Thermodynamics

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Thermodynamics

• Chapter 18

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The First Law of Thermodynamics
q heat w work
E q w heat work Energy
Everything that is not part of the system (sys)
is the surroundings (surr), and vice versa
Euniv Esys Esurr
The total energy of the universe is constant and,
therefore, energy cannot be created or
destroyed.
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Thermodynamics
One of the main objectives of thermodynamics is
to predict whether or not a reaction will occur
when reactants are brought together under
specific conditions. A reaction that does occur
under the specific set of conditions is called a
spontaneous reaction. The reverse of a
spontaneous reaction cannot occur under the same
set of conditions. What kind of rule can we come
up with to characterize spontaneous
processes? Perhaps spontaneous processes decrease
energy.
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Spontaneous Physical and Chemical Processes

• A waterfall runs downhill
• A lump of sugar dissolves in a cup of coffee
• At 1 atm, water freezes below 0 0C and ice melts
  above 0 0C
• Heat flows from a hotter object to a colder
  object
• A gas expands in an evacuated bulb
• Iron exposed to oxygen and water forms rust

18.2
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Sign of H Cannot Predict Spontaneous Change
All combustion reactions are spontaneous and
exothermic
CH4 (g) 2 O2 (g) CO2 (g) 2
H2O(g)
Horxn -802 kJ
Iron rusts spontaneously and exothermically
2 Fe(s) O2 (g) Fe2O3
(s)
Horxn -826 kJ
Ionic compounds form spontaneously from their
elements with a large release of heat
Horxn -411 kJ
Na(s) Cl2 (g) NaCl(s)
At ordinary pressure, water freezes below 0C but
melts above 0C. Both processes are spontaneous,
but the first is exothermic and the second
endothermic.
H2O(l) H2O(s) Horxn -6.02
kJ
(exothermic spontaneous at T lt 0oC) H2O(s)
H2O(l) Horxn 6.02 kJ
(endothermic
spontaneous at T gt 0oC)
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A Spontaneous, Endothermic Chemical Reaction
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Does a decrease in enthalpy mean a reaction
proceeds spontaneously?
Spontaneous reactions
18.2
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spontaneous
nonspontaneous
No energy change! Probability factor.
18.2
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Entropy and the Second Law of Thermodynamics
Entropy (S)Entropy is a measure of the
randomness or disorder of a system
A system with relatively few equivalent ways to
arrange its components, such as a crystalline
solid or a deck of cards in a specific sequence,
has relatively small disorder and low entropy. A
system with many equivalent ways to arrange its
components, such as a gas or a shuffled deck of
cards, has relatively large disorder and high
entropy.
Sdisorder gt Sorder
Ssys Sfinal - Sinitial
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Entropy (S) is a measure of the randomness or
disorder of a system.
DS Sf - Si
If the change from initial to final results in an
increase in randomness
Sf gt Si
DS gt 0
For any substance, the solid state is more
ordered than the liquid state and the liquid
state is more ordered than gas state
Ssolid lt Sliquid ltlt Sgas
DS gt 0
18.2
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Processes that lead to an increase in entropy (DS
gt 0)
18.2
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Predicting Relative Entropy Values
Problem Choose the member with the higher
entropy in each of the following pairs, and
justify your choice. (a) 1 mol NaCl(s) or 1
mol NaCl(aq) (b) 1 mol O2 and 2 mol H2 or 1
mol H2O (c) 1 mol H2O(s) or 1 mol H2O(g)
(d) Liptons noodle soup at 24oC or at 95oC
Is the entropy change greater or less than zero
for (a) freezing ethanol (b) evaporating a
beaker of bromine (c) dissolving urea in water
(d) cooling N2 gas
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How does the entropy of a system change for each
of the following processes?
(a) Condensing water vapor
Randomness decreases
Entropy decreases (DS lt 0)
(b) Forming sucrose crystals from a
supersaturated solution
Randomness decreases
Entropy decreases (DS lt 0)
(c) Heating hydrogen gas from 600C to 800C
Randomness increases
Entropy increases (DS gt 0)
(d) Subliming dry ice
Randomness increases
Entropy increases (DS gt 0)
18.2
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Entropy and the Second Law of Thermodynamics
The Second Law of Thermodynamics - the entropy
of the universe (system surroundings) always
increases in a spontaneous process and remains
unchanged in an equilibrium process. Any process
in which the entropy of the universe would
decrease is forbidden. DSuniv DSsys DSsurr gt
0 spontaneous DSuniv DSsys DSsurr lt 0
forbidden
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Entropy Changes in the System (DSsys)
S0(CO) 197.9 J/Kmol
S0(CO2) 213.6 J/Kmol
S0(O2) 205.0 J/Kmol
18.3
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Entropy Changes in the System (DSsys)
When gases are produced (or consumed)

• If a reaction produces more gas molecules than
  it consumes, DS0 gt 0.
• If the total number of gas molecules diminishes,
  DS0 lt 0.
• If there is no net change in the total number of
  gas molecules, then DS0 may be positive or
  negative BUT DS0 will be a small number.

The total number of gas molecules goes down, DS
is negative.
18.3
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Entropy Changes in the Surroundings (DSsurr)
Exothermic Process DSsurr gt 0
Endothermic Process DSsurr lt 0
18.3
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Third Law of Thermodynamics
The entropy of a perfect crystalline substance is
zero at the absolute zero of temperature.
18.3
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Gibbs Free Energy
DSuniv DSsys DSsurr gt 0
Spontaneous process
DSuniv DSsys DSsurr 0
Equilibrium process
For a constant-temperature process
Gibbs free energy (G)
DG DHsys -TDSsys
DG lt 0 The reaction is spontaneous in the
forward direction.
DG gt 0 The reaction is nonspontaneous as
written. The reaction is
spontaneous in the reverse direction.
DG 0 The reaction is at equilibrium.
18.4
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Entropy and Free Energy
Gibbs free energyThis is a function that
combines the systems
enthalpy and entropy
G H - TS
The change in free energy of a system at constant
temperature and pressure is given by the Gibbs
equation
The Second Law can be stated in terms of G for
the system.
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Reaction Spontaneity and the Signs of Ho,
So, and Go
Ho So -T So Go
Description
- -
- 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
An endothermic reaction can be spontaneous only
if there is an entropy increase (an increase in
disorder).
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Effect of Temperature on Reaction Spontaneity
The temperature at which a reaction occurs
influences the magnitude of the T S term. By
scrutinizing the signs of H and S, we can
predict the effect of temperature on the sign of
G and thus on the spontaneity of a process
at any temperature.
Temperature-independent cases (opposite signs)
1. Reaction is spontaneous at all temperatures
Ho lt 0, So gt 0 2. Reaction is
nonspontaneous at all temperatures Ho gt 0,
So lt 0
2 H2O2 (l) 2 H2O(l) O2 (g) Ho
-196 kJ and So 125 J/K
3 O2 (g) 2 O3 (g)
Ho 286 kJ and So - 137 J/K
Temperature-dependent cases (same signs)
3. Reaction is spontaneous at higher temperature
Ho gt 0 and So gt 0

4. Reaction is
spontaneous at lower temperature Ho lt 0 and
So lt 0
2 N2O(g) O2 (g) 4 NO(g) Ho
197.1 kJ and So 198.2 J/K
2 Na(s) Cl2 (g) 2 NaCl(s) Ho -
822.2 kJ and So - 181.7 J/K
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18.4
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What is the standard free-energy change for the
following reaction at 25 0C?
DG0 -6405 kJ
lt 0
spontaneous
18.4
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There is an important relationship between the
Gibbs Free Energy change and the equilibrium
constant. Go -RT ln K Homework -
pages 598 - 601 1, 2, 5, 6, 7, 10, 14, 15, 16,
19, 22, 44, 66
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The Relationship Between Go and K at 25oC
Go (kJ) K
Significance
200 9 x 10 -36
Essentially no forward reaction 100
3 x 10 -18 reverse reaction goes
to 50 2 x 10 -9
completion. 10 2 x
10 -2 1 7 x 10 -1 0
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Forward and reverse reactions -1
1.5 proceed to same
extent. -10 5 x 101 -50
6 x 108 -100 3
x 1017 Forward reaction goes to
-200 1 x 1035
completion essentially no
reverse reaction.
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DG DH - TDS
18.4
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Temperature and Spontaneity of Chemical Reactions
DH0 177.8 kJ
DS0 160.5 J/K
DG0 DH0 TDS0
At 25 0C, DG0 130.0 kJ
DG0 0 at 835 0C
18.4
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DG0 - RT lnK
18.4