting Ice?
• Freezing of Liquid Water?
• Rusting of Iron?
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Spontaneous Processes
Processes that are
spontaneous in
one direction are
nonspontaneous in
the reverse
direction.
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Reversible Processes
Spontaneous Processes
Processes that are spontaneous at one
temperature may be nonspontaneous at other
temperatures.
‹ Above 0°C it is spontaneous for ice to melt.
‹ Below 0°C the reverse process is spontaneous.
In a reversible process
the system changes in
such a way that the
system and surroundings
can be put back in their
original states by exactly
reversing the process.
‹
Changes are
infinitesimally small in a
reversible process.
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1
What Contributes to
Spontaneity?
Irreversible Processes
• Change in Enthalpy (∆H):
• Is heat/energy absorbed or given off
• Change in Entropy (∆S):
• Entropy is a measure of randomness or
disorder
Irreversible processes cannot be undone by
exactly reversing the change to the system.
‹ All Spontaneous processes are irreversible.
• Temperature of the Reaction (T):
GHS Honors Chem
GHS Honors Chem
‹
Enthalpy’s Contribution to Spontaneity
• Many spontaneous processes proceed with
a DECREASE in energy, and are Exothermic
(produces heat) at 250C and 1 atm. (STP)
• 2H2(g) + O2(g) Æ 2H2O(l) ∆H = - 571.6 kJ
But it’s not a direct
correlation, or a
perfect fit …
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Enthalpy’s Contribution to Spontaneity
• Endothermic (takes in heat) reactions that
are non-spontaneous at room temp, often
become spontaneous at higher
temperatures
So ∆H’s
contribution is tied
to the temperature
of the reaction …
GHS Honors Chem
• What is the temperature, or better said, the
kinetic energy of the particles in the reaction
Enthalpy’s Contribution to Spontaneity
‹
Endothermic (takes in heat) reactions that
are non-spontaneous at room temp, often
become spontaneous at higher
temperatures (Increase in energy often
increases spontaneity).
CaCO3(s) Æ CaO(s) + CO2 ∆H = +17803kJ
‹
The decomposition of limestone occurs at
1100 K and 1atm. At 250C & 1atm this
reaction does not occur.
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What about Entropy?
What is it?
Entropy can be thought of as
a measure of the randomness
of a system.
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2
What about Entropy?
What is it?
What are the variables
that affect Entropy?.
• First law of thermodynamics: Energy in the
Universe is a constant. Energy cannot be
created nor destroyed.
• Second law of thermodynamics: Randomness or
disorder in the Universe is increasing.
• What is Entropy (S)? It is measure of molecular
randomness or disorder. Change in entropy is it
denoted by the symbol ∆S.
GHS Honors Chem
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Entropy on the Molecular Level
What Increases Entropy?
• Adding Particles: adding more
particles increases the
collisions, and the randomness
of motion
• Adding Energy/Increasing
Temperature: velocity of
particle motions is increased.
• Increasing Volume: particles
are allowed to roam in greater
space, more random motion
How is Entropy Affected by Physical
States?
‹
Entropy increases with the freedom of motion
of molecules: S(g) > S(l) > S(s)
http://www.wwnorton.com/chemistry/tutorials/ch13.htm
GHS Honors Chem
GHS Honors Chem
What happens to Entropy in
Solutions?
Dissolution of a solid:
• Ions have more
entropy
• But, some water
molecules have less
entropy (they are
grouped around ions).
Third Law of Thermodynamics
•
•
At absolute zero, a pure substance exists as a
perfect crystal, with no molecular
movements (no kinetic energy).
The entropy of a pure crystalline substance at
absolute zero is 0.
Usually, there is an overall increase in S.
(The exception is very highly charged ions that make a
lot of water molecules align around them.)
GHS Honors Chem
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3
For a chemical reaction, that change in
entropy (∆S) under standard conditions is
calculated as:
Standard Molar Entropies
•
•
Now I know you’re wondering … where do
I get these standard Molar Entropy
Values??
GHS Honors Chem
Whip out your Thermochemical Tables!!
These are molar
entropy values of
substances in
their standard
states.
Standard
entropies tend to
increase with
increasing molar
mass.
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Trends in Standard Entropies
• Larger and more complex molecules
have greater entropies.
Entropy Worksheet
• Note for pure elements:
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How Are Enthalpy (∆H) & Entropy
(∆S) Related?
Introducing Gibbs Free Energy
Gibbs Free Energy & Spontaneity
• Gibbs Free Energy is defined as the
energy in a system that is available
to do useful work.
Can the sign of ∆G tell us whether a reaction
is spontaneous?
Or
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A + B
1.
2.
Æ
C
If ∆G is negative, the forward reaction
is spontaneous.
If ∆G is positive, the reaction is
spontaneous in the reverse direction.
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Calculating ∆G
1.
‹
Calculating ∆G
Standard free energies of formation, ∆Gf°
are analogous to standard enthalpies of
formation, ∆Hf°. These are located on your
Thermochemical Data Chart.
∆G can be calculated from ∆H and ∆S:
• This relationship holds true as long as the
temperature and pressure remain constant
during the reaction.
• There are two parts to the free energy equation:
• ∆H°— the enthalpy term
• T∆S° — the entropy term
• The temperature dependence of free energy
comes from the entropy term.
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GHS Honors Chem
The Effect of Temperature on
∆G and Spontaneity
By knowing the sign (+ or -) of ∆S and
∆H, we can get the sign of ∆G and
determine if a reaction is spontaneous.
• High T favors the Entropy term
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• Low T favors the Enthalpy term
Calculating ∆G Problem
Calculating ∆G
• The ∆Hrxn can be calculated from ∆Hf at
standard temperature and pressure
• The ∆Srxn is also calculated from the ∆S’s at
standard temperature and pressure.
• The Gibbs Equation allows you to calculate the
∆G, and verify reaction spontaneity, at other
Temperatures.
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Gibbs Free Energy & Spontaneity
1.
C3H6(l) + O2(g) Æ
CO2(g)
+
H2O(g)
2.
• Using the standard enthalpies and entropies of
formation, determine whether the reaction is
spontaneous at -20oC.
• Balance first ….
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3.
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If ∆G is negative,
the forward
reaction is
spontaneous.
If ∆G is 0, the
system is at
equilibrium.
If ∆G is positive,
the reaction is
spontaneous in
the reverse
direction.
5
Gibbs Free Energy Worksheet
&
Free Energy Practice Problems
GHS Honors Chem
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