Entropy
Entropy
Entropy is defined
as a state of
disorder or
randomness.
In general the
universe tends to
move toward
release of energy
and greater entropy.
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Entropy
The statistical
interpretation of
thermodynamics was
pioneered by James
Clerk Maxwell (1831–
1879) and brought to
fruition by the Austrian
physicist Ludwig
Boltzmann (1844–1906).
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Entropy
Spontaneous chemical
processes often result in a
final state is more Disordered
or Random than the original.
The Spontaneity of a
chemical process is related to
a change in randomness.
Entropy is a thermodynamic
property related to the
degree of randomness or
disorder in a system.
Reaction of potassium
metal with water. The
products are more
randomly distributed
than the reactants
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Entropy and
Thermodynamics
According to the second law or
thermodynamics the entropy of the
universe is always increasing.
This is true because there are many more
possibilities for disorder than for order.
Entropy
Royal
Flush
Nothing
hand
Entropy is Disorder
Disorder in a system can take many forms.
Each of the following represent an increase in
disorder and therefore in entropy:
1. Mixing different types of particles. i.e.
dissolving salt in water.
2. A change is state where the distance between
particles increases. Evaporation of water.
3. Increased movement of particles. Increase in
temperature.
4. Increasing numbers of particles. Ex.
2 KClO3  2 KCl + 3O2
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Entropy States
The greatest increase in entropy is usually
found when there is an increase of
particles in the gaseous state.
The symbol for the change in disorder or
entropy is given by the symbol, DS.
The more disordered a system becomes
the more positive the value for DS will be.
Systems that become more ordered have
negative DS values.
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Entropy, S
The entropy of a substance depends
on its state:
S (gases) > S (liquids) > S (solids)
So (J/K-1mol-1)
H2O (liquid)
69.95
H2O (gas)
188.8
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Entropy and States of Matter
S˚(Br2 liquid) < S˚(Br2 gas)
S˚(H2O solid) < S˚(H2O liquid)
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Entropy, Phase & Temperature
S increases
slightly with T
S increases a
large amount
with phase
changes
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Entropy and Temperature
The Entropy of a substance increases with
temperature.
Molecular motions
of heptane, C7H16
Molecular motions of
heptane at different temps.
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Entropy - Boltzman
Ludwig Boltzman
S = k Ln W
Entropy is proportional
to the number of degrees
of freedom or possible
configurations in a
system.
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Standard Entropy Values
The standard entropy, DSo, of a substance
is the entropy change per mole that occurs
when heating a substance from 0 K to the
standard temperature of 298 K.
Unlike enthalpy, absolute entropy changes
can be measured.
Like enthalpy, entropy is a state function.
The change in entropy is the difference
between the products and the reactants
DSo = S So (products) - S So (reactants)
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Standard Entropy Values
Some standard enthalpy values
The amount of entropy in a
pure substance depends on
the temperature, pressure,
and the number of molecules
in the substance.
Values for the entropy of
many substances at have
been measured and tabulated.
The standard entropy is also
measured at 298 K.
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Gibbs Free Energy
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Spontaneity
A chemical reaction is spontaneous if it
results in the system moving form a less
stable to a more stable state.
Decreases in enthalpy and increases in
entropy move a system to greater stability.
The combination of the enthalpy factor and
the entropy factor can be expressed as the
Gibbs Free Energy.
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Gibbs Free Energy
The standard free energy change is defined
by this equation
DGo = DHo – T DSo
Where
DHo = the enthalpy change
DSo = the entropy change
T = Kelvin temperature
A chemical reaction is
spontaneous if it results
in a negative free energy change.
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Gibbs Free Energy
Possible Combinations for free energy change:
DGo = DHo – T DSo
DG
DH
DS
DH-TDS
Always
Spontaneous
Never
Spontaneous
< 0 (-) < 0 (-) > 0 (+)
Always (-)
> 0 (+) > 0 (+) < 0 (-)
Always (+)
Spontaneous at
High Temperature
< 0 (-)
> 0 (+) > 0 (+) > 0 (+)
Spontaneous at
Low Temperature
> 0 (+)
< 0 (-) < 0 (-)
< 0 (-)
(-) if T large
(+) if T small
(+) if T large
(-) if T small
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Free Energy Problem 1
A certain chemical reaction is exothermic with a
standard enthalpy of - 400 kJ mol-1. The entropy
change for this reaction is +44 J mol-1 K-1.
Calculate the free energy change for this reaction
at 25 oC. Is the reaction spontaneous?
Solution
Convert the entropy value to kJ. 44 J mol-1 K-1 = 0.044
kJ mol-1 K-1
DG = - 400 kJ mol-1 – (298 K)(0.044 kJ mol-1 K-1)
DG = - 400 kJ mol-1 – 13.1 kJ mol-1
DG = - 413.1 kJ mol-1 .
Since DG is negative the
reaction is spontaneous.
Note. Because DH <0 and DS >0, this reaction is spontaneous
at all temperatures.
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Free Energy Problem 2
A certain chemical reaction is endothermic with a
standard enthalpy of +300 kJ mol-1. The entropy
change for this reaction is +25 J mol-1 K-1.
Calculate the free energy change for this reaction
at 25 oC. Is the reaction spontaneous?
Solution
Convert the entropy value to kJ. 25 J mol-1 K-1 = 0.025
kJ mol-1 K-1
DG = + 300 kJ mol-1 – (298 K)(0.025 kJ mol-1 K-1)
DG = + 300 kJ mol-1 – 7.45 kJ mol-1
DG = + 292.55 kJ mol-1 .
Since DG is positive the
reaction is non-spontaneous.
Note. Because DH >0 and DS >0, this reaction is nonspontaneous at low temperatures. It the temperature were
substantially increased it would become spontaneous.
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