Transcript thermodynamics - CHM152-SP10

THERMODYNAMICS
Internal Energy
Enthalpy
Entropy
Free Energy
Chapter 17 (McM)
Chapter 20 Silberberg
Goals & Objectives
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See the following Learning
Objectives on page 914.
Understand these Concepts:
20.1-22.
Master the Skills:
20.1-10.
Thermodynamics
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the study of the changes in energy
and the transfers of energy that
accompany chemical and physical
processes.
Addresses three fundamental
questions
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Will 2 or more substances react when they
are mixed under specified conditions?
If they do react, what energy changes and
transfers are associated with their
reaction?
If a reaction occurs, to what extent does it
occur?
Thermodynamics
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Used to determine if a reaction can
occur under specified conditions.
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spontaneous reaction--can occur under
the specified conditions
nonspontaneous reaction--do not occur
to a significant extent under the
specified conditions
First Law of Thermodynamics
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The internal energy of an isolated
system is constant.
The total amount of energy in the
universe is constant.
Some Thermodynamic Terms
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System--the substances involved in
the chemical and physical changes
under investigation
Surroundings--the rest of the
universe
Universe--the system and its
surroundings
Types of Thermodynamic Systems
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Open system--can exchange both
matter and energy with its
surroundings
Closed system--has a fixed amount
of matter but can exchange energy
with its surroundings
Isolated system--has no contact
with its surroundings
Thermodynamic State of a
System
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defined by a set of conditions that
completely specifies all the
properties of the system
State functions--the properties of a
system( pressure, temperature,
energy, e.g.) whose values depend
only on the state of the system
Changes in Internal Energy,DE
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Internal energy represents the total
energy of a system.
DE = q(heat flow) + w(work)
Work is usually defined as PDV
If the work term is 0 (no work
done) then at constant volume DE =
q
Limitations of the First Law of Thermodynamics
DE = q + w
Euniverse = Esystem + Esurroundings
DEsystem = -DEsurroundings
DEsystem + DEsurroundings = 0 =
DEuniverse
The total energy-mass of the universe is constant.
However, this does not tell us anything about the direction of
change in the universe.
Enthalpy
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The change in enthalpy (DH) is
measured at constant P.
At constant P: DH = q
Figure 20.1
A spontaneous endothermic chemical reaction.
water
Ba(OH)2.8H2O(s) + 2NH4NO3(s)
Ba2+(aq) + 2NO3-(aq) + 2NH3(aq) + 10H2O(l)
DH0rxn = +62.3 kJ
Enthalpy Change
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DH = SDHfo (products) SDHfo(reactants)
where DHfo is the standard molar
enthalpy of formation and DH is the
enthalpy change for the reaction.
Enthalpy Change
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Calculate the enthalpy change for
the following reaction at 298K.
C3H8(g) + 5O2(g) ----> 3CO2(g) +
4H2O(l)
The Second Law of
Thermodynamics
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In spontaneous changes the
universe tends toward a state of
greater disorder.
In thermodynamics, entropy is a
measure of the degree of disorder.
Entropy tends to increase.
The Second Law of
Thermodynamics
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Likely
The Second Law of
Thermodynamics
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Unlikely
The Concept of Entropy (S)
Entropy refers to the state of order.
A change in order is a change in the number of ways of
arranging the particles, and it is a key factor in determining the
direction of a spontaneous process.
more order
solid
more order
crystal + liquid
more order
crystal + crystal
less order
liquid
gas
less order
ions in solution
less order
gases + ions in solution
Entropy
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Entropy can be indirectly measured.
Absolute standard molar entropy
values can be found in the
textbook.
An increase in entropy corresponds
to an increase in disorder.
When DS is _______, disorder
increases.
When DS is _______, disorder
decreases.
Entropy
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The Third Law of Thermodynamics
states that the entropy of a
pure,perfect,crystalline substance at
0K is zero.
The following relationship applies to
entropy changes.
DS = SSo(products) - SSo(reactants)
Figure 20.4
Random motion in a crystal
The third law of
thermodynamics.
A perfect crystal has
zero entropy at a
temperature of
absolute zero.
Ssystem = 0 at 0 K
Changes in Entropy
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Calculate the entropy change for
the following reaction at 298K.
Indicate whether disorder increases
or decreases.
2NO2(g) -----> N2O4(g)
Free Energy Change, DG
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If heat is released in a chemical
reaction, some of the heat may be
converted to useful work. Some of
it may be used to increase the order
of the universe. If the system
becomes more disordered, then
more energy becomes available
than indicated by enthalpy alone.
The Gibbs Free Energy Change
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At constant T and P
DG = DH - TDS
When DG is > 0, the reaction is
nonspontaneous
When DG is = 0, the reaction is at
equilibrium
When DG is < 0, the reaction is
spontaneous
Gibbs Free Energy Change
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The following relationship exists for
standard molar Gibbs free energy
Gibbs Free Energy Change changes:
DGo = SDGfo(products) SDGfo(reactants)
Gibbs Free Energy Change
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Calculate the Gibbs free energy
change for the following reaction at
298K. Indicate whether the
reaction is spontaneous or
nonspontaneous under these
conditions.
C3H8(g) + 5O2(g) ----> 3CO2(g) +
4H2O(l)
Table 20.1 Reaction Spontaneity and the Signs of DH0, DS0, and DG0
DH0
DS0
-TDS0
DG0
-
+
-
-
Spontaneous at all T
+
-
+
+
Nonspontaneous at all T
+
+
-
+ or -
Spontaneous at higher T;
nonspontaneous at lower T
-
-
+
+ or -
Spontaneous at lower T;
nonspontaneous at higher T
Description
The Gibbs Helmholtz Equation
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Calculate DSo for the following reaction
at 298K.
C3H8(g) + 5O2(g) ----> 3CO2(g) +
4H2O(l)
From previous examples we found
DHo = -2219kJ and
DGo = -2107kJ
Indicate whether disorder increases or
decreases
The Relationship Between DGo
and the Equilibrium Constant
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The standard free energy change
for a reaction is DGo. This is the
free energy change that would
accompany the complete conversion
of all reactants, initially present in
their standard states, to all products
in their standard states. DG is the
free energy change for other
concentrations and pressures.
The Relationship Between DGo
and the Equilibrium Constant
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The relationship between DG and
DGo is
DG = DGo + RTlnQ where
R = universal gas
constant(8.314J/moleK)
T = temperature in K
Q = reaction quotient
The Relationship Between DGo
and the Equilibrium Constant
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When a system is at equilibrium,
DG = 0 and Q = K. Then:
0 = DGo + RTlnK
Rearranging gives
DGo = -RTlnK
Table 20.2 The Relationship Between DG0 and K at 250C
DG0(kJ)
K
100
3x10-18
50
2x10-9
10
2x10-2
1
7x10-1
0
1
-1
1.5
-10
5x101
-50
6x108
-100
3x1017
-200
1x1035
Essentially no forward reaction;
reverse reaction goes to completion
Forward and reverse reactions
proceed to same extent
Forward reaction goes to
completion; essentially no reverse
reaction
REVERSE REACTION
9x10-36
FORWARD REACTION
200
Significance
The Relationship Between DGo
and the Equilibrium Constant
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Calculate the value for the
equilibrium constant, Kp, for the
following reaction at 298K.
N2O4(g)
=
2NO2(g)
At 25oC and 1.00 atmosphere
pressure, Kp=4.3x10-13, for the
decomposition of NO2. Calculate
DGo at 25oC.