Bài giảng Chapter 17 Free Energy and Thermodynamics

Chapter 17
Free Energy
and
Thermodynamics
2011, NKMB Co., Ltd.
Chemistry, Julia Burdge, 2
st
Ed.
McGraw Hill.
Mr. Truong Minh Chien ;
/>
Don’t get ahead of the game
•
There is a lot of theory in this chapter
•
Keep terms separate (5), and 4 equations.

Universe, System, Surrounds, Enthalpy, and Entropy
are all different, and you must know each
•
Memorize the 3 Laws of Thermodynamics
•
You can break a California State Law but you
can NOT break a Thermodynamic Law
2
Chemistry, Julia Burdge, 2
nd
e., McGraw Hill.
3
First Law of Thermodynamics
•

you can’t 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
•
∆E
universe
= 0 = ∆E
system
+ ∆Ε
surroundings
•
Think of a match burning, crumbling paper, etc
Chemistry, Julia Burdge, 2
nd
e., McGraw Hill.
4
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 + work done

∆E = q + w

∆E = ∆H + P∆V
•
∆E is a state function

internal energy change independent of how done
Chemistry, Julia Burdge, 2
nd
e., McGraw Hill.
Tro, Chemistry: A Molecular Approach 5
Energy Tax
•
you can’t break even!
•
to recharge 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
∆E = q + w
∆E = ∆H + P∆V
Tro, Chemistry: A Molecular Approach 6
Heat Tax
fewer steps
generally results
in a lower total
heat tax
Tro, Chemistry: A Molecular Approach 7
Thermodynamics and Spontaneity
•
thermodynamics predicts whether a process will
proceed under the given conditions

spontaneous process

nonspontaneous processes require energy input to go
•
spontaneity is determined by comparing the free
energy of the system before the reaction with the free
energy of the system after reaction.

if the system after reaction has less free energy than
before the reaction, the reaction is
thermodynamically favorable.
•
spontaneity ≠ fast or slow
Tro, Chemistry: A Molecular Approach 8
Comparing Potential Energy

The direction of
spontaneity can
be determined by
comparing the
potential energy
of the system at
the start and the
end.
Tro, Chemistry: A Molecular Approach 9
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 the opposite direction
Tro, Chemistry: A Molecular Approach 10
Thermodynamics vs. Kinetics
Tro, Chemistry: A Molecular Approach 11
Diamond → Graphite
Graphite is more stable than diamond, so the conversion of
diamond into graphite is spontaneous – but don’t worry, it’s

so slow that your ring won’t turn into pencil lead in your
lifetime (or through many of your generations).
Tro, Chemistry: A Molecular Approach 12
Factors Affecting Whether a
Reaction Is Spontaneous
•
The two factors that determine the thermodynamic
favorability are the enthalpy and the entropy.
•
The enthalpy is a comparison of the bond energy
of the reactants to the products.

bond energy = amount needed to break a bond.

∆H
•
The entropy factors relates to the
randomness/orderliness of a system

∆S
•
The enthalpy factor is generally more important
than the entropy factor
Tro, Chemistry: A Molecular Approach 13
Enthalpy
•
related to the internal energy
•
∆H generally kJ/mol
•

stronger bonds = more stable molecules
•
if products more stable than reactants, energy released

exothermic

∆H = negative
•
if reactants more stable than products, energy absorbed

endothermic

∆H = positive
•
The enthalpy is favorable for exothermic reactions and
unfavorable for endothermic reactions.
•
Hess’ Law ∆H°
rxn
= Σ(∆H°
prod
) - Σ(∆H°
react
)
Substance
∆
H°
kJ/mol
Substance
∆

H°
kJ/mol
Al(s) 0 Al
2
O
3
-1669.8
Br
2
(l) 0 Br
2
(g) +30.71
C(diamond) +1.88 C(graphite) 0
CO(g) -110.5 CO
2
(g) -393.5
Ca(s) 0 CaO(s) -635.5
Cu(s) 0 CuO(s) -156.1
Fe(s) 0 Fe
2
O
3
(s) -822.16
H
2
(g) 0 H
2
O
2
(l) -187.8

H
2
O(g) -241.82 H
2
O(l) -285.83
HF(g) -268.61 HCl(g) -92.30
HBr(g) -36.23 HI(g) +25.94
I
2
(s) 0 I
2
(g) +62.25
N
2
(g) 0 NH
3
(g) -46.19
NO(g) +90.37 NO
2
(g) +33.84
Na(s) 0 O
2
(g) 0
S(s) 0 SO
2
(g) -296.9


Tro, Chemistry: A Molecular Approach 15
Entropy

•
entropy is a thermodynamic function that increases as the
number of energetically equivalent ways of arranging the
components increases, S
•
S generally J/mol
•
S = k ln W

k = Boltzmann Constant = 1.38 x 10
-23
J/K

W is the number of energetically equivalent ways, unitless
Tro, Chemistry: A Molecular Approach 16
W (microstates possible)
Energetically
Equivalent States for
the Expansion of a
Gas
Tro, Chemistry: A Molecular Approach 17
Macrostates → Microstates
This macrostate can be achieved through
several different arrangements of the particles
These microstates
all have the same
macrostate
So there are 6
different particle
arrangements that

result in the same
macrostate