Contributed by:
The highlights are:
1. Entropy and entropy changes
2. Second law of thermodynamics
3. Third law of thermodynamics
4. Gibbs free energy
5. Free energy changes
6. Free energy and temperature
1.
First Law of Thermodynamics
• You will recall from Chapter 5 that
energy cannot be created nor
destroyed.
• Therefore, the total energy of the
universe is a constant.
• Energy can, however, be converted
from one form to another or transferred
from a system to the surroundings or
vice versa. Chemical
Thermodynamics
2.
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
Chemical
Thermodynamics
3.
Spontaneous Processes
Processes that are
spontaneous in one
direction are
nonspontaneous in
the reverse
direction.
Chemical
Thermodynamics
4.
Spontaneous Processes
• Processes that are spontaneous at one
temperature may be nonspontaneous at other
temperatures.
• Above 0C it is spontaneous for ice to melt.
• Below 0C the reverse process is spontaneous.
Chemical
Thermodynamics
5.
Reversible Processes
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.
Chemical
Thermodynamics
6.
Irreversible Processes
• Irreversible processes cannot be undone by
exactly reversing the change to the system.
• All Spontaneous processes are irreversible.
• All Real processes are irreversible.
Chemical
Thermodynamics
7.
Entropy
• Entropy (S) is a term coined by Rudolph
Clausius in the 19th century.
• Clausius was convinced of the
significance of the ratio of heat
delivered and the temperature at which
it is delivered, q
T
Chemical
Thermodynamics
8.
Entropy
• Entropy can be thought of as a measure
of the randomness of a system.
• It is related to the various modes of
motion in molecules.
Chemical
Thermodynamics
9.
Entropy
• Like total energy, E, and enthalpy, H,
entropy is a state function.
• Therefore,
S = Sfinal Sinitial
Chemical
Thermodynamics
10.
Entropy
• For a process occurring at constant
temperature (an isothermal process):
qrev = the heat that is transferred when the
process is carried out reversibly at a constant
temperature.
T = temperature in Kelvin.
Chemical
Thermodynamics
11.
Second Law of Thermodynamics
The second law of thermodynamics:
The entropy of the universe does not
change for reversible processes
and
increases for spontaneous processes.
Reversible (ideal):
Irreversible (real, spontaneous):
Chemical
Thermodynamics
12.
Second Law of Thermodynamics
“You can’t break even”
Reversible (ideal):
Irreversible (real, spontaneous):
Chemical
Thermodynamics
13.
Second Law of Thermodynamics
The entropy of the universe increases (real,
spontaneous processes).
But, entropy can decrease for individual systems.
Reversible (ideal):
Irreversible (real, spontaneous):
Chemical
Thermodynamics
14.
Entropy on the Molecular Scale
• Ludwig Boltzmann described the concept of
entropy on the molecular level.
• Temperature is a measure of the average
kinetic energy of the molecules in a sample.
Chemical
Thermodynamics
15.
Entropy on the Molecular Scale
• Molecules exhibit several types of motion:
Translational: Movement of the entire molecule from
one place to another.
Vibrational: Periodic motion of atoms within a molecule.
Rotational: Rotation of the molecule on about an axis or
rotation about bonds.
Chemical
Thermodynamics
16.
Entropy on the Molecular Scale
• Boltzmann envisioned the motions of a sample of
molecules at a particular instant in time.
This would be akin to taking a snapshot of all the
molecules.
• He referred to this sampling as a microstate of the
thermodynamic system.
Chemical
Thermodynamics
17.
Entropy on the Molecular Scale
• Each thermodynamic state has a specific number of
microstates, W, associated with it.
• Entropy is
S = k lnW
where k is the Boltzmann constant, 1.38 1023 J/K.
Chemical
Thermodynamics
18.
Entropy on the Molecular Scale
• more particles
-> more states -> more entropy
• higher T
-> more energy states -> more entropy
• less structure (gas vs solid)
-> more states -> more entropy
Chemical
Thermodynamics
19.
Entropy on the Molecular Scale
• The number of microstates and,
therefore, the entropy tends to increase
with increases in
Temperature.
Volume (gases).
The number of independently moving
molecules.
Chemical
Thermodynamics
20.
Entropy and Physical States
• Entropy increases with
the freedom of motion
of molecules.
• Therefore,
S(g) > S(l) > S(s)
Chemical
Thermodynamics
21.
Solutions
Dissolution of a solid:
Ions have more entropy
(more states)
But,
Some water molecules
have less entropy
(they are grouped
around ions).
Usually, there is an overall increase in S.
(The exception is very highly charged ions that Chemical
Thermodynamics
make a lot of water molecules align around them.)
22.
Entropy Changes
• In general, entropy
increases when
Gases are formed from
liquids and solids.
Liquids or solutions are
formed from solids.
The number of gas
molecules increases.
The number of moles
increases. Chemical
Thermodynamics
23.
Third Law of Thermodynamics
The entropy of a pure crystalline
substance at absolute zero is 0.
Chemical
Thermodynamics
24.
Third Law of Thermodynamics
The entropy of a pure crystalline
substance at absolute zero is 0.
No stereotypes,
Entropy: labels, or
Smiles for genres can
rationalize this.
stab wounds Fueled by the
2004 decay of the
world, order
and chaos
unite, Entropy
is born... Chemical
Music to make
Thermodynamics
your head
25.
Standard Entropies
• These are molar entropy
values of substances in
their standard states.
• Standard entropies tend
to increase with
increasing molar mass.
Chemical
Thermodynamics
26.
Standard Entropies
Larger and more complex molecules have
greater entropies.
Chemical
Thermodynamics
27.
Entropy Changes
Entropy changes for a reaction can be
calculated the same way we used for H:
S° for each component is found in a table.
Note for pure elements:
Chemical
Thermodynamics
28.
Practical uses: surroundings & system
Entropy Changes in Surroundings
• Heat that flows into or out of the system
also changes the entropy of the
surroundings.
• For an isothermal process:
Chemical
Thermodynamics
29.
Practical uses: surroundings & system
Entropy Changes in Surroundings
• Heat that flows into or out of the system also changes
the entropy of the surroundings.
• For an isothermal process:
• At constant pressure, qsys is simply
H for the system.
Chemical
Thermodynamics
30.
Link S and H: Phase changes
A phase change is isothermal
(no change in T).
Entropysystem
For water:
Hfusion = 6 kJ/mol
Hvap = 41 kJ/mol
Chemical
If we do this reversibly: Ssurr = –Ssys Thermodynamics
31.
Practical uses: surroundings & system
Entropy Change in the Universe
• The universe is composed of the system and
the surroundings.
Therefore,
Suniverse = Ssystem + Ssurroundings
• For spontaneous processes
Suniverse > 0
Chemical
Thermodynamics
32.
Practical uses: surroundings & system
Chemical
= – Gibbs Free Energy Thermodynamics
33.
Practical uses: surroundings & system
= – Gibbs Free Energy
Make this equation nicer:
Chemical
Thermodynamics
34.
Practical uses: surroundings & system
…Gibbs Free Energy
TSuniverse is defined as the Gibbs free
energy, G.
For spontaneous processes: Suniverse > 0
And therefore: G < 0
G is easier to determine than Suniverse.
Chemical
Use G to decide if a process is spontaneous.
Thermodynamics
35.
Gibbs Free Energy
1. If G is negative, the
forward reaction is
spontaneous.
2. If G is 0, the system
is at equilibrium.
3. If G is positive, the
reaction is spontaneous
in the reverse direction.
Chemical
Thermodynamics
36.
Standard Free Energy Changes
Standard free energies of formation, Gf
are analogous to standard enthalpies of
formation, Hf.
G can be looked up in tables,
or
calculated from S° and H.
Chemical
Thermodynamics
37.
Free Energy Changes
Very key equation:
This equation shows how G changes with
temperature.
(We assume S° & H° are independent of T.)
Chemical
Thermodynamics
38.
Free Energy and Temperature
• There are two parts to the free energy
equation:
H— the enthalpy term
TS — the entropy term
• The temperature dependence of free
energy comes from the entropy term.
Chemical
Thermodynamics
39.
Free Energy and Temperature
By knowing the sign (+ or -) of S and H,
we can get the sign of G and determine if a
reaction is spontaneous.
Chemical
Thermodynamics
40.
Free Energy and Equilibrium
Remember from above:
If G is 0, the system is at equilibrium.
So G must be related to the equilibrium
constant, K (chapter 15). The standard free
energy, G°, is directly linked to Keq by:
Chemical
Thermodynamics
41.
Free Energy and Equilibrium
Under non-standard conditions, we need to use
G instead of G°.
Q is the reaction quotiant from chapter 15.
Note: at equilibrium: G = 0.
away from equil, sign of G tells which way rxn goes
Chemical
spontaneously. Thermodynamics
42.
Gibbs Free Energy
1. If G is negative, the
forward reaction is
spontaneous.
2. If G is 0, the system
is at equilibrium.
3. If G is positive, the
reaction is spontaneous
in the reverse direction.
Chemical
Thermodynamics
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